Universe - The Definitive Visual Guide (September 2012)

["THE MOON 149 FAR SIDE southern hemisphere FAR SIDE southern hemisphere Mare Orientale South Pole\u2013 Aitken Basin TYPE Multi-ring basin AGE 3.8 billion years TYPE Impact crater DIAMETER 560 miles AGE 3.9 billion years (900 km) DIAMETER 1,550 miles (2,500 km) This multi-ring basin is half the size ROOK AND CORDILLERA MOUNTAINS The South Pole\u2013Aitken Basin is an SOUTH POLE of the near side\u2019s Mare Imbrium. It Orientale is surrounded by two huge circular immense impact crater, lying almost The massively cratered, cold lunar South Pole lies on the eastern limb of the far mountain ranges. The outer range is called entirely on the far side of the Moon. can only be glimpsed tangentially from Earth. side, and from Earth the Montes Montes Cordillera (above right) and the inner It stretches from just above the South NASA\u2019s Clementine mission provided the first Rook, the innermost eastern portion one is called Montes Rook (lower left). Pole to beyond the Aitken Crater, detailed map of the region in 1994. of the three distinct rings, can be which is close to the center of the clearly seen.This giant lunar bull\u2019s-eye below that depth should have been far side. South Pole\u2013Aitken is a the deep crater were richer in iron was formed by a massive asteroid, and excavated, but this deep rock has not staggering 1,550 miles (2,500 km) in than normal lunar surface material. two theories have been proposed to been found. Alternatively, the seismic diameter, and is over 7.4 miles (12 km) Iron oxide and titanium oxide explain the rings.The first has the shocks generated by the massive deep. It is one of the largest craters in abound. Impact geophysicists are impact excavating a deep transient impact could have briefly turned the the solar system, and is comparable in convinced that a normal impact could crater.The cracked inner walls of this surrounding rocks into a fluidized size to the Chryse Basin on Mars. It is not have produced a crater this large crater would have been unable to powder.Tsunami-type waves moved about 70 percent of the diameter of without digging up large amounts of support the weight of surrounding out through the pulverized rock but Moon.The asteroid that produced it rock from the mantle that lies below crust, so the rock slumped into the quickly became frozen, resulting in would have been over 60 miles the lunar crust. It may be that the hole, guided by a series of concentric three clearly visible mountain rings. (100 km) across. crater was produced by a low-velocity fault systems that account for the collision, with the impactor coming rings that remain. Not only was most Even though the basin was first into the surface at a low angle. Huge of the crater filled in, but the break- discovered in 1962, detailed amounts of material would have been up of subsurface rock allowed lava investigation only started when the from far below the lunar surface to Galileo spacecraft imaged the Moon blasted from the lunar surface seep up and fill in the central regions. in 1992, while on its way to Jupiter. and would have moved off However, the highland crust is about The South Pole\u2013Aitken Basin looked around the Moon\u2019s orbit. 37 miles (60 km) thick, and rock from darker than the rest of the far-side In the subsequent highland rocks, indicating that the 10 million years, this lower-crustal rocks at the bottom of debris would have collided with the Aitken Crater Moon, producing many new craters. South Pole LARGEST KNOWN IMPACT CRATER The South Pole\u2013 Aitken Basin, the largest impact scar on the Moon, is outlined on this relief map from the altimeter on NASA\u2019s Lunar Reconnaissance Orbiter. The lowest areas are shown in blue, while the highest areas are shown in red and brown. EXPLORING SPACE LOOKING FOR WATER LUNAR BULL\u2019S-EYE gamma-ray communications The South Pole-Aitken is one of the THE SOLAR SYSTEM Mare Orientale\u2019s concentric rings spectrometer antennae lowest regions on the Moon, and can be seen in this composite Lunar parts of it never see the Sun.Water Reconnaissance Orbiter image. extendable solar panels seeping up from cracks in the mantle, booms or released by an impact, will not be neutron able to escape from these \u201ccold traps.\u201d spectrometer In 1998, Lunar Prospector found hydrogen, thought to be from the LUNAR PROSPECTOR breakup of water ice, within these traps. Both the Chandrayaan-1 mission (2008) and LCROSS (2009) also indicate the presence of water in this region.","150 MARS MARS 38\u201339 Gravity, motion, and orbits MARS IS THE OUTERMOST of the four rocky planets.Also known 64\u201365 Celestial cycles as the Red Planet because of its rust-red color, it is named after 68\u201369 Planetary motion the Roman god of war. Its varied surface features include deep 100\u2013101 The history of the solar system canyons and the highest volcanoes in the solar system. 102\u2013103 The family of the Sun Although Mars is now a dry planet, a large body of evidence indicates that liquid water once flowed across its surface. ORBIT axis of rotation tilts 60\u00b0 from Mars has an elliptical orbit, so at its closest direction vertical approach to the Sun (perihelion) it receives of sunlight water ice 45 percent more solar radiation than at the concentrated at cold lower farthest point (aphelion).This means that the south polar 60\u00b0 latitudes surface temperature can vary from -195\u02daF region (-125\u02daC) at the winter pole, to 77\u02daF (25\u02daC) exposed to water ice during the summer. At 25.2\u02da, the current axial sunlight is still present ice-free at equator tilt of Mars is similar to that of Earth and, like water ice builds up Earth, Mars experiences changes in seasons as equator 45\u00b0 at colder the north pole, and then the south pole, points receives more north pole toward the Sun during the course of its orbit. sunlight than Throughout its history, Mars\u2019s axial tilt has at 60\u00b0 tilt water ice concentrated fluctuated greatly due to various factors, around north polar region including Jupiter\u2019s gravitational pull.These fluctuations have caused significant changes in climate.When Mars is heavily tilted, the poles are more exposed to CHANGES IN AXIAL TILT ice-free 35\u00b0 the Sun, causing water Water-ice distribution during a equator 25\u00b0 ice to vaporize and Martian winter in the northern build up around the hemisphere varies with the axial colder lower latitudes. tilt. The translucent white areas At a lesser tilt, water ice shown here represent thin ice that becomes concentrated melts during the summer, whereas at the colder poles. the thick white ice remains. axis tilts from NORTHERN NORTHERN vertical by SPRING WINTER 25.2\u00b0 EQUINOX SOLSTICE APHELION PERIHELION SPIN AND ORBIT 155 million 128 million Mars\u2019s orbit is miles (249 miles (207 highly eccentric million km) million km) compared to that of Earth, which means that Sun its distance from the Sun varies more during a Martian year. A Martian day is 42 minutes longer than an Earth day. NORTHERN Mars spins on SUMMER its axis every SOLSTICE 24.63 hours Mars orbits Sun NORTHERN in 687 Earth days FALL EQUINOX THE SOLAR SYSTEM STRUCTURE rock crust small, probably solid iron core Mars is a small planet, about half the size of Earth, and farther away from the Sun. Its size and distance mean that it has cooled more rapidly mantle of than Earth, and its once-molten iron core is probably now solid. Its silicate rock relatively low density compared to the other terrestrial planets indicates that the core may also contain a lighter element, such as sulfur, in the MARS INTERIOR form of iron sulfide.The small core is surrounded by a thick mantle, Mars has a distinct crust, composed of solid silicate rock.The mantle was a source of volcanic mantle, and core. The activity in the past, but it is now inert. Data gathered by the Mars core is much smaller in Global Surveyor spacecraft has revealed that the rocky crust is about proportion to Earth\u2019s, and 50 miles (80 km) thick in the southern hemisphere, whereas it is only has probably solidified. about 22 miles (35 km) thick in the northern hemisphere. Mars has the same total land area as Earth, since it has no liquid water on its surface.","MARS 151 MARS PROFILE AVERAGE DISTANCE FROM THE SUN ROTATION PERIOD 141.6 million miles (227.9 million km) 24.63 hours SURFACE TEMPERATURE ORBITAL PERIOD (LENGTH OF YEAR) \u2013195\u00baF to 77\u00baF (\u2013125\u00baC to 25\u00baC) 687 Earth days MASS (EARTH = 1) 0.11 DIAMETER 4,213 miles (6,780 km) GRAVITY AT EQUATOR (EARTH = 1) 0.38 VOLUME (EARTH = 1) 0.15 SIZE COMPARISON MARS EARTH NUMBER OF MOONS 2 OBSERVATION Mars is visible to the naked eye. It is brightest when at its closest to Earth, which is approximately once every two years. It then has an average magnitude of \u20132.0. ATMOSPHERE AND WEATHER Mars has a very thin atmosphere, which exerts an average pressure on the surface of about 6 millibars (0.6 percent of the atmospheric pressure on Earth).The atmosphere is mostly carbon dioxide, and it appears pink because fine particles of iron oxide dust are SAND DUNES suspended in it.Thin clouds of Looking down into a small impact crater frozen carbon dioxide and water in a southern upland area called Noachis Terra, NASA\u2019s Mars Reconnaissance Orbiter ice are present at high altitudes, captured these rippling sand dunes. The and clouds also form on high dunes were sculpted by Martian winds and peaks in the summer. Mars is a are shown here in enhanced color. The image cold, dry planet\u2014the average is about 0.6 miles (1 km) across. surface temperature is -81\u00b0F (-63\u00b0C)\u2014where it never rains, but in the winter clouds at the polar regions cause ground frosts. Mars has highly dynamic weather systems. In the southern spring and summer, warmer winds from the south blow into the northern hemisphere, stirring up local clouds of dust that can reach 3,000 ft (1,000 m) in height and last for weeks.The high-level winds can also create powerful dust storms that cover vast areas of the planet (see below). Mars also has low-level prevailing winds, which have sandblasted its surface for centuries, creating distinctive landforms (see photograph, above). carbon dioxide (95.3%) oxygen, carbon monoxide, ATMOSPHERIC and trace gases (0.4%) COMPOSITION The thin atmosphere of argon (1.6%) Mars is dominated by carbon nitrogen (2.7%) dioxide, with tiny amounts of nitrogen and argon and other gases, and some traces of water vapor. EVOLUTION OF A STORM SYSTEM THE SOLAR SYSTEM SCARRED SURFACE 1 On June 30, 1999, a storm 2 A giant, turbulent cloud of 3 Expanding rapidly, the storm 4 Six hours after the first image This mosaic of Viking Orbiter was taken, the storm was still images shows Mars\u2019s distinct red system developed over the north orange-brown dust was raised by swirled over the white ice cap gathering strength. coloration and reveals the vast extent of the Valles Marineris, polar region of Mars. high surface winds. (center, top). a system of valleys more than 2,500 miles (4,000 km) long.","","","THE LABYRINTH OF MARS Noctis Labyrinthus (which is Latin for \u201clabyrinth of the night\u201d) is a complex system of steep- walled canyons at the western end of the giant Valles Marineris rift valley on Mars (see p.156). It is thought to result from faulting when the giant volcanoes on the Tharsis ridge of Mars caused the crust to bulge in this area. Landslides can be seen on the valley floors.","","156 MARS TECTONIC FEATURES Mars has two areas of markedly different terrain. Much of the northern hemisphere is characterized by relatively smooth and low-lying volcanic plains.The older southern landscape is typically cratered highland.The boundary between the two is an imaginary circle tilted by about 30\u00b0 to the equator.The planet\u2019s major tectonic features are found within a region that extends roughly 30\u00b0 each side of the equator. It contains WESTERN FLANK OF OLYMPUS MONS Mars\u2019s main volcanic center, the Tharsis region, Tectonic features on Mars take on familiar and the Valles Marineris, the vast canyon system forms but are on a much grander scale than that slices across the center of the planet. those on Earth. This escarpment on the side of Olympus Mons is 4.3 miles (7 km) high. THARSIS MONTES THARSIS MONTES Pavonis Mons Ascraeus Mons TYPE Shield volcano TYPE Shield volcano AGE 300 million years AGE 100 million years DIAMETER 235 miles DIAMETER 285 miles (375 km) (460 km) A huge bulge in the western PIT CHAIN Ascraeus Mons is the northernmost of CALDERA hemisphere, commonly known as the A chain of pits lies in a shallow trough on the the three Tharsis Montes volcanoes. The caldera on the summit is made up of Tharsis Bulge, contains volcanoes of lower east flank. The pits and trough formed The three lie on the crest of the eight major depressions and has a nested various sizes and types, from large because the ground was either moved apart by Tharsis Bulge and form a line in a appearance (center). Its deepest point is tectonic forces or uplifted by molten rock. southwest\u2013northeast direction.The over 1.9 miles (3 km) below the rim. shields to smaller domes. line marks the position of a major rift Olympus Mons dominates Montes mountain range. Pavonis zone, long since buried under lava. the region. But three other Mons, situated on the equator, is the The three volcanoes grew by the volcanoes, which anywhere middle of the three. It is a shield gradual buildup of thousands of else would be considered volcano with a broad base and sloping individual and successive lava flows enormous, are also found sides and is similar to those found that came to the surface through the here.The three form a line in Hawaii on Earth.The volcano\u2019s rift zone. Ascraeus is the tallest of the and together make the Tharsis summit stands 4.3 miles (7 km) above three, rising about 11 miles (18 km) the surrounding plain and has a single above the surrounding plain. It has a CHANNELS caldera within a larger, shallow large number of lines and channels all These deep channels on the depression. Hundreds of narrow lava round the rim of the caldera, showing volcano\u2019s southern flank may have flows are seen to emanate from the the paths taken by flowing lava. started out as subsurface lava rim of the caldera, and others can be tubes whose roofs collapsed as traced back to pits situated close by. THARSIS MONTES pits developed over them. Arsia Mons TYPE Shield volcano AGE 700 million years DIAMETER 295 miles 475 km) Arsia Mons is second only to the LAYERED OUTCROP mighty Olympus Mons in terms of This outcrop of layered rock lies in a pit on volume. It is the southernmost of the volcano\u2019s lower west flank. The layers the three Tharsis Montes volcanoes, are thought to consist mostly of volcanic rock and its summit rises more than 5.6 formed by successive lava flows. miles (9 km) above the surrounding THE SOLAR SYSTEM SUMMIT DEPRESSION plain. Like the other two, it has a The summit caldera lies within summit caldera bigger than any a shallow depression that is known on Earth. Arsia Mons almost twice the caldera\u2019s size measures 75 miles (120 km) across and has faulted sides. and is surrounded by arc-shaped faults. Lava flows fan out down the volcano\u2019s shallow slopes.The lava is of basaltlike composition and of low viscosity, and the flows are shorter nearer the summit than on the lower flanks. CLOUDY SUMMIT Water-ice clouds hang over the volcano\u2019s summit\u2014a common sight every Martian afternoon in the Tharsis region.","MARS 157 THARSIS REGION on Earth. Olympus is one of the giant Different areas of its floor are EXPLORING SPACE shield volcanoes of the Tharsis region, associated with different periods of Olympus Mons which is home to the greatest number activity.The largest central area, which MARTIAN of volcanoes on Mars, including the is marked by ring-shaped faults, is METEORITES TYPE Shield volcano planet\u2019s youngest.Volcanoes evolve more recent, at 140 million years old. AGE 30 million years over long periods of time and can be Solidified basaltic lava covers the DIAMETER 403 miles inactive for hundreds of millions of The caldera is surrounded by a Tharsis region. Pieces of lava that (648 km) years. Olympus Mons is considered surface of wide terraces formed by lava flowed on the Martian surface as the youngest of the shield volcanoes. flows, crossed by thinner flows.These recently as 180 million years ago Olympus Mons is unquestionably The summit has a complex caldera. are encircled by a huge scarp, up to are now on Earth. Impactors hit the largest volcano in the Solar 3.7 miles (6 km) high.Vast plains, Mars and ejected them, and after System. Its height, of about termed aureoles, extend from the north journeys lasting millions of years, 15 miles (24 km), makes it the and west of the summit like petals from they fell to Earth as meteorites. tallest, and its volume is over 50 a flower.These regions of gigantic They include the Shergotty times that of any shield volcano ridges and blocks, whose origins meteorite (right), remain unexplained, extend outward which landed for up to 600 miles (1,000 km). in Shergahti, India, on August LAVA FLOWS 25, 1865. These lava flows and a collapsed lava tube (top left) on the southwest flank have been peppered by tiny impact craters. MIGHTY OLYMPUS This massive volcano is named after the mountaintop home of the gods and goddesses of Greek mythology. Broad lava-flow terraces surround the caldera at the volcano\u2019s summit. COMPLEX CALDERA LANDSLIDE CLIFFS THE SOLAR SYSTEM In this bird\u2019s-eye view of the Olympus Mons is bounded on all sides by 32-mile- (52-km-) wide nested steep cliffs, thought to have been caused caldera on the summit of Olympus by landslides. This close-up taken by Mars Mons, five roughly circular areas Reconnaissance Orbiter shows an area of of caldera floor can be seen. the cliffs about 0.6 miles (1 km) wide on the northern side of the volcano.","158 MARS VALLES