Universe - The Definitive Visual Guide (September 2012)

["\u201cHow vast those Orbs must be, and how inconsiderable this Earth, the Theatre upon which all our mighty Designs, all our Navigations, and all our Wars are transacted, is when compared to them.\u201d Christiaan Huygens THE SOLAR SYSTEM IS the region of space that falls within the gravitational influence of the Sun, an ordinary yellow star that has shone steadily for almost 5 billion years. After the Sun itself, the most significant objects in the solar system are the planets\u2014a group of assorted rocky, gaseous, and icy worlds that follow independent, roughly circular orbits around their central star. Most of the planets are orbited in turn by moons, while a huge number of smaller lumps of rock and ice also follow their own courses around the Sun\u2014though largely confined in a few relatively crowded zones. Myriad tiny particles flow around all these larger bodies\u2014 ranging from fragments of atoms blown out by the Sun to motes of dust and ice left in the wake of comets. Our local corner of the universe has been studied intensively from the time of the first stargazers to the modern era of space probes, yet it is still a source of wonder and surprise. SOLAR FLARE On the broiling surface of the Sun, a cataclysmic release of magnetic energy triggers a solar flare\u2014a violent outburst of radiation and high-energy particles that will reach Earth within hours. THE SOLAR SYSTEM","100 THE HISTORY OF THE SOLAR SYSTEM THE HISTORY OF THE SOLAR SYSTEM 22\u201323 The scale of the universe THE SOLAR SYSTEM IS THOUGHT to have 1 SOLAR NEBULA FORMS 24\u201327 Celestial objects begun forming about 4.6 billion years 34\u201337 Radiation ago from a gigantic cloud of gas and The solar nebula started as a huge 38\u201339 Gravity, motion, and orbits dust, called the solar nebula.This cloud cloud of cold gas and dust, many times 68\u201369 Planetary motion contained several times the mass of the larger than our present solar system. present-day Sun. Over millions of years, Its initial temperature would have been about \u2013382\u00b0F (\u2013230\u00b0C). From the start, the solar nebula was probably spinning very slowly. it collapsed into a flat, spinning disk, which had a dense, hot central region.The central part of the disk eventually became the Sun, while the planets and everything else in the solar system formed from a portion of the remaining material. THE FORMATION OF THE SIX STEPS TO FORM A SOLAR SYSTEM SOLAR SYSTEM Shown here is an outline of the nebular hypothesis\u2014the most widely accepted No one knows for certain what caused the great cloud of gas and theory for how the solar system formed. dust, the solar nebula from which the solar system formed, to start It provides a plausible explanation for collapsing.What is certain is that gravity somehow overcame the many of the basic facts about the solar forces associated with gas pressure that would otherwise have kept it system. For example, it explains why the orbits of most of the planets lie roughly in the same plane and why the planets all orbit in the same direction. expanded. As it collapsed, the cloud flattened into a pancake-shaped disk with a bulge at its center. Just as an ice skater spins faster as she PIERRE-SIMON DE LAPLACE pulls in her arms, the disk began to rotate faster and faster as it contracted.The central region also became hotter and denser. In the parts of the disk closest to this hot central region, only rocky particles and metals could remain in solid form. Other materials were vaporized. In due course, these rocky and metallic particles gradually came Pierre Laplace (1749\u20131827) was a French together to form planetesimals (small bodies of rock, up to several mathematician who developed the nebular miles in diameter) and eventually the inner rocky planets\u2014Mercury, hypothesis\u2014the idea, originally proposed by Venus, Earth, and Mars. In the cooler outer regions of the disk, a similar the German philosopher Immanuel Kant, process occurred, but the solid particles that came together to form that the solar system originated from the planetesimals contained large amounts of various ices, such as water, contraction of a huge gaseous nebula.Today, ammonia, and methane ices, as well as rock.These materials were this hypothesis provides the most widely destined eventually to form the cores of the gas-giant planets\u2014 accepted theory for how the solar system Jupiter, Saturn, Uranus, and Neptune. formed. Another of Laplace\u2019s contributions to science was to analyze the complex 6 REMAINING DEBRIS inner solar forces of gravitational attraction between system the planets. He investigated how these Radiation from the Sun blew away might affect the stability of the solar most of the remaining gas and other system and concluded that the system is unaccreted material in the planetary inherently stable. solar system. Some of the leftover planetesimals in the outer part of the IDA disk formed the vast and remote Oort The ring of planetesimals Cloud of comets. between Mars and Jupiter failed to form a planet, possibly THE SOLAR SYSTEM because of the gravitational influence of Jupiter. Instead they formed a belt of asteroids, including this asteroid, Ida. frozen cometary nuclei","THE HISTORY OF THE SOLAR SYSTEM 101 dense, hot central ROCK AND ICE PARTICLES region (protosun) As they orbited the protosun, grains of dust and ice collided at low velocities and protoplanetary disk became stuck together. Over tens of millions of years, these particles grew to form the planetesimals. 3 RINGS AND PLANETESIMALS Instabilities in the rotating disk caused regions within it to condense into rings under the influence of gravity. Very gradually, planetesimals (small objects made of rock or rock and ice) formed in these rings through the accretion of much smaller particles. 2 FORMATION OF THE PROTOSUN Under the influence of gravity, the solar nebula condensed into a dense central region (the protosun) and a diffuse outer region (the protoplanetary disk). As it contracted, the cloud began to spin faster and flattened out, and its central region heated up. GAS AND DUST 4 ROCKY PLANETS planetesimals The cloud consisted mainly of forming within hydrogen and helium gas, together The planetesimals attracted each other rings with grains of dust containing some by gravity and collided to build planets. metals and substances such as water, Closest to the protosun, where it was hot inner region methane, and ammonia. extremely hot, only rocky material and of disk metals could withstand the heat, so the VENUS planets formed in this region are made Venus and the other inner mainly of these materials. rocky planets were formed in a molten state, because the collisions that led to their formation generated a huge amount of heat. Later, they partly solidified. 5 GAS GIANTS accreting planetesimals In the outer part of the disk, the bodies formed from planetesimals were made THE BIRTH OF THE PLANETS of rock and ice; they became big enough to attract large amounts of After tens of millions of years of planetesimal formation, the final stages gas around them. Soon after these gas of planet construction are thought to have happened relatively quickly, giants formed, the protosun became about 4.56 billion years ago. Once the planetesimals were a few miles a full-fledged star. in diameter, their gravity was strong enough to attract more and more material in a runaway process. Many of the planetesimals came together cooler outer Sun begins GAS-PLANET FORMATION to form Moon-sized bodies called protoplanets, which finally THE SOLAR SYSTEM part of disk producing In the gas giants, such as Jupiter energy by and Saturn, a core of rock and ices underwent a series of dramatic collisions to form gas giant forming nuclear fusion formed first. These cores then the rocky inner planets and the cores of the outer attracted, and became enveloped gas-giant planets.The latter, containing both rock by, large amounts of gas. and ice, were massive enough also to pull in vast amounts of hydrogen, helium, and other gases, which accreted onto the planetary cores to form dense atmospheres. Many of the leftover planetesimals are thought to have become comets and asteroids. Pluto may have formed from material not used in the gas giants, or it may have been captured by the solar system at a later time.","102 THE FAMILY OF THE SUN THE FAMILY OF THE SUN URBAIN LE VERRIER 22\u201323 The scale of the universe THE SOLAR SYSTEM CONSISTS OF the Sun, eight Urbain Le Verrier 24\u201327 Celestial objects recognized planets, over 140 moons, and countless (1811-1877) was a 34\u201337 Radiation small bodies such as asteroids ands comets. Its inner French mathematician 38\u201339 Gravity, motion, and orbits region contains the Sun and the rocky planets\u2014 and astronomer who, after 68\u201369 Planetary motion Mercury,Venus, Earth, and Mars. Beyond this lies studying irregularities in a ring of asteroids, called the Main Belt, and the the orbit of Uranus, The Milky Way 226\u201329 gas giant planets Jupiter, Saturn, Uranus, and predicted the existence of the planet Neptune, Neptune. Next is a huge region populated by Pluto and other ice and calculated its position in 1846. He asked the dwarfs and finally a vast cloud of comets. In total, the solar system is German astronomer Johann Galle to look for about 9.3 trillion miles (15 trillion km) across; the planets occupy a Neptune, and within an hour the planet had been found. zone extending just 3.25 billion miles (6 billion km) from the Sun. ORBITS IN THE SOLAR SYSTEM THE SUN Most orbits of objects in the solar system have the shapes of ellipses The Sun's diameter at the (stretched circles). However, for most of the planets, these ellipses are close equator is 864,900 miles to being circular. Only Mercury has an orbit that differs very markedly (1.4 million km), and its from being circular. All the planets and nearly all asteroids orbit the Sun equatorial rotation period in the same direction, which is also the direction in which the Sun spins is about 25 Earth days on its own axis.The orbital period (the time it takes to orbit the Sun) increases with distance from the Sun, from 88 Earth days for EARTH Mercury to nearly 250 Earth years for Pluto, following a Orbits the Sun in 365.26 mathematical relationship first discovered by the German Earth days at an average astronomer Johannes Kepler in the early 17th century distance of 92.9 million miles (see p.68). As well as having longer orbits to (149.6 million km) complete, the planets farther from the Sun move much more slowly. JUPITER Orbits the Sun in 11.86 Earth years at an average distance of 483.4 million miles (778.4 million km) THE SOLAR SYSTEM URANUS MERCURY Orbits the Sun in 84.01 Orbits the Sun in 88 Earth Earth years at an average days at an average distance of distance of 1.8 billion 36.0 million miles (57.9 million km) miles (2.9 billion km) PLANET ORBITS All the orbits of the planets, and the Asteroid Belt, lie roughly in a flat plane known as the ecliptic plane. Only Mercury and the dwarf planet Pluto orbit at significant angles to this plane (7.0\u00b0 and 17.1\u00b0, respectively). The planets and their orbits are not shown to scale here.","THE FAMILY OF THE SUN 103 THE GAS GIANTS THE ROCKY PLANETS The four large planets immediately The four inner planets of the solar system are also called the rocky planets.They are much smaller than the gas giants, have few or no moons, beyond the Asteroid Belt are called the and no rings. All four were born in a molten state due to the heat of the collisions that led to their formation.While molten, the materials gas giants.These planets have many from which they are made became separated into a metallic core and a rocky mantle and crust.Throughout their later history, all these planets properties in common. Each has a core suffered heavy bombardment by meteorites that left craters on their surfaces, although on Earth these craters have largely become hidden by composed of rock and ice.This is various geological processes. In some other respects, the rocky planets surrounded by a liquid or semi-solid are quite diverse. For example,Venus has a dense mantle containing hydrogen and helium, atmosphere consisting mainly of carbon dioxide, or, in the case of Uranus and Neptune, a while Mars has a thin atmosphere composed of the combination of methane, ammonia, and same gas. In contrast, Mercury has virtually no water ices. Each has a deep, often stormy atmosphere and Earth\u2019s is rich in nitrogen and oxygen. atmosphere composed mainly of URANUS AND RINGS hydrogen and helium. All four have a Uranus has 11 major rings and a blue significant magnetic field, but Jupiter\u2019s is coloration caused by the presence of exceptional, being 20,000 times stronger methane in its atmosphere (this is a than that of Earth. Each of the gas giants Hubble infrared image). Its spin axis is tilted over on its side. is orbited by a large number of moons, several dozen in the case of Jupiter. Finally, all four gas giants have ring systems made of grains of rock or ice.These rings may have been present since the planets formed, or they may be the fragmented remains of moons that were broken up by the gas giants\u2019 powerful gravitational fields. 6-mile- (10-km-) wide impactor GOSSES BLUFF CRATER MARS VENUS This impact crater in a central desert Orbits the Sun in 687 Earth Orbits the Sun in 224.7 region of Australia resulted from an days at an average distance Earth days at an average asteroid 0.6 miles (1 km) wide that of 141.6 million miles distance of 67.2 million smashed into Earth\u2019s surface (227.9 million km) miles (108.2 million km) 142 million years ago. MAIN BELT IMPACTOR STRIKES back of impactor Lies between the orbits of front of continues Mars and Jupiter and is a impactor forwards source of meteorites; some collapses asteroids orbit the Sun outside the Main Belt EXPLOSION ON IMPACT crater 60 miles rocks (100 km) wide and blast into 7.5 miles (12 km) atmosphere deep SATURN Orbits the Sun in 29.46 Earth years at an average distance of 886 million miles (1.4 billion km) CRATER FORMATION steep sides crater up to fall in 150 miles (240 km) wide NEPTUNE CRATER COLLAPSE THE SOLAR SYSTEM Orbits the Sun in 164.8 DEEP IMPACT Earth years at an average This sequence shows what typically distance of 2.8 billion happens when a 6-mile- (10-km-) miles (4.5 billion km) wide projectile hits a rocky planet or moon. The crater formed is much larger than the impactor. The latter usually vaporizes on impact, though some melted or shattered remnants may be left at the site.","104 THE SUN THE SUN 31 Nuclear fission and fusion THE SUN IS A 4.6-BILLION-YEAR-OLD main-sequence 34\u201337 Radiation star. It is a huge sphere of exceedingly hot plasma 67 Solar eclipses (ionized gas) containing 750 times the mass of all the solar system\u2019s planets put together. In its core, Stars 232\u201333 nuclear reactions produce helium from hydrogen The life cycles of stars 234\u201337 and generate colossal amounts of energy.This energy is gradually carried outward until it Star formation 238\u201339 eventually escapes from the Sun\u2019s surface. Main-sequence stars 250\u201351 INTERNAL STRUCTURE The Sun has three internal layers, although there are no sharp boundaries between them. At the center is the core, where temperatures and pressures are extremely high. In the core, nuclear fusion turns the nuclei of hydrogen atoms (protons) into helium nuclei at the rate of about 600 million tons per second. Released as byproducts of the process are energy, in the form of photons of electromagnetic (EM) radiation, and neutrinos (particles with no charge and almost no mass).The EM radiation travels out from the core through a slightly cooler region, the radiative zone. It takes about 1 million years to find its way out of this zone, as the photons are continually absorbed and reemitted by ions in the plasma. Farther out, the energy wells up in a convective zone\u2014where huge flows of rising hot plasma occur next to areas of falling cooler plasma\u2014and is transferred to a surface layer called the photosphere. There it escapes as heat, light, and other forms of radiation. chromosphere is an irregular layer of atmosphere above the photosphere photosphere is the Sun\u2019s visible surface convective zone is a region where energy is carried by convection cells radiative zone, where energy travels in the form of photons core, where nuclear reactions occur THE SUN\u2019S STRUCTURE The Sun\u2019s interior consists of the core, the radiative zone, and the convective zone. Light and heat escape into space at the photosphere. The Sun is composed principally of hydrogen (71 percent by mass) and helium (27 percent). SUN PROFILE THE SOLAR SYSTEM AVERAGE DISTANCE FROM EARTH ROTATION PERIOD (POLAR) VIOLENT SUN This composite image taken by the SOHO 93.0 million miles (149.6 million km) 34 Earth days observatory shows both the Sun\u2019s surface and its corona. When the corona image SURFACE TEMPERATURE ROTATION PERIOD (EQUATORIAL) was taken, billions of tons of matter were being blasted through it into space. 