Chapter 1 THE PLANET One by one they race around the Sun: Mercury, swiftest of all, its surface scarred by countless craters; cloud-covered Venus, suffocated beneath air so hot and thick it would kill any life that tried to arise; Earth, beautiful blue, a world of flowing water, mammoth oceans, and vibrant life; Mars, an orange desert with giant volcanoes, dried-up riverbeds, and poles capped by ice; Jupiter, mightiest of all, whose stormy atmosphere boasts a red whirlwind larger than the entire Earth; Saturn, a giant golden globe encircled by bright white rings; Uranus, a green world so faint and distant it eluded the ancients; Neptune, Uranus's twin, a turquoise giant of storms and high winds; and tiny Pluto, icy cold, patrolling the edge of the Sun's planetary domain. On their ceaseless journey around the Sun, most of these worlds carry others with them: the Earth, for example, anchors the Moon, whose light brightens the night, whose gravity stirs the seas. Dodging the planets and their satellites are thousands of asteroids, most between the orbits of Mars and Jupiter, and trillions of comets, which can unfurl majestic tails if they near the Sun. The Sun The Sun rules the solar system, hoarding over seven hundred times more mass than all the many worlds that dance around it. The Sun's mass produces the gravity that holds these objects captive, even from as far away as two light years. The Sun has more than just mass, however; it also emits huge amounts of light that warms and brightens its planets. The Sun generates this energy from nuclear power, fusing nuclei of hydrogen, the lightest element in the universe, into nuclei of helium, the second lightest element. This reaction transforms a tiny amount of mass into pure energy. Each second, the Sun converts 600 million tons of hydrogen into helium. This nuclear reaction, at the Sun's center, releases deadly x-rays and gamma rays. But as they try to escape the Sun's core, the x-rays and gamma rays bounce to and fro against the Sun's own material and lose energy. As a result, a typical x-ray or gamma ray takes 30,000 years to reach the solar surface. By the time the once-lethal radiation finally wins the battle, it is so exhausted that it emerges as the relatively low-energy visible light which changes night into day. Threading the Sun are magnetic fields that generate sunspots on its surface and flares erupting above it. The number of spots waxes and wanes approximately every eleven years. From 1645 645 to 1715, however, nearly all spots vanished. During this period, called the Maunder minimum, Earth cooled-probably because the Sun faded. Surrounding the Sun's surface is the corona, hot but tenuous gas extending millions of miles, and blasting through the corona is the solar wind, a stream of charged particles that shoots beyond Pluto. As a star, the Sun distinguishes itself from the planets by generating its own light, whereas the planets merely reflect the light from the Sun. Nevertheless, the Sun and its planets formed at the same time, 4.6 billion years ago. Somewhere in our Galaxy, a cloud of gas and dust collapsed under the weight of its gravity. Much of this material fell into what was to become the Sun, but some remained in orbit, in a swirling disk. Dust and ice in the disk stuck together and grew into asteroids and comets, which then collided with one another to give birth to the nine planets. These still revolve around the Sun in nearly the same plane and in the same direction, counterclockwise as viewed from above the Earth's north pole. Different substances condensed out of the gas at different distances from the Sun. The inner part of the disk spun fast, so friction heated it; therefore, in the inner disk, only hardy substances with high melting points -- rock and iron -- condensed and formed planets. Thus, the solar system's four inner planets -- Mercury, Venus, Earth, and Mars-became worlds of rock and iron. They are small because the disk contained little of these materials. In the outer solar system, however, the disk was cool, so ice also condensed, which was far more common than the rock and iron. As a result, four outer planets -- Jupiter, Saturn, Uranus, and Neptune -- grew into giants, and their great gravity stole some of the hydrogen and helium gas that pervaded the disk, further augmenting their sizes. Scattered about the solar system was debris left over from the planets' formation -- asteroids, comets, and Pluto. Mercury The Sun's first planet is something of an acquired taste. Mercury drapes