MARINERIS MESA The Noctis Labyrinthus region This small mesa (a marks the western end of the system. Valles Marineris flat-topped hill) lies in This is a roughly triangular area of northwestern Candor intersecting rift valleys that form TYPE Canyon system Chasma in the central a mazelike arrangement.The eastern AGE About 3.5 billion Valles Marineris. Light- end of Valles Marineris is bounded years toned outcrops of layered by chaotic terrain of irregular LENGTH Over 2,500 miles sedimentary rock are appearance. Here, smaller canyons (4,000 km) exposed on the top. These and depressions give way to outflow may have formed from canyons.These carried ancient rivers Valles Marineris is the largest feature material deposited in a of water out of Marineris toward the formed by tectonic activity on Mars. lake in the chasma. Darker lowland region, Chryse Planitia, to It consists of a system of canyons that windblown ripples cover the north.This whole area has seen stretches for over 2,500 miles the surrounding plains. extensive water erosion; millions of (4,000 km), is up to 430 miles (700 cubic miles of material have been km) wide, and averages 5 miles (8 removed by water action. km) in depth.The Grand Canyon in Arizona is dwarfed by comparison; it The system\u2019s canyons are described is only about one-tenth as long and as chasma (plural chasmata) and are one-fifth as deep.Valles Marineris lies given identifying names.The main just south of the Martian equator, and chasma in the western part of the the system trends, very roughly, west system is Ius.The central complex is to east.The trend follows a set of made up of three parallel canyons, fractures that radiates from the Tharsis Bulge at Marineris\u2019s western end. which is a primarily water-eroded LAYERED DEPOSITS canyon. But water, as well as wind, has A detail of the floor of western Candor The origins of the system date played its part in the development of Chasma shows layered sedimentary rock. back a few billion years to when the the Marineris system. Buffeting Up to 100 layers have been counted, each canyons were formed by faulting.This winds, flowing water, and the collapse about 30 ft (10 m) thick. The layers may be contrasts with the Grand Canyon, of unstable walls have all widened and made from material deposited in an impact deepened the canyons. crater before the chasma formed. THE SOLAR SYSTEM SCARRED PLANET Valles Marineris extends about a quarter of the way around Mars. All but the extreme western end of the canyon system is shown here. This image covers the area from the equator (top) to 20\u00b0 south.","MARS 159 Ophir and Candor and, to their EASTERN EOS CHASMA EXPLORING SPACE south, Melas Chasma.The long Water flowed through Coprates Chasma stretches out to the this broad chasma, out of MARS EXPRESS\u2019S east, where it meets the broader Eos the Valles Marineris and STEREO CAMERA Chasma.The name of the whole into a series of valleys system,Valles Marineris, means and channels. The High Resolution Stereo \u201cValleys of the Mariner.\u201dThe Mariner Camera onboard Mars Express in this case is the Mariner 9 mission Mars and took began its two-year program to that mapped the entire surface of the first close-up map the entire Martian surface images of this in January 2004. Its nine charge- LOOKING WEST THROUGH OPHIR CHASMA area. More recent craft, such as coupled device sensors record data Over billions of years, Ophir Chasma has Mars Global Surveyor and Mars one line at a time. Downward, widened as its walls have collapsed and Express, have provided more detailed backward, and forward views are slumped downward, covering the floor coverage. For example, their surveys used to build up 3-D images.The with debris. Super Resolution Channel provides have revealed more detailed information. layered rock in the canyon walls that Digital Unit includes camera head could be a profile Camera Control of the different Processor lava flows that built the plains DUST STORM instrument frame Super that the canyons Wind blows through the canyons carrying dust. provides mechanical Resolution Channel cut through. The pinkish dust cloud at the bottom of this stability Rocks on the floor image is moving north across the junction of may have formed Ius Chasma and Melas Chasma. The higher, from windborne bluish-white clouds are water ice. dust layers or by deposits in ancient lakes that once filled the canyons. THE SOLAR SYSTEM","160 MARS THARSIS REGION BROKEN CRUST were produced STRESSED LANDSCAPE In this perspective in the process of the A fault system cutting across an Acheron Fossae view across the highly Tharsis uplift, and crustal ancient impact crater is evidence of the deformed area of cracks formed as hot mantle stress felt by the Martian crust. The TYPE Fault system Acheron Fossae, curved material pushed upward onto the crater floor has since been resurfaced AGE Over 3.5 billion years faults trending to the surface rock layer.The brittle Martian by material from outside the area. LENGTH 695 miles northwest dominate the scene. crust broke along zones of weakness (1,120 km) when the tension of the uplift and is located about 600 miles became too strong.The eroded walls, Acheron Fossae is a relatively high (1,000 km) north of Olympus Mons. the subdued nature of the high- area that has seen intense tectonic Acheron is part of a network of standing hills, and the wind activity in the past. It marks the fractures that radiates out from the etching on the flat surfaces northern edge of the Tharsis Bulge Tharsis Bulge\u2014a huge region of all confirm this is ancient uplift and volcanic activity. It can be terrain.This type of CLIFF FACE compared to the East African Rift on feature gets its name This is a close-up of the bright, Earth (see p.130), where continental from the Latin fossa, steep slopes of a scarp or cliff. plates have spread apart.The huge meaning \u201ctrough.\u201d Dark streaks on the cliff face may curved faults in the Tharsis Bulge In Greek mythology, be formed as the dust mantle that Acheron is the river covers the region gives way and that flows into Hades, produces a dust avalanche. the Underworld. GRABENS AND HORSTS The planetary crust has fallen between parallel faults to form grabens up to 1 mile (1.7 km) deep; remnants of the preexisting heights are termed horsts. CIMMERIA TERRA AONIA TERRA Apollinaris Patera Claritas Fossae TYPE Patera volcano TYPE Fault system AGE 900 million years AGE Over 3.5 billion years DIAMETER 184 miles LENGTH 1,275 miles (296 km) (2,050 km) This is an example of a type of MESAS AND TROUGHS Claritas Fossae is a series of roughly LAVA BLANKET volcano that was first identified on A group of mesas was created by pitting and northwest-to-southeast-trending The eastern part of Claritas Fossae (bottom) Mars. Known as patera volcanoes, erosion of the surface in an area north of linear fractures, which forms the meets the western part of Solus Planum they have very gentle slopes (with Apollinaris Patera. Windblown dust has filled southern end of the Tharsis Bulge. It (top). The lava from Solus has flowed over angles as low as 0.25\u00b0). Apollinaris the troughs between the mesas. is located south of the equator at the some of the older fractured terrain of Claritas Patera is one of the largest on the western end of the Valles Marineris. and surrounds some of the higher ground. planet, situated on the northern edge major volcano that is isolated from the The region is about 95 miles (150 km) of Cimmeria Terra, a few degrees two major volcanic regions of Tharsis, wide at its northern end and 340 as a result of enormous stresses south of the equator. It is the only to the northeast, and Elysium, to the miles (550 km) wide in the south. associated with the formation of the northwest. Apollinaris is a broad, Individual fractures range from a few Tharsis Bulge. As the crust pulled to tens of miles across.They formed apart, blocks of crust dropped roughly shield-shaped between two faults to form features volcano, reminiscent of an FAULTS IN CLARITAS FOSSAE called grabens. Crustal blocks that upturned saucer. It is only Running from the volcanic Tharsis Bulge, the remained in place or were thrown about 3 miles (5 km) high linear features in this image of Claritas Fossae up are termed horsts. Claritas Fossae and has a caldera about coincide with fractures in the Martian crust separates two volcanic plains: that of 50 miles (80 km) across. It produced by stretching forces. Solis Planum to the east and Daedalia appears to have been formed Planum to the west. by both effusive and THE SOLAR SYSTEM explosive activity. Lava flows are clearly visible beyond the summit. A cliff surrounding the caldera area is visible on the northern side, but has disappeared on the opposite side. It is buried under a fan of material whose surface is marked by broad channels. The fan material could have formed from flowing lava or volcanic rock fragments. SPLIT-LEVEL CALDERA The caldera has two different floor levels. It is partially hidden here by a patch of blue-white clouds. The summit area is pocked with impact craters.","161 FEATURES FORMED BY WATER Both liquid and solid water have formed and shaped surface features on Mars. Giant channel-like valleys emerge fully formed out of the landscape. Some of these were cut during catastrophic floods, others were formed by water flowing more gradually through networks of river valleys, and others still were carved by glaciers. Some features suggest Mars once had seas, although the evidence is inconclusive. However, any REULL VALLIS potential rivers and seas have long since vanished, and Long, wide river channels are etched only water ice remains, most markedly in the two ice into the surface, revealing that huge plateaus that cap the planet. volumes of water flowed across Mars billions of years ago. PLANUM BOREUM The North Polar Cap on Mars is This is when carbon LAYERED DEPOSITS an ice-dominated mound that stands dioxide in the atmosphere Layers of ice at the Martian north pole attest North Polar Region several miles above the surrounding condenses into frost and to past variations in the planet\u2019s climate. terrain. It consists of a virtually snow, and not only covers The layers are exposed at the edge of the ice TYPE Polar ice cap permanent cap of water ice, which is the water-ice cap, but also sheet, which slopes downhill from the bottom either covered by or free of a deposit the surrounding region, to the top in this image. The thickness of the AGE Under 2.5 billion of carbon-dioxide ice, depending on down to latitudes of about ice is about 0.6 miles (1 km). years the time of the Martian year.The cap 65\u00b0 north.When spring is roughly circular but\u2014as is also the turns to summer and millions of years to form, building DIAMETER 685 miles case for the South Polar Cap\u2014its the Sun is permanently in up at the rate of about 0.04 in (1,100 km) bright ice forms a distinctive swirling, the polar sky, its warmth (1 mm) per year. A study of these loosely spiral pattern when seen from evaporates the carbon layers will reveal the history of Two bright, white polar caps stand above (see p.163). dioxide and turns some of the Martian climate. out against the otherwise dark surface the water ice directly into of Mars.The one roughly centered on The entire region is in darkness vapor.The polar cap shrinks the North Pole is officially named for about six months during the until just water ice remains. Planum Boreum\u2014the Northern Plain\u2014 Martian winter. The cap is not made although it is generally exclusively of ice but consists referred to as the POLAR POLYGONS of layers of ice and layers of North Polar Cap. Both Polygon-shaped dusty sediment. Frost grains this and its southern structures, similar to form around small particles of counterpart are easy those found in Earth\u2019s dust during winter dust storms to detect from Earth, polar regions, pattern in much the same way that but spacecraft have also parts of Mars\u2019s polar hailstones form on Earth.These flown over the poles, landscape. On Earth, cover the ground until the frost allowing monitoring they form as a result is evaporated in the warmer of daily, seasonal, and of stresses induced by months, leaving a layer of dust. longer-term change. repeated freezing and The yards-deep layers take thawing of water. CLIFFS NEAR THE NORTH POLE THE SOLAR SYSTEM This close-up view of the Martian North Polar Cap shows water ice close to cliffs about 1.2 miles (2 km) high. Dark material in the caldera-like structures and dune fields could be volcanic ash.","162 MARS UTOPIA PLANITIA result of bursting are on a far LUNAR PLANUM Utopia Planitia bubbles of volcanic grander scale, the Kasei Valles TYPE Lowland plain gas. A thin layer of size of a town or TYPE Outflow channel AGE 2\u20133.5 billion years frost was also seen small city.The land AGE 3\u20133.5 billion years DIAMETER 2,000 miles by the craft, first in areas are 3\u201312 LENGTH 1,105 miles (3,200 km) (1,780 km) mid-1977, when miles (5\u201320 km) Utopia is one of the enormous Kasei, which takes its name from the lava-covered plains of the northern it covered the across, and the Japanese word for Mars, is the largest hemisphere.The giant Elysium outflow channel. Not only is it long, volcanoes are at its eastern perimeter. surface for about cracks between but parts of its upper reaches are over From above, it is possible to see that 125 miles (200 km) across and in places complex albedo patterns, polygonal 100 Earth days, and them are hundreds it is over 2 miles (3 km) deep.The fractures, and craters mark the vast catastrophic flooding that formed Kasei rolling plain. Down on the surface, then when it built of yards wide. was greater than any other known the landscape is uniformly flat and flood event on Mars, or Earth. Kasei rock-strewn. At least that is the view up again in May Earth\u2019s mud cracks originates in Lunar Planum, directly in northeastern Utopia, where Viking north of central Valles Marineris (see 2 landed on September 3, 1976. 1979, one Martian form as ground p.158), then flows across the ridged Angular boulders of basaltic rock plain to Chryse Planitia.Along its route cover the landing site, close to Mie year (23 Earth POLYGON TROUGH FLOOR dries through lie streamlined islands, isolated as the Crater. Small holes in the rocks are a months) later. This close-up of one of the huge surface water evaporation. water flow split and then rejoined. cracks that isolate polygon-shaped areas The Martian These giant polygons are not of land reveals bright, evenly spaced cracks could have unique to Utopia; windblown ripples of sediment. a similar origin. they are also seen Certainly, Mars on other northern plains such as has experienced the large-scale floods Acidalia and Elysium (below).They of water required. And dust-covered are polygon-shaped chunks of flat cemented rock found by Viking 2 land separated by huge cracks, or seemed to be held together by salts troughs, and are reminiscent of mud left behind as briny water vaporized. cracks seen in dried-up ponds on However, it has also been suggested Earth. Earth\u2019s polygonal patterns are that the polygons formed in other book- to table-sized; the Martian ones ways\u2014for example, in cooling lava. ICE ON THE ROCKS PLATEAU EDGE A coating of water ice covers the This image shows the steep edge of a valley volcanic rocks and soil at the Viking 2 in northern Kasei Valles. The plateau to the landing site. The ice layer is incredibly left is about 0.8 miles (1.3 km) higher than thin, no more than a thousandth of an the floor of the valley, similar to the depth inch (a fraction of a millimeter) thick. of the Grand Canyon on Earth. THE SOLAR SYSTEM ELYSIUM PLANITIA the coast of Earth\u2019s Antarctica.These XANTHE TERRA \u201cice plateaus\u201d are surrounded by bare Elysium Planitia rock.They formed when water Nanedi Valles flooded through a series of fractures TYPE Lowland plain in the Martian crust, creating a sea TYPE Outflow channel similar in size to Earth\u2019s North Sea. As AGE Under 2.5 billion the water froze, floating pack ice broke AGE 2\u20133.5 billion years years into rafts.These were later covered in dust from the nearby volcanoes, LENGTH 315 miles DIAMETER 1,860 miles and this coating protected them. (508 km) (3,000 km) Unprotected ice between the rafts vaporized into the atmosphere, leaving This major outflow channel lies in a The Elysium Planitia is an extensive bare rock around the ice plateaus. relatively flat area.There is no visible lava-covered plain just north of the source for the channel in the south, equator. It has been suggested that an ICE PLATEAUS AND IMPACT CRATERS but its snakelike route northward, area almost directly south of the great The darker-toned ice plateaus are a few across the cratered plains of Xanthe volcano Elysium Mons is a dust- tens of miles across. The relatively small Terra, is clearly seen before the channel covered frozen sea. It is dominated by number of impact craters in this area comes to a sudden stop. Nanedi Vallis irregular blocky shapes that look like appears to have undergone different the rafts of segmented sea ice seen off suggests a young surface. stages of flow. Initially, the meandering river almost created some oxbow lakes.Then, areas of riverbed drained and became the terraces now seen stranded between the main channel and the cratered plain above. A gully down the center of the channel indicates a final flow of water. TERRACING Nanedi Vallis formed by water flow over a long period. Terracing is evident in this image, and a portion of the narrow central channel is just visible (top right).","MARS 163 TERRA MERIDIANI global view of Mars UNIQUE METEORITE forms in liquid water, is exposed and but achieved This basketball-sized easily found on the surface.The Meridiani Planum prominence as the rock has an iron-nickel hematite could have been produced landing site and composition. It is not a from iron-rich lavas, but it is believed TYPE Highland plain exploration ground Martian rock but a that water was involved.This area is AGE Over 3.5 billion years for the Opportunity meteorite\u2014the first to dry now but was once soaking wet DIAMETER 680 miles rover.The plain is be found on a planet and could well have been the site of (1,100 km) about 15\u00b0 due west other than Earth. an ancient lake or sea about 3.7 billion of Schiaparelli years ago. Eroded layered outcrops In the westernmost portion of Terra Crater (see p.164). Smaller impact Volcanic basalt is found within the beyond the landing site support this Meridiani and just south of the craters pepper the area.They range area, but the region is of greatest theory and point to a deep and long- equator lies the high plain Meridiani from Airy, just 26 miles (41 km) across, interest because it contains ancient lasting volume of water as large as Planum. It does not stand out in the to much smaller bowl-shaped craters, layered sedimentary rock that includes Earth\u2019s Baltic Sea. At this time, Mars such as the 72-ft- (22-m-) wide Eagle the mineral hematite. Some of this must have been a much warmer and Crater where Opportunity landed. mineral, which on Earth almost always wetter place than it is today. HEAT SHIELD AND SCORCH MARK Subsurface pale dirt was spattered onto the plain when Opportunity landed. The remains of the craft\u2019s discarded heat shield are at left and center. PROMETHEI TERRA types seen on Mars. In the collapsed PLANUM AUSTRALE region at the southern base of the Reull Vallis volcano Hadriaca Patera, for example, South Polar Region it is a fully formed outflow channel. TYPE Outflow channel But small tributaries also feed into TYPE Polar ice cap AGE 2\u20133.5 billion years the main channel, as they would in a DIAMETER 587 miles runoff channel. And the main channel AGE Under 2.5 billion (945 km) has the features of a fretted years channel\u2014a wide, flat floor and steep Reull Vallis is one of the larger walls. Reull Vallis takes its name from DIAMETER 900 miles channels of the southern hemisphere. the Gaelic word for \u201cplanet.