9,932\u00b0F (5,500\u00b0C) 25 Earth days MASS (EARTH = 1) 333,000 CORE TEMPERATURE SIZE COMPARISON 27 million \u00b0F (15 million \u00b0C) EARTH DIAMETER AT EQUATOR THE SUN 864,900 miles (1.4 million km) OBSERVATION The Sun has an apparent magnitude of \u201326.7 and should never be observed directly with the naked eye or any optical instrument. It can be observed safely only through special solar filters.","105 STUDYING THE SUN FROM SPACE Since 1960, a series of space probes and satellites have been launched by NASA and other organizations with the aim of collecting data about the Sun. Some of the most important missions are listed below. 1960\u201368 PIONEERS 5 TO 9 (USA) These were a series of probes that successfully orbited the Sun and studied the solar wind, solar flares, and the interplanetary magnetic field. 1974, 1976 HELIOS 1 AND 2 (USA AND GERMANY) The two Helios probes were put into orbits that involved high-velocity passes close to the Sun\u2019s surface. They measured the solar wind and the Sun\u2019s magnetic field. 1980 SOLAR MAXIMUM MISSION (USA) This studied the Sun at its most active, collecting X-rays, gamma rays, and ultraviolet radiation produced by flares, sunspots, and prominences. 1990 ULYSSES (USA AND EUROPE) The first space probe to be sent into an orbit over the Sun\u2019s poles, Ulysses has studied the solar wind and the Sun\u2019s magnetic field over its polar regions. 1991 YOHKOH (JAPAN, USA, AND UK) Yohkoh was an Earth-orbiting satellite that for 10 years observed high-energy radiation (X-rays and gamma rays) produced by solar flares, as well as pre-flare conditions. YOHKOH SOHO 1995 SOHO (USA AND EUROPE) This solar observatory follows a special \u201chalo\u201d orbit around the Lagrangian point 930,000 miles (1.5 million km) from Earth in the direction of the Sun. SOHO (solar and heliospheric observatory) studies the Sun\u2019s interior and events at its surface. 1998 TRACE (USA) Trace is a satellite in Earth\u2019s orbit that studies the corona and a thinner layer in the Sun\u2019s atmosphere called the transition region. The objective of TRACE (transition region and coronal explorer) is to better understand the connection between the Sun\u2019s magnetic field and coronal heating. 2006 STEREO (USA) The Solar Terrestrial THE SOLAR SYSTEM Relations Observatory consists of twin spacecraft that observe the Sun from different directions, giving all-round coverage of solar eruptions and the solar wind. 2010 SDO (USA) NASA\u2019s Solar Dynamics Observatory monitors the Sun to improve our understanding of its activity and to make better predictions of how this activity will affect Earth. SDO","106 THE SUN CORONAL MASS SURFACE EJECTION is a bubble of plasma The visible surface of the Sun is called the ejected from the Sun into space photosphere. It is a layer of plasma (ionized CORONA gas) about 60 miles (100 km) thick and is hundreds of times hotter than appears granulated or bubbly.The bumps, the photosphere which are about 600 miles (1,000 km) GRANULATION is the mottling of wide, are the upper surfaces of convection the surface caused by cells that bring hot plasma up from the convection cells Sun\u2019s interior. Other significant features of FACULAE are intensely the photosphere are sunspots, which are bright active regions that cooler regions that appear dark against are associated with the their brighter, hotter surroundings. SUNSPOTS appearance Sunspots and related phenomena, such as Each sunspot has a dark central of sunspots solar flares (tremendous explosions on the region, the umbra, and a lighter Sun\u2019s surface) and plasma loops, are periphery, the penumbra. Away thought to have a common underlying from the sunspots, the Sun\u2019s cause\u2014they are associated with strong surface looks granulated. Each magnetic fields or disturbances in these granule is the top of a convection cell in the Sun\u2019s interior. fields.The magnetic fields result from the fact that the Sun is a rotating body that consists largely of electrically charged particles (ions in its plasma). Different parts of the Sun\u2019s convective zone rotate at different rates (faster at the equator than the poles), causing the magnetic field lines to become twisted and entangled over time. Sunspots are caused by concentrations of magnetic field lines inhibiting the flow of heat from the interior where they intersect the photosphere. Other types of disturbance are caused by twisted field lines popping out of the Sun\u2019s surface, releasing tremendous energy, or by plasma erupting as loops along magnetic field lines.The amount of sunspot and related activity varies from a minimum to a maximum over an 11-year cycle. SPICULES PROMINENCE are short-lived is a dense cloud jets of gas that of gas, suspended are 6,000 miles above the Sun\u2019s (10,000 km) long surface by magnetic field loops, that may persist for days or even weeks SOLAR ACTIVITY FIRST OBSERVATION OF A SOLAR QUAKE PHOTOSPHERE This ultraviolet image The base of the photosphere has a temperature of the Sun was obtained of 10,300\u00b0F (5,700\u00b0C) but its upper layers are by an instrument cooler and emit less light. Here, the edge of the onboard the SOHO solar Sun\u2019s disk looks darker because light from it has observatory. It shows emanated from these cooler regions. the Sun\u2019s chromosphere THE SOLAR SYSTEM (the layer just above the 1 In July 1996, by analyzing data 2 The quake, equivalent to an 3 The seismic waves looked like 4 Over the course of an hour, the photosphere) and various protuberances, including obtained by an instrument on the earthquake of magnitude 11, was ripples on a pond but were 2 miles waves traveled a distance equal to a huge solar prominence, as well as a number of SOHO observatory, scientists recorded caused by a solar flare, visible as the (3 km) high and reached a maximum 10 Earth diameters before fading active regions on the solar surface. The image a solar quake for the first time. white blob with a \u201ctail\u201d to its left. speed of 248,600 mph (400,000 km\/h). into the fiery background. also shows a coronal mass ejection with a bright central area of ultraviolet emission.","THE SUN 107 JOSEPH VON ATMOSPHERE CHROMOSPHERE FRAUNHOFER The Sun\u2019s chromosphere is As well as forming its visible surface, the photosphere is the visible here as an irregular, A German physicist and optical lowest layer of the Sun\u2019s atmosphere. Above it are three more thin red arc adjacent to the instrument maker, Joseph von atmospheric layers.The orangey-red chromosphere lies much brighter photosphere. Fraunhofer (1787\u20131826) is best above the photosphere and is about 1,200 miles (2,000 km) Also apparent is a flame- known for his investigation of dark deep. From the bottom to the top, its temperature rises from like protuberance from lines in the Sun\u2019s spectrum. Now 8,100\u00b0F (4,500\u00b0C) to about 36,000\u00b0F (20,000\u00b0C).The the chromosphere into known as Fraunhofer lines, they chromosphere contains many flamelike columns of plasma the corona. correspond to wavelengths of light called spicules, each rising up to 6,000 miles (10,000 km) absorbed by chemical elements high along local magnetic field lines and lasting for a few CORONA in the outer parts of the Sun\u2019s minutes. Between the chromosphere and the corona is a The outermost layer atmosphere. Fraunhofer\u2019s thin, irregular layer called the transition region, within which of the Sun, the corona observations were later used to the temperature rises from 36,000\u00b0F (20,000\u00b0C) to about extends outward into help determine the composition 1.8 million \u00b0F (1 million \u00b0C). Scientists are studying this space for millions of the Sun and other stars. region in an attempt to understand the cause of the of miles from the temperature increase.The outermost layer of the solar chromosphere. It is atmosphere, the corona, consists of thin plasma. At a great most easily observed distance from the Sun, this blends with the solar wind, a during a total eclipse stream of charged particles (mainly protons and electrons) of the Sun, as here. flowing away from the Sun across the solar system.The corona is extremely hot, 3.6 million \u00b0F (2 million \u00b0C), for reasons that are not entirely clear, although magnetic phenomena are believed to be a major cause of the heating. Coronal mass ejections (CMEs) are huge bubbles of plasma, containing billions of tons of material, that are occasionally ejected from the Sun\u2019s surface through the corona into space. CMEs can disturb the solar wind, which results in changes to aurorae in Earth\u2019s atmosphere (see p.74). CORONAL MASS MAGNETIC ERUPTION Hot plasma explodes into EJECTION the atmosphere, following This image of a coronal magnetic field lines. In this mass ejection (top left) TRACE image, colors represent was taken by the SOHO temperature, with blue being solar observatory, using the coolest, red the hottest. a coronagraph\u2014an instrument that blocks direct sunlight by means of an occulter (the central smooth red area in the image). The white circle represents the occulted disk of the Sun. NORTHERN LIGHTS POST-FLARE LOOPS THE SOLAR SYSTEM When charged particles from the solar These three images of a magnetically active wind reach Earth, they can cause solar region, taken by the TRACE satellite, aurorae. This photograph of the aurora span a period of 2.5 hours. The loops in the borealis was taken in Manitoba, Canada. Sun\u2019s corona probably followed a solar flare and consist of plasma heated to exceedingly high temperatures along magnetic field lines.","MAGNETIC DISTURBANCE Active regions are areas on the Sun where magnetic fields burst through the Sun\u2019s visible surface, causing dark sunspots and bright areas called faculae. Active Region 1429, seen here, appeared in March 2012 and fired off flares and coronal mass ejections that caused spectacular aurorae on Earth. In this false-color image, the dark regions are the hottest and brightest.","","110 MERCURY MERCURY 38\u201339 Gravity, motion, and orbits MERCURY IS THE SECOND-SMALLEST planet in the solar 68\u201369 Planetary motion system, the closest planet to the Sun, and the richest in iron. 100\u2013101 The history of the Solar System The surface environment is extremely harsh.There is hardly 102\u2013103 The family of the Sun any shielding atmosphere, and the temperature rises to a blistering 800\u00b0F (430\u00b0C) during the day, then plummets to an air-freezing \u2013290\u00b0F (\u2013180\u00b0C) at night. No other planet experiences such a wide range of temperatures. Its surface has been churned up by meteoritic bombardment and is dark and dusty. ORBIT With the exception of Pluto, TRANSIT OF MERCURY Mercury has the most eccentric Mercury passes directly of all the planetary orbits. At between Earth and the perihelion it is only 28.6 million Sun about 13 times a miles (46 million km) from the century. This row of dots Sun, but at aphelion it is 43.3 is a multiple exposure of million miles (69.8 million km) Mercury\u2019s transit across the Sun in 2006. away.The plane of Mercury\u2019s equator coincides with the plane of its orbit (in other SPIN AND ORBIT Mercury rotates three times in two words, its axis of rotation is almost vertical).This means that the planet has no seasons, and that some orbits (in other words, there are three craters close to the poles never receive any sunlight Mercurian \u201cdays\u201d in two Mercurian and are permanently cold.The orbit is inclined at 7\u00b0 \u201cyears\u201d). This unusual spin\u2013orbit to the plane of the Earth\u2019s orbit. Because Mercury coupling means that for an observer orbits inside the Earth\u2019s orbit, it displays phases, just standing on Mercury there would be like the Moon (see p.62). an interval of 176 Earth days between one sunrise and the next. axial tilt is planet spins on its axis almost vertical every 58.65 Earth days Sun PERIHELION 29 million APHELION miles (46 million km) 43 million miles(70 million km) Mercury orbits Sun in 88 Earth days EXPLORING SPACE perihelion moves with each EINSTEIN AND MERCURY orbit THE SOLAR SYSTEM Mercury\u2019s perihelion position moves Sun slightly more than Isaac Newton\u2019s orbital path theories of motion predict. In the 19th century, it was proposed that a planet (called Vulcan) inside Mercury\u2019s orbit produced this effect. In his general theory of relativity of 1915, the German physicist Albert Einstein suggested that space near the Sun was curved and correctly predicted the exact amount by which the perihelion would move. MERCURY\u2019S WOBBLY ORBIT aphelion POCKMARKED PLANET Mercury\u2019s perihelion advances by about 1.55\u00b0 Mercury\u2019s heavily cratered surface, every century, which is 0.012\u00b0 more than is seen here from the Messenger probe expected given the gravitational influence of in 2009, resembles the highland areas of nearby planets. the Moon. The planet also has large expanses of younger, smooth, lightly cratered plains, rather like the lunar maria.","MERCURY 111 crust of STRUCTURErocky, silicate silicate rock mantle iron The very high density of Mercury indicates that it is rich in iron.This core iron sank to the center some 4 billion years ago, producing a huge core, 2,235 miles (3,600 km) in diameter.There is a possibility that a thin layer of the outer core is still molten.The solid rocky mantle is about 340 miles (550 km) thick and makes up most of the outer 25 percent of the planet.This outer mantle has slowly cooled, and during the last billion years volcanic eruptions and lava flows have ceased, making the planet tectonically inactive.The mantle and the thin crust mainly consist of the silicate mineral anorthosite, just like the old lunar highlands.There are no iron oxides. Unlike MERCURY INTERIOR on other planets, it seems that all the iron has Compared to the other rocky gone into the core, which produces a planets, Mercury is very rich in metals magnetic field with a strength that is about and poor in heat-producing radioactive one percent of Earth\u2019s