itself in a gray surface that bears crater after crater, prompting one critic to call it a world only a confirmed crater counter could love. A poll once asked space enthusiasts to name the worst spacecraft ever launched; one respondent said, "Anything having to do with Mercury." Still, Mercury's ugly face conceals an intriguing soul. As the nearest planet to the Sun, Mercury represents an extreme that can elucidate the formation of planets in general. Precisely because of this position, though, Mercury shed its secrets slowly. As viewed from Earth, Mercury never strays far from the blinding Sun, so simply seeing the planet challenges observers: Mercury appears only briefly at dawn or dusk, hugging the horizon, where the Earth's turbulent air distorts the view of celestial bodies. Nevertheless, ancient people not only knew this elusive planet but noted how it flitted from dawn to dusk and back again. They therefore named it for the swift messenger of the gods. Stymied by Mercury's proximity to the Sun, astronomers learned little. They knew it was small -- just over a third the size of Earth -- and suspected it was hot. They also knew it orbited the Sun in just 88 days, making Mercury's year a quarter as long as Earth's. Only in 1965, however, did they discover its rotation period, 58.6 days. Before then, they thought it spun every 88 days, the same as its year. This equality between day and year would have meant that one hemisphere forever faced the Sun and fried, while the other forever faced away and froze. In 1974 and 1975, the Mariner 10 spacecraft flew past Mercury three times and delivered most of what is now known about the planet. Even though Mercury's day does not equal its year, the planet exhibits an enormous temperature range, from 800 degrees Fahrenheit at hottest-sufficient to melt lead-to -300 degrees Fahrenheit at coldest. The extreme temperature range stems from Mercury's long day and absence of an appreciable atmosphere to insulate itself. High temperature, in turn, destroys any hope of atmosphere, since heat imparts high speeds to airborne molecules and causes them to escape the planet's weak gravity. Because of Mercury's heat, scientists in 1991 were astonished to discover water ice at the planet's poles. This ice hibernates in crater floors that never see the Sun. Mercury has a huge iron core, which sets it apart from the look-alike Moon and even the Earth. All the worlds of the inner solar system contain iron and rock. Their proportions can be inferred from planetary densities, since iron is over twice as dense as rock. The Moon has such a low density that it must have little it any iron core, while Earth is so dense that its iron core accounts for a third of its mass. Mercury's iron core, however, makes up 70 percent of the planet's mass. Perhaps Mercury once had more rock but a giant asteroid blasted most of it away. Indeed, Mercury's harsh, crater-scarred surface testifies to a brutal history. Its largest impact basin, the appropriately named Caloris Basin ("basin of heat"), spans 800 miles, bigger than Texas. So great was the Caloris impact that it disrupted land on the opposite side of the planet. Venus The Sun's second planet, Venus, at first looks more inviting. It is certainly more prominent. Unlike evasive Mercury, Venus proudly decorates the changing colors of twilight as a brilliant morning or evening star that outshines every other heavenly body but the Sun and Moon. Venus can even cast a shadow. Because of its radiance, ancient people named it for the goddess of love and beauty -- and modern people occasionally think it a UFO. Venus once evoked visions of life, for in some ways the planet so resembles our own that it was called Earth's twin. Venus nearly equals Earth's size, mass, and density. It passes closer to Earth than does any other planet, which is one reason Venus looks so bright. And like Earth it has an atmosphere and clouds. The clouds also contribute to Venus's brightness, for they reflect most of the sunlight striking them. The clouds cloak Venus's surface, so scientists -- and science fiction writers -- were free to speculate about what the clouds hid. Clouds suggested water, so some scientists pictured dense tropical jungles, warm and steamy, teeming with creatures large and small, while others thought a vast ocean covered the entire planet, broken only by a few craggy peaks here and there. The real Venus, though, is a hot, dry desert smothered beneath a hot, thick atmosphere. Because Venus is so close to Earth, it was the