\u201d (1,450 km) It extends across the northern part of Promethei Terra, to the east of Hellas MERGING CHANNELS The South Polar Cap, known formally Basin (see p.165). Reull is thought to Reull Vallis (top left) is joined by a tributary, as Planum Australe (Southern Plain), have had a complex evolution because Teviot Vallis (right). The parallel structures is an ice-dominated mound a few it exhibits the characteristics in the fretted channel floor were possibly miles high. It consists of three of all three channel caused by a glacial flow of loose debris different parts. First is the bright polar mixed with ice. cap that is roughly centered on the South Pole.This is a permanent cap of water ice with a covering of SPIDERS ON MARS carbon-dioxide ice. Next are the These spiderlike features in the south polar scarps, made primarily of water ice, region of Mars were cut by dry ice (frozen which fall away from the cap to the carbon dioxide) as it turned to gas in the spring. surrounding plains.Thirdly, hundreds The channels are 3\u20136 ft (1\u20132 m) deep. of square miles of permafrost encircle the region.The permafrost is water carbon-dioxide ice mixed into the soil and frozen to frost covering the hardness of solid rock.The South Polar Cap shrinks and grows with the seasons like the North Polar Cap (see THE SOLAR SYSTEM p.161).Yet surprisingly the southern cap does not get warm enough in the summer to lose its carbon- dioxide ice covering. Dust storms that block out the Sun may keep the cap cooler than expected. water ice, SOUTH POLAR CAP no carbon Carbon-dioxide frost (shown as pink) dioxide covers over the water-ice cap (green-blue). Scarps of water ice at the edge of the cap slope toward the surrounding plains.","164 MARS IMPACT CRATERS The Martian surface is scarred by tens of thousands of craters, about 1,000 of which have been given names.They range from simple bowl craters, less than 3 miles (5 km) across, to basins hundreds of miles wide.The oldest craters are found in the southern hemisphere and have been eroded throughout their lifetimes. Their floors have been filled and their rims degraded, and the craters have become characteristically shallow. ANCIENT GEOLOGICAL FEATURE Smaller, fresher-looking craters have formed on top of Large impact craters such as Hale them.The ejecta has been distributed by flowing across (right) have had their central peaks the surface rather than being flung through the air. and terraced walls continuously eroded for up to 4 billion years. MERIDIANI PLANUM ARABIA TERRA Victoria Crater Schiaparelli Crater TYPE Crater TYPE Large crater AGE Under 100 million AGE About 4 billion years years DIAMETER 293 miles DIAMETER 0.5 miles (471 km) (800 m) Victoria is a small impact crater about CAPE ST. VINCENT This crater takes its name from the two-thirds the size of the Arizona At Cape St. Vincent, a rocky outcrop on the astronomer Giovanni Schiaparelli meteorite crater on Earth (see p.221). northern rim of Victoria, layers of bedrock are (see p.220), who spent much of his Victoria\u2019s beautifully scalloped edges topped by looser material thrown out by the working life studying Mars. It is a have been eroded by winds, gradually impact that formed Victoria. highly circular crater, as are most increasing its diameter, and like many Martian craters, although a significant Martian craters its floor is covered crater was explored by the Mars rover number are elliptical\u2014a rarity on with dunes of wind-blown dust.The Opportunity over one Martian year (or the Moon and Mercury. Schiaparelli two Earth years, from 2006 to 2008). straddles the equator and is the largest crater in the Arabia Terra. It is an old WIND EROSION Half of that time was spent crater, formed by an impacting body These layers of ancient rock sediments on driving along part of the crater\u2019s when the planet was young, and the floor of an impact crater lying within the rim before it carefully edged shows signs of degradation.The rim has northwestern rim of Schiaparelli have been down a slope into the interior at been smoothed down and in parts is eroded and exposed by the wind. an opening called Duck Bay. For completely missing. Any central peak the next Earth year, it examined in the crater has been SHALLOW CRATER rocky outcrops along the crater\u2019s obliterated. Material has Here, color is used to walls with the instruments on its been deposited within indicate altitude. The crater robot arm, finally driving out the crater, and smaller floor is at the same height again to resume its trek across craters have formed as much of the surrounding the Martian surface. across the whole area. terrain. Higher deposits are Wind continues to in green. The degraded rim DUNE-FILLED CRATER shape the landscape by (yellow) is only about 0.75 Rippling sand dunes cover the erosion and by moving miles (1.2 km) above the floor. floor of Victoria, as seen in this surface material. enhanced-color view from the Mars Reconnaissance Orbiter. THE SOLAR SYSTEM TERRA TYRRHENA MESAS Smooth-topped hills Huygens Crater (mesas) on the crater floor are left behind TYPE Multi-ringed crater as a former smooth layer of material is AGE About 4 billion years eroded to reveal a more rugged surface. DIAMETER 292 miles (470 km) time.The pattern of markings is Huygens is one of the largest impact reminiscent of craters in the heavily cratered dendritic drainage southern highlands of Mars. It was systems on Earth, formed during the period of intense which from above bombardment within the first look like the trunk and branches of a 500 million years of the planet\u2019s early tree. Dark material within this crater\u2019s history.The age of craters such as drainage channels was carried either Huygens is determined by counting by the draining water or by the wind. the number of craters that overlay their rims. Huygens has a second ring EASTERN RIM inside its mountainous rim.This has In this perspective view across Huygens\u2019s been filled by material carried into eastern rim (foreground) to the surrounding the ring.The rim is heavily eroded, terrain, a branchlike network of drainage and markings on it suggest that channels flows away from the rim, and small, surface water has run off it at some more recently formed craters can be seen.","165 HELLAS PLANITIA ROCK OUTCROPS Layered sedimentary Hellas Planitia rocks, which formed long after Hellas, lie TYPE Basin in an eroded region AGE About 4 billion years northeast of the DIAMETER 1,365 miles crater basin. Darker (2,200 km) windblown ripples mark the surface. ANCIENT BASIN The Hellas Basin is the largest impact Argyre Planitia (below). Over the past The original crater floor has been crater on Mars and possibly the 3.5\u20134 billion years, Hellas Basin has covered by volcanic and wind-borne largest in the solar system. It is the had its floor filled by lava and its deposits, and it also shows signs of dominant surface feature in the features changed by wind, water, and change by water and glacial ice. Dust southern hemisphere. It is not fresh crater formation. Despite all this, storms continue to shape the surface. immediately apparent that Hellas is some of its original features are still an impact crater. Indeed, its official visible. Its overall shape and the name, Hellas Planitia, indicates that remains of its rim can still be seen, as it is a large, low-lying plain.This can inward-facing, arc-shaped cliffs designation dates from over a century lying up to several hundred miles ago, when the Martian surface was beyond the rim.These are possibly observed only through Earth-based the remnants of multiple rings. telescopes and the true nature of this ERODED RIM vast, shallow feature This perspective view shows the northern was not known. rim\u2014the mountain range formed around the Hellas is the Greek crater as the planet\u2019s crust was lifted up at word for Greece. the time of impact. Whole portions of the rim are missing to the northeast and southwest. Particularly large craters that have been subsequently altered are termed basins.They are analogous to the maria on Earth\u2019s Moon.The term basin is also applied to the second-largest Martian crater, Isidis Planitia, and the third-largest, SIRENUM TERRA ARGYRE PLANITIA AONIA TERRA Nansen Crater Argyre Planitia Lowell Crater TYPE Large crater TYPE Basin TYPE Multi-ringed crater AGE About 4 billion years AGE About 4 billion years AGE About 4 billion years DIAMETER 50 miles DIAMETER 500 miles DIAMETER 126 miles (81 km) (800 km) (203 km) Martian impact craters were first Argyre is the third-largest crater on FROST IN THE SOUTHERN HILLS Erosion has changed Lowell since its identified in 22 images returned by Mars. Its floor has been flooded by Frost (mainly of carbon dioxide) covers an formation early in Mars\u2019s history.The Mariner 4 in 1966. Nansen Crater volcanic lava, and it has been heavily area of cratered terrain in the Charitum edges of both its outer rim and inner was among the first and was named eroded by wind and water. It is Montes in early June 2003, at which time ring have been smoothed out, and its after the Norwegian explorer Fridtjof speculated that in the distant past, the south polar frost cap had been retreating fine-grained ejecta soil has been Nansen. New craters continue to be water drained into the basin from the southward for about a month. blown around.The crater\u2019s appearance added to the list as a result of surveys south polar ice cap. Channels entering continues to undergo long-term by spacecraft.The Viking orbiter the basin at its southeastern edge and changes, but it also changes on a recorded this image of Nansen in 1976. CRATER DUNE FIELD others leading out from its northern short-term basis. Frost covers the The crater shows signs of erosion; its Argyre\u2019s floor and rugged highland rim edge reveal the water\u2019s route.The path crater\u2019s face in the winter months as walls have been nibbled by the wind. contain smaller craters. Some of these show cuts through the mountain ranges that the frost line extends north from the Smaller, sharply defined craters have signs of erosion. This one lying in the define the basin: the Charitum south polar region. punctuated the surrounding northwestern part of Argyre Basin contains Montes to the south and the terrain. A more recent crater has Nereidum Montes to the north. formed inside Nansen. Its central a dark dune field. dark floor could be volcanic basalt. THE SOLAR SYSTEM CRATER WITHIN A CRATER LOWELL IN WINTER","166 MARS MERIDIANI PLANUM Endurance Crater TYPE Bowl crater AGE Under 4 billion years DIAMETER 420 ft (130 m) This small and inconspicuous crater robotic rover Opportunity.The small Antarctic, is an almost circular crater BURNS CLIFF has been explored and investigated to craft just happened to land within bounded by a rim of rugged cliffs. Its This portion of the crater\u2019s southern inner a greater extent than almost any other roving distance of this football-field- inner walls slope down to the crater wall is called Burns Cliff. Forty-six crater on Mars. In early 2004, it did not sized crater when it made its scheduled floor, 66\u2013100 ft (20\u201330 m) below. Layers Opportunity images taken in November 2004 even have a name, but by the end of landing in the Meridiani Planum in of bedrock line the crater, some of combine to make this 180\u00b0 view. The wide- that year its rim, slopes, and floor had Mars\u2019s northern hemisphere. which are exposed; loose material and angle camera makes rock walls bulge all been imaged and examined by the sand dunes cover the rest of the floor. unrealistically toward the viewer. Endurance, named after the ship that carried Irish-born British Opportunity spent approximately Opportunity made its way down the explorer Ernest Shackleton to the six months exploring Endurance.The inner slope, examining rocks and soil rover started by traveling round the along its route. It headed toward the SAND DUNES southern third of the crater\u2019s rim; here crater\u2019s center but got less than halfway The center of the crater floor is covered by it crossed a region named Karatepe before doubling back; any farther and small sand dunes. The reddish dust has and traveled along the edge of Burns it might have gotten stuck in the sandy formed flowing tendrils, which are a few Cliff. It then retraced its route to enter terrain. It then exited the crater to inches to a yard or so deep. the crater on its southwestern limb. THE SOLAR SYSTEM DRAMATIC PANORAMA This approximately true-color view across Endurance Crater was taken by Opportunity\u2019s panoramic camera as the rover perched on the western rim. A dune field lies in the center of the crater.","MARS 167 move off across the adjoining WOPMAY ROCK EXPLORING SPACE flat plain, Meridiani Planum. The 3-ft- (1-m) wide rock Wopmay (below) is one of the MARTIAN BLUEBERRIES The exposed layers of rock in loose rocks on the crater floor. walls such as Burns Cliff reveal what The image coloring highlights Dark round pebbles nicknamed lies beneath the Martian surface, and bluish dots in the rock, which blueberries were found both what geological processes occurred are iron-rich spheres. The rover within and on the terrain outside there in the past.The composition of left wheel tracks in the soil Endurance Crater.The name is, rocks on the crater floor, including (left) as it drove away however, misleading; the pebbles, from Wopmay. which appear bluer than their surroundings, are in fact dark gray. those named The half-inch blueberries are rich Escher,Virginia, and in the mineral hematite, which is Wopmay, was analyzed and also found on Earth. Hematite the finer-grained floor material was usually forms in lakes and hot scrutinized. All the findings suggest springs on Earth, and this supports that water has affected the rocks both the idea that this part of Mars has before and after Endurance had a watery past. A second type was formed. of round pebble that is lighter- colored and rougher-textured has been nicknamed popcorn. EVIDENCE OF WATER A mixture of blueberries and popcorn lies on top of a rock called Bylot inside Endurance Crater. THE SOLAR SYSTEM","MARTIAN DUNE Mars is a dusty planet, and winds blow the dust around to form fantastic shapes, reminiscent of those seen in deserts on Earth. Shown here, on the floor of an old Martian crater called Arkhangelsky, is a barchan dune. Barchan dunes are arc-shaped, with two horns that point downwind and a steep slope between. This false-color image was taken by NASA\u2019s Mars Reconnaissance Orbiter.","","170 ASTEROIDS ASTEROIDS 34\u201337 Radiation ASTEROIDS ARE REMNANTS OF a failed SATURN\u2019S ORBIT 38\u201339 Gravity, motion, and orbits attempt to form a rocky planet that would 100\u2013101 The history of the solar system have been about four times as massive as EROS Earth.They are dry, dusty objects and Orbital period Meteorites 222\u201323 far too small to have atmospheres. 