magnetic field. elements. Its huge iron core is probably solid. ATMOSPHERE oxygen (52%) sodium (39%) potassium and other gases (1%) Mercury has a very thin helium (8%) temporary atmosphere because the planet\u2019s mass is too small for an atmosphere to persist. Mercury is ATMOSPHERIC COMPOSITION very close to the Sun, so daytime temperatures are extremely high, Oxygen is the most abundant gas, followed reaching 810\u00b0F (430\u00b0C).The escape velocity is less than half that of by sodium and helium. However, loss and Earth, so hot, light elements in the atmosphere, such as helium, regeneration of the gases is continuous, and quickly fly off into space. All the atmospheric gases therefore need the atmospheric composition can vary constant replenishment. Mercury\u2019s atmosphere was analyzed by an drastically over time. ultraviolet spectrometer onboard the Mariner 10 spacecraft in 1974. Oxygen, helium, and hydrogen were detected in this way, and northern DAY 1 hemisphere subsequently atmospheric sodium, potassium, and calcium have been detected by Earth-based telescopes.The hydrogen and helium are captured from the solar wind of gas that is constantly escaping from the Sun.The other elements originate from the planet\u2019s surface and are intermittently kicked up into the tenuous atmosphere by the impact of ions from Mercury\u2019s magnetosphere and micrometeorite direction southern particles from the solar system dust cloud. The atmospheric gases are of sunlight hemisphere much denser on the cold night side of the planet than on the hot day side, as the molecules have less energy to escape. DAY 2 MERCURY\u2019S SODIUM TAIL cloud of night side Pressure exerted by sunlight pushes sodium of Mercury sodium atoms away from Mercury, vapor forming a \u201ctail\u201d some 25,000 miles DAY 3 (40,000 km) long. Mercury and the Sun are off to the left in this false- colored view of Mercury\u2019s sodium tail. Emissions from this tail have previously been observed with Earth- based telescopes, but this image from a spectrometer on board Messenger is the most detailed image yet. MERCURY PROFILE AVERAGE DISTANCE FROM THE SUN ROTATION PERIOD sodium cloud THE SOLAR SYSTEM has disappeared 36 million miles (57.9 million km) 59 Earth days TEMPORARY ATMOSPHERE SURFACE TEMPERATURE ORBITAL PERIOD (LENGTH OF YEAR) Thin clouds of sodium suddenly appear over some regions of Mercury and then just as \u2013290\u00b0F to 810\u00b0F (\u2013180\u00b0C to 430\u00b0C) 88 Earth days quickly disappear, as seen in these false-color observations made by the Kitt Peak Solar DIAMETER 3,029 miles (4,875 km) MASS (EARTH = 1) 0.055 Observatory, USA. The clouds might be VOLUME (EARTH = 1) 0.056 produced by meteorite impacts \u2013 the freshly NUMBER OF MOONS 0 GRAVITY AT EQUATOR (EARTH = 1) 0.38 cratered surface releases sodium vapor when it is next heated by sunlight. Another possibility OBSERVATION SIZE COMPARISON MERCURY is that ionized particles actually hit Mercury\u2019s EARTH surface and release sodium from the regolith. Never more than 28\u00b0 away from the Sun in the sky, Mercury is always seen at dawn or dusk. It is the most difficult of the nearby planets to spot and is visible only for a few days each month.","","","114 VENUS VENUS 38\u201339 Gravity, motion, and orbits VENUS IS THE SECOND PLANET FROM THE SUN and 68\u201369 Planetary motion Earth\u2019s inner neighbor.The two planets are virtually 100\u2013101 The history of the solar system identical in size and composition, but these are very 102\u2013103 The family of the Sun different worlds. An unbroken blanket of dense clouds permanently envelops Venus. Underneath lies a gloomy, lifeless, dry world with a scorching surface, hotter than that of any other planet. Radar has penetrated the clouds and revealed a landscape dominated by volcanism. ORBIT Venus\u2019s orbital path is the least elliptical of all the planets. It is almost a perfect circle, so there is little difference between the planet\u2019s aphelion and perihelion distances.Venus takes 224.7 Earth days to complete one orbit. As it orbits the Sun,Venus spins extremely slowly on its axis\u2014 slower than any other planet. It takes 243 Earth days for just one spin, which means that a Venusian day is longer than a Venusian year. However, the time between one sunrise and the next on Venus is 117 Earth days.This is because the planet is traveling along its orbit as it spins, and so any one spot on the surface faces the Sun every 117 Earth days.Venus\u2019s slow spin is also in the opposite direction from most other planets.Venus does SPIN AND ORBIT not have seasons as it moves through its orbit. Venus is tipped over by This is because of its almost circular path and 177.4\u00b0. This means its the planet\u2019s small axial tilt.Venus\u2019s orbit lies spin axis is tilted by just inside that of the Earth, and about every 19 2.6\u00b0 from the vertical. As months Venus moves ahead of Earth on its a result, neither of the inside track and passes between our planet and planet\u2019s hemispheres nor the Sun. At this close encounter,Venus is poles points markedly toward the Sun during within 100 times the distance to the Moon. the course of an orbit. spins on its axis every 243 Earth days South Pole APHELION PERIHELION 67.6 million miles 66.8 million miles (108.9 million km) (107.5 million km) Sun axis tilts from vertical by 2.6\u02da orbits the Sun in planet is tilted by silicate crust 224.7 Earth days 177.4\u02da so the North Pole is at the bottom rocky mantle STRUCTURE of the globe molten iron and nickel outer core THE SOLAR SYSTEM Venus is one of the four terrestrial planets and the most similar of the group to Earth. It is a dense, rocky world just smaller than solid iron and nickel inner core Earth and with less mass.Venus\u2019s Earth-like size and density leads scientists to believe that its internal structure, its core VENUS INTERIOR dimensions, and the thickness of its mantle are also similar to Venus was formed from the same material Earth\u2019s. So Venus\u2019s metal core is thought to have a solid inner as Earth about 4.5 billion years ago and has part and a molten outer part, like Earth\u2019s core. In contrast to differentiated into distinct layers in much the Earth,Venus has no detectable magnetic field.The planet spins same way. A substantial part of the core has extremely slowly compared to Earth, far too slowly to produce solidified; the exact amount still molten is unknown. the circulation of the molten core that is needed to generate a magnetic field.Venus\u2019s internal heat\u2014generated early in the planet\u2019s history and from radioactive decay in the mantle\u2014is lost through the crust by conduction and volcanism. Heat melts the subsurface mantle material, and magma is released onto the surface.","VENUS 115 TERRIBLE BEAUTY VENUS PROFILE Venus\u2019s thick, reflective clouds cause AVERAGE DISTANCE FROM THE SUN ROTATION PERIOD the planet to shine brightly, so that from a distance it looks beguiling and 67.2 million miles (108.2 million km) 243 Earth days beautiful, which is why it was named after the Roman goddess SURFACE TEMPERATURE ORBITAL PERIOD (LENGTH OF YEAR) of love and beauty. Close up, it is a different story; no 867\u00b0F (464\u00b0C) 224.7 Earth days human could survive on this planet. DIAMETER 7,521 miles (12,104 km) VOLUME (EARTH = 1) 0.86 MASS (EARTH = 1) 0.82 GRAVITY AT EQUATOR (EARTH = 1) 0.9 NUMBER OF MOONS 0 SIZE COMPARISON OBSERVATION EARTH VENUS Venus is the brightest planet in Earth\u2019s sky and is surpassed in brightness only by the Sun and the Moon. Its maximum magnitude is \u20134.7. It is seen in the early morning or early evening sky. carbon dioxide 96.5% nitrogen and trace gases 3.5% ATMOSPHERE COMPOSITION OF ATMOSPHERE Along with carbon dioxide and Venus\u2019s carbon-dioxide-rich nitrogen, Venus\u2019s atmosphere atmosphere stretches up from the contains traces of other gases, ground for about 50 miles (80 km). such as water vapor, sulfur dioxide, and argon. A deck of clouds with three distinct layers lies within the atmosphere.The lowest layer is the densest and contains large droplets of sulfuric acid.The middle layer contains fewer droplets, and the top layer has small droplets. Close to the planet\u2019s surface, the atmosphere moves very slowly and turns with the planet\u2019s spin. Higher up, in the cloudy part of the atmosphere, fierce winds blow westward.The clouds speed around Venus once every four Earth days.The clouds reflect the majority of sunlight reaching Venus back into space, and so this is an overcast, orange-colored world.Venus\u2019s equator receives more solar heat than its polar regions.Yet the surface temperature at the equator and the poles varies by only a few degrees from 867\u00b0F (464\u00b0C), as do the day and night temperatures.The initial difference generates cloudtop winds that transfer the heat to about 80 percent the polar regions of sunlight in one large reflects away circulation cell. cloud deck As a result,Venus stretches from has no weather. about 28 miles (45 km) to reflected light about 43 miles means cloud (70 km) above surface is bright the ground and easy to see carbon dioxide thick layers of in atmosphere sulfuric acid clouds holds in heat stop most sunlight from reaching the surface THE SOLAR SYSTEM infrared radiation is absorbed by carbon dioxide and cannot escape into space MIDDLE CLOUD LAYER 20 percent of In this infrared image of Venus, heat sunlight reaches from the lower atmosphere shines rocky surface through the sulfuric acid clouds. The colors indicate the relative GREENHOUSE EFFECT transparency of the middle cloud Venus\u2019s thick cloud layers trap heat and help produce the planet\u2019s layer: white and red are thin clouds; high surface temperature in the same way that glass traps heat in a black and blue are thick. greenhouse. Only 20 percent of sunlight reaches the surface. Once there, it warms up the rock. Heat in the form of infrared radiation is then released, but it cannot escape and adds to the warming process.","","","","","120 VENUS ATLA REGIO Sapas Mons TYPE Shield volcano AGE Under 500 million years HEIGHT 1 mile (1.5 km) Rising 1 mile (1.5 km) above the CRATER ON EASTERN FLANK down the volcano\u2019s flanks.The DOUBLE SUMMIT surrounding terrain and with a Bright lava flows from Sapas Mons have flows commonly overlap, and In this Magellan image of Sapas Mons taken diameter of about 135 miles (217 km), stopped short of an impact crater on the many originate from the flanks from directly overhead, the two flat-topped Sapas Mons is one of Venus\u2019s shield volcano\u2019s eastern side. The flows, which are rather than the summit.The mesas, which give the volcano the appearance volcanoes.These are shaped like a tens of miles long, cover some of the ejecta summit has two mesas with flat of a double summit, appear dark against the shield or inverted plate, with a broad and therefore are younger than the crater. to slightly convex tops. Nearby bright lava flows. The area shown covers base and gently sloping sides, and are are groups of pits up to 0.6 miles about 400 miles (650 km) from top to bottom. like those found on Earth\u2019s Hawaiian Mons, which is 4 miles (6 km) high, (1 km) wide that are believed to Islands. Sapas Mons is located in the and the largest Venusian volcano, Maat have formed when underground Atla Regio, a broad volcanic rise just Mons, which is 5 miles (8 km) high. chambers of magma drained away north of Venus\u2019s equator with an Sapas Mons is covered in lava flows and the surface collapsed.The shield average elevation of 2 miles (3 km). and grew in size as the layers of lava volcanoes are mainly named after The region is believed to have accumulated.The flows near the goddesses: Sapas was a Phoenician formed as a result of large volumes of summit appear bright in Magellan goddess; Ozza, a Persian one; and molten rock welling up from the radar images, suggesting that these are Maat, an Egyptian. planet\u2019s interior. It is home to some rougher than the dark flows farther particularly large shield volcanoes, which are linked by complex systems of fractures.These include Ozza THE SOLAR SYSTEM SAPAS SUMMIT The bright feature in the foreground is the summit of Sapas Mons, and Maat Mons is the volcano rising up behind it. The vertical scale has been exaggerated to emphasize the surface features.","VENUS 121 LINEAR RIDGES APHRODITE TERRA Ridges 20\u201340 miles (30\u201360 km) long lie along a northern slope of Ovda Regio. Miralaidji Corona Dark lava, or possibly windblown dirt, fills the spaces between the ridges. TYPE Corona AGE Under 500 million years DIAMETER 186 miles (300 km) APHRODITE TERRA 15 and 16.Tesserae are raised plateau- this landscape. Magma, which may This large volcanic feature was shaped regions with chaotic and have welled up from the planet\u2019s formed by a plume of magma rising Ovda Regio complex patterns of crisscrossing interior, has flowed across part of under the Venusian surface.The lines. In places, the planet\u2019s crust has the region, and ridges formed by magma partially melted the crustal TYPE Highland terrain been fractured into mile-sized blocks. compression have filled with lava. rock, which was raised up above the Elsewhere there are folds, faults, and Ovda Regio is named after a Marijian surrounding land to produce the AGE Under 500 million belts of ridges and grooves hundreds (Russian) forest goddess. corona, a blisterlike formation with years of miles long.These are best seen radial faulting.The coronae on Venus along Ovda Regio\u2019s boundaries, range in size from about 30 to DIAMETER 3,279 miles where curving ridges and troughs 1,600 miles (50 to 2,600 km) across (5,280 km) have developed.There is also and are circular to elongate in shape. evidence that volcanic activity has They are named after fertility Ovda Regio is a highland area in played its part in the shaping of goddesses. Miralaidji is an Aboriginal Venus\u2019s equatorial region. It forms fertility goddess. the western part of Aphrodite Terra, HIGHLANDS AND LOWLANDS Venus\u2019s most extensive highland Tessera ridges run between RADIAL FAULTING system, which rises 2 miles (3 km) the Ovda Regio highland above the mean surface level. Ovda (right) and lowland lava Regio is one of a handful of highland flows (left). Some of the regions on Venus that displays a type highland depressions of complex ridge terrain known as have been partially filled tessera, a form of terrain, that was first by smooth material. identified in images taken by Veneras APHRODITE TERRA APHRODITE TERRA LADA TERRA LAVA CHANNEL Part of a 745-mile- Dali Chasma Artemis Corona Lada Terra (1,200-km-) long channel, carved TYPE Fault TYPE Corona TYPE Highland terrain through Lada Terra AGE Under 500 million AGE Under 500 million AGE Under 500 million by high-temperature, years years years very fluid lava, runs LENGTH 1,291 miles DIAMETER 1,614 miles LENGTH 5,350 miles from west to east (2,077 km) (2,600 km) (8,615 km) across the center of this image. The Dali Chasma lies in western Artemis is more than twice as big as Lada is the second largest of three Quetzalpetlatl, Aphrodite Terra. It is a system of the next largest corona on Venus, highland regions on Venus. It is in Eithinoha, and canyons and deep troughs coupled Heng-o. A nearly circular trough, the south polar region of the planet, Otygen. Lava has with high mountains that makes a Artemis Chasma, which has a raised largely south of latitude 50\u00b0S, and flowed over and cut through the broad, curving cut through more than rim, marks its boundary.Within it are comparatively little is known about it. northern part of the region. All three 1,200 miles (2,000 km) of the planet\u2019s complex systems of fractures, volcanic Lada Terra includes some typical tessera terras on Venus are named after surface. Along with the Diana flows, and small volcanoes. Artemis, terrain of crisscrossing troughs and goddesses of love: Aphrodite is named Chasma system, it connects the Ovda like other coronae, could have been ridges.Volcanic activity has also affected after the Greek goddess; Ishtar (see and Thetis highland regions with the formed by hot material welling up the area. Lada includes three large p.118), the Babylonian goddess; and large volcanoes at Atla Regio.The under the surface. But its large size coronae (blisterlike features), called Lada, the Slavic goddess. mountain ranges associated with the and the surrounding trough mean that other forces, such as the pulling apart canyons rise for of the crust and surface, were involved. 