first planet to receive a spacecraft, Mariner 2 in 1962. This and subsequent missions painted a portrait of a hellish world. The atmosphere is over ninety times thicker than Earth's: its clouds consist not of water, as on Earth, but of sulfuric acid -- battery acid; and its atmosphere is full of carbon dioxide, the same gas we get rid of when we breathe. The carbon dioxide produces a terrible green-house effect that makes Venus, not Mercury, the hottest planet in the solar system: sunlight penetrates the atmosphere (if you stood on Venus, it would look like a heavily overcast day on Earth) and heats the surface, but the carbon dioxide prevents the heat from escaping. As a result, the surface of Venus is 860 degrees Fahrenheit. Venus and Earth actually possess similar quantities of carbon dioxide, but little of Earth's seeps into the air. Instead, it resides in corals and carbonate rocks, like limestone, because life incorporates it into shells, and rainfall washes it from the atmosphere. Venus has neither life nor rain to remove atmospheric carbon dioxide, so whereas carbon dioxide accounts for only 0.035 percent of Earth's atmosphere, it makes up 96.5 percent of Venusian air. A few spacecraft have plunged through the thick atmosphere and briefly glimpsed Venus's rocky surface. Other spacecraft, such as Magellan, have circled the planet from afar and peered at the surface by radar. These images reveal a world ruled by lava-volcanoes, lava flows, and lava plains. A seven-mile-high mountain range, Maxwell Montes, rises near the planet's north pole. Venus possesses the longest river channel in the solar system, Baltis Vallis, which stretches over 4,200 miles, greater than the distance between Anchorage and Miami; but it was carved by lava, not water. Billions of years ago, Venus may have had water, too, perhaps even oceans, which may have given rise to life. At that time, the Sun was fainter and Venus cooler, so ancient Venus may have resembled modern Earth. Rain removed carbon dioxide from the atmosphere, keeping Venus mild. As the Sun brightened, though, Venus heated up and its oceans evaporated. Rain no longer fell, so carbon dioxide gas accumulated and the planet grew even hotter. No wonder, then, that scientists worry about another planet whose atmospheric carbon dioxide abundance is increasing -- the Earth. Earth Once regarded as the supreme center of the universe around which all else revolved, the Earth is now known to be the Sun's third and most precious planet, a world that speeds around the Sun at 67,000 miles per hour and carries a cargo unique in the solar system: a vibrant web of millions of different living species. Earth is the largest of the four inner planets. It has a large iron core enveloped by a rocky mantle and crust. Currents in the core generate a magnetic field that shelters life from the solar wind. Interaction between the solar wind and the magnetic field creates aurorae, the northern and southern lights. Earth has a rich supply of liquid water, the substance in which terrestrial life probably first arose. Water blankets 71 percent of the Earth's surface, most in oceans, but some in rivers, lakes, and ice sheets. Surrounding the Earth is a thin atmosphere of nitrogen (78 percent) and oxygen (21 percent). The atmosphere not only powers life but also insulates Earth from wild day-to-night temperature swings; shields Earth from the countless meteoroids that bombard it; maintains the ozone layer that protects land-based life from the Sun's deadly ultraviolet rays; and bears greenhouse gases -- especially carbon dioxide and water vapor -- that raise the Earth's temperature 60 degrees Fahrenheit, thereby transforming what would be a barren ice-covered planet into a luxurious world brimming with life. No other planet in the solar system abounds with so much atmospheric oxygen, and billions of years ago neither did Earth itself. At that time, the atmosphere was primarily carbon dioxide, water vapor, and nitrogen, all vented from volcanoes. The carbon dioxide and water vapor kept the Earth warm even though the Sun was then fainter. As the Sun brightened, rainfall removed the carbon dioxide from the atmosphere, so Earth remained mild. Meanwhile, another phenomenon began to alter the air. The first life arose in the oceans some 4 billion years ago, and photosynthesis consumed carbon dioxide and produced oxygen. The oxygen at first combined with oceanic rocks, but about 2 billion years ago, with the rocks saturated, the oxygen bubbled into the atmosphere. Some