1.76 years Over 200,000 have been discovered, although over a JUPITER\u2019S ORBIT billion are predicted to exist.The astronomers who discover asteroids have the right to name them. ORBITS TROJANS Both groups of Trojans follow Jupiter\u2019s orbit Most asteroids are found in a concentration known as the EARTH\u2019S ORBIT Main Belt, which lies between Mars and Jupiter, about 2.8 times farther from the Sun than Earth.Typically, they take APOLLO between four and five years to orbit the Sun.The orbits are Orbital slightly elliptical and of low inclination. Even though the period 1.81 asteroids are all orbiting in the same direction, collisions at years velocities of a few miles per second often take place. So as time passes, asteroids tend to break up. Some asteroids CERES ICARUS SUN have been captured into rather strange orbits.The Trojans Orbital period have the same orbital period as Jupiter and tend to be 4.6 years Orbital either 60\u00b0 in front or 60\u00b0 behind that planet.Then there period are the Amor and Apollo asteroid groups (named after MAIN BELT 1.12 individual asteroids), with paths that cross the orbits of years Mars and Earth, respectively. Aten asteroids have such small orbits that they spend most of their time inside Earth\u2019s orbit.These three groups are classed as near- Earth asteroids.They can be dangerous, having the potential to hit Earth and cause a great deal of damage. Fortunately, this happens very rarely. THE SOLAR SYSTEM MARS\u2019S ORBITASTEROID PATH To picture stars, the Hubble Space Telescope scans the sky, keeping the stars stationary in the image frame. Asteroids, being much closer than the stars and in orbit around the Sun, form streaky trails (the blue line) during the exposure time. STRUCTURES direction AMOR of orbits Orbital period At the dawn of the solar system, there existed quite a 5.3 years few asteroids nearly as large as Mars.The radioactive CERES decay of elements within the asteroidal rock melted VESTA ASTEROID SHAPES AND SIZES these large bodies, and, during their fluid stage, IDA The largest asteroids, such as Ceres and gravity pulled them into a spherical shape Vesta, are nearly spherical, whereas before they cooled. Many of these have since smaller asteroids, such as Ida, are been broken up or reshaped by collisions irregularly shaped. All asteroids have with other asteroids. Smaller asteroids, craters on their surfaces, but some areas which cooled more efficiently than larger have been sandblasted and smoothed by ones, did not reach melting point and a multitude of minor collisions. retained a uniform rocky-metallic composition and their original irregular shape.There are three main compositional classes of asteroids.The vast majority are either carbonaceous (C-type) or silicaceous (S-type).The next most populated class is metallic (M-type).These classes correspond to carbonaceous chondrite (stony) meteorites, stony-iron meteorites, and iron meteorites.","ASTEROIDS 171 TROJANS ASTEROID ORBITS FRANZ XAVER VON ZACH Orbital period Asteroids tend to stay close to the plane of 11.87 years the solar system, and they orbit in the same Franz Xaver von Zach (1754\u20131832) was a direction as the planets. A few individual Hungarian baron and the director of the direction asteroid orbits are shown here, together with Seeberg Observatory in Germany. He of orbits the Main Belt. Asteroids often cross paths, became convinced that there was a missing suggesting that collisions are common. As planet orbiting between Mars and Jupiter. time passes, more and more asteroids are In September 1800, he organized a group produced, but the average size gets of 24 astronomers to help him look. smaller and smaller. Popularly known as the Celestial Police, they divided the celestial zodiac into 24 parts and started searching but were beaten by a nose by Giuseppe Piazzi\u2019s accidental discovery of the asteroid Ceres in 1801. The Police were surprised by how small Ceres was, and then were surprised again when more and more asteroids were found in similar orbits. HIDALGO COLLISIONS Orbital period 13.7 years The effect of a collision between asteroids depends on the sizes of the bodies involved. If a very small asteroid hits a larger one, it will produce a crater on the surface.This crater will be about 10 times the size of the incoming body. Since asteroids are much smaller than planets, the material blasted out of the crater will escape and move off into an independent orbit around the Sun.This orbit will, however, be very similar to that of the impacted asteroid, and there is a good chance that the ejected material will hit the cratered asteroid again. A bigger impactor can break up the asteroid that it hits. But so much energy is used to do this that the resulting fragments cannot escape from the gravitational field, and they will all fall back to form an irregular ball of rubble. Subsequent minor impacts will break up the surface, covering the asteroid in a rocky, dusty layer. A casual observer would not realize that the underlying asteroid is actually in pieces. A large impactor will not only shatter the asteroid, but the fragments will also escape.These will form a family of asteroids that eventually spreads out around the orbit of the original body. CRATERING ADONIS crater forms Orbital period 2.6 years asteroid forms dusty ball impacting of rubble asteroid is less than 1\/50,000th of size of larger body FRACTURING rocky body fractures impacting asteroid asteroid breaks ASTEROID COLLISIONS THE SOLAR SYSTEM is 1\/50,000th of size into fragments of There are far more small asteroids of larger body rock and dust than large ones. For every asteroid rocky body shatters more than 6 miles (10 km) along SHATTERING into pieces its longest axis, there are 1,000 longer than 0.6 miles (1 km) and impacting asteroid is family of asteroids forms a million longer than 0.06 miles more than 1\/50,000th (0.1 km). So cratering is much of size of larger body more common than fracturing, which in turn is much more common than shattering. Asteroids that are shattered are likely to be already fractured.","172 ASTEROIDS ASTEROIDS Mainly moving between the orbits of Mars and Jupiter, asteroids are the remnants of a planet-formation process that failed.Today\u2019s asteroid belt contains only about 100 asteroids that are larger than 125 miles (200 km) across. But there are 100,000 asteroids greater than about 12.5 miles (20 km) across and a staggering 1 billion that are over 1.25 miles (2 km) along their longest axis. Ceres, the first asteroid to be discovered, in EROS 1801, is now also called a dwarf planet (see Only asteroids bigger than about p.175). Ceres contains about 25 percent 215 miles (350 km) in diameter are of the mass of all the asteroids combined. spherical. Eros is an irregularly shaped fragment of a much larger body. MAIN-BELT ASTEROID MAIN-BELT ASTEROID as it passed within 2,050 miles NEAR-EARTH ASTEROID (3,300 km) on its way to Comet 951 Gaspra 5535 Annefrank Wild 2. Annefrank turned out to be 4179 Toutatis twice as large as had been predicted AVERAGE DISTANCE TO SUN AVERAGE DISTANCE TO SUN 206 million miles from Earth-based observations.The AVERAGE DISTANCE TO SUN 234 million miles (331 million km) brightness that is detected from Earth (376 million km) 206 million miles (331 million km) ORBITAL PERIOD 3.29 years is proportional to the reflectivity ORBITAL PERIOD 4.03 years ORBITAL PERIOD 3.29 years ROTATION PERIOD Not known multiplied by the surface area, but ROTATION PERIOD 5.4 and 7.3 days ROTATION PERIOD 7.04 hours LENGTH 3.7 miles (6 km) astronomers had used too high a value LENGTH 2.65 miles (4.26 km) LENGTH 11.2 miles (18 km) DATE OF DISCOVERY March 23, 1942 for the reflectivity.The asteroid was DATE OF DISCOVERY January 4, 1989 DATE OF DISCOVERY July 30, 1916 named after the famous diarist Anne Annefrank orbits in the inner regions Frank, who died during the Holocaust. Toutatis was named after a Celtic god Until 1991, asteroids could be glimpsed of the Main Belt of asteroids and is a only from afar. In October of that year, member of the Augusta family. On SURFACE BRIGHTNESS (who, incidentally, appears in the a much closer view was obtained when November 2, 2002, Annefrank was False colors (left) are used to the Galileo spacecraft flew within imaged by NASA\u2019s Stardust spacecraft highlight differences in brightness Asterix comic books). A typical near- 1,000 miles (1,600 km) of Gaspra, over the surface of the asteroid taking 57 color images. Gaspra is a (above). The variations are mainly Earth asteroid, it sweeps past the planet silicate-rich asteroid.The surface is due to dusty soil layers reflecting very gray, with some of the recently different amounts of sunlight nearly every four years. In September exposed crater edges being bluish and in different directions. some of the older, low-lying areas 2004, it came as close as just four appearing slightly red. times the distance IRREGULAR SHAPE of the Earth to the Moon.Toutatis is an S-class asteroid, similar to a stony- iron meteorite in composition. It tumbles in space rather like a rugby ball after a botched pass, spinning around two axes, with periods of 5.4 and 7.3 days. RADAR IMAGE MAIN-BELT ASTEROID MAIN-BELT ASTEROID MAIN-BELT ASTEROID 2867 \u0160teins 21 Lutetia 253 Mathilde AVERAGE DISTANCE TO SUN AVERAGE DISTANCE TO SUN 227 million miles AVERAGE DISTANCE TO SUN 246 million miles (365 million km) (396 million km) 220 million miles (354 million km) ORBITAL PERIOD 3.80 years ORBITAL PERIOD 4.31 years ORBITAL PERIOD 3.63 years ROTATION PERIOD 8.17 hours ROTATION PERIOD About 418 hours ROTATION PERIOD 6.05 hours DIAMETER 75 miles (121 km) LENGTH 41 miles (66 km) DIAMETER 4.14 miles (6.67 km) DATE OF DISCOVERY November 15, 1852 DATE OF DISCOVERY November 12, 1885 DATE OF DISCOVERY November 4, 1969 THE SOLAR SYSTEM Some asteroids are not solid but consist Lutetia was the second of two asteroids ROSETTA\u2019S VIEW Mathilde was visited by the NEAR of rock fragments with gaps between. visited by the European Space Agency\u2019s Lutetia was photographed by Shoemaker space probe in 1997 but, One such example\u2014termed a \u201crubble Rosetta space probe, the first being Rosetta at its closest approach because it spins very slowly, only about pile\u201d\u2014is \u0160teins, which the Rosetta 2867 \u0160teins (see left). At of 1,970 miles (3,170 km) in the half of the surface was imaged. It is a space probe showed to be shaped like a over 60 miles (100 km) top image. Craters and grooves primitive carbonaceous asteroid with a cut diamond.The impact that produced across, Lutetia is one of on the asteroid\u2019s surface are density much lower than that of most its largest crater is thought to have the larger asteroids and visible in the close-up (left). rocks, suggesting that it is full of holes. fractured the asteroid. is also one of the most Mathilde is probably a compacted dense, suggesting that it Some of its craters have been partly or pile of rubble. DIAMOND IN THE SKY contains large amounts completely buried by landslides set off of iron and might once by the vibrations from later impacts. WEDGE-SHAPED CRATER have had a molten core. Lutetia seems to be a link between Rosetta\u2019s images, taken in small \u201crubble pile\u201d asteroids and July 2010, showed hundreds of craters terrestrial planets such as Earth. up to 34 miles (55 km) wide and boulders as large as 1,000 ft (300 m) across on Lutetia\u2019s battered surface. Lutetia may have been almost spherical before parts of it were chipped off.","ASTEROIDS 173 MAIN-BELT ASTEROID family. Asteroidal families were DACTYL discovered by the Japanese astronomer At just 1 mile (1.6 km) long, Dactyl is tiny. 243 Ida Hirayama Kiyotsugu in 1918. He Its orbit around Ida is nearly circular, found that there were groups of with a radius of about 56 miles (90 km) AVERAGE DISTANCE TO SUN 266 million miles asteroids with very similar orbital and an orbital period of about 27 hours. (428 million km) parameters.The individual members were strung out on one orbit and asteroid like Gaspra (see opposite), but ORBITAL PERIOD 4.84 years formed a stream of minor bodies in observations revealed that its density is the inner Solar System (see p.170). too low and it is more likely to be a ROTATION PERIOD 4.63 hours Koronis is the most prominent C-type asteroid. It has about five member of Ida\u2019s family. times more craters per unit area than LENGTH 37 miles (60 km) Gaspra, indicating that its surface is Ida is famous because the Galileo considerably older.The most exciting DATE OF DISCOVERY September 29,1884 spacecraft imaged it in detail as it outcome of the Galileo flyby was the flew within 6,800 miles (11,000 km) discovery that Ida has its own moon, Ida was one of 119 asteroids discovered during August 1993, on its way to Dactyl.This binary system is thought by the Austrian astronomer Johann Jupiter. Because Ida makes a complete to have been formed during the Palisa, who, together with Max Wolf rotation every 4 hours 36 minutes, asteroid collision and breakup that of Heidelberg, Germany, was a Galileo was able to image most of the created the Koronis family. pioneer in the use of photography to surface during the flyby. Ida was produce star maps and hunt for minor originally thought to be an S-type planets (another name for asteroids). Ida is a member of the Koronis IDA AND ITS MOON THE SOLAR SYSTEM Dactyl was the first asteroid satellite to be discovered. Ann Harch, a Galileo mission member, noticed it when examining images that had been stored on the spacecraft when it passed Ida six months earlier.","174 ASTEROIDS MAIN-BELT ASTEROID Rheasilvia has a mountainous central to the lava that spews out METEORITE peak. It overlaps an older and slightly of Hawaiian volcanoes. In This meteorite, which 4 Vesta smaller crater, which is 230 miles its early life,Vesta melted landed in Western (375 km) wide. Some of the fragments and then resolidified, with Australia in October AVERAGE DISTANCE TO SUN 219 million miles of Vesta\u2019s crust produced by the denser material sinking to the 1960, originated from Vesta. (353 million km) cratering process are still trailing Vesta centre. It now has a layered on similar orbits. Others have hit structure like the rocky planets, with heavily cratered, with grooves running ORBITAL PERIOD 3.63 years Earth and been recognized as strange a low-density crust lying above layers around the equator, possibly fractures meteorites with a composition similar of pyroxene and olivine and an iron from the south-polar impact. One ROTATION PERIOD 5.34 hours to the igneous rock basalt. Six percent core.Vesta is thought to be the only of the asteroid\u2019s most distinctive of Earth\u2019s recent meteorite falls are remaining differentiated asteroid in features was a chain of three craters DIAMETER 330 miles (530 km) Vesta-like in their mineralogical the Main Belt. It is one of the densest nicknamed the Snowman, seen in the makeup.The composition is similar asteroids known, with a density image below. From top to bottom, DATE OF DISCOVERY March 29, 1807 similar to that of Mars. In July 2011, the individual craters are called NASA\u2019s Dawn spacecraft went into Minucia, which is 13 miles (21.5 km) Vesta is one of the largest asteroids, orbit around Vesta, beginning a year- wide; Calpurnia, 30 miles (50 km) with a surface that reflects, on average, long study of the asteroid.Vesta\u2019s wide; and Marcia, 36 miles (58 km) 42 percent of the incoming light.This surface turned out to be old and wide.The Dawn spacecraft\u2019s makes it the brightest asteroid in the instruments are studying the surface night sky and the only one that is SOUTH-POLAR IMPACT BASIN composition of Vesta, with the visible to the unaided eye. Most A giant impact basin called Rheasilvia at objective of matching meteorites asteroids of Vesta\u2019s size are expected the south pole of Vesta is seen here in a to specific areas on the asteroid. to be nearly spherical, but Vesta\u2019s false-color image taken by NASA\u2019s Dawn One likely source is the south-polar shape has been distorted by a massive spacecraft. It is 300 miles (500 km) across mountain, which rises 13 miles impact at the south pole that created and has a central peak (shown in red) and a (22 km) within the Rheasilvia a huge basin called Rheasilvia, which rim that is 9 miles (15 km) high. impact basin. measures some 300 miles (500 km) across\u2014almost as wide as Vesta itself. Like many large craters on the Moon, SNOWMAN CRATERS The three craters that form the \u201cSnowman\u201d are at the bottom left in this image. The smoother surface around them is thought to be a blanket of debris thrown out by the impacts. THE SOLAR SYSTEM","ASTEROIDS 175 MAIN-BELT ASTEROID of Mars and Jupiter (see pp.170\u201371). Using spectroscopes to analyze the EXPLORING SPACE By 1800, some of Europe\u2019s leading light reflected from their surfaces, 1 Ceres astronomers had started to look for astronomers found that asteroids had PIAZZI\u2019S TELESCOPE objects in this gap, and Piazzi made different colors, due to different AVERAGE DISTANCE TO SUN 257 million miles the first discovery. About a year later, compositions, and this led to the Known as the Palermo Circle, (414 million km) a German doctor and astronomer, establishment of a classification system this telescope was made between Heinrich Olbers, was observing the (see p.170). Ceres was classified as a 1787 and 1789 by Jesse Ramsden ORBITAL PERIOD 4.60 years path of Ceres in an attempt to carbonaceous, or C-type, asteroid. of London, England, the greatest produce a more accurate estimate European instrument maker of ROTATION PERIOD 9.08 hours of its orbital parameters when he In 2006, the new category of the 18th century. Its lens has an discovered a second asteroid.This was dwarf planets was introduced to aperture of 3 in (75mm).The DIAMETER 590 miles (950 km) given the number 2 and named Pallas. describe objects that are rounded circular altitude scale and the The orbits of Ceres and Pallas were in shape but have not swept their horizontal azimuth scale are both DATE OF DISCOVERY January 1, 1801 found to cross, and Olbers concluded orbits clear of other bodies (see read using microscopes. In that they were fragments of a planet p.209). Ceres was placed in this its day, it was the most southerly Ceres was discovered by accident in that had broken up. As the century category. However, it remains the European telescope, being on top 1801 by Giuseppe Piazzi, the director progressed, more and largest member of the asteroid belt, of the Royal Palace in Palermo, of the Palermo Observatory in Italy, more asteroids were Sicily.While measuring star while he was compiling a catalog discovered that so it can be said to have a dual positions with this telescope, Piazzi of fixed stars (see panel, right). One of were smaller identity as both a dwarf planet discovered the first asteroid, Ceres. the \u201cstars\u201d had moved during the night, and fainter and an asteroid. In 2015 and this turned out to be the first than the Ceres is due to be visited