2\u20132.5 miles (3\u20134 km) above THE SOLAR SYSTEM the surrounding terrain.The canyons are 1.2\u20132.5 miles (2\u20133 km) deep. TROUGHS RIDGE BELT In this view along Bright and dark lava flows from the Dali Chasma, the Ammavaru Volcano, which is part of the raised 200 miles (300 km) to the left of rim of the 600-mile- this image, cut across a ridge belt (1,000-km-) wide to form a massive pool of lava. Latona Corona is visible on the left. ARTEMIS CORONA AND ARTEMIS CHASMA","122 IMPACT CRATERS Meteorite impact craters on Venus range in size from 4 miles (7 km) to 168 miles (270 km) across.The largest are multiple-ringed, those of intermediate size have central peaks, and the smaller ones have smooth floors.The smallest of all\u2014simple, bowl-like craters that are common on the Moon and Mars\u2014are scarce on Venus, because the thick atmosphere filters out the small asteroids that would create them. Venusian craters are young and in many cases in pristine condition. CUNITZ CRATER The last volcanic resurfacing of Venus could have occurred as This typical impact crater has recently as 500 million years ago, so its craters must have mostly a dark basin 30 miles (48 km) formed since then, and there has been little geological activity or wide, with mountainous weathering to affect them. Individual craters on Venus are named central peaks and a bright after women of note or are given female first names. ejecta blanket around it. ISHTAR TERRA ISHTAR TERRA GUINEVERE PLANITIA Wanda Crater Cleopatra Crater Jeanne Crater TYPE Central-peak crater TYPE Double-ring crater TYPE Central-peak crater AGE Under 500 million AGE Under 500 million AGE Under 500 million years years years DIAMETER 21.6 km DIAMETER 65 miles DIAMETER 12 miles (13.4 miles) (105 km) (19.4 km) Wanda Crater is in the northern part Cleopatra Crater is named after the MYSTERY CRATER An asteroid traveling from the of the Akna Montes mountain range. legendary Egyptian queen. It is The dark inner basin, the rim, and the southwest smashed obliquely into It was mapped first in 1984, by the located on Maxwell Montes,Venus\u2019s surrounding ejecta revealed in this Magellan the Guinevere Planitia and created Venera 15 and 16 missions, and highest mountain range, and stands image from 1990 convinced astronomers that Jeanne Crater. Ejecta pushed Magellan studied it again a few years out as a smooth, eyelike feature against Cleopatra is an impact crater. out of the impact later.The crater has a large, rugged the rough mountainous terrain. basin produced peak in the center of its smooth, lava- Cleopatra was imaged by the Venera images from Magellan revealed an a distinctive 15 and 16 spacecraft and the Arecibo inner basin and rough ejecta deposits, triangular shape. flooded floor. About radio telescope in the mid-1980s. It providing conclusive proof that Lobes formed to one-third of all was one of several circular features Cleopatra is an impact crater. the northwest of Venusian craters have that resembled both an impact crater the crater as such peaks. Material and a volcanic feature.The data of the molten material from the mountain time revealed a feature of apparently produced by ridges seems to have great depth, without the rim deposits the impact collapsed into the typical of impact craters. As a result, flowed downhill. crater\u2019s western edge. Cleopatra was classified as a volcanic caldera. However, high-resolution TRIANGULAR EJECTA CENTRAL PEAK BETA REGIO APHRODITE TERRA Balch Crater Riley Crater TYPE Central-peak crater TYPE Central-peak crater AGE Under 500 million AGE Under 500 million years years DIAMETER 25 miles (40 km) DIAMETER 16 miles (25 km) THE SOLAR SYSTEM Most impact craters on Venus have Riley Crater, named after 19th- remained unchanged since they were century botanist Margaretta Riley, is formed and have sharply defined one of the few Venusian craters to rims. However, a relatively small have been precisely measured. number have been modified by Comparison of images from different volcanic eruptions and other kinds of angles shows that the 16-mile- (25- tectonic activity. Balch Crater is one km-) wide crater\u2019s floor lies 1,880 ft of these. Its circular form was split in (580 m) below the surrounding plain, two as the land was wrenched apart the rim is 2,009 ft (620 m) above it, during the formation of a deep rift and the peak is 1,737 ft (536 m) high. valley.The rift, which is up to 12.4 miles (20 km) wide, created a division HALF CRATER that runs from north to south through A rift valley separates most of the Balch the crater\u2019s center.The western half of Crater (left) from its smaller eastern part, the crater remains intact, but most just visible on the opposite side of the of the eastern half was destroyed. A fault. The original central peak is the central peak and an ejecta blanket are bright patch in the crater\u2019s western part. visible in the western half.The crater was initially called Somerville, but is OBLIQUE VIEW OF RILEY CRATER now named after American economist and Nobel laureate Emily Balch.","VENUS 123 APHRODITE TERRA LAVINIA PLANITIA THREE CRATERS 48.8 km wide) and Saskia lies at the Aglaonice (39.6 Mead Crater Saskia Crater lower left of this 300- miles\/63.7 km mile- (500-km-) wide wide), which lie TYPE Multi-ringed crater TYPE Central-peak crater segment of Lavinia within a few Planitia. Above it are AGE Under 500 million AGE Under 500 million the Danilova and hundred miles of years years Aglaonice craters. Saskia, have been produced from DIAMETER 168 miles DIAMETER 23 miles pushed down by the radar data (270 km) (37.1 km) energy released collected by during the impact. Mead is the largest impact crater on Saskia is a middle-sized crater, and its The original crater Magellan. Raw Venus\u2014although compared to craters ejecta pattern is typical for its size. rim has collapsed and formed terraced radar images on the Moon and Mercury, it is not The ejecta blanket extends all the way walls.The incoming object must have (such as the one very large. Mead is a multiple-ringed around the crater\u2019s basin, suggesting been about 1.6 miles (2.5 km) across above) do not show features as they crater whose inner ring is the rim that the impacting body smashed into to produce a crater of this size. Images would appear to the naked eye. of the crater basin.This encloses a the surface at a high angle.The crater of Saskia and other craters, such as the Instead, brightness varies according to smooth, flat floor, which hides a has central peaks, formed as the similarly sized Danilova (30.3 miles\/ the smoothness of the surface\u2014 possible central peak.The crater floor planet\u2019s surface recoiled after being rough areas appear light, while was flooded at the time of impact as a smooth ones look dark. result of impact melt or by volcanic lava being released from below the surface.This explains why a crater of Mead\u2019s size is so shallow; there is a drop of only about 0.6 miles (1 km) between the crater rim and the crater center. LARGEST CRATER SASKIA CRATER IN 3-D Mead has two The color in this 3-D perspective view distinct rings. Ejecta of Saskia is based on the color images lies between them of the Venusian surface recorded by the and beyond the outer Venera 13 and 14 spacecraft. ring. The vertical bands running through the picture are a result of image processing. LAVINIA PLANITIA LADA TERRA Stein Crater Field Alcott Crater TYPE Crater field TYPE Degraded crater AGE Under 500 million AGE Under 500 million years years DIAMETER 41 miles (66 km) DIAMETER 8.7 miles (14 km), 6.8 miles (11 km), and 5.6 miles (9 km) Small asteroids heading for Venus\u2019s AINO PLANITIA CRATER AND OUTFLOW Alcott is one of the few craters on THE SOLAR SYSTEM surface can be broken up by the A 372-mile- (600-km-) long, radar-bright flow Venus that have been modified by planet\u2019s dense atmosphere.The Addams Crater of once-molten debris stretches to the east volcanic activity not associated with resulting fragments continue heading of Addams Crater. the crater\u2019s production. Many craters toward the surface, striking it TYPE Central-peak crater have floors flooded with lava that simultaneously within a relatively to the east.The molten material came up through the surface as the small area to form a crater field.The AGE Under 500 million flowed downhill for about 370 miles crater basin was formed. In Alcott\u2019s Stein field consists of three small years (600 km) from the impact site.The case, lava erupted elsewhere and then craters.The two smallest ones overlap. Magellan spacecraft found this area to flowed over the crater. About half of Material ejected by all three craters DIAMETER 54 miles ( 87 km) be radar bright\u2014that is, it bounced the crater\u2019s rim is still visible, along extends mainly to the northeast, back a large portion of the radio with ejecta from the original impact suggesting that the fragments struck The large, circular Addams Crater waves that Magellan transmitted to it, lying to the south and east. A channel from the southwest. Material melted measures almost 55 miles (90 km) which suggests it has a rugged surface. where lava once flowed touches the by the impacts has formed flow across, but it is its long tail that makes Venus\u2019s high surface temperature of southwest edge of the crater. this crater unusual. An asteroid hit about 867\u00b0F (464\u00b0C) allows ejecta to deposits, also lying the ground from the northwest and remain molten for a longer time than MODIFIED BY LAVA STEIN TRIPLETS to the northeast. created a crater basin with an ejecta if it were on Earth. However, once blanket stretching out beyond about the material cools below about three-quarters of the crater rim. 1,800\u00b0F (1,000\u00b0C) it becomes so Additionally, impact-melt ejecta and viscous it stops flowing.The crater is lava extend out from about a third of named after the American social the rim, creating a mermaid-style tail reformer Jane Addams.","124 EARTH EARTH 38\u201339 Gravity, motion, and orbits EARTH IS THE THIRD-CLOSEST PLANET to the Sun. 56\u201357 Life in the universe The largest of the four rocky planets, it formed 64\u201367 Celestial cycles approximately 4.56 billion years ago. Earth\u2019s 68\u201369 Planetary motion internal structure is similar to that of its planetary 74\u201375 Lights in the sky neighbors, but it is unique in the solar system 103 The rocky planets in that it has abundant liquid water at its surface, an oxygen-rich atmosphere, and is known to support life. Earth\u2019s surface is in a state of constant dynamic change as a result of processes occurring within its interior and in its oceans and atmosphere. ORBIT Earth orbits the Sun at an average speed of 67,000 mph (108,000 km\/h), in a counterclockwise direction when viewed from above its North Pole. Like the other planets, Earth orbits the Sun along an elliptical path. As a result, about 7 percent more solar radiation currently reaches Earth\u2019s surface in January than in July.The plane of Earth\u2019s orbit around the Sun is called the ecliptic plane. Earth\u2019s spin axis is not perpendicular to this plane but is tilted at an angle of 23.5\u00b0.The eccentricity of Earth\u2019s elliptical orbit around the Sun (the degree to which it varies from a perfect circle) changes over a cycle of about 100,000 SPIN AND ORBIT years, and its axial tilt varies over a 42,000- Earth is about 3 percent year cycle. Combined with a third cycle\u2014 closer to the Sun at a wobble in the direction in which the spin perihelion (in January) axis points in space, called precession (see than at aphelion (in p.64)\u2014these variations are believed to play a July). Its axial tilt part in causing long-term cycles in Earth\u2019s combined with its elliptical orbit gives rise climate, such as ice ages. to seasons (see p.65). NORTHERN axis tilts from NORTHERN Earth spins on SUMMER the vertical SPRING its axis once SOLSTICE by 23.5\u00b0 EQUINOX every 23.93 hours APHELION PERIHELION 94.5 million miles 91.4 million miles (152.1 million km) (147.1 million km) Sun NORTHERN WINTER NORTHERN Earth orbits the SOLSTICE FALL Sun in 365.26 days EQUINOX solid, rocky crust STRUCTURE mantle of THE SOLAR SYSTEM Earth\u2019s rotation causes its equatorial regions to bulge solid silicate out slightly, by about 13 miles (21 km) compared to rock the poles. Internally, Earth has three main layers.The central core has a diameter of about 4,350 miles molten iron-nickel outer core (7,000 km) and is made mainly of iron with a small amount of nickel. It has a central solid part, which solid iron-nickel has a temperature of about 8,500\u00b0F (4,700\u00b0C), and inner core an outer liquid part. Surrounding the core is the mantle, which contains rocks rich in magnesium and EARTH INTERIOR iron and is about 1,700 miles (2,800 km) deep. At Earth\u2019s center is a hot dense core. Earth\u2019s crust consists of many different types of rocks Surrounding the core are the mantle and and minerals, predominantly silicates, and is differentiated the thin, rocky outer crust, which supports into continental crust and a thinner oceanic crust. Earth\u2019s biosphere, with its oceans, atmosphere, plants, and animals.","EARTH 125 WATER WORLD EARTH PROFILE Viewed from space, what clearly makes Earth unique is the abundance of surface water\u2014 AVERAGE DISTANCE FROM THE SUN ROTATION PERIOD in the oceans, lakes, atmosphere, and polar ice caps. The presence of 93.0 million miles (149.6 million km) 23.93 hours surface water has been a key factor in the development AVERAGE SURFACE TEMPERATURE ORBITAL PERIOD (LENGTH OF YEAR) of life on Earth. 