of the oxygen formed ozone, which guarded the surface from solar ultraviolet radiation and allowed life to advance from sea to land. The Moon The Earth's nearest celestial neighbor may also have helped. No other small planet has such a large satellite. Mercury and Venus are moonless, and the moons of Mars are minuscule. But the Moon is over a quarter the size of Earth, and lunar tides may have pushed life from the oceans onto the land. In the past, the Moon was closer and these tides were stronger. The Moon then revolved around the Earth faster. Today the Moon revolves only once a month; the word "month" derives from "Moon." The Moon rotates as fast as it revolves, so the same side always faces Earth. The Moon has long been linked to madness, or "lunacy." Even today, people sometimes superstitiously blame the Moon, especially the full Moon. However, most of the time that one hears "There must be a full Moon tonight," the Moon is not actually full. The Moon affects people in other ways, for a beautiful crescent suspended in a twilit sky can stir our hearts. The Moon itself, though, is dead. The lunar near side consists of two types of terrain: the cratered highlands, which are ancient, dating back some 4 billion years, and the smoother maria, or lunar seas, which formed somewhat later, when lava overflowed these regions. In contrast, the Moon's far side consists almost exclusively of cratered highland terrain. The Moon boasts the solar system's largest impact basin, the South Pole-Aitken Basin, which is 8 miles deep and 1,600 miles across. Although the rocks gathered by Apollo astronauts are incredibly dry, polar craters, perpetually shielded from sunlight, seem to harbor ice, just as Mercury's poles do. Not only does the Moon's cratered face testify to the solar system's violent past, but so does the Moon's very existence. According to the leading theory for the Moon's origin, a Mars-sized object smashed into the young Earth and splattered material from the Earth's mantle into orbit, where it conglomerated into a large satellite. This is why the Moon's composition matches that of the Earth's mantle: much rock, little iron. Mars The fourth planet from the Sun looks like a drop of blood, prompting the ancients to name it for the god of war. Mars is indeed ruddy for the same reason blood is. Blood turns red when the iron-bearing compound hemoglobin joins with oxygen, and Mars is reddish because the same two elements join to form iron oxide, or rust, on the Martian deserts. Mars is best known for its Martians, conjured up by imaginative Earthlings. Foremost among these was wealthy Bostonian Percival Lowell, who in 1894 founded an observatory in Arizona to study the neighbor world. Lowell thought he saw canals the Martians had built to ferry water from the planet's polar caps to its equator. Lowell's visions excited the public but antagonized other astronomers; nevertheless, long after Lowell died, many scientists believed that Mars at least had plants. In 1965, even these modest hopes were dashed when the Mariner 4 spacecraft flew past the planet and revealed a cratered landscape that looked like the lifeless Moon. This and later missions, including several that landed on Mars, depicted a frozen world with an atmospheric pressure less than 1 percent of that on Earth. There is almost no oxygen to breathe and no ozone to protect the surface from the Sun's ultraviolet rays. Still, Mars offers far friendlier surface conditions than any other planet but Earth. Furthermore, certain red planet trails uncannily mirror the Earth's. For example, Mars spins once every 24 hours, 37 minutes, and the planet's axis tilts 25.2 degrees, so Martian days and seasons resemble terrestrial ones, except the latter last nearly twice as long, since Mars takes nearly twice as long to orbit the Sun, 68/ days. Mars exhibits a north-south asymmetry: the northern hemisphere is fairly smooth, the southern hemisphere heavily cratered. The two hemispheres recall the two types of lunar terrain, the smooth lunar seas, or maria, and the cratered highlands. The northern hemisphere must be younger, since lava flows have eradicated most of its craters. Yet the old southern hemisphere, battered and cratered though it is, preserves signs of a kinder, gentler Mars: valleys carved by running water, indicating that billions of years ago, Mars was warmer and wetter. During that ancient epoch, Mars may have sprouted life, whose fossils may still exist. If Mars was once a warm, wet world where life flowered, what went