known asteroid, Ceres. Some 100 years first two. by a NASA space probe before, Johannes Kepler (see p.68) had called Dawn, which suspected that there was a \u201cmissing\u201d will fly there after planet in the gap between the orbits having studied Vesta (see opposite). CERES IN THE HYADES When viewed from Earth, Ceres, highlighted here in the Hyades star cluster in the constellation Taurus, moves nearly a quarter of a degree (half a Moon diameter) per day against the background stars. Hence its motion can easily be noticed from night to night, although a pair of binoculars or a small telescope is needed to see it. NEAR-EARTH ASTEROID Hayabusa, which the agency launched across. Large impacts broke the ELONGATED ASTEROID in May 2003 (see panel, below). At asteroid into smaller pieces, which This image of Itokawa was taken by the 25143 Itokawa the time of launch, the asteroid bore then gently reassembled to form the Hayabusa probe, which landed in the smooth only the reference number 1998SF36, low-density rubble-pile structure area near the middle to collect dust samples. ttgt(198 million km) but it was named in honor of Hideo that we see today. The asteroid lacks obvious impact craters, in ORBITAL PERIOD 1.52 years Itokawa (1912\u201399), known as the contrast to most others visited by spacecraft. ROTATION PERIOD 12.1 hours father of Japanese rocketry, while the Rocks that are up to 165 ft (50 m) LENGTH 0.34 miles (0.54 km) probe was on its way. wide are dotted over Itokawa\u2019s surface, ROUGH SURFACE DATE OF DISCOVERY September 26, 1998 while the narrow neck near the Large rocks can be seen strewn over The Hayabusa probe reached middle is smoother and covered Itokawa\u2019s surface in this close-up. These Itokawa is a small, irregularly shaped Itokawa in September 2005 and with dust. In November 2005, the rocks are probably fragments of an earlier asteroid of the type known as a spent two months surveying the Hayabusa space probe gently touched breakup of the asteroid that have since \u201crubble pile,\u201d meaning that it is not asteroid before attempting a landing. down on the smooth part. It collected collected together again. a solid, coherent body.The Japan The asteroid\u2019s gherkinlike shape, numerous microscopic particles of Aerospace Exploration Agency 1,110 ft (540 m) long and 690 ft the asteroid\u2019s dust, which it brought (JAXA) chose it as the target for (210 m) across at its narrowest, gives it back to Earth.The sample return a sample-collection mission called the appearance of two separate masses capsule blazed through the Earth\u2019s stuck together. Astronomers think atmosphere to land at Woomera, that Itokawa was once much bigger, Australia, in June 2010.The capsule perhaps up to 12 miles (20 km) was retrieved and opened under sterile conditions in a laboratory in EXPLORING SPACE Japan.The asteroid\u2019s surface rocks THE SOLAR SYSTEM proved to be rich in the mineral THE HAYABUSA MISSION olivine, similar to common types of chondrite meteorites. Hayabusa was a Japanese probe sent to rendezvous with, and bring back samples from, the near-Earth asteroid Itokawa. Hayabusa is the Japanese name for the peregrine falcon. It was intended to swoop down on the asteroid like a hawk, to take samples, and then return them to Earth. It spent 30 minutes on the asteroid\u2019s surface before taking off again with dust samples. ASTEROID APPROACH An artist\u2019s impression shows the Japanese space probe Hayabusa approaching asteroid Itokawa in November 2005.","176 ASTEROIDS NEAR-EARTH ASTEROID 433 Eros AVERAGE DISTANCE TO SUN 136 million miles (218 million km) ORBITAL PERIOD 1.76 years ROTATION PERIOD 5.27 hours LENGTH 19.25 miles (31 km) DATE OF DISCOVERY August 13, 1898 Lying in near-Earth orbit, outside the Main Belt, Eros is usually closer to the Sun than to Mars (see p.170). Its orbit also brings it close to Earth; COMPUTER MODELS at the last close approach, in 1975, The gravity on Eros is about 1\/2,000th of that Eros came within 14 million miles on Earth, but varies by nearly a factor of two (22 million km) of the planet.The from place to place. The colors in this image orbit is unstable, and Eros has a one- represent the rate at which a rock would roll in-ten chance of hitting either Earth downhill. It would roll fastest in the red areas, or Mars in the next million years. In and it wouldn\u2019t move at all in the blue areas. 1960, Eros was detected by radar, and infrared measurements taken in 160,000 images were the 1970s indicated that recorded.They revealed the surface was not just an irregularly shaped bare rock but was body, which had heavily covered by a thermally cratered 2-billion-year- insulating blanket of old areas lying next to dust and rock fragments. relatively smooth Eros was the first regions. Even though asteroid to be orbited the gravitational field is by a spacecraft and the CLOSING IN very small, several first to be landed on. It was chosen for NEAR Shoemaker took this thousand boulders larger image from a height of 3,770 ft than 50 ft (15 m) across close study because (1,150 m) shortly before it have fallen back to the it is big and nearby. touched down on Eros. surface after being On February 14, ejected by impacts, and 2000, the Near Earth Asteroid some surface dust has rolled down the Rendezvous (NEAR) spacecraft slopes to form sand dunes a few yards (renamed NEAR Shoemaker in (meters) deep. Laser measurements of March 2000) went into orbit around the NEAR\u2013Eros distance as the space- Eros. It landed 363 days later. About craft orbited have not only produced an accurate map of the asteroid\u2019s shape ROCK AND REGOLITH but also indicated that the interior is Some of the rocks nearly uniform, with a density about and regolith on Eros\u2019s the same as that of Earth\u2019s crust. Eros surface have a red is not a pile of rubble like Mathilde; coloring. The longer it is a single, solid lump of rock. their exposure to The spacecraft\u2019s gamma-ray minor impacts and spectrometer worked for two weeks the solar wind, the after touchdown. Eros was found to redder they appear. be silicate-rich and highly reflective. THE SOLAR SYSTEM THE SADDLE Four images have been combined to produce this view of the \u201csaddle\u201d region at the south of the asteroid. This 6-mile- (10-km-) wide scoop, which has been named Himeros, is relatively boulder-free, unlike the region at the lower right of the frame.","177THE SOLAR SYSTEM THE COSMIC POTATO Eros is elongated and irregular\u2014like a cosmic potato. Its shape is the result of a series of vigorous collisions. Large impacting objects have created craters all over Eros, and dust impacts have sandblasted the surface, smoothing it off. This view of Eros was taken looking down on the north polar region.","178 JUPITER JUPITER 38\u201339 Gravity, motion, and orbits JUP ITER IS THE LARGEST AND M OST M ASSIVE of all the 68\u201369 Planetary motion planets. It has almost 2.5 times the mass of the other 100\u2013101 The history of the solar system eight planets combined, and over 1,300 Earths 103 Gas giants could fit inside it. Jupiter bears the name of the most important of all the Roman gods (known as Zeus in Greek mythology).The planet has the largest family of moons in the solar system, its members named after Jupiter\u2019s lovers, descendants, and attendants. ORBIT Jupiter is the fifth planet from the Sun. It lies approximately five times as far away as Earth, but its distance from the Sun is not constant. Its orbit is elliptical and there is a difference of 47.3 million miles (76.1 million km) between its aphelion and perihelion distances. Jupiter\u2019s spin axis tilts by 3.1\u00b0, and this means that neither of the planet\u2019s hemispheres point markedly toward or away from the Sun as it moves around its orbit. Consequently, Jupiter does not have obvious seasons.The planet spins quickly around its axis, more quickly than any other planet. Its rapid spin throws material in its equatorial region outward.The result is a bulging equator and a slightly squashed appearance. spins on its axis tilts from the axis once vertical by 3.1\u00b0 every 9.93 hours APHELION PERIHELION 507.1 million miles 459.9 million (816.6 million km) miles (740.5 million km) Sun Jupiter orbits SPIN AND ORBIT the Sun in 11.86 The rotation period of just less than Earth years 10 hours and orbital period of nearly 12 Earth years means that there are about STRUCTURE 10,500 Jovian days in one Jovian year. THE SOLAR SYSTEM Although it is the most massive planet (318 times the gaseous hydrogen mass of the Earth), Jupiter\u2019s great size means that its and helium density is low. Its composition is more like the Sun\u2019s than any other planet in the solar system. Jupiter\u2019s outer layer of hydrogen and helium is in a gaseous form in the liquid hydrogen outer part of the planet, where the temperature and helium is about -166\u02daF (-110\u02daC). Closer to the center, the pressure, density, and temperature inner layer of increase.The state of the hydrogen and metallic hydrogen helium changes accordingly. By about 4,350 miles (7,000 km) deep, at about core of rock, metal, and 3,600\u02daF (2,000\u02daC), hydrogen acts more hydrogen compounds like a liquid than a gas. By 8,700 miles (14,000 km), at about 9,000\u02daF JUPITER INTERIOR (5,000\u02daC), hydrogen has compacted to At the heart of Jupiter lies a relatively metallic hydrogen and acts like a molten small, dense, and probably solid core. metal. Deep inside, at a depth of about The core is surrounded by layers of 37,260 miles (60,000 km), is a solid core of metallic, liquid, and gaseous material, rock, metal, and hydrogen compounds.The which is predominantly hydrogen. core is small compared to Jupiter\u2019s great size but is about 10 times the mass of Earth.","JUPITER 179 GAS GIANT JUPITER PROFILE Jupiter\u2019s surface is not solid. Each light or dark band and every big or small swirl or spot is a part of the planet\u2019s cloudy atmosphere. AVERAGE DISTANCE FROM THE SUN ROTATION PERIOD 483.6 million miles (778.3 million km) 9.93 hours CLOUDTOP TEMPERATURE ORBITAL PERIOD (LENGTH OF YEAR) \u2013160\u00baF (\u2013110\u00baC) 11.86 Earth years MASS (EARTH = 1) 318 DIAMETER 88,846 miles (142,984 km) CLOUD-TOP GRAVITY (EARTH = 1) 2.53 VOLUME (EARTH = 1) 1,321 SIZE COMPARISON NUMBER OF MOONS 64 EARTH JUPITER OBSERVATION Jupiter is bright and easy to spot. It has a maximum magnitude of \u20132.9. Even at its faintest it is brighter than Sirius, the brightest star in the sky. Jupiter is best seen at opposition, which occurs once every 13 months. MAGNETIC FIELD Jupiter has a magnetic field\u2014it is as if the planet had a large bar magnet deep inside it.The field is generated by electric currents within the thick layer of metallic hydrogen, and the axis joining the magnetic poles is AURORA tilted at about 11\u00b0 to the spin axis. This striking electric-blue The field is stronger than that of aurora centered on Jupiter\u2019s any other planet. It is about 20,000 north magnetic pole was imaged by the Hubble Space times stronger than Earth\u2019s magnetic Telescope in 1998. field and has great influence on the volume of space surrounding Jupiter. Solar wind particles (see p.107) streaming from the Sun plow into the field.They are slowed down and rerouted to spiral along the field\u2019s magnetic lines of force. Some particles enter Jupiter\u2019s upper atmosphere around its magnetic poles.They collide with the atmospheric gases, which radiate and produce aurorae. Other charged particles (plasma) are trapped and form a disklike sheet around Jupiter\u2019s magnetic equator. Electric currents flow within this sheet. High- energy particles are trapped and form radiation belts, similar to, but much more intense than, the Van Allen belts round Earth (see p.125). Jupiter\u2019s magnetic field is shaped by the solar wind, forming a vast region called the magnetosphere. Its size varies with changes in pressure of the solar wind, but the tail is thought to have a length of about 370 million miles (600 million km). axis of rotation solar wind direction of magnetic force lines deflected plasma sheet axis of THE SOLAR SYSTEM magnetic field northern horn solar wind JUPITER\u2019S MAGNETOSPHERE bow shock tail Jupiter\u2019s magnetosphere\u2014the bubblelike southern horn region around Jupiter dominated by the turbulence magnetic equatorial plane planet\u2019s magnetic field\u2014is enormous; it is radiation belt one thousand times the volume of the Sun, solar wind deflected magnetosheath and its tail stretches away from the planet as far as Saturn\u2019s orbit. This slice through the magnetosphere reveals its structure.","180 JUPITER north polar region ATMOSPHERE rising air forms air flow diverted zone of white to the east by the Jupiter\u2019s atmosphere is dominated by ammonia clouds Coriolis effect North Temperate Belt hydrogen, with helium being the next red-brown cloud belt most common gas.The rest is made up CLOUD FORMATION storm system of simple hydrogen compounds\u2014 such Clouds of different compounds form at different altitudes in North Tropical as methane, ammonia, and water\u2014and the atmosphere. Convection Zone (includes currents move the mixture of the paler bands more complex ones such as ethane, gas upward. Water is first to above and below) reach the altitude where it is acetylene, and propane. It is cool enough to condense to form clouds. Higher up, these compounds that condense descending where it is cooler, red-brown cooler air ammonium hydrosulfide to form the different-colored clouds form, and highest of all, where it is coolest, are North clouds of the upper atmosphere westward air the white ammonia clouds. Equatorial and help give Jupiter its distinctive flow Belt banded appearance.The temperature Equatorial Zone of the atmosphere increases toward the South planet\u2019s interior. As gases condense at different Equatorial Belt temperatures, different types of clouds form at specific altitudes. All the while, the gas in Jupiter\u2019s equatorial region is heated by the Sun, and this water vapor at rises and moves toward the polar regions. Cooler lower altitude air flows from the polar regions at a lower altitude to take its place, creating in effect a large circulation cell.This hemisphere-wide circulation transfer would be Great Red straightforward if Jupiter were stationary. It is not\u2014it Spot rotates, and speedily at that, and a force known as the South Tropical Zone (includes the Coriolis effect (see p.126) deflects the north\u2013south flow paler band above) into an east\u2013west flow. As a result, the large circulation cell is split into many smaller cells of rising and falling air.These are seen on Jupiter\u2019s surface as alternating bands of color. South Jupiter\u2019s white bands of cool rising air are called zones.The Temperate Belt red-brown bands of warmer falling air are known as belts. BELTS AND ZONES hydrogen helium with traces of methane COMPOSITION OF This mosaic of images taken by the Cassini (89.8%) and ammonia (10.2%) ATMOSPHERE spacecraft at a distance of 6 million miles Hydrogen dominates, but (10 million km) shows the colorful bands of south it is the trace compounds Jupiter\u2019s upper atmosphere as they would polar region that color Jupiter\u2019s upper appear to the human eye. atmosphere. MOONS TRIPLE ECLIPSE Three shadows were cast onto Jupiter\u2019s Jupiter has over 60 known moons, over two-thirds of which have been surface on March 28, 2004 as its three discovered since January 2000. Only 50 of the moons have been given largest moons passed between the planet names, and several have yet to have their orbit confirmed.The recent and the Sun. Io is the white circle in the discoveries are typically irregularly shaped rocky bodies a few miles center, its shadow to its left. Ganymede is across, and are thought to be captured asteroids. By contrast, Jupiter\u2019s the blue circle at upper right, and its shadow four largest moons are spherical bodies that were formed at the same lies on Jupiter\u2019s left edge. Callisto\u2019s shadow time as Jupiter. Collectively known as the Galilean Moons (see p.182), is on the upper-right edge, but the moon they were the first moons to be discovered after Earth\u2019s Moon. As they itself is out of view, to the right of the planet. orbit Jupiter, passing between it and the Sun, their shadows sweep across the planetary surface; from within the shadow, the Sun appears + Lysithea 163.9 eclipsed. A triple eclipse happens just once or twice a decade. Elara 164.2 JUPITER\u2019S MOONS Callisto 26.3 Themisto 105.0 Himalia 160.3 Europa 9.4 THE SOLAR SYSTEM Ganymede 15.0 Leda 156.2 1 25 50 75 100 125 150 175 2 radius Io 5.91 + Adrastea 1.80 + S\/2003 J9 313.9 S\/2003 J19 318.9 Thebe 3.11 Kallichore 313.3 Arche Metis 1.79 Amalthea 2.54 320.7 Scale in radii of Jupiter Moons are not to scale and increase in size 1 1 radius = 44,397 miles (71,492 km) for magnification purposes only","JUPITER 181 WEATHER Jupiter has no notable seasons, and the planet\u2019s THE GREAT RED SPOT temperature is virtually uniform. Its polar This giant storm, which is regions have temperatures similar to those bigger than Earth, is constantly of its equatorial regions because of internal changing its size, shape, and color. It rotates counterclock- heating. Jupiter radiates about 1.7 times more wise every six to seven days. heat than it absorbs from the Sun.The excess is infrared heat left from when the planet was formed. Most of Jupiter\u2019s weather occurs in the part of its atmosphere that contains its distinct white and red-brown cloud layers and is dominated by clouds, winds, and storms.The rising warm air and descending cool air within the atmosphere produce winds that are channeled around the planet, both to the east and west, by Jupiter\u2019s fast spin.The wind speed changes with latitude; winds within the equatorial region are particularly strong and reach speeds in excess of 250 mph (400 km\/h).The solar and infrared heat, the wind, and Jupiter\u2019s spin