59\u00b0F (15\u00b0C) 365.26 days DIAMETER 7,926 miles (12,756 km) MASS (EARTH = 1) 1 VOLUME (EARTH = 1) 1 GRAVITY AT EQUATOR (EARTH = 1) 1 NUMBER OF MOONS 1 MAGNETIC FIELD Earth has a substantial magnetic field, which is thought to be caused by a swirling motion of its liquid metal outer core.This motion is driven by a combination of Earth\u2019s rotation and convection currents within the outer core.The magnetic field behaves as though a large bar magnet were present within the Earth, tilted at an angle to its axis of rotation.The lines of the magnetic field converge at two points on Earth\u2019s surface called the north and south magnetic poles.The location of these points slowly changes over time. Currently, the north magnetic pole is north of Canada in the Arctic Ocean, while the south magnetic pole is north of JAMES VAN ALLEN eastern Antarctica, in the Southern Ocean.The James Van Allen (1914\u20132006) is magnetic field extends into an American physicist who, in space, forming a protective the 1950s, designed and built blanket around the planet by instruments for American satellites. deflecting high-speed streams In 1958, a Van Allen-designed of charged particles that flow instrument carried by the first US satellite, Explorer 1, detected two toward Earth in the solar wind large, doughnut-shaped belts of (see p.107). A few of the radiation around Earth, which carry particles escape deflection and trapped charged particles.The belts become trapped within two are named after Van Allen. regions surrounding Earth called the Van Allen radiation belts (see panel, right). Studies of iron-rich minerals in Earth\u2019s crust have shown that at variable time intervals (from less than 100,000 to millions of years) Earth\u2019s north and south magnetic poles switch. magnetic axis magnetic equatorial plane direction of magnetic force lines solar wind magnetosphere tail THE SOLAR SYSTEM bow shock Van Allen belts EARTH\u2019S MAGNETOSPHERE The imaginary surface at which Earth\u2019s magnetic field first deflects the solar wind is called the bow shock. Behind it is a region of space dominated by the magnetic field, in the sense that the field prevents solar wind particles from entering. Despite its elongated shape, this region is called the magnetosphere.","126 EARTH ATMOSPHERE AND THERMOSPHERE HEIGHT WEATHER ABOVE The thermosphere SEA LEVEL Earth is surrounded by the atmosphere, a layer of gases extends to over 375 many hundreds of miles thick.This atmosphere is miles (600 km) above 130 km thought to have arisen partly from gases spewed out by the Earth\u2019s surface. 81 miles ancient volcanoes, although its oxygen content\u2014so Temperature rises vital to most forms of life\u2014was created mainly by rapidly in the lower 120 km plants.Through the effects of gravity, the atmosphere thermosphere due to 75 miles absorption of solar energy and then aurora 110 km is densest at Earth\u2019s surface and rapidly thins with increases gradually 68 miles altitude.With increasing altitude, there are also with altitude, changes in temperature and a progressive drop in reaching as high as 100 km atmospheric pressure. For example, at a height of 3,100\u00b0F (1,700\u00b0C). 62 miles 19 miles (30 km), the pressure is just 1 percent that at sea level.Within the lowest layer of the meteor burning up 90 km atmosphere, the troposphere, continual changes in the atmosphere 56 miles occur in temperature, air flow (wind), humidity, and precipitation, known as weather.The basic MESOSPHERE ice crystals on 80 km cause of weather is the fact that Earth absorbs more meteoric dust 50 miles of the Sun\u2019s heat at the equator than the poles.This This layer extends up produces variations in atmospheric pressure, which to about 50 miles ozone layer absorbs 70 km create wind systems.The winds drive ocean currents (80 km). Temperatures harmful radiation 43 miles and cause masses of air with different temperatures and fall through the from the Sun moisture content to circulate over the planet\u2019s surface. mesosphere to as low 60 km Earth\u2019s rotation plays a part in causing this atmospheric as \u2013135\u00b0F (\u201393\u00b0C). 37 miles circulation because of the Coriolis effect (below). STRATOSPHERE 50 km ATMOSPHERIC LAYERS 31 miles The four main layers in Earth\u2019s The stratosphere is a atmosphere are distinguished by calm layer stretching 40 km different temperature characteristics. up to about 30 miles 25 miles No boundary exists at the top of the (50 km) above sea atmosphere. Its upper regions level. The temperature 30 km progressively thin out and rises to 27\u00b0F (\u20133\u00b0C) at 19 miles merge with space. the top of this layer. 20 km TROPOSPHERE 12 miles initial direction of moving air This layer extends to 10 km deflection to right 5 miles (8 km) above 6 miles (Northern Hemisphere) the poles and 10 miles sea level (16 km) above the equator. It contains 75 percent of the total all weather occurs direction of spin THE CORIOLIS EFFECT mass of the atmosphere. Temperatures in the lowest level The Coriolis effect causes fall to as low as \u201362\u00b0F (\u201352\u00b0C) at the top. of the atmosphere deflections of air moving across Earth\u2019s surface. It is a nitrogen 78.1% argon and trace gases 1% consequence of the fact that objects at different latitudes deflection to move at different speeds left (Southern around Earth\u2019s spin axis. Hemisphere) COMPOSITION OF ATMOSPHERE oxygen 20.9% Nitrogen and oxygen make up 99 percent of dry air by volume. About 0.9 percent is argon, and the destructive boundary, plate dragged constructive boundary, rest consists of tiny amounts of other gases. The where tectonic along by where plates diverge and plates converge convection current new crust is created atmosphere also contains variable amounts (up to circular motion of 4 percent) of water vapor. convection current plate in collision upper mantle lithospheric descends lower mantle tectonic plate into mantle MOVING PLATES PLATE TECTONICS Earth\u2019s plates move Earth\u2019s crust and the top part of its mantle are joined in a structure called the lithosphere.This is broken up mantle plume rises outer core relative to each other as a into several solid structures called plates, which \u201cfloat\u201d on from lower mantle result of convection currents underlying semi-molten regions of the mantle and move relative within the mantle. The currents to each other. Most plates carry both oceanic crust and some thicker THE SOLAR SYSTEM continental crust, although a few carry only oceanic crust.The scientific cause parts of the mantle to rise, theory concerning the motions of these plates is called plate tectonics, and the phenomena associated with the movements are called tectonic features. move sideways, and then sink again, dragging the plates along as they do so. Most tectonic features, which include ocean ridges, deep sea trenches, high North American Plate mountain ranges, and volcanoes, result from processes occurring at plate boundaries.Their nature depends on the type of crust on either side of the Eurasian Plate boundary and whether the plates are moving Pacific Plate toward or away from each other. Tectonic features TECTONIC PLATES occurring away from plate boundaries include Earth\u2019s surface is broken Plate boundary volcanic island chains, such as the Hawaiian into seven large plates, such Indian Plate islands.These are caused by magma (molten rock) as the Eurasian plate, and Australian Plate upwelling from \u201chot spots\u201d in the mantle, causing a many smaller ones, such as the series of volcanoes to form on the overlying plate. Indian plate. Each continent is embedded in one or more plates.","EARTH 127 SANDY DESERT SURFACE FEATURES Deserts cover about 20 percent of Earth\u2019s From space, the flatter areas of Earth\u2019s land land surface, but only surface (apart from the areas dominated by a small proportion ice) appear either dark green or various are occupied by sand shades of yellow-brown.The green areas are dunes, like these in forests and grasslands, which comprise a the Sahara Desert. major component of Earth\u2019s biosphere (the planet\u2019s life-sustaining regions).The yellow- brown areas are mainly deserts, which have been created over long periods by various weathering and erosional processes. Like the other rocky planets, Earth has suffered many thousands of meteorite impacts over its history (see p.103). But because Earth\u2019s surface is so dynamic, the evidence for most of these impacts has disappeared, removed by erosion or covered up by depositional processes. RAINFOREST WATER Forests cover 30 percent of Earth\u2019s land surface and range from the cold, dark Water is a dominant feature of Earth\u2019s surface. Overall, about boreal forest of the far north to the dense 97 percent of the water is in oceans (which cover 75 percent of rainforests of the humid tropics. the surface), 2 percent is in ice sheets and glaciers, less than 1 per- cent is in ground water (underground and in rocks), and the rest is in rivers, lakes, and the atmosphere.The presence of liquid water has been key to the development of life on Earth, and the heat capacity of the oceans has been important in keeping the planet\u2019s temperature relatively stable. Liquid water is also responsible for most of the erosion and weathering of Earth\u2019s continents, a process unique in the solar system, although it is believed to have occurred on Mars in the past. KINGDOMS OF LIFE clouds carry water flows ice melts water water inland downhill in rivers to form returns meltwater to land loss of water water returns streams as snow from lakes by to land as rain Biologists use various systems for classifying living organisms, but the most widely loss of water evaporation used is the five-kingdom system. This classifies organisms mainly on the basis of from plants by their cell structure and method of obtaining nutrients and energy. However, not all transpiration scientists accept this system as satisfactory, and some have proposed switching to an eight-kingdom system or one with 30 kingdoms grouped into three superkingdoms. water evaporates from sea and ANIMALS VERTEBRATE condenses to form clouds Animals are multicellular organisms that contain muscles or other contractile structures allowing some method of movement. They acquire nutrients, and so gain energy, by ingesting food. Many animals, including mammals, are vertebrates (they possess a backbone), but a far larger number are invertebrates (without a backbone). PLANTS FLOWERING PLANT frozen water accumulates Plants are multicellular organisms that obtain energy from in glaciers sunlight through the process of photosynthesis. Their cells contain special pigments for absorbing light energy and are water seeps into enclosed by cell walls made of cellulose. ground and flows to sea FUNGI TOADSTOOL THE GLOBAL WATER CYCLE Earth\u2019s water is in a state of continuous Fungi acquire nutrients by absorption from other living water returns movement, passing between the oceans organisms or dead and decaying organic material. They to sea via rivers and lakes, the atmosphere, and the land have no means of locomotion. They range from yeasts and streams in a cycle of connected processes. (microscopic unicellular organisms) to multicellular forms with large fruiting bodies, such as mushrooms. LIFE ON EARTH THE SOLAR SYSTEM PROTISTS PARAMECIUM Evidence in ancient rocks points to the presence of simple, bacteria-like organisms on Earth some 3.8 billion years ago. However, the prevailing Protists are microscopic, mainly single-celled organisms scientific view is that life started on Earth long before that, as a result of whose cells contain nuclei. Some gain energy from complex chemical reactions in the oceans or atmosphere.These reactions sunlight, others ingest food like animals. eventually led to the appearance of a self-replicating and self-repairing molecule, a precursor of DNA (deoxyribonucleic acid). Once life, in this MONERANS MYCOBACTERIUM rudimentary form, had started, processes such as mutation and natural selection inevitably led, over the vast expanses of geological time, to a Monerans are the simplest, smallest, most primitive, and collection of life forms of increasing diversity and complexity. Life spread most abundant organisms on Earth. The two main groups from the seas to the land and to every corner of the planet. Currently, are bacteria and blue-green algae (cyanobacteria). Earth is teeming with life in astonishing abundance and diversity. Monerans are single-celled but their cells contain no distinct nucleus. Most reproduce by splitting in two.","","ISLAND VORTICES These twisting cloud patterns are caused as low-level winds blow over the Cape Verde islands (seen in the upper right of this image), in the Atlantic Ocean off the coast of West Africa. They are seen here in a natural-color image from the MODIS instrument on board NASA\u2019s Earth- watching Terra satellite. Such repeating patterns of swirls are technically termed von K\u00e1rm\u00e1n vortex streets.","130 TECTONIC FEATURES Most of Earth\u2019s tectonic features are associated with plate boundaries. At constructive (or divergent) boundaries, plates move apart and new crust is added. Examples are mid-ocean ridges and the East African Rift. At destructive (or convergent) boundaries, two plates push against each other, producing a range of features, depending on THE SAN ANDREAS FAULT the nature of the crust on each plate. This fault in California, known for Many plate boundaries are associated producing earthquakes, marks a with an increased frequency of transform boundary where two volcanism, earthquakes, or both. plates push past each other. AFRICA east and ASIA southwest with the development of a THE NORTHERN RED SEA constructive plate boundary, which The Gulf of Aqaba (center right), a branch East African Rift is formed as ascending magma of the Red Sea, forms part of the northern creates new crust and pushes the arm of the East African Rift. The Gulf of LOCATION Extending from plates on either side of the rift apart. Suez (center) is a side branch of the rift. Mozambique northward The main section of the East through East Africa, the African Rift runs (in two branches) Red Sea, and into Lebanon through eastern Africa. Over tens of millions of years, rifting in this TYPE Series of rift faults region has caused extensive faulting, the collapse of large chunks LENGTH 5,300 miles of crust, and associated features (8,500 km) such as volcanism and a series of lakes in the subsided sections. As The East African Rift provides an rifting continues, it is anticipated example of the geological process of that a large area of eastern Africa rifting\u2014the stretching and tearing apart of a section of continental crust will eventually split by a plume of hot off as a separate island. magma pushing up A northern arm of underneath it. the rift valley runs Rifting is associated up the Red Sea and eventually reaches OL DOINYO LENGAI Lebanon, in the north. This active volcano in This coincides with a northern Tanzania sits divergent boundary in the middle of the that is pushing Arabia east African part of away from Africa. the East African Rift. BLACK SMOKERS ATLANTIC OCEAN basin.The discovery in the 1960s of Hydrothermal vents are this spreading of the Atlantic sea floor\u2014 underwater geysers Mid-Atlantic Ridge evidenced by the fact that crustal located near mid-ocean material near the ridge is younger ridges. The hot water LOCATION Extending from than that farther away\u2014led to general spewed out by some vents, the Arctic Ocean to the acceptance of the theory of continental called \u201cblack smokers,\u201d is Southern Ocean drift.The ridge is a site of extensive discolored by the dark earthquake activity and volcanism, mineral iron sulfide. TYPE Slow-spreading along with many seamounts (isolated mid-ocean ridge underwater mountains).Where the THE SOLAR SYSTEM volcanoes break the ocean surface, LENGTH 10,000 miles they have formed islands such as (16,000 km) Iceland and the Azores. The Mid-Atlantic Ridge is the SURTSEY longest mountain chain on Earth Between 1963 and 1967, and one of its most active volcanic a massive and dramatic regions, albeit mainly underwater. submarine eruption, from a The ridge sits on top of the Mid- section of the Mid-Atlantic Atlantic Rise, a bulge that runs the Ridge to the south of length of the Atlantic Ocean floor. Iceland, produced the new Both rise and ridge coincide with island of Surtsey. plate boundaries that divide the North and South American plates, on the west, from the Eurasian and African plates, on