wrong? Mars resides farther from the Sun's heat than does the Earth, which partially explains the frigid Martian climate. When Mars was young, however, its volcanoes spewed carbon dioxide and water vapor, triggering a greenhouse effect that warmed the planet, allowing water to flow. But Mars was doomed. Volcanoes derive their strength from a planet's internal heat, which in turn depends on the planet's size. Although Earth and Mars were both born hot, Mars is only half as big, so its interior cooled faster, just as a small, freshly baked roll cools faster than a large loaf of bread. As the Martian interior cooled, the volcanoes shut down, the atmosphere thinned, the greenhouse effect diminished, and the planet froze, killing any life. Today, Martian air is still mostly carbon dioxide, but so thin that the greenhouse effect lifts the temperature only 10 degrees Fahrenheit. As if to prove their past power, huge volcanoes still tower above the red plains of Mars. The largest, which dwarf Mount Everest, cluster together in the Tharsis bulge, a blister on the Martian sphere. The formation of Tharsis apparently cracked the surface, for running away from Tharsis is a long canyon, Valles Marineris, which if transported to Earth would stretch from Cincinnati to San Francisco. Circling the red planet are two moons named for the horses that escorted the chariot of Mars, Phobos ("fear") and Deimos ("terror"). These small, cratered worlds, irregularly shaped, were first seen by American astronomer Asaph Hall in 1877, but Irish author Jonathan Swift had actually written of both a century and a half before, in his 1726 satire Gulliver's Travels. Even earlier, German astronomer Johannes Kepler had predicted two moons for Mars, reasoning that since Earth had one and Jupiter was then known to have four, Mars should have a number between, two or three. He chose two, as did Swift, who probably meant to ridicule Kepler's logic. Both moons lie much closer to Mars than the Moon does to Earth, and they therefore circle the planet much more quickly. Phobos, the larger and nearer, orbits every 7.7 hours, faster than Mars spins, so a Martian colonist would see the satellite rise not in the east but in the west. More distant Deimos takes 30 hours to revolve. Both moons are dark, like most asteroids, suggesting that Mars snatched them from the neighboring asteroid bell. Asteroids Asteroids are small, rocky bodies. The first and largest, six-hundred-mile-wide Ceres, was discovered in 1801 and thought to be a planet. But then a second asteroid, Pallas, was found in 1802, followed by a third and fourth, Juno and Vesta, in 1804 and 1807. Today thousands have been catalogued, most lying between the orbits of Mars and Jupiter. They are victims of Jupiter's immense gravity. When the Sun was young, still surrounded by a disk of gas and dust, Jupiter's gravity prevented the material inside its orbit from aggregating into a full-fledged planet. Although science-fiction movies sometimes portray asteroid belts choked with deadly debris, asteroids are actually so far apart that a million spacecraft could fly blind through the asteroid belt and likely not suffer a single collision. Nevertheless, in 1972, when the first spacecraft trespassed into the realm of the asteroids, scientists feared the craft would smash into smaller material, dust and pebbles, too faint for astronomers to see. Fortunately, this zone turned out to be fairly clean, and several spacecraft have successfully traversed the asteroid belt and reached the outer planets. In 1991 and 1993, one such spacecraft, Jupiter bound Galileo, photographed asteroids Gaspra and Ida, and in 1997, the Near Earth Asteroid Rendezvous (NEAR) spacecraft sailed past Mathilde. These asteroids proved heavily cratered and irregularly shaped, like the moons of Mars. This is to be expected, since asteroids get hit by other asteroids and most have too little gravity to pull themselves into spheres. Ida even has its own moon, a tiny world just a mile across. It is the smallest moon ever found. Not all asteroids stay within the asteroid belt. Some venture closer to the Sun and even cross the orbit of Earth. Human beings owe their existence to one such asteroid that collided with Earth 65 million years ago and killed off the dinosaurs, thereby letting new forms of life take their place. But what gave life can also take it away: a similar impact today might wipe out civilization. Other asteroids, called Trojans, lie on the other side of the asteroid belt from Earth, at Jupiter's