combine to produce regions of turbulent motion, including circular and oval cloud structures, which are giant storms.The smallest of these storms are like the largest hurricanes on Earth.They can be relatively short- lived and last for just days at a time, RED TRIO but others endure for years. Jupiter\u2019s RINGS In 2008, the Great Red Spot was most prominent feature, the Great Red accompanied by two smaller red spots. Spot, is an enormous high-pressure Jupiter\u2019s ring system was revealed for the first time in an image taken One, called Red Spot Jr., had formed in storm that may have first been sighted by Voyager 1 in 1979. It is a thin, faint system made of dust-sized 2006. The third, and smallest, one was from Earth over 340 years ago. particles knocked off Jupiter\u2019s four inner moons.The system consists later absorbed by the Great Red Spot. of three parts.The main ring is flat and is about 4,350 miles (7,000 km) wide and less than 18 miles (30 km) thick. Outside this is the flat EXPLORING SPACE collision course with the planet. In gossamer ring, which is 528,000 miles (850,000 km) wide and July 1994, the fragments hurtled stretches beyond Amalthea to Thebe\u2019s orbit. On the inside edge of the DEATH OF A COMET into Jupiter\u2019s atmosphere, each main ring is the 12,400-mile- (20,000 km-) thick doughnut-shaped impact being followed by an halo. Its tiny dust grains reach down to Jupiter\u2019s cloudtops. Comet Shoemaker\u2013Levy 9 was erupting fireball of hot gas. discovered orbiting Jupiter in March 1993. Unusually, it wasn\u2019t a single HEADING FOR DESTRUCTION object but a string of 22 cometary Shoemaker\u2013Levy 9 fragments orbit Jupiter in chunks. Astronomers calculated May 1994, just weeks before they slammed that in July 1992 the comet had into the planet\u2019s atmosphere. A cloud of gas been torn apart by Jupiter\u2019s and dust surrounds each fragment. gravitational pull, and they realized that the fragments were on a + Helike 293.5 Harpalyke 295.3 Thelxinoe 296.5 Hermippe 297.2 Thyone JUPITER\u2019S MAIN RING Euanthe 294 298.1 Jupiter\u2019s main ring was imaged by Galileo with the Sun behind the planet. From this position, small S\/2003 J16 293.7 Praxidike 295.8 Orthosie 296.1 Iocaste 297.5 Ananke 297.7 particles within the ring and in Jupiter\u2019s upper atmosphere stand out. The halo and gossamer ring are revealed only if the main ring is overexposed. S\/2003 J3 256.5 Euporie 271.2 S\/2003 J18 289.5 S\/2003 J15 307.7 Kore 349.7 S\/2003 J2 399.6 Herse 307.7 Carpo 239.2 S\/2003 J12 265.8 S\/2010 J2 284.0 Mneme 288.1 THE SOLAR SYSTEM 200 225 250 275 300 325 350 375 400 425 Pasithee 322.1 Kalyke 329.8 Sponde 333 Cyllene 335.7 S\/2003 J5 336.8 S\/2003 J10 339.2 Eukelade 343.5 Megaclite 333 Pasiphae 330.4 Aoede 333 Sinope 334.9 Callirhoe 337.1 Autonoe 337.4 Hegemone 342.8 + Chaldene 324.2 Aitne 324.5 Isonoe Erinome Taygete 326.7 S\/2003 J23 324.8 325.6 329.4 Kale 323.4 Eurydome 324.8 S\/2003 J4 325.4 S\/2010 J1 326.1 Carme 327.3","182 JUPITER\u2019S MOONS GALILEAN MOON Jupiter\u2019s moons fall into three categories: the four inner moons; the Europa four large Galilean moons; and the rest, the small outer moons. DISTANCE FROM JUPITER 416,630 miles (670,900 km) The inner and Galilean moons orbit in the usual direction\u2014that ORBITAL PERIOD 3.55 Earth days is, the same direction as Jupiter\u2019s spin (counterclockwise viewed from DIAMETER 1,939 miles (3,122 km) above the north pole). Most of the outer Europa is an ice-covered ball of rock, which has been studied for about 400 SO NEAR AND YET SO FAR moons travel in the opposite direction, years but whose intriguing nature was Io, one of the largest of Jupiter\u2019s suggesting that they originated from an only fully revealed once the Galileo 64 moons, appears close to its asteroid that fragmented after it was space probe started its study in 1996. planet, but the two are almost three captured by Jupiter\u2019s gravitational field. The probe was named after the Italian times the diameter of Jupiter apart. scientist Galileo Galilei, who observed Europa, along with the three other INNER MOON INNER MOON INNER MOON moons that collectively bear his name, in January 1610, from Padua, Italy.The Metis Adrastea Thebe German astronomer Simon Marius (1573\u20131624) is believed to DISTANCE FROM JUPITER 79,460 miles (127,960 km) DISTANCE FROM JUPITER 80,100 miles (128,980 km) DISTANCE FROM JUPITER 137,800 miles (221,900 km) ORBITAL PERIOD 6 hours 58 minutes DIAMETER 25 miles (40 km) ORBITAL PERIOD 7 hours 9 minutes ORBITAL PERIOD 16 hours 5 minutes DAYTIME TEMPERATURE LENGTH 68 miles (110 km) Infrared observations Metis, the closest moon to Jupiter, is LENGTH 16 miles (26 km) reveal heat radiation from irregular in shape and lies within the The most distant of the inner moons, Europa\u2019s surface at planet\u2019s main ring. It was discovered The small, Thebe is named after an Egyptian midday. Temperatures at on March 4, 1979 by the Voyager 1 irregularly king\u2019s daughter who was a grand- the equator (shown here probe. Metis is named after the first shaped Adrastea daughter of Io.The moon, which was as yellow) reach about wife of Zeus, who was swallowed by is the second discovered on March 5, 1979 by \u2013225\u02daF (\u2013140\u02daC). Farther him when she became pregnant. moon out from Voyager 1, lies within the outer part away from the equator, Jupiter and lies of the Gossamer Ring (see p.181). the surface temperatures JUPITER\u2019S CLOSEST MOON within its main ADRASTEA are even lower. ring. For each THEBE SHOWING IMPACT CRATER orbit of Jupiter, Adrastea spins once on its axis, so the same side of the moon always faces the planet.This synchronous rotation is also exhibited by Adrastea\u2019s three closest neighbors, Metis, Amalthea, and Thebe. Adrastea was discovered by Voyager 2 in July 1979, and is named after a nymph of Crete into whose care, according to Greek mythology, the infant Zeus was entrusted. INNER MOON OUTER MOON Amalthea Themisto DISTANCE FROM JUPITER 112,590 miles (181,300 km) DISTANCE FROM JUPITER 4.66 million miles (7.5 million km) ORBITAL PERIOD 11 hours 46 minutes ORBITAL PERIOD 130 Earth days LENGTH 163 miles (262 km) DIAMETER 5 miles (8 km) The largest of Jupiter\u2019s inner moons and the third from the planet,Amalthea In November 2000, astronomers at is named after the nurse of newborn the Mauna Kea Observatory, Hawaii, Zeus.The irregularly shaped moon carried out a systematic search for lies within the Gossamer Ring and new moons and identified 11 small is believed to be a source of ring moons. Observations recorded on material. Meteoroids from outside the subsequent nights revealed that one of Jovian system collide with Amalthea the 11, since named Themisto, was a and the other inner moons, chipping moon that had been discovered by off flecks of dust, which then become American astronomer Charles Kowal part of the ring system. Amalthea\u2019s on September 30, 1975, but then lost. unexpected discovery on September 9, THE SOLAR SYSTEM 1892, over 280 years after the four much larger Galilean moons had been discovered, was headline news. BARNARD\u2019S TELESCOPE Amalthea was the last of Jupiter\u2019s moons to be discovered by direct visual observation (as opposed to photography). Its discoverer, American Edward Barnard, used a 36-in (91-cm) refractor telescope, which is now preserved at the Lick Observatory in California. BATTERED SURFACE THEMISTO REDISCOVERED The circular feature in this image is Pan, which, This digital image is one of a series that with a diameter of about 56 miles (90 km), is shows Themisto (highlighted) and its changing the largest impact crater on Amalthea. The position against the background stars, which bright spot below Pan is associated with led to its rediscovery in November 2000. another, smaller crater, Gaea (bottom).","JUPITER 183 have observed the moons first, but it EXPLORING SPACE was Galileo who published his findings and brought the moons to the attention KEEP EUROPA CLEAN of the scientific and wider community. PWYLL CRATER TERRAIN MODEL After a six-year journey from Earth, Jupiter\u2019s fourth-largest satellite is a the Galileo space probe spent eight fascinating world. It is a little smaller This three-dimensional model of the 16-mile- years studying the Jovian system than Earth\u2019s Moon, but much and made 11 close flybys of Europa. brighter, since its icy surface reflects (26-km-) wide Pwyll Crater (above) was made The decision was made to destroy five times as much light. A liquid sea the probe because NASA wanted may lie below Europa\u2019s water-ice by combining images (see example, left) taken to avoid an impact with Europa crust, which is just tens of miles thick. and the potential contamination of This watery layer, which is estimated from different angles and then applying color. its subsurface ocean, which could to be 50\u2013105 miles (80\u2013170 km) possibly harbor life.With little fuel deep and to contain more liquid than Unusually, the crater floor (blue) is the same left, Galileo was put on a collision Earth\u2019s oceans combined, could be a course with Jupiter.The probe haven for life. Below lies a rocky height as the moon\u2019s surface, and the central disintegrated in the planet\u2019s mantle surrounding a metallic core. atmosphere on September 21, 2003. OVERHEAD VIEW peak (red) is much higher than the crater\u2019s rim. fractures in crust The surface appears to be geologically crust that has broken up and floated high-gain young and consists of smooth ice plains, into new positions. Round or oblong, antenna disrupted terrain, and regions criss- city-sized dark spots freckle the surface. GALILEO crossed by dark linear structures that Known as lenticulae, these form as can be thousands of miles long.The large globules of warm and slushy ice nuclear-powered push up from underneath and briefly generators provided mottled appearance of the melt the surface ice. Exactly how the electricity disrupted terrain comes from dark lines formed is unclear, but volcanically heated water and ice and Pwyll Crater other kinds of tectonic activity were involved.Tidal forces fractured the IMAGE OF THE FAR SIDE crust, and liquid or icy water erupted This is how the far side of through the crack to freeze almost Europa would appear to the instantly on the surface. human eye. Bright plains in the polar areas (top and In Greek myth, Europa was the girl bottom) sandwich a darker, who was seduced by Zeus in the form disrupted region of the crust. of a white bull and carried off to Crete. ICY SURFACE THE SOLAR SYSTEM This area of Europa\u2019s northern hemisphere shows features typical of the moon\u2019s icy surface. Brown grooves and ridges slice across a blue-gray water-ice surface freckled by lenticulae. The colors in this mosaic of images taken by Galileo have been enhanced to reveal detail.","184 JUPITER GALILEAN MOON Io DISTANCE FROM JUPITER 261,800 miles (421,600 km) ORBITAL PERIOD 1.77 Earth days DIAMETER 2,262 miles (3,643 km) Io is a little larger and denser than Earth\u2019s Moon, and orbits Jupiter at a distance only slightly greater than the Moon\u2019s from Earth. But there the similarities end. Io is a highly colored world of volcanic pits, calderas and JUPITERSHINE Sunlight reflected off Jupiter illuminates Io\u2019s vents, lava flows, and high-reaching western side. The eastern side is in shadow except for a burst of light beyond the limb plumes.The moon\u2019s nature was where the plume of the volcano Prometheus is lit. The yellowish sky is produced by sodium revealed first by the two Voyager probes atoms surrounding Io scattering the sunlight. and then more fully explored by the Galileo mission. Prior to Voyager 1\u2019s arrival in March 1979, scientists expected to find a cold, impact-cratered moon. Instead, it found the most volcanic body in the solar system. Io has a thin silicate crust that surrounds a molten silicate layer. Below this lies a comparatively large iron-rich core that extends about halfway to the surface. Io orbits Jupiter quickly, every 42.5 hours or so. As it orbits, it is subjected to the strong gravitational pull of Jupiter on one side and the lesser pull of Europa on the other. Io\u2019s surface flexes as a ring of sulfur- consequence of the dioxide snow varying strength and direction of the pull it experiences.The flexing is accompanied by friction, which produces the heat that keeps part of Io\u2019s interior molten. It is this material that erupts through the surface and constantly renews it. The evidence of such volcanism is seen all over Io. Over 80 major active volcanic sites and more than 300 vents have been identified. Features known as plumes are also found at the surface; these fast-moving and long- lived columns of cold gas Culann and frost grains are more Patera like geysers than volcanic VOLCANIC ACTIVITY Tohil Mons explosions.They are created In this color-enhanced as superheated sulfur dioxide shoots Galileo image, the dark spots on Io\u2019s surface through fractures in Io\u2019s crust.The are active volcanic centers. The dark eruptive material in the plumes falls slowly area of Prometheus at center left is encircled back to the surface as snow, and leaves by a pale yellow ring of sulfur-dioxide snow circular or oval frost deposits. Plume deposited by the volcano\u2019s plume. material also spreads into space surrounding Io and supplies a at the volcanic hot spots can be over CULANN PATERA 2,240\u02daF (1,230\u02daC), the highest surface Colorful lava flows THE SOLAR SYSTEM doughnut-shaped temperatures in the solar system stream away from the outside the Sun. Elsewhere the surface irregularly shaped green- body of material is cold, reaching just -244\u02daF (-153\u02daC). floored volcanic crater of Culann Patera (right of that has formed Simon Marius (see p.182) suggested center). The reasons for the names of the Galilean moons. Io the varied colors are along Io\u2019s orbital is named after one of Zeus\u2019s loves, uncertain. The diffuse red whom he changed into a cow to hide material is thought to be path. Temperatures her from his jealous wife. Hera was a compound of sulfur not fooled and sent a gadfly to torment deposited from a plume of TOHIL MONS Io forever. Other surface features are gas. The green deposits Non-volcanic named after people and places from may be formed when mountains are also the Io myth or from Dante\u2019s Inferno, sulfur-rich material coats found on Io. Here, the or after fire, sun, volcano and thunder warm silicate lava. sunlit peak of the gods, goddesses, and heroes. 185-mile- (300-km-) wide Tohil Mons rises 3.4 miles (5.4 km) above Io\u2019s surface.","185THE SOLAR SYSTEM PELE ERUPTS In this Voyager 1 image from 1979, a 185-mile- (300-km-) high plume rises above Pele, the first active volcanic site discovered on Io. Io\u2019s low gravity allows the gas to rise high above the moon before falling back to the surface. Named after the Hawaiian volcano goddess, Pele was still active almost 20 years later.","186 JUPITER GANYMEDE In this color-enhanced view, frosts at polar latitudes appear pale mauve. A distinct, dark area is called a regio, and Nicholson Regio, visible lower left, is the third-largest, at 2,425 miles (3,900 km) across. THE SOLAR SYSTEM GALILEAN MOON Ganymede URUK SULCUS with patches of carbon-dioxide ice, was formed from a This computer-generated is generally smoother and marked by Ganymede 60:40 mix of rock and ice.This perspective shows the area Uruk fewer craters. It is crisscrossed by ridges differentiated, and today the moon Sulcus, named after a Babylonian and grooves produced by the tectonic DISTANCE FROM JUPITER 664,470 miles has an iron-rich core surrounded by city. Icy material can be seen on stretching of the moon\u2019s surface. (1.07 million km) a lower mantle of rock, an upper the crests of the parallel ridges. mantle of ice, and an icy crust of Sulcus is the term for the grooved SIPPAR SULCUS ORBITAL PERIOD 7.15 Earth days contrasting dark and bright areas.The and ridged regions of bright terrain. This depression within dark terrain is pockmarked by impact Sippar Sulcus appears DIAMETER 3,267 miles (5,262 km) craters, suggesting that it is an older the icy surface in the distant past.The to be an old caldera surface. Circular bright areas termed dark terrain is also characterized by (a volcano\u2019s collapsed Ganymede is the largest moon in the palimpsests are the smoothed-out and long depressions about 4 miles (7 km) underground reservoir) solar system, bigger than both Pluto filled-in remains of craters formed on wide, called furrows.These may have containing frozen lava. and Mercury, and three-quarters the formed as subsurface ice flowed into size of Mars. It is named after the INFRARED MAPPING recently formed beautiful young boy in Greek myth The infrared image on the left, craters and material who was taken to Olympus by Zeus taken by Galileo, locates dragged across the and became cupbearer to the gods. surface water-ice\u2014the surface created the brighter the shading, the bow-shaped troughs. greater the amount. The colors of the right-hand The bright image indicate the location terrain, which is of minerals (red) and the size rich in water ice of ice grains (shades of blue).","JUPITER 187 GALILEAN MOON TINDR CRATER The craters are named after heroes and VALHALLA REGION The partial collapse of heroines of northern myths, and the This photograph of part of the Valhalla Basin, Callisto the rim of this 47-mile- large, ringed features, such as theValhalla lit by sunlight streaming in from the left, (76-km-) wide crater and Basin (see below), after homes of the shows a 6-mile- (10-km-) wide fault scarp, DISTANCE FROM JUPITER 1.17 million miles its pitted floor is gods or heroes. About 1,600 miles part of Valhalla\u2019s ring system. The smallest (1.88 million km) probably the result of (2,600 km) across, theValhalla Basin was craters visible are about 510 ft (155 m) across. erosion by ice. probably formed by a large meteorite ORBITAL PERIOD 16.69 Earth days strike