the east. These are constructive plate boundaries, where new ocean crust is formed by magma upwelling from Earth\u2019s mantle. As this crust forms, the plates on either side are pushed away from the ridge at a rate of 0.4\u20134 in (1\u201310 cm) a year, widening the Atlantic","131 FIERY ARENAL Arenal is one of the most active volcanoes in Costa Rica\u2014a region where the small Cocos Plate is subducted under the neighboring Caribbean Plate. PACIFIC OCEAN smaller plates in the Pacific colliding OKMOK VOLCANO The volcanic Aleutian Islands were Pacific Ring of Fire with neighboring plates along a series created as the Pacific Plate was driven under the oceanic crust of LOCATION Pacific Ocean of destructive plate boundaries.The the North American Plate. This rim, from Chile to New volcano is on the island of Umnak. Zealand main driving force for this activity is TYPE Series of MOUNT FUJI destructive boundaries the creation of new crust by a large In the northwest Pacific, the subduction of LENGTH 20,000 miles the Pacific Plate under the Eurasian Plate is (32,000 km) mid-ocean ridge in the eastern Pacific responsible for creating the islands of Japan, the site of volcanoes such as Mount Fuji, The Ring of Fire is a huge arc of (the East Pacific Rise). Here, new which last erupted in 1707. volcanic and seismic (earthquake) activity around the rim of the Pacific material is continually added to the Ocean. It stretches from the western coasts of South America and North Pacific and Nazca plates, and to the America, across the Aleutian Islands of Alaska, and down the eastern edge of small Cocos Plate, forcing them Asia, to the northeast of Papua New Guinea, and finally to New Zealand. toward the edges of the Pacific. More than half of the world\u2019s active volcanoes above sea level are part of Across much of its northern and the ring.The Ring of Fire results from the Pacific Plate and other western edges, the oceanic crust of large mountain ranges, interspersed with volcanoes, along much of the MOUNT RUAPEHU the Pacific Plate is subducted (forced western coast of the Americas.These At the southwest corner of the Ring of Fire is include the Cascade Range in New Zealand. Here, steam rises from the underneath) by the oceanic crust of Washington State, home of the active country\u2019s tallest volcano, Ruapehu, between volcano Mount St. Helens, and the eruptions that occurred in 1995 and 1996. other plates, forming deep-sea Andes in South America, Earth\u2019s longest and most active land trenches.This predisposes these mountain range. regions to earthquakes, and the subducted crust also melts deep down to create hot magma, which reaches the surface through volcanoes.The result has been the formation of many highly volcanic island arcs in these regions\u2014examples being the Aleutian Islands, the Kurile Islands, the islands of Japan, and the Mariana Islands. THE SOLAR SYSTEM On the eastern side of the Pacific, the situation is somewhat different. Here, parts of the Pacific, Nazca, and Cocos plates are being subducted below continental THE ANDES crust. Deep-sea On the western edge trenches have of South America, also formed here, subduction of the but instead of Nazca Plate under island arcs, the the South American plate collisions Plate has created have led to the the Andes, another formation of highly active region.","132 EARTH ASIA south Himalayas LOCATION Running southeast from northern Pakistan and India across Nepal to Bhutan TYPE Continent\u2013continent collision LENGTH 2,400 miles (3,800 km) The Himalayas are the highest mountain range on Earth, as well as one of the youngest. If the TIBETAN RANGE The Kailas Range is a central region of the neighboring Karakoram Range is Himalayas.Today, because the Himalayas, close to the border between Tibet and India. Here the mountains are viewed included, the Himalayas contain Himalayas are still rising, earthquakes from the Tibetan Plateau, which is itself about 3 miles (5 km) above sea level. Earth\u2019s 14 highest mountain peaks, and accompanying landslides remain each with an altitude of over 5 miles a common occurrence. (8 km), including its highest mountain, The mountains form a number of Mount Everest.These peaks are still distinct ranges.Traveling northward, being uplifted at the from the high plains rate of some 20 in of the Ganges, the (50 cm) per century first of these are the by the continent\u2013 Siwalik Hills, a line of continent collision gravel deposits carried that originally down from the high formed them. mountains. Here, However, the there are subtropical mountains are forests of bamboo weathered and and other vegetation. eroded at almost the Farther north are the same rate, with the Lesser Himalayas, debris carried away which rise to heights by great rivers, such EASTERN HIMALAYAS of about 3,000 ft as the Ganges and In this satellite view of an eastern (5,000 m) and are Indus to the south. region of the Himalayas, which traversed by extends into China, the snow- numerous deep The collision that covered high-altitude regions are gorges formed by brought about both the Himalayas and clearly delineated. swift-flowing streams. the Tibetan Plateau Farthest north are the to its north occurred between 50 and Great Himalayas, between 20,000 and 30 million years ago when tectonic 29,000 ft (6,000 and 8,800 m) tall plate movements caused India\u2014at and containing the highest peaks. that time an island continent\u2014to This region is heavily crash into Southeast Asia. For millions glaciated and contains of years before the collision, the floor lakes filled with of the ocean between India and Asia glacial meltwater. (called Tethys) was consumed by subduction under the Eurasian Plate. But once the ocean closed, first the continental margins between India MOUNT EVEREST and Asia, and finally the continents At 29,035 ft (8,850 m), themselves, collided.The crust from Everest is the highest both was thickened, deformed, and peak on Earth. Satellite metamorphosed, and parts of both studies show it is being continents and the floor of the Tethys uplifted by a fraction of Ocean were pushed up to form the an inch per year. THE SOLAR SYSTEM GLACIAL LAKES Many of the higher areas of the Himalayas are covered in glaciers and dotted with lakes dammed by glacial moraines. In the left foreground is the Tsho Rolpa Glacier Lake in northeast Nepal, which, at 15,092 ft (4,600 m), is one of the highest lakes on Earth.","133 ROOF OF THE WORLD In this photograph, taken from a NASA Space Shuttle, the snow-covered Himalayas, on the left, are bordered by Earth\u2019s largest upland region, the vast and lake-spattered Tibetan Plateau. THE SOLAR SYSTEM","134 FEATURES FORMED BY WATER Some of the most obvious and striking features of Earth\u2019s surface are large bodies and flows of liquid water, such as oceans, seas, lakes, and rivers. In addition to these, there are landforms caused by the erosional or depositional power of liquid water, which include gorges, river valleys, and coastal features ranging from beaches to eroded headlands. Ice, too, has had a major impact on Earth\u2019s appearance. Ice-formed features include existing GRAND CANYON bodies of ice, such as glaciers and ice-sheets, and Carved over millions of years by landforms, such as U-shaped valleys, sculpted by the Colorado River, the Grand the movement of past glaciers. Canyon is Earth\u2019s largest gorge. NORTH AMERICA northeast SOUTH AMERICA north BRAIDING Over its course, the Great Lakes Amazon River Amazon frequently braids into channels, LOCATION Straddling the NIAGARA FALLS LOCATION Flows from creating many border of the US and The greatest drop in water level within the the Peruvian Andes, temporary islands. Canada Great Lakes system is at Niagara Falls, across Brazil to the between lakes Erie and Ontario. Here, the Atlantic Ocean Ucayali, high in TYPE System of water plunges a spectacular 167 ft (51 m). the Andes of freshwater lakes TYPE River southern Peru. carved basins were filled with The Ucayali flows AREA 94,480 square miles meltwater left by a retreating ice sheet. LENGTH 3,995 miles north from this (244,767 square km) Originally, several of today\u2019s lakes were (6,430 km) area, turns east, united in one huge lake, but following and joins another major tributary, the The Great Lakes of North America are post-glacial uplift in the region, they The Amazon is the greatest river on Mara\u00f1\u00f3n River, where it becomes a system of five connected lakes that took on their present form about Earth, whether measured by the area the Amazon proper.The river then together form the largest body of fresh 10,000 years ago.The lake surfaces vary of the planet\u2019s land surface that it meanders for thousands of miles across water on Earth.The lakes\u2014named, in height above sea level, from 600 ft drains or by the volume of water that the Amazon Basin, a vast flat area that from west to east, Superior, Michigan, (183 m) at Lake Superior to 246 ft it discharges every year. Overall, the contains Earth\u2019s largest rainforest, Huron, Erie, and Ontario\u2014contain 20 (75 m) at Lake Ontario. Sprinkled Amazon accounts for nearly 20 percent percent of Earth\u2019s surface fresh water across the lakes are thousands of islands, of all river water discharged into merging with and drain a basin of approximately including Isle Royale on Lake Superior, Earth\u2019s oceans.The numerous tributaries 289,900 square miles (751,100 square source of the Amazon along the way. Just km).They are connected to each other which is itself big enough to has been established as east of Manaus, at by short rivers, a strait, and canals, and hold several lakes. a headwater of the its confluence with drain into the Atlantic Ocean via the Apur\u00edmac River, a the Negro River, the St. Lawrence River.The Great Lakes tributary of the Amazon is already began to form at the end of the last 10 miles (16 km) wide, MEETING THE ATLANTIC while still 1,000 miles ice age when glacier- The mouth of the Amazon (1,600 km) from the occupies the whole top sea.At its mouth, the part of this image, which Amazon discharges into covers an area of tens of the Atlantic Ocean at thousands of square the incredible rate of miles. Rio Par\u00e1, the about 200 billion gallons estuary of a separate of water (770 billion major river, the Tocantins, liters) every hour. can be seen at bottom. THE SOLAR SYSTEM LAKES HURON AND SUPERIOR MEANDERING TRIBUTARY In this photograph taken from a NASA The Tigre is a tributary of the Amazon Space Shuttle, the largest lake, Superior, in Peru. Here, it meanders through the is on the right, and appears partly iced Peruvian rainforest, over 1,860 miles over. Lake Huron is on the left. (3,000 km) from the Amazon\u2019s mouth.","135 ASIA west Caspian Sea THE VOLGA DELTA LOCATION On the borders OIL EXTRACTION The huge triangular delta of the of Azerbaijan, Iran, Some of Earth\u2019s largest oil reserves underlie Volga River is visible in the bottom of Kazakhstan, Russia, and the Caspian Sea. The greatest concentration this image, with the Caspian Sea Turkmenistan of proven reserves and extraction facilities is stretching out beyond it to the south. in its northeastern section. TYPE Saline inland sea throughout history in line with AREA 143,000 square discharges from the Volga, which in miles (371,000 square km) turn have depended on rainfall levels in the Volga\u2019s vast catchment basin The Caspian Sea is the largest inland in Russia.Today, the Caspian Sea body of water on Earth. It contains contains about 18,800 cubic miles salty rather than fresh water, so it can (78,200 cubic km) of water\u2014about be appropriately described either as one-third of Earth\u2019s inland surface a salt lake or as an inland sea.The water. Its salinity (saltiness) varies from Caspian was once joined, via another 1 percent in the north, where the inland sea, the Black Sea, to the Volga flows in, to about 20 percent in Mediterranean. However, several Kara-Bogaz-Bol Bay, a partially cut- million years ago it was cut off from off area on its eastern shore. those other seas when water levels fell during an ice age.The sea has no outflow other than by evaporation, but it receives considerable inputs of water from the Volga River (supplying three-quarters of its inflow) and from the Ural,Terek, and several other rivers. Its surface level has changed ANTARCTICA such as some regions within the THE BEARDMORE GLACIER Transantarctic Range, are known to This huge glacier drains the East Antarctic Antarctic Ice Sheet be rich sources of meteorites (see Ice Sheet into the Ross Ice Shelf. At pp.220\u201321). Meteorites continually 260 miles (415 km) in length, it is one of LOCATION Covering most fall onto the ice sheet and become the longest glaciers on Earth. of Antarctica buried in it. But in a few places, where there is an upward flow of Antarctic Ice Sheet is shrinking due TYPE Continental ice and some evaporation, they to global warming. Scientists agree ice-sheet concentrate again at the surface. that the West Antarctic Ice Sheet has For some years, there have been been showing a general pattern of AREA 5.3 million sq. miles retreat for over 10,000 years, but (13.7 million sq. km) concerns that the West think there is only a small risk that it will collapse within the next Earth\u2019s largest glacier, the SATELLITE VIEW few centuries. Antarctic Ice Sheet, is an This radar image shows the immense mass of ice whole of Antarctica, with the that covers almost all larger, eastern section of its of the continent of ice sheet on the left. The gray Antarctica and holds area around its coast is partly over 70 percent of ice shelf and partly sea ice. Earth\u2019s fresh water.The ice-sheet has two distinct THE LARSEN ICE SHELF parts, separated by a range of Around the coast of Antarctica, glaciers mountains called the Transantarctic and ice streams merge to form platforms Range.The West Antarctic Ice Sheet of floating ice called ice shelves. These has a maximum ice thickness of are home to large colonies of penguins. 