distance from the Sun, 60 degrees ahead of and behind the planet. Jupiter The ancients named Jupiter well. Because of the planet's brilliance (among planets, normally second only to Venus) and its slow, majestic movement (it takes twelve years to orbit the Sun), the planet suggested the king of the gods. The ancients knew nothing of Jupiter's gargantuan size, but it harbors over twice the mass of all the other planets combined, 318 Earth masses. The king of the goods is also the king of the planets. Jupiter stands apart from the inner planets not only because of its great size but also because of its unearthly composition. Whereas the Earth and the other inner planets are dense rock-iron worlds, Jupiter consists of more ethereal material. The planet is a gas giant, composed primarily of hydrogen and helium, the lightest and most common elements in the universe. On Earth they are gases, but Jupiter's enormous weight squeezes most of the hydrogen into a metal that could conduct electricity. The hydrogen and helium envelop a rock-water core. Though this core has roughly ten times the mass of the Earth, it holds only a few percent of Jupiter's total mass. Jupiter radiates more energy than it receives from the Sun. Some of Jupiter's excess energy is left over from its birth, and some arises because its helium is sinking and giving off energy, like water falling over a dam. This energy helps keep Jupiter's atmosphere stormy, turbulent, and colorful. Furthermore, Jupiter spins faster than any other planet, once every 9 hours and 55 minutes, which also stirs up the atmosphere. The planet's banded disk sports a red spot bigger than Earth that has persisted for centuries. Because of Jupiter's rapid rotation and its fluffy hydrogen-helium composition, the planet is flattened, its equatorial diameter exceeding its polar diameter by thousands of miles. Although Jupiter is lifeless, in 1992 George Wetherill of the Carnegie Institution of Washington suggested that both it and its neighbor Saturn may be responsible for intelligent life on Earth, because the mighty gravity of Jupiter and Saturn long ago ejected trillions of comets out of the solar system -- leaving relatively few of these deadly objects to strike the Earth, and sufficient time between major impacts for intelligence to evolve. Two years later, as if to confirm this idea, Jupiter took a direct hit from Comet Shoemaker-Levy 9, which left the quardian world scarred for months. Jupiter rules a miniature solar system, a retinue of at least sixteen satellites. The four largest, which Galileo Galilei spotted in 1610, gave him ammunition to shoot down the Catholic Church's assertion that all worlds circled Earth. From innermost to outermost, the four Galilean satellites are Io, Europa, Ganymede, and Callisto (mnemonic: 1 Eat Graham Crackers). Ganymede is larger than either Mercury or Pluto: in fact, it is the largest moon in the solar system. Of the four Galilean satellites, to is the most fiery. In 1979, the Voyager 1 spacecraft discovered nine erupting volcanoes on the moon, eight of which were still going strong when Voyager 2 flew by four months later. The volcanoes spew so prolifically that their lava quickly erases craters to owes its volcanic strength to Jupiter, whose tides squeeze, stretch, and heat the moon's interior. Europa, the smallest Galilean satellite, holds not fire but ice. Beneath its icy surface may lurk an ocean of liquid water. If the ocean floor bears heat vents powered by Jovian tides, those vents could provide energy for life, which would blossom not in the light but in the dark. On Earth heat vents beneath the ocean nourish creatures that never see the Sun. Still, life in Europa's hypothetical ocean remains speculative and may prove just as chimerical as the beings once envisioned on Venus and Mars. Twelve other moons circle Jupiter, four inside the Galilean orbits, eight outside. One of the inner moons, egg-shaped Amalthea, orbits close to 10 and is colored reddish by its volcanoes. The outer moons may be captured asteroids or comets. The four most distant of these all revolve backward, suggesting that they indeed did not form with Jupiter. In addition to moons, Jupiter possesses a ring, but one so faint that it hardly rivals those of Saturn. Saturn Saturn once marked the solar system's frontier, the farthest planet astronomers knew. It takes 29 1/2 years to complete a cycle, longer than any other classical planet. Though named for the somber god of time, Saturn seems the most cheerful of planets, thanks