early in Callisto\u2019s history, which MYTHS AND STORIES toward its crust.The surface, scarred fractured the moon\u2019s cold, brittle crust, DIAMETER 2,994 miles (4,821 km) by craters and multi-ringed structures allowing ice that was previously below CALLISTO created by meteorite impacts, bears few the surface to flood the impact site. The most distant, second-largest, and signs of geological activity. Callisto Callisto was a beautiful follower darkest of the Galilean moons, Callisto does not appear to have been shaped of the huntress Artemis, who was is still brighter than Earth\u2019s Moon since by plate tectonics or cryovolcanism, seduced by Zeus and bore him a its surface contains ice that reflects where ice behaves like volcanic lava, son. According to one myth, Zeus\u2019s sunlight. Callisto has undergone little although the ice has eroded the rock jealous wife, Hera, turned Callisto internal change since its formation. Its in places, causing crater rims to be into a bear. One day, Callisto came original mix of rock and ice is only worn away and sometimes collapse. across her son, Arcas, now grown. partly differentiated, so that the moon Fearful for his life, Arcas was only is rockier toward its center and icier SCARRED SURFACE stopped from killing Callisto by This is the only complete global color image Zeus, who raised a whirlwind that dark areas of Callisto obtained by Galileo. The surface is lack ice carried the pair up into the uniformly cratered, and the bright impact sky. Callisto became scars are easily visible against its the constellation Ursa otherwise dark, smooth surface. Major and Arcas formed Bo\u00f6tes. ice on crater rim and floor shines brightly MULTI-RING BASIN THE SOLAR SYSTEM The multi-ringed Valhalla Basin dominates Callisto\u2019s surface. The bright, ice-covered central zone is about 370 miles (600 km) across. It is surrounded by rings, which are troughs about 30 miles (50 km) apart.","188 SATURN SATURN 38\u201339 Gravity, motion, and orbits SATURN IS THE SECOND-LARGEST PLANET and the sixth 68\u201369 Planetary motion from the Sun\u2014it is the most distant planet normally visible 100\u2013101 The history of the solar system to the naked eye. A huge ball of gas and liquid, Saturn has 102\u2013103 The family of the Sun a bulging equator and an internal energy source.With a composition dominated by hydrogen, it is the least dense of all the planets. A spectacular system of rings encircles the planet itself, and it also has a large family of moons. ORBIT NORTHERN axis tilts from SPRING EQUINOX the vertical Saturn takes 29.46 Earth years to by 26.7\u00b0 complete one orbit of the Sun. It is NORTHERN spins on its SUMMER SOLSTICE axis every NORTHERN 10.66 hours WINTER SOLSTICE tilted to its orbital plane by 26.7\u00b0, a little more than Earth\u2019s axial tilt. This means that as Saturn moves APHELION PERIHELION along its orbit, the north and south 938 million poles take turns pointing toward miles (1.51 838 million the Sun.The changing orientation billion km) miles (1.35 billion km) Sun of Saturn to the Sun is seen from Earth by the apparent opening and closing of the planet\u2019s ring system.The rings are seen edge-on, for example, at the Saturn orbits the Sun in start of an orbital period.Then an increasing 29.46 Earth years portion of the rings is seen from above as the North Pole NORTHERN FALL EQUINOX tips toward the Sun.The rings slowly close up and disappear from view as the North Pole starts to tip away until, 14.73 Earth years (half an orbit) later, they appear edge-on again. Now the South Pole tips SPIN AND ORBIT Saturn spins on its axis as it sunward and the rings are seen increasingly from below.They close up orbits. The rapid spin flings material outward, with the result once again as the South Pole turns away, until they are seen edge-on that Saturn is about 10 percent wider at its equator than its poles. once more, as the orbit is completed.The strength of the Sun at Saturn Its bulging equator is bigger than that of any other planet. is only about 1 percent of that received on Earth, but it is enough to generate seasonal smog. Saturn is at perihelion at the time the South Pole is facing the Sun. STRUCTURE Saturn\u2019s mass is only 95 times that of Earth, yet 764 Earths could fit inside it.This is because Saturn is composed mainly of the lightest elements, hydrogen and helium, which are in both gaseous and liquid states. Saturn is the least dense of all the planets. If it were possible to put Saturn in an ocean of water, it would float.The planet has no discernible surface: its outer layer is gaseous atmosphere. Inside the planet, pressure and temperature increase with depth and the hydrogen and helium molecules are forced closer and closer together until they become fluid. Deeper still, the atoms are stripped of their electrons and act as a liquid metal. Electric currents within this region generate a magnetic field 71 percent the strength of Earth\u2019s (see p.125). atmosphere outer layer of liquid hydrogen and helium inner layer of liquid metallic hydrogen and helium THE SOLAR SYSTEM core of rock RINGLEADER and ice Girdled by its bright system of rings, Saturn has a hazy, muted appearance in this Cassini image, which shows the planet in its natural colors. SATURN\u2019S INTERIOR A thin, gaseous atmosphere surrounds a vast shell of liquid hydrogen and helium. The central core is about 10\u201320 times the mass of Earth.","189 ATMOSPHERE Saturn\u2019s atmosphere forms the planet\u2019s visible surface. It is seen as a pale-yellow cloud deck with muted bands of various shades, which lie parallel to the planet\u2019s equator. Its upper clouds have a temperature of about \u2013220\u00b0F (\u2013140\u00b0C).The atmospheric temperature decreases with height, and as different compounds condense into liquid droplets at different temperatures, clouds of different composition form at different levels. Saturn is believed to have three cloud layers.The highest, visible layer is made of ammonia ice crystals; beneath this lies a layer of ammonium hydrosulfide; water-ice clouds, so far unseen, form the lowest layer.The upper atmosphere absorbs ultraviolet light, and the temperature rises here, leading to the production of a thin layer of smoggy haze; it is this layer that gives the planet its indistinct, muted appearance.The smog builds up on the hemisphere that is tilted toward the Sun. Saturn radiates almost twice the amount of energy it receives from the Sun.The extra heat is generated by helium rain droplets within the planet\u2019s metallic shell.These convert motion energy to heat energy as they fall toward the planet\u2019s center.The heat is transported COMPOSITION OF ATMOSPHERE through the lower atmosphere The trace gases include methane, and, along with the planet\u2019s ammonia, and ethane. It is not known rotation, generates Saturn\u2019s winds. which elements or compounds color the atmosphere\u2019s clouds and spots. hydrogen 96.3% helium and trace gases 3.7% JANUARY 28, 2004 SATURN PROFILE JANUARY 26, 2004 AVERAGE DISTANCE FROM THE SUN ROTATION PERIOD THE SOLAR SYSTEM CHANGING SOUTH POLAR AURORA Solar-wind particles in the upper atmosphere 888 million miles (1.43 billion km) 10.66 hours produce aurorae. The brightening of the aurora JANUARY 24, 2004 on January 28 corresponds with the arrival at CLOUDTOP TEMPERATURE ORBITAL PERIOD (LENGTH OF YEAR) Saturn of a disturbance in the solar wind. \u2013220\u00b0F (\u2013140\u00b0C) 29.46 Earth years DIAMETER 74,898 miles (120,536 km) MASS (EARTH = 1) 95 GRAVITY AT CLOUDTOPS (EARTH = 1) 1.07 VOLUME (EARTH = 1) 763.59 SIZE COMPARISON NUMBER OF MOONS 62 EARTH SATURN OBSERVATION Saturn is visible to the naked eye for about 10 months of the year. It appears like a star and takes about 2.5 years to pass though one zodiacal constellation. A telescope is needed to make out the ring system.","190 SATURN BANDS AND SPOTS WEATHER WHITE STORM ON SATURN Bands of clouds, spots, and ribbonlike In December 2010, a white features move across Saturn\u2019s visible Giant upper-atmosphere storms composed of white storm cloud appeared in surface. The spots look small but can ammonia ice can be seen from Earth when they rise Saturn\u2019s northern hemisphere. be thousands of miles across. through the haze.They occur once every 30 years or This image shows the storm, so, when it is midsummer in the northern hemisphere, which grew until it extended all but, as yet, there is no accepted explanation for the the way around the planet, storms.The last of these \u201cGreat White Spots\u201d was three weeks after it broke out. discovered on September 25, 1990. It spread around the planet, almost encircling the equatorial region over about a month. Smaller, different-colored oval spots and ribbonlike features have been observed on a more regular basis. In 2004, Cassini revealed a region then dominated by storm activity, nicknamed \u201cstorm alley\u201d.Wind speed and direction on the planet are determined by tracking storms and clouds. Saturn\u2019s dominant winds blow eastward, in the same direction as the planet\u2019s spin. Near the equator, they reach 1,200 mph (1,800 km\/h). C ring B ring MOONS DIONE Cassini produced this image Saturn has more than 60 known mooms. Most of these have been discovered since of the moon Dione against 1980, through exploration by the Voyager and Cassini probes and by improved the backdrop of Saturn\u2019s Earth-based observing techniques. Future observations are expected to confirm clouds in December 2005. the presence of more moons.Titan is the largest and was the first to be discovered, This true-color view reveals in 1655. It is a unique moon, being the only one in the solar system to have a the variations in brightness substantial atmosphere. Saturn\u2019s moons are mixes of rock and water ice. Some have of the moon\u2019s icy surface. ancient, cratered surfaces, and others show signs of resurfacing by tectonics or ice Dione is Saturn\u2019s fourth- volcanoes.The moons are mostly named after mythological giants.The first to be largest moon. discovered were named after the Titans, the brothers and sisters of Cronus (Saturn) in Greek mythology. More recent discoveries have Gallic, Inuit, and Norse names. SATURN\u2019S MOONS Hyperion 24.6 Iapetus 59.1 THE SOLAR SYSTEM Titan 20.3 75 100 125 Calypso 4.88 Tethys 4.88 Helene 6.26 1 25 50 Dione 6.26 150 175 2 radius + Telesto 4.88 Polydeuces 6.26 Rhea 8.75 + Kiviuq 189 + S\/2009 S1 1.94 Aegaeon 2.78 Ijiraq 190 Daphnis 2.26 Anthe 3.28 + Atlas 2.28 Pandora 2.35 Mimas 3.08 Pallene 3.50 Pan Janus 2.51 2.22 Prometheus 2.28 Epimetheus 2.51 Methone 3.22 Enceladus 3.95","SATURN 191 RINGS Saturn\u2019s visible rings are the most extensive, massive, and spectacular in the Solar System. From Earth, they appear as a band of material whose appearance changes according to Saturn\u2019s position.The rings are, in fact, collections of separate pieces of dirty water ice following individual orbits round Saturn.The pieces range from dust grains to boulders several yards across.They are very reflective, so the rings are bright and easy to see. Individual rings are identified by letters, allocated in order of discovery. The readily seen rings are the C, B, and A rings.These are bounded by others, made of tiny particles, that are almost transparent.The thin F ring, the broader G ring, and the diffuse E ring lie outside the main rings. The D ring, inside C, completes the system.The rings change slowly over time, and moons orbiting within the system shepherd particles into rings and maintain gaps such as the MAIN RING SYSTEM Encke gap. Far beyond the visible COMPOSITIONAL DIFFERENCES This mosaic of six images shows the main system is a huge, doughnut-shaped In this ultraviolet image of the outer portion of the C ring rings in natural color and reveals the ring. Almost impossible to see, it was (left) and inner B ring (right), red indicates the presence of ringlets within the Cassini Division. The discovered in 2009 by the infrared dirty particles and turquoise indicates purer ice particles. distance from the inner edge of C to the F glow of its cool and sparse dust. ring is about 40,500 miles (65,000 km). Encke gap F ring Cassini A ring Division PROMETHEUS AND THE F RING INVISIBLE RINGS Saturn\u2019s innermost moons orbit within the Saturn\u2019s largest ring is invisible to the eye. It consists ring system and interact with it. Some act of dust and was discovered at infrared wavelengths as shepherd moons, confining particles by the Spitzer Space Telescope. Above is an artist\u2019s within specific rings. Prometheus (just representation of the ring, which lies between about below the rings in this image) and Pandora 3.7 and 11.2 million miles (6 million and 18 million km) work in this way on either side of the F ring. from Saturn. At the top is a Spitzer image of part of the ring. The ring is tilted about 27 degrees from Saturn\u2019s PANDORA main ring plane. The small shepherd moon Pandora orbits just beyond the F ring. It is visible as a white dot in this view taken by Cassini on February 18, 2005. Phoebe 215 Paaliaq 252 Albiorix 272 Bebhionn Siarnaq 301 Tarvos 303 Kari 367 S\/2006 S3 Loge 383 Hyrrokkin 396 284 367 THE SOLAR SYSTEM Skathi 260 S\/2007 Erriapus 292 Skoll Tarqeq 297 Fenrir 373 Surtur Ymir 383 Fornjot 417 S2 278 293 380 200 225 250 275 300 325 350 375 400 425 + Bergelmir 321 Jarnsaxa 321 S\/2004 S12 Farbauti 339 Aegir 344 330 Suttungr 323 Bestia S\/2004 337 S7 349 S\/2004 S17 323 Hati 328 Thyrmr 340 + S\/2004 S13 305 Narvi S\/2006 311 S1 312 Scale in radii of Saturn Moons (and rings) are not to scale and increase Mundilfari S\/2007 1 1 radius = 37,448 miles (60,268 km) in size for magnification purposes only 310 S3 315 Greip 306","192 SATURN SATURN\u2019S MOONS The moons of Saturn are divided into three groups.The first consists of the major moons, which are large and spherical.The second group, the inner moons, are smaller and irregularly shaped. Members of both these groups orbit within or outside the ring system.The third set of moons lies far beyond the other two\u2014the most distant orbit over 15 million miles (25 million km) from Saturn. These irregularly shaped moons are tiny, just a few miles to tens of miles across.They have inclined orbits, which suggests that DWARFED BY SATURN they are captured objects. From Earth, Saturn\u2019s moons appear as Saturn\u2019s moons, such as Tethys (top) and little more than disks of light, but Voyager and Cassini revealed Dione (below), are not only small compared many of them as worlds in their own right. to their host planet but, with the exception of Titan, they are all smaller than Earth\u2019s Moon. INNER MOON INNER MOON (below left), which orbit BATTERED SURFACE just beyond the F ring, Epimetheus (left) and its Prometheus Epimetheus are such a pair.They co-orbital moon, Janus, are swap orbits every four believed to be the remnants DISTANCE FROM SATURN 86,539 miles DISTANCE FROM SATURN 94,089 miles years, taking turns being of a larger object that was (139,353 km) (151,422 km) slightly closer to Saturn. broken apart by an impact. Epimetheus is a lumpy ORBITAL PERIOD 0.61 Earth days ORBITAL PERIOD 0.69 Earth days moon, just 17 miles same way on the (28 km) longer than it inner side of the ring. LENGTH 84 miles (136 km) LENGTH 81 miles (130 km) is wide or deep, and it is The existence of one of 16 moons that Epimetheus was Prometheus is a small, elongated moon Occasionally, moons orbit a planet lie within the ring suspected in 1967 but was not orbiting just inside the multi-stranded within about 30 miles (50 km) of each system. Epimetheus is in synchronous confirmed until February 26, 1980. It F ring. Along with Pandora, it is a other.They are described as co-orbital rotation\u2014that is, it keeps the same was one of eight moons discovered in \u201cshepherd\u201d of the F ring. Cassini since they virtually share an orbit.The face toward Saturn at all times Voyager data that year.The moon is two moons Epimetheus and Janus because its rotation and orbital named after a Titan, the family of images of periods are the same. As it orbits giants in Greek mythology who once Prometheus and CO-ORBITAL MOON Saturn, it works as a shepherd moon, ruled the Earth. Prometheus was one the F ring show Epimetheus orbits against the backdrop of confining the ring particles within the of Epimetheus\u2019s five brothers. them to be linked Saturn\u2019s rings, which are seen nearly edge-on F ring. Prometheus (left) works the by a fine thread in this view taken by Cassini\u2019s narrow-angle of material, camera on February 18, 2005. produced as Prometheus pulls particles out of the ring.The moon\u2019s long axis points SHEPHERD MOON toward Saturn. INNER MOON Janus DISTANCE FROM SATURN 94,120 miles (151,472 km) ORBITAL PERIOD 0.69 Earth days LENGTH 126 miles (203 km) THE SOLAR SYSTEM Heavily cratered and irregularly INNER MOON not found by chance but were shaped, Janus orbits Saturn just beyond identified in images taken as part of the F ring and only 30 miles (50 km) Pallene a search for new moons within this farther away than its co-orbital moon, region around Saturn.The contrast of Epimetheus. Its existence was first DISTANCE FROM SATURN 131,000 miles the images was enhanced to increase reported in December 1966, and it (211,000 km) visibility.Twenty-eight images, was named after the Roman god including the one on the right, Janus, who could look forward and ORBITAL PERIOD 1.14 Earth days acquired over a period of 9.25 hours back at the same time.Yet it was only together make a movie showing confirmed as a moon in February 1980, DIAMETER 3 miles (5 km) Pallene as it progresses along its after Voyager 1 data had been studied. orbital path around Saturn. Two small moons orbiting between BEYOND THE F RING the major moons Mimas and S\/2004 S2 Enceladus were discovered in 2004 in Pallene is a tiny world, just 2.5 miles (4 km) data collected by the Cassini probe. As long, which has only been seen as a bright with all such discoveries, the moons dot. The