2.2 miles (3.5 km), and its base lies mainly below sea level.The larger THE SOLAR SYSTEM East Antarctic Ice Sheet is over 2.8 miles (4.5 km) thick in places with a base above sea level. Both parts of the ice sheet are domed, being slightly higher at their centers and sloping gently down toward their edges. A few areas around the edges of the ice sheets,","136 THE MOON THE MOON 38\u201339 Gravity, motion, and orbits EVEN THOUGH IT HAS ONLY 1.2 percent of the mass 64\u201367 Celestial cycles of Earth, the Moon is still the fifth-largest planetary 102\u2013103 The family of the Sun satellite in the solar system.When full, it is the brightest object in our sky after the Sun, and its Meteorite impacts 221 gravity exerts a strong influence over our planet. However, the Moon is too small to retain a substantial atmosphere, and geological activity has long since ceased, so it is a lifeless, dusty, and dead world.Twelve men have walked on its surface and over 838 lb (380 kg) of lunar rock have been collected, but scientists are still not sure exactly how the Moon formed. ORBIT same face always Earth points at The Moon has an elliptical orbit around Earth the Earth, so the distance between the two bodies varies. At its closest to Earth (perigee), the Moon is 10 percent closer DAY 1 than when at its farthest point (apogee). Moon rotates The Moon takes 27.32 Earth days to spin counterclockwise on its axis, which is the same time it takes to orbit the Earth.This is known as synchronous rotation (see right) and keeps direction of DAY 8 one side of the Moon permanently facing Moon\u2019s orbit Earth\u2014although eccentricities in the SYNCHRONOUS ROTATION Moon\u2019s orbit called librations allow a few For each orbit of Earth, the Moon spins regions of the far side to come into view. once on its axis. As a result, it always Because the Earth is moving around the keeps the same face toward Earth. Sun, the Moon takes 29.53 Earth days to return to the same position relative to the Sun in Earth\u2019s sky, completing its cycle of phases (see p.66).This is also the length of a lunar day (the time between successive sunrises on the Moon). axis tilts from the Moon spins on SPIN AND ORBIT vertical by 6.7\u02da its axis every The Moon\u2019s orbital 27.32 Earth days path is tilted at an APOGEE angle to Earth\u2019s 251,966 miles PERIGEE equator, causing its (405,500 km) 225,744 miles path across the sky to (363,300 km) vary in an 18-year cycle. Earth\u2019s equator Tidal forces mean that the Moon is slowing down Earth\u2019s rotation, while the Moon moves away from the Earth at a rate of about 1 in (3 cm) each year. Moon orbits Earth in 27.32 Earth days STRUCTURE rocky mantle THE SOLAR SYSTEM The lunar crust is made of calcium-rich, granite-like rock. It is about 30 miles (48 km) thick on the near side and 46 miles (74 km) thick on possible small the far side. Because of the Moon\u2019s history of meteorite bombardment, metallic core the crust is severely cracked.The cracks extend to a depth of 15 miles crust of (25 km); below that, the crust is completely solid.The Moon\u2019s rocky granite-like rock mantle is rich in silicate minerals but poor in metals such as iron.The upper mantle is solid, rigid, and stable. Radioactive decay of minor MOON INTERIOR components of the lunar rock means that the temperature increases The density of the Moon is much with depth.The lower mantle lies about 600 miles (1,000 km) below less than that of the whole Earth, but the crust, and here the rock gradually becomes partially molten.The is similar to that of Earth\u2019s mantle. It is average density of the Moon indicates that it might have a small iron possible that the Moon is entirely made core.The Apollo missions measured the velocities of shock waves traveling of solid rock and has no metallic core at all. through the Moon, but the results proved inconclusive. Further seismic evidence is needed to confirm the existence of a metallic core.","THE MOON 137 THE LUNAR SURFACE MOON PROFILE This Apollo 16 image is centered on the AVERAGE DISTANCE FROM EARTH ROTATION PERIOD boundary between the near and far sides of the Moon\u2014a view never seen 238,900 miles (384,400 km) 27.32 Earth days before the era of spaceflight. At least 4 billion years of SURFACE TEMPERATURE LENGTH OF A DAY ON THE MOON asteroid bombardment has saturated the lunar \u2013240\u00baF to 240\u00baF (\u2013150\u00baC to 120\u00baC) 29.53 Earth days surface with craters. DIAMETER 2,160 miles (3,476 km) MASS (EARTH = 1) 0.012 VOLUME (EARTH = 1) 0.02 GRAVITY AT EQUATOR (EARTH = 1) 0.165 NUMBER OF MOONS 0 SIZE COMPARISON MOON OBSERVATION EARTH The amount of the sunlit Moon visible from Earth varies throughout the month, starting with a thin crescent in the western sky just after sunset. The month ends with a thin crescent moon visible in the east just before dawn. ATMOSPHERE The Moon has a very thin, tenuous atmosphere with a total mass of about 22,000 lb (10,000 kg).This is the same as the amount of gas released by a landing Apollo spacecraft.The surface temperature varies by about 480\u02daF (270\u02daC) over a lunar day, and the quantity of gas near the surface is 20 times greater during the cold lunar night than during the heat of the day.The Moon\u2019s gravity is just one-sixth of ATMOSPHERIC COMPOSITION Earth\u2019s, and the lunar atmosphere is The neon, hydrogen, and helium escaping all the time. However, the have been captured from the atmosphere is also constantly being solar wind. The argon is derived replenished by the solar wind. from the radioactive decay of potassium in the lunar rocks. neon 29% helium 25.8% hydrogen 22.6% argon 20.6% trace gases HISTORY OF THE MOON No one knows exactly how the Moon was formed, but most astronomers agree with the giant-impact theory, which hypothesizes that the process was set in motion about 4.5 billion years ago, when a massive asteroid hit the young Earth (see below). During the first 750 million years of its life, the Moon went through a period of heavy meteorite bombardment, which cracked the crust and created craters all over the surface. About 3.5 billion years ago, the rate of bombardment slowed and there followed a period of considerable volcanic activity. Lava from 60 miles (100 km) below the surface oozed up through cracks in the crust and filled large, low-lying craters.The lava solidified, producing the dark, flat basaltic areas called maria.This volcanic activity stopped about 3.2 billion years ago, and since then the Moon has been relatively dead. Many of the features formed in the early days of the Moon\u2019s history have been destroyed by subsequent impacts. One of the most recent large craters is Copernicus, which was produced about 900 million years ago. FORMATION OF EARTH\u2019S MOON 1 In a glancing collision between a 2 The ejected material formed a 3 The majority of the ejected 4 Rocks grew by mutual collisions THE SOLAR SYSTEM material went into a circular orbit until a single body dominated the ring, Mars-sized asteroid and Earth, a huge massive cloud of gas, dust, and rock. around Earth, forming a clumpy, sweeping up the remaining material. dense, doughnut-shaped ring. The Moon was born. amount of silicate material was jetted Heat was radiated away and the away from Earth\u2019s mantle. cloud quickly began to cool.","138 THE MOON MYTHS AND STORIES LUNAR INFLUENCES inertial tidal bulge gravitational Moon\u2019s force pull of orbit WEREWOLVES Although the Moon is much smaller than Moon Many myths and old folk tales Earth, its gravity still exerts an influence.The attribute strange powers to the Moon. Some say that a full moon Moon\u2019s gravitational attraction is felt most can turn people mad (the origin of the word \u201clunacy\u201d), and many strongly on the side of Earth facing the tidal bulge cultures, from Eurasia to the Moon, and this pulls water in the oceans Americas, share a belief that when toward it. Inertia (the tendency of objects Earth\u2019s spin causes the moon is full some humans can with mass to resist forces acting upon be transformed into vicious them) attempts to keep the water in place, TIDAL BULGES tidal bulges to sweep werewolves.The superstition is but because the gravitational force is Gravitational interaction between Earth over surface widespread and ancient\u2014even the greater, a bulge of water is pulled toward Babylonian King Nebuchadnezzar the Moon. On the opposite side of Earth, and the Moon creates two bulges in Earth\u2019s (c. 630-c. 562 bc) imagined that he had become a werewolf. oceans (exaggerated here). As Earth spins on its axis, the bulges of water sweep over the surface, creating tides. the water\u2019s inertia is stronger than the TIDAL RANGE Moon\u2019s gravity, so a second bulge of water is created. As The magenta in this Earth rotates, the bulges sweep over the planet\u2019s surface, satellite image of creating daily changes in sea level called tides.The time of Morecambe Bay on the the high tide changes according to the Moon\u2019s position in northwest coast of the sky.The height of the tides changes during the lunar England reveals the cycle, but the actual height also depends on local geography. inlets and mud flats In shallow coastal bays, the tidal range can be huge. that are left exposed at low tide. SURFACE FEATURES The surface of the Moon has been pulverized by meteorites and is covered by a rough, porous blanket of rubble several yards thick. This debris ranges in size from particles of dust to huge lumps of rock dozens of yards across.The soil (or regolith) consists of fine- grained, fragmented bedrock, the size of the MOON ROCK grains getting progressively larger with This 6-in- (15-cm-) wide rock depth. Since there is no wind or rain, the formed as lava from the interior surface material does not move far, and its rose to the Moon\u2019s surface and composition can change considerably from solidified. The small holes were formed as gas bubbles escaped. LAVA TUBE place to place.The thickness also varies\u2014in Over 3 miles (5 km) wide and hundreds of miles long, this rille is a collapsed tubelike young mare regions it is about 16 ft (5 m) thick, but this increases to structure through which lava once 32 ft (10 m) in the old highlands. Micrometeorite impacts continuously flowed. Moonquakes caused by nearby impacts may have erode exposed rocks, and they are also damaged by cosmic rays and caused the roof to fall in. solar-flare particles.The topmost layer of soil is saturated with hydrogen ions absorbed from the solar wind. THE SOLAR SYSTEM TRACKS IN THE SOIL Lunar Rover tire tracks lead away from the Apollo 15 module \u201cFalcon,\u201d nestling near Hadley Rille in 1971. Over a million or more years, they will eventually be erased by meteorite bombardment.","THE MOON 139 CRATERS SUNRISE OVER COPERNICUS CRATER The vast majority of lunar craters are produced by impacts. Asteroids usually strike the Moon at velocities of about 45,000 mph (72,000 km\/h).The resulting crater is about 15 times larger than the impacting body. Unless the asteroid nearly skims the surface on entry, the resultant crater is circular.Three types are formed.Those smaller than 6 miles (10 km) across are bowl-shaped, having a depth of around 20 percent of the diameter. Craters between 6 and 90 miles (10\u2013150 km) in diameter have outer walls that have 1 Just after dawn in the crater, the 2 Halfway through the morning, small 3 At noon, the Sun is overhead, and slumped into the initial crater pit.There is often a low eastern Sun casts long shadows, central mountainous peak produced by the recoil of which emphasize the variation in shadows enhance the ejecta blanket the scene appears much flatter and the underlying stressed rocks.The crater depth is a height between the floor and rim. outside the crater. The temperature washed out. The temperature is now inside the crater is rising. more than 212\u00b0F (100\u00b0C). few miles, and much excavated material falls back into the crater just after the impact. Craters wider than 90 miles (150 km) EUGENE SHOEMAKER contain concentric rings of Gene Shoemaker (1928\u20131997) was an American astrogeologist mountains, created as rebounding who studied terrestrial and lunar meteorite impact craters and dreamed material rippled out from the of going to the Moon. Addison\u2019s disease prevented that. Instead, he center before solidifying. Such taught the Apollo astronauts to be field geologists. In 1969, he joined craters were so deep that hot a team at Palomar, magma flooded to the surface searching for near- Earth asteroids. After and filled the bottom of the Shoemaker died, some of his crater with lava. ashes were carried to RAY CRATERS the Moon Material ejected from a crater during an aboard the impact is often confined to narrow jets. Lunar Where this material hits the surface, it Prospector plows up the lunar soil, and this disturbed space probe region then reflects more sunlight than its in 1999. surroundings. From Earth, these appear as rays. The rays around Tycho Crater (far right) extend for thousands of miles. MAPPING THE MOON Some ancient Greeks thought that the Moon was like the Earth and that its dark areas were water.This belief continued into the 17th century, when the dark patches were given aquatic names such as mare (sea) and oceanus (ocean) on the first proper maps. Palus Putredinis (the Marsh of Decay) and Sinus Iridum (the Bay of Rainbows) are evocative examples. Italian astronomer Galileo Galilei was the first to realize that the height of surface features could be added LUNA 3 to maps by noting how the shadow lengths changed On October 7, 1959, during the lunar day.The first photographic atlas the Soviet Union\u2019s appeared in 1897, but the real leap forward came Luna 3 space probe with the advent of spaceflight. In 1959, the Soviet imaged the far side of the Moon. It had never Union sent the Luna 3 space probe behind the been seen before. Moon to photograph the far side. NASA\u2019s five LUNAR ORBITER IV This superb wide-angle image of the half-lit Lunar Orbiter spacecraft imaged 99 percent of the lunar surface Mare Imbrium was one of 546 images taken by NASA\u2019s orbiter on May 11\u201326, 1967, from in 1966\u201367, paying special attention to potential Apollo landing a height of about 2,485 miles (4,000 km). sites. In the 1990s, the Moon\u2019s mineral composition was surveyed THE SOLAR SYSTEM by Clementine and Lunar Prospector. Since 2009, Lunar GALILEO SKETCHES Reconnaissance Orbiter (LRO) has Galileo\u2019s first telescopic observations of the been engaged in a detailed Moon were made on November 30, 1609. The mapping project. pictures, published in Sidereus Nuncius in 1610, emphasized the roughness of the surface. SMART-1 During its approach phase, ESA\u2019s SMART-1 spacecraft took this image of an illuminated region of the far side, near the lunar north pole, on November 12, 2004, from a distance of about 37,250 miles (60,000 km).","","","EARTHRISE FROM APOLLO 8 In December 1968, the three-man crew of Apollo 8 became the first humans to orbit the Moon. They also became the first to see Earth rise over the Moon\u2019s cratered surface, as in this image taken through the spacecraft\u2019s window. Apollo 8\u2019s pictures of Earth helped emphasize how small and fragile our home planet is and strongly influenced the environmental movement.","","144 FEATURES OF THE MOON From afar, the Moon is clearly divided into two types of terrain.There are large, dark plains called maria (Latin for \u201cseas\u201d) and also brighter, undulating, heavily cratered highland regions.The whole surface was initially covered with craters, most of which were produced during a time of massive bombardment.The rate at which asteroids have been striking the Moon has decreased over the last 4 billion years. Around 4 billion years ago, the Moon was also MOSAIC OF THE NORTH POLE volcanically active. Lava rose to the surface through The lunar North Pole is partially hidden cracks and fissures, filling the lower parts of the large from view from Earth and is best imaged craters to produce the dark plains.The plains reflect by orbiting spacecraft. Galileo took a only about 4 percent of the sunlight that hits them, series of photographs of the region on whereas the mountains reflect about 11 percent. December 7, 1992 on its way to Jupiter. NEAR SIDE northern hemisphere about 54\u00b0F (30\u00b0C) higher than that of NEAR SIDE northern hemisphere the surrounding terrain.Young craters Aristarchus Crater contain many large boulders.These Mare Crisium take a long time to heat up during TYPE Impact crater the day and also a long time to cool TYPE Lava-filled impact AGE About 300 million down at night. As time passes, the crater (sea) years boulders are broken up by small DIAMETER 23 miles (37 km) impacting asteroids, so this thermal AGE 3.9 billion years difference eventually disappears. DIAMETER 350 miles (563 km) This young crater has a series of Mare Crisium has an extremely nested terraces, which were produced smooth floor, which varies in height by by concentric slices of rock in the less than 290 ft (90 m).The lava that wall slipping downward.This both flooded Crisium had extremely low widened the