to its bright, resplendent rings, discovered in the 1600s. As the solar system's other gas giant, Saturn resembles a smaller Jupiter. It is mostly hydrogen and helium surrounding a rock-water core. Whereas Jupiter has 318 Earth masses, Saturn has 95. Because of its lower mass and weaker gravity, Saturn cannot compress its hydrogen and helium as much as Jupiter does, and thus Saturn's density is so low that if the planet were placed in an ocean large enough, it would float. And like Jupiter, Saturn emits more energy than the Sun provides, perhaps because helium is separating from the lighter hydrogen and falling toward Saturn's core, thereby releasing energy and warmth. Saturn's golden globe occasionally displays white spots, but the planet lacks the stormy turbulence that marks its big brother. Saturn more than compensates by adorning itself with stunning rings. From one side of Saturn to the other, the three brightest rings span 170,000 miles -- nearly three-fourths of the distance between the Moon and Earth. The rings consist of countless ice particles. They may be the wreckage of a moon or comet that strayed too close to Saturn and got torn apart by its gravity, which pulled harder on the near side of the luckless object than on the far side. Or the rings may be the remains of a small moon that got smashed to bits by a comet. The rings are so thin that they disappear from terrestrial view when the planet turns exactly equator -- on to Earth. These ring-plane crossings occur twice during Saturn's year, or about once every fifteen Earth years. The next happens in 2009. During these ring-plane crossings, astronomers can better see Saturn's surroundings and glimpse its moons, most of which were first spotted at such times. The moons number at least eighteen, more than revolve around Jupiter. The king of the Saturnian moons is the appropriately named Titan, a world larger than Mercury and Pluto and the second largest moon in the solar system, after Jupiter's Ganymede. Titan first made its unusual nature known in 1944, when American astronomer Gerard Kuiper reported methane gas around the satellite. Although it seemed thin, the atmosphere was the first ever found around a mere moon. In 1980, Voyager I discovered that Titan's true atmosphere is actually thicker than Earth's. Furthermore, it consists mostly of the predominant gas surrounding Earth, nitrogen. The previously known methane exists, too, but constitutes only a few percent of Titan's air. Unfortunately, the atmosphere also contains orange haze that blocked Voyager's view of the surface, but in 2004 the Saturn-bound Cassini spacecraft will send a probe to sail through the atmosphere and possibly land on the surface. Titan is especially intriguing because in some ways it resembles ancient Earth: it has nitrogen and organic compounds, but no atmospheric oxygen. Although Titan is certainly lifeless-its frigid surface is -290 degrees Fahrenheit -- the moon preserves some of the chemical conditions that prevailed on Earth billions of years ago. Titan even has water. The difference is that Earth was warm enough to have liquid water, in which life developed, whereas Titan is so cold that its water froze. Most of Saturn's other satellites are worlds made chiefly of water ice and lie inside Titan's orbit. These include small "shepherd moons" whose gravitational tugs tend and sculpt some of the rings. Three especially strange moons reside outside Titan's orbit-Hyperion, Iapetus, and Phoebe. Hyperion is a misshapen world that resembles a biscuit rather than a sphere. Iapetus is half white, half black. And Phoebe, which lies much farther out than its peers, is probably a captured asteroid or comet, because it revolves around Saturn backward. Phoebe is dark, and its dust may rain down on Iapetus and darken one side of that moon. Uranus and Neptune The next two planets, Uranus and Neptune, are twin worlds. They have similar natures, and even their discoveries were intertwined. Uranus, twice as far from the Sun as Saturn, is dimly visible to the naked eye. Astronomers had actually seen the green world prior to its discovery, but all mistook it for a star. In 1781, William Herschel, a professional musician and amateur astronomer in England, noticed an object that did not lo Excerpted from Magnificent Universe by Ken Croswell All rights reserved by the original copyright owners. Excerpts are provided for display purposes only and may not be reproduced, reprinted or distributed without the written permission of the publisher.