large, bright object is Saturn, which were initially identified by numerical has been overexposed in an attempt to designations (S\/2004 S1 and S\/2004 record new, small, faint moons. S2).The two moons are now known as Methone and Pallene.They were","SATURN 193 MAJOR MOON Mimas DISTANCE FROM SATURN 115,208 miles (185,520 km) ORBITAL PERIOD 0.94 Earth days DIAMETER 246 miles (396 km) Mimas is the first of the major moons out from Saturn, and it orbits the planet in the outer part of the ring system. It is in synchronous rotation, so the same side of the moon always faces the planet, in the same way that one side of the Moon always faces Earth. Mimas is a round moon, but it is not a perfect sphere\u2014this icy object is about 19 miles EMPHASIZING DIFFERENCES (30 km) longer than it is False color highlights slight wide and deep. Its surface differences in surface composition is covered in deep, bowl- on Mimas, for example the bluish shaped impact craters. terrain near the crater Herschel, Many of those greater than possibly caused by impact ejecta, about 12 miles (20 km) and greenish terrain elsewhere. across have central peaks. One crater, Herschel, dwarfs the rest and is the moon\u2019s most prominent feature. It is about 80 miles (130 km) wide, almost 6 miles (10 km) deep, and has a prominent central peak. If the impacting body that formed the crater had been much bigger, it might have smashed the moon apart.The crater is named after the astronomer William Herschel, who discovered Mimas on July 18, 1789. It was the sixth of Saturn\u2019s moons to be discovered and the first of two discovered by Herschel. Mimas is named after a Titan (see p.190). GIANT CRATER THE SOLAR SYSTEM The crater Herschel lies on the moon\u2019s leading hemisphere (the side that points in the direction in which it is moving) and is about one-third of the diameter of Mimas itself. The impact that formed Herschel must have come close to shattering the moon. TRUE BLUE Mimas drifts against the backdrop of Saturn\u2019s northern hemisphere in this true-color view. Scattering of sunlight in the relatively cloud- free area gives the planet a bluish hue. The dark lines cutting across the atmosphere are shadows cast by Saturn\u2019s rings.","194 SATURN MAJOR MOON BLUE WALLS A false-color view reveals long Enceladus fractures (in blue) on the moon\u2019s icy surface. The walls of the fractures are DISTANCE FROM SATURN 147,898 miles WATER JETS thought to expose ice with coarser (238,020 km) Ice and water vapor spray out from texture than on the smooth surface. so-called tiger stripes near the south ORBITAL PERIOD 1.37 Earth days pole of Enceladus, as shown here in MAJOR MOON this image from the Cassini probe. DIAMETER 313 miles (504 km) Tethys Mimas (see p.193) is about the same Enceladus orbits within the broad size but is inactive. Craters are DISTANCE FROM SATURN 183,093 miles E ring of Saturn. Its orbit lies within concentrated in some regions, and (294,660 km) the densest part of the ring, which elsewhere there are grooves, fractures, ORBITAL PERIOD 1.89 Earth days suggests that Enceladus could be and ridges. Images processed to DIAMETER 660 miles (1,062 km) supplying the ring with material. accentuate color differences have The moon is in synchronous rotation revealed previously unseen detail. The Italian\u2013French astronomer with Saturn.The frosty surface of The blue color seen in some fracture Giovanni Cassini discovered Tethys on Enceladus is highly reflective and walls could be due to the exposure of March 21, 1684. Nearly 200 years later, makes this moon particularly bright, solid ice, or because the composition it was discovered that Tethys shares its the brightest in the solar system.The or size of particles in the orbit with two far smaller moons: surface terrain suggests that this frigid buried ice is different from Telesto (left) and Calypso. Its surface moon has experienced a long history that on the surface. Enceladus shows that Tethys has undergone of tectonic activity and resurfacing. was discovered by William tectonic change and resurfacing. The extent of the geological change Herschel on August 28, 1789. Two features stand out. A 248-mile- is surprising in such a small world\u2014 (400-km-) wide impact crater called BAGHDAD SULCUS Odysseus dominates the leading SMOOTH PLAINS This is a closeup of part of Baghdad hemisphere. Large but shallow, its This region of Sulcus, the longest of several original bowl shape has been flattened smooth plains has a linear structures popularly termed by ice flows.The second large feature band of chevron- tiger stripes in the south polar is the Ithaca Chasma on the side of shaped features region of Enceladus. Tethys facing Saturn.This vast canyon running across its system extends across half of the center, cut across moon. It may have been formed by at the top by a tensional fracturing as a result of the system of crevasses. impact that produced the Odysseus Crater, or when Tethys\u2019s interior froze INNER MOON ICY MOON and the moon expanded in size and The pale, icy disk of Tethys was imaged by stretched its surface. Telesto Cassini on January 18, 2005 as it orbited below Saturn\u2019s south polar region, where ITHACA CHASMA DISTANCE FROM SATURN 183,093 miles fierce storms were raging. This canyon system, (294,660 km) which is up to 2.5 miles (4 km) ORBITAL PERIOD 1.89 Earth days deep, runs from the lower left of the LENGTH 20 miles (32.5 km) prominent Telemachus Crater (top right). Telesto shares an orbit within the E ring with two other moons: Calypso, which is about the same size as Telesto; and the much larger Tethys (right).Telesto moves along the orbit 60\u00b0 ahead of Tethys, and Calypso follows 60\u00b0 behind Tethys.The positions taken on the orbit by the two smaller moons are called the Lagrange points. In these positions, the two small moons can maintain a stable orbit balanced between the gravitational pull of Saturn and that of Tethys.Telesto and Calypso were discovered in 1980; Calypso by Earth- based observation and Telesto in Voyager images.The probe revealed two irregularly shaped moons. SMOOTH MOON Telesto\u2019s surface appears less cratered than most of Saturn\u2019s other moons in this image taken by Cassini from a distance of only 9,000 miles (14,500 km).","195 MAJOR MOON IMPACT CRATERS The well-defined central peaks of Dione\u2019s Dione largest craters are visible in this Voyager image. Dido Crater lies just left of center, DISTANCE FROM SATURN 234,505 miles with Romulus and Remus just above it and (377,400 km) Aeneas Crater near the upper limb. ORBITAL PERIOD 2.74 Earth days it is the second-densest of Saturn\u2019s moons.The terrain displays evidence DIAMETER 698 miles (1,123 km) of tectonic activity and resurfacing. There are ridges, faults, valleys, and Dione is the most distant moon depressions.There are also craters, within Saturn\u2019s ring system, but it is which are more densely distributed in not alone in the outer reaches of the some regions than others\u2014Dione\u2019s E ring.Two other moons, Helene and leading face, for example, has more Polydeuces, follow the same orbit, than the trailing face.The largest Helene is ahead of Dione by 60\u00b0 crater is over 124 miles (200 km) and Polydeuces follows 60\u00b0 behind. across. Dione also has bright streaks Helene was discovered in March on its surface.These wispy features are 1980; Polydeuces was discovered in composed of narrow, bright, icy lines. Cassini data some 24 years later, just after the probe arrived at Saturn. Giovanni Cassini discovered Dione in 1684, on the same day that he discovered Tethys (opposite). Dione has a higher proportion of rock in its rock\u2013ice mix than most of the other moons (only Titan has more), and so DIONE\u2019S FAR SIDE MAJOR MOON ANCIENT SURFACE Impact craters scar the surface of the side of Two large impact basins (top center) are Dione that is permanently turned away from Rhea visible in this enhanced-color image of Rhea\u2019s Saturn because it is in synchronous rotation. heavily cratered surface. The great age of Areas of wispy terrain are visible on the left DISTANCE FROM SATURN 327,487 miles (527,040 km) these basins is indicated by the many of this image. smaller craters upon them. ORBITAL PERIOD 4.52 Earth days Rhea froze early in its history and DIAMETER 949 miles (1,527 km) became frigid. Its ice would then have behaved like hard rock. Rhea\u2019s craters, Vast sweeps of ancient cratered terrain for example, are freshly preserved in cover large parts of Rhea. At first its icy crust.The craters on other icy glance, the landscape resembles that moons, such as Jupiter\u2019s Callisto (see seen on Earth\u2019s Moon, although p.187), have collapsed in the soft, icy Rhea\u2019s surface is bright ice.There is crust. Rhea is the first of Saturn\u2019s some evidence of resurfacing, moons to lie beyond the ring system. although not as much as expected for It is named after the Titan Rhea, such a large moon. Rhea is Saturn\u2019s who was the mother of Zeus in second-largest moon, but other, Greek mythology. smaller moons, such as its inner neighbors Dione and Tethys, show more resurfacing. It is thought that HEAVILY CRATERED Rhea\u2019s icy surface is heavily cratered, suggesting that it dates back to the period immediately following the formation of the planets. This image shows the region around the moon\u2019s North Pole. The largest craters are several miles deep. CLIFFS OF ICE FRESH ICE THE SOLAR SYSTEM A Cassini close-up of Dione\u2019s wispy An enhanced-color view terrain reveals that it is formed from of the surface of Rhea lines of ice cliffs created by tectonic shows blue patches of fractures, rather than deposits of ice freshly uncovered ice. and frost as was previously thought. The ice is thought to have been exposed when debris in orbit around Rhea struck the surface along the equator.","196 SATURN MAJOR MOON chemistry of the atmosphere has EXPLORING SPACE similarities to that of the young Titan Earth, before life began.The first THE HUYGENS PROBE chance to see the surface and test DISTANCE FROM SATURN 758,073 miles the atmosphere came in 2005, when The European probe Huygens traveled to (1.22 million km) Cassini turned its attention to Titan, Titan onboard NASA\u2019s Cassini spacecraft. and Huygens plunged through Once there, it separated from the larger craft ORBITAL PERIOD 15.95 Earth days the atmosphere to the surface and parachuted into Titan\u2019s haze. During (see panel, right). its 2.5-hour descent, Huygens tested the DIAMETER 3,200 miles (5,150 km) atmosphere, measured the speed of the The nitrogen-rich atmosphere buffeting winds, and took images of the Titan was discovered in 1655 by the extends for hundreds of miles above moon\u2019s surface. An instrument recorded Dutch scientist Christiaan Huygens. Titan. Layers of yellow-orange, the first surface touch on January 14, It is the second-largest moon in the smoglike haze high within it are the 2005, sending back evidence of a thin, solar system after Jupiter\u2019s Ganymede result of chemical reactions triggered hard crust with softer material beneath. (see p.186) and is by far the largest of by ultraviolet light. Methane clouds Saturn\u2019s moons.This Mercury-sized form much closer to the surface. HUYGENS AND CASSINI body is also one of the most These rain methane onto Titan, where The shield-shaped Huygens probe (right) is attached fascinating. A veil of smoggy haze it forms rivers and lakes. It then to Cassini\u2019s frame in preparation for the launch from shrouds the moon and permanently evaporates and forms clouds, and the Cape Canaveral, Florida, in October 1997. obscures the world below.Titan is cycle, which is reminiscent of the water intriguing, not least because the cycle on Earth, continues. that its surface is shaped by Earth-like processes\u2014 TITAN\u2019S ATMOSPHERE Titan is the tectonics, erosion, and Infrared and ultraviolet densest of winds\u2014and perhaps ice data combined reveals Saturn\u2019s moons: volcanism. No liquid aspects of the atmosphere. it is a 50:50 methane was detected Areas where methane mix of rock on the initial flybys, but absorbs light appear and water ice drainage channels and dark orange and green. The with a surface elliptical regions, thought high atmosphere is blue. temperature of to be evaporated lakes, \u2013292\u00b0F (\u2013464\u00b0C). showed where fluids had It is a gloomy been. Linear features POLAR LAKES world because nicknamed \u201ccat scratches\u201d Lakes of liquid methane and the smog blocks were also identified. ethane have formed at Titan\u2019s 90 percent of the north pole. The lakes are shown incident sunlight. Cassini revealed in blue. The largest are bigger than the Great Lakes of North America, but much shallower. ORANGE AND PURPLE HAZE The upper atmosphere consists of separate layers of haze; up to 12 have been detected in this ultraviolet, true-color image of Titan\u2019s night-side limb. THE SOLAR SYSTEM ICY SURFACE TITAN REVEALED The surface of Titan is shown in this image Infrared observations taken by the Huygens lander in 2005. The that cut through Titan\u2019s pebbles in the foreground are up to about clouds reveal bright highlands, 6 in (15 cm) across and are thought to dark lowlands, and a possible be composed of frozen water. impact basin near the disk\u2019s center.","SATURN 197 INNER MOON surface is cratered, and there are segments of cliff faces. Its shape and Hyperion scarred surface suggest Hyperion could be a fragment of a once-larger DISTANCE FROM SATURN 919,620 miles object that was broken by a major (1.48 million km) impact. Even Hyperion\u2019s discovery was unusual. Astronomers in the US ORBITAL PERIOD 21.28 Earth days and England found it independently and within two days of LENGTH 224 miles (360 km) each other in September Nothing about Hyperion is typical. 1848. First, it is an irregularly shaped moon with an average width of about 174 miles (280 km).This makes it one of the largest nonspherical bodies in the solar system. Second, it follows an elliptical orbit just beyond Saturn\u2019s largest moon,Titan (opposite). And, as it orbits, it rotates chaotically: its rotation axis wobbles, and the moon appears to tumble as it travels. Its STRANGE CRATERS Hyperion has a strange, spongy appearance, resulting from its low density and weak gravity. MAJOR MOON DARK COATING OUTER MOON CRATER FLOOR Iapetus, the two-toned moon of Saturn, Debris covers the Iapetus is seen in this closeup from the space Phoebe floor of this impact probe Cassini. One side of Iapetus crater. The streaks DISTANCE FROM SATURN 2.21 million miles consists of bright ice, while the other DISTANCE FROM SATURN 8.05 million miles inside the crater (3.56 million km) is covered with a dark coating. (12.95 million km) indicate where loose ejecta has slid down ORBITAL PERIOD 79.33 Earth days LANDSLIDE IN CASSINI REGIO ORBITAL PERIOD 550 Earth days toward the center. Land has collapsed down a 9-mile- (15-km-) DIAMETER 913 miles (1,469 km) high scarp, which marks the edge of a huge DIAMETER 132 miles (213 km) compared to most impact crater, into a smaller crater. The long moons). Half the Most of Saturn\u2019s inner and major distance traveled by the material along the Phoebe was discovered in 1898 and, outer moons moons orbit in the equatorial plane floor indicates that it could be fine-grained. until 2000, was thought to be Saturn\u2019s orbit this way. Phoebe is by far the (also the plane of the rings). Iapetus is only outer moon. Many others are largest outer moon; the others are, at an exception, its orbit being inclined on the bright side. Although the now known to exist. Phoebe has a most, only 12 miles (20 km) across. by 14.72\u00b0 to the equatorial plane. Cassini probe revealed more of the long orbital period and follows a From Cassini images, it appears to be Other moons follow orbits with moon\u2019s heavily cratered surface, the highly inclined orbit, a characteristic an ice-rich body coated with a thin greater inclination, but these are the origin of the dark material remains of the outer moons. Phoebe\u2019s orbit is layer of dark material. much smaller, outer moons. Iapetus is a mystery. It has been suggested that inclined by 175.3\u00b0 and so it travels Saturn\u2019s most distant major moon. It the material erupted from the moon\u2019s in a retrograde manner (backward BLASTED PHOEBE is also in synchronous rotation. interior, or that it is ejecta from Phoebe\u2019s impact craters were impacts on a more distant moon, such Erginus revealed by Cassini in June Iapetus was discovered by Italian as Phoebe (right). A unique feature 2004. They are named astronomer Giovanni Cassini while he after the Argonauts in THE SOLAR SYSTEM was working from Paris on October revealed by Cassini Greek mythology. 25, 1671. He noticed that Iapetus has has provided another The largest, Jason, a naturally dark leading hemisphere mystery. It is not is about 60 miles and a bright trailing hemisphere.The known whether a (100 km) across. dark region is called Cassini Regio 800-mile- (1,300-km-) and is covered in material as dark long ridge that Jason as coal, in contrast to the icy surface coincides almost exactly with the Iphitus SURFACE COMPOSITION moon\u2019s equator is False colors represent a folded mountain Euphemus Iapetus\u2019s vastly different belt or material that surface compositions. Bright erupted through a Butes blue signifies an area rich crack in the surface. in water ice, dark brown Eurydamas indicates a substance rich in organic material, and the Canthus yellow region is composed of a mix of ice and organic Oileus chemicals.","SATURN FROM ABOVE This is Saturn as it can never be seen from Earth\u2014looking down from above, with the full sweep of the rings encircling the planet. In a complex interplay of light and shade, the shadow of the rings inscribes a dark line around the planet, while the inky black of the planet\u2019s shadow falls across the rings. This image, looking toward the unlit side of the rings, was taken by the Cassini probe."]