crater and made it viscosity and became like a still pond considerably shallower, as the initially before it solidified.The Soviet Luna deep central region was filled with 24 probe was the last mission to bring material from the rim. Aristarchus back rock samples from the Moon. In was mapped by the Apollo Infrared 1976, it returned to Earth with a core Scanning Radiometer. During the of rock weighing 6 oz (170 g), which night, the temperature in the crater is was collected from Crisium\u2019s floor. LUNAR ORBITER 5 IMAGE OVAL CRATER This view of Aristarchus, taken from The Mare Crisium, which can be seen directly above, underlines the crater\u2019s with the naked eye from Earth, is nearly circularity and reveals the extensive circular in shape. Over 95 percent of surrounding blanket of hummocky ejecta. lunar craters are completely circular. NEAR SIDE northern hemisphere NEAR SIDE northern hemisphere the very fluid lava that RICH IN TITANIUM flooded them.The This Galileo image has Montes Apenninus Mare Tranquillitatis Mare Tranquillitatis been color-coded (Latin for \u201cSea of according to the titanium TYPE Mountain range TYPE Sea Tranquillity\u201d) lies just content of the rock. The AGE 3.6 billion years north of the lunar blue Tranquillitatis region AGE 3.9 billion years DIAMETER 542 miles equator and joins onto is rich in titanium, whereas (873 km) the southeast part of the orange Serenitatis LENGTH 249 miles( 401 km) the Mare Serenitatis region at the lower right The surfaces of lunar maria are much (Sea of Serenity). is titanium-poor. The Lunar Apennine mountains form darker than highland rock and are also Together, the two seas a ring around the southeastern edge considerably younger.This means that form one of the Moon\u2019s most several points, but only flooded with of the Mare Imbrium impact basin. they are relatively smooth and contain prominent features.The basin in lava about 3.6 billion years ago. The They consist of crustal blocks rising only a few impact craters.Their low which the \u201csea\u201d formed is very Sea of Tranquillity was famously the more than 1.9 miles (3 km) above the reflectivity is due to the chemistry of ancient, predating the formation of landing place of US astronauts Neil flat lava plain, pushed up by the shock the Imbrium Basin 3.9 billion years Armstrong and Buzz Aldrin on their wave from the Imbrium impact.The ago. It overlaps with other basins at 1969 Apollo 11 mission. mountain chain stretches for some 375 miles (600 km), though its southern end is partially buried beneath lava flows. LUNAR MOUNTAINS BEFORE TOUCHDOWN The Apennines lie in The flat, desolate plain of the Sea of the lower right of this Tranquillity stretches away to the north Apollo 15 image. The in this view from the Apollo 11 lunar dark area to their left is module taken just before landing. Palus Putredinis.","THE MOON 145 NEAR SIDE northern hemisphere CRATER CHAINS peppered with NEAR SIDE southern hemisphere The material secondary craters Copernicus Crater excavated by an formed by Alphonsus Crater impact showers boulders thrown TYPE Impact crater down on the out during the TYPE Impact crater AGE 900 million years surrounding lunar impact. Fine, AGE 4.0 billion years DIAMETER 57 miles (91 km) surface, producing light gray rock DIAMETER 80 miles (117 km) long chains of particles ejected This young ray crater has massive secondary craters. during the NASA\u2019s Ranger 9 spacecraft was terraced walls.The crater floor is crater\u2019s formation deliberately crash-landed into the below the general level of the surrounding walls. were collected by Alphonsus Crater on March 24, surrounding plain, and lies 2.3 miles Copernicus is an intermediate-sized the Apollo 12 1965, taking television pictures as it (3.7 km) below the top of the crater with high central peaks.These astronauts near their landing site. Such approached.The crater formed in an mountains were formed when the particles were responsible for forming impact, but the dark patches and LUNAR ORBITER 2 IMAGE rock directly below the crater the rays that surround the crater.The fractures Ranger 9 found on its floor Copernicus Crater\u2019s terraced walls and central rebounded after being compressed by high reflectivity of the rays is due to are thought to be a result of volcanic peaks were revealed by NASA\u2019s second Lunar the explosion caused by the impacting the ejecta churning up the lunar activity\u2014probably explosive Orbiter in 1966. asteroid.The vicinity of Copernicus is regolith (rough material reflects more eruptions. Because of these features, light than smooth material). Alphonsus was considered a possible landing site for later Apollo missions. NEAR SIDE southern hemisphere NEAR SIDE southern hemisphere THREE MINUTES BEFORE IMPACT Rupes Altai Humboldt Crater NEAR SIDE southern hemisphere TYPE Cliff TYPE Impact crater Tycho Crater AGE 4.2 billion years LENGTH 315 miles AGE About 3.8 billion TYPE Impact crater (507 km) years AGE 100 million years DIAMETER 52 miles (85 km) DIAMETER 120 miles (189 km) Altai is by far the longest cliff on the This crater is remarkable because its Lying in the southern highlands,Tycho THREE FILTERED IMAGES OF TYCHO Moon. It is about 1.1 miles (1.8 km) lava-filled floor is crisscrossed with a is one of the most perfect walled The Ultraviolet\/Visual camera onboard the high.The energy that is released series of radial and concentric fractures craters on the Moon, with a central Clementine spacecraft was equipped with a during an impact does more than just (or rilles). On closer inspection, some mountain peak towering 1.8 miles series of filters. Differing color combinations excavate a crater and lift material out look like collapsed tubes through (3 km) above a rough infilled inner revealed the variability in the physical and to form walls and an ejecta blanket. which lava once flowed, others like region. Surveyor 7 landed on the chemical structure of the crater rock. Violent seismic shock waves radiate rift valleys. Lunar volcanic activity north rim of Tycho\u2019s ejecta blanket away from the impact point. An lasted for over 500 million years. Lava in January 1968. About 21,000 YOUNGEST LARGE LUNAR CRATER? obstacle such as a mountain can halt would seep up into a crater and then photographs were taken, and the Although Tycho is one of the youngest lunar these waves and the lunar crust may cool, shrink, crack, and sink. It would soil was chemically analyzed.The craters (Giordano Bruno may be younger), it then buckle, forming a long cliff. Altai then be covered by more lava.The final highland soil was found to be mainly still formed in the age of the dinosaurs. was created by the Nectaris impact. basaltic infill would have many layers. made of calcium-aluminum silicates, in contrast to the maria material, which is iron-magnesium silicate. ALTAI CRACKS ON THE FLOOR OF HUMBOLDT THE SOLAR SYSTEM ESCARPMENT The curving Rupes Altai\u2014 310 miles (500 km) long\u2014runs from top to bottom in this image. The crater at top left of the picture is Piccolomini.","146 THE MOON NEAR SIDE northern hemisphere HIGHLAND MASSIFS The Taurus-Littrow HARRISON SCHMITT The flat-based Taurus- Valley is surrounded Taurus-Littrow Valley Littrow Valley can be by steep-sided Harrison \u201cJack\u201d Schmitt (b. 1935) seen in the center of this mountains, known was born in New Mexico. He TYPE Valley image, nestling between as massifs. Moon studied geology at Caltech and the rugged, blocky mountains are Harvard University.While working AGE About 3.85 billion mountains known different from those for the US Geological Survey, years prosaically as the North, found on Earth. On he joined a team instructing South, and East massifs. Earth, the crustal astronauts in the art of field LENGTH 18.6 miles (30 km) plates collide, geology. In June 1965, Schmitt was was produced about producing huge In December 1972, the last crewed 3.9 billion years ago, and much of the mountain ranges selected as a scientist-astronaut mission to the Moon landed in the basaltic rock dates from that time, like the Alps and Himalayas.These by NASA and was later dark-floored Taurus-Littrow Valley when the crater was flooded with new mountains are subsequently chosen to be the lunar at the edge of the basalt-filled Mare lava.The third type of rock was found eroded by rain and ice.The Moon\u2019s module pilot for Apollo 17. Serenitatis.The range of geological on the top of nearby hills.This was crust is not broken into plates. In December 1972, he features was impressive, and the barium-rich granite and had been Nothing moves. All the Moon became the first and Apollo 17 astronauts found three ejected from one of the surrounding mountains are produced by only geologist to distinct types of rock in the region. large craters. Most of the material impacts, and the mountains walk on the One piece of crushed magnesium near the landing site was extremely around the Taurus-Littrow Moon. One of olivine was 4.6 billion years old and dark and consisted of cinders and ash Valley are the remains of the highlights of had crystallized directly from the ejected billions of years ago from old crater walls. Part of the Apollo 17 melted shell of the just-formed nearby volcanic vents and fissures. the valley floor just to mission was his Moon.The nearby Serenitatis crater the north of South discovery of orange glass within the lunar rock. THE SOLAR SYSTEM SHORTY CRATER Harrison Schmitt stands by the Lunar Rover, parked to the left of the 356-ft- (110-m-) wide Shorty Crater. Behind Schmitt, 4 miles (6 km) away, is Family Mountain, one of the Taurus- Littrow range named by the astronauts. Near the rover, patches of orange soil can be seen.","THE MOON 147 Massif was covered with a light continuously bombarded by asteroids, EXPLORING SPACE mantle of regolith a few yards thick. the number of craters per unit area This had been produced by a rock increases with time.There are MOON GLASS avalanche, possibly triggered when the relatively few craters on the Taurus- area was bombarded by boulders Littrow valley floor, which was taken The lunar regolith contains large ejected when the nearby Tycho Crater to indicate that the surface is even amounts of volcanic glass.This was formed. As the Moon is being younger than the Apollo 12 landing occurs as glazings on rock fragments and also as tiny teardrop- and site. One crater in dumbbell-shaped droplets. Colors the valley, Shorty, range from green and wine-red was once thought through to orange and opaque.The to be a volcanic orange glass found near Shorty was vent, but more typical of high-titanium lunar detailed analysis of glasses, but it was also rich in zinc. its raised rim and central mound ORANGE SOIL IN SHORTY CRATER indicated that, like The glassy orange surface soil was excavated millions of other by an impact about 20 million years ago. It was lunar craters, it was actually formed about 3.6 billion years ago. produced by an impacting asteroid. SPLIT ROCK This house-sized boulder was ejected from an impact crater in the Mare Serenitatis and then rolled down into the valley. Scoop marks can be clearly seen where some samples have been taken from its surface. THE SOLAR SYSTEM","148 THE MOON FAR SIDE northern hemisphere FAR SIDE southern hemisphere Pascal Crater Van de Graaff Crater TYPE Impact crater TYPE Double impact AGE About 4.1 billion crater years DIAMETER 71 miles (115 km) AGE About 3.6 billion years LENGTH 155 miles (250 km) This is one of 300 lunar craters named FAR SIDE southern hemisphere DARK FLOOR Less than one percent of the lunar after mathematicians. It honors the If Tsiolkovsky had been formed earlier in craters are noncircular.Van de Graaff Frenchman Blaise Pascal. The image Tsiolkovsky Crater lunar history, the volcanic activity would have is typical of such irregular craters, below was taken in 2004, with the been greater and more of the crater floor which are produced on the rare camera looking directly down into TYPE Impact crater would have been filled with lava. occasions when the impacting the crater.The Sun is low in the sky, asteroid hits the surface at an angle below the bottom of the picture.The AGE About 4.2 billion southern rim of the crater.The first of less than 4\u00b0.Van de Graaff is also tiny craters around Pascal are bowl- years images of the lunar far side was special because it is both magnetic shaped and young, with circular rims obtained in October 1959 by the and has the highest concentration of much sharper than the older rim of DIAMETER 123 miles Soviet spacecraft Luna 3. Resolution natural radiation. Most of the ancient Pascal.The larger crater\u2019s rim was (198 km) was low, but the features that could be lunar magnetic field decayed away initially eroded by slumping and rock seen were nevertheless given names, over 3 billion years ago. However, slides, and is now being worn down such as Mare Moscoviense and Sinus there are still a few magnetic further by more recent impacts. Astronautarum. Only a few craters anomalies (magcons), of which Van could be made out, including this de Graaff and nearby Aitken are the PASCAL AND ITS YOUNGER NEIGHBORS Only half the size of Mare Crisium, strongest. Magcons were discovered this far-side crater is special because one. Konstantin by small magnetometer sub-satellites FAR SIDE southern hemisphere only half the interior basin has been Tsiolkovsky was a released by Apollos 15 and 16. filled with lava.The central peak is Russian rocketry Korolev Crater also unusually offset pioneer who not only IRREGULARLY SHAPED CRATER from the center of the designed a liquid TYPE Ringed impact crater.There have been hydrogen\/liquid crater extensive rock oxygen rocket but also AGE About 3.7 billion avalanches down the suggested the multi- years stage approach to DIAMETER 250 miles ORBITER 3 IMAGE spaceflight.The crater (405 km) The crest of the rim of was penciled in as a Tsiolkovsky Crater runs possible landing site for to the upper right of this one of the post-Apollo image. The diagonal 17 missions, which banding to its right is were canceled. probably the result of a large avalanche down the slope of the rim. THE SOLAR SYSTEM Sergei Korolev led the Soviet space EXPLORING SPACE effort in the 1950s and 1960s and was responsible for the early Sputnik NUCLEAR CRATER and Vostok spacecraft. He has two craters named after him, one on It is very difficult to estimate the the Moon and the other on Mars. relationship between the size of a Korolev is one of only 10 craters on crater and the size of the asteroid the lunar far side that are more than that produced it. Usually the crater 125 miles (200 km) across. It is is about 20 times bigger. Only in double-ringed and pocked with controlled nuclear explosions can smaller craters.The outer ring is an exact relationship between 252 miles (405 km) in diameter.The energy release and crater size be inner ring is much less distinct. It is established. Sedan Crater in the only half the height of the outer ring Nevada Desert (below) is bowl- and its diameter is half that of the shaped and 1,200 ft (368 m) outer ring.Together with Hertzsprung across. It was produced by a and Apollo, Korolev forms a trio of subsurface nuclear blast equivalent huge ringed formations on the lunar to 100 kilotons of TNT in July far side.The lunar crust varies in 1962. It is very similar to small thickness, and it reaches its maximum lunar impact craters such as those thickness of 66 miles (107 km) in the within Korolev. region around the Korolev Crater. INDESTRUCTIBLE? This Orbiter 1 image shows that later impacts have done little to obliterate the huge Korolev Crater."]