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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 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.
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.
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.
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 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.
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 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.
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 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 look like a star. Thinking it a comet, he reported the find to professional astronomers, who discovered it to be a planet-the first new planet sighted in the heavens in all of recorded history. It was named for Uranus, the father of Saturn and grandfather of Jupiter.
Almost as soon as Uranus was discovered, however, it spelled trouble, for it sometimes moved too quickly, other times too slowly. Eventually astronomers suspected that a still more distant planet was to blame, a planet whose gravity yanked Uranus and sped it up or slowed it down. English astronomer John Couch Adams and French astronomer Urbain Leverrier independently predicted the unseen world's position, and in 1846 German astronomers Johann Galle and Heinrich d'Arrest found it, close to where Adams and Leverrier had predicted. The planet was blue-green, and it was named Neptune, for the god of the sea.
Once shrouded in mystery -- even their rotation periods went unknown until the 1980s-Uranus and Neptune received close up scrutiny from the Voyager 2 spacecraft, which flew past Uranus in 1986 and Neptune in 1989. Both planets are giants, but considerably smaller than Jupiter and Saturn: Uranus has 15 and Neptune 17 Earth masses, about one-twentieth the mass of Jupiter. Yet all four giant planets have thick hydrogen-helium envelopes surrounding rock-water cores weighing roughly 10 Earth masses. This similarity points to their greatest difference: on Jupiter and Saturn the hydrogen and helium dominate, whereas on Uranus and Neptune the rock-water core accounts for most of the mass.
Uranus and Neptune have similar colors-Uranus is green and Neptune turquoise blue. On Uranus, the green results from methane gas, which constitutes a small part of its atmosphere. Methane removes red light and leaves green. Neptune's bluer color arises both from methane and from blue particles in its clouds. Both planets spin fast, but Uranus lies on its side as it turns. Perhaps a giant asteroid hit Uranus and knocked it over.
Because Uranus and Neptune reside far from the Sun, they take a long time to revolve. Uranus circles the Sun only once every 84 Earth years, so a human being on Uranus would celebrate at most one birthday. Neptune takes nearly twice as long, completing an orbit every 165 years. Surprisingly, even though Neptune is a billion miles farther from the Sun's warmth, the two planets share exactly the same temperature, -353 degrees Fahrenheit. This similarity stems from one of their few differences: Neptune, like Jupiter and Saturn, radiates more energy than it receives from the Sun, whereas Uranus, for some reason, does not. As a result, Neptune's atmosphere is livelier. When Voyager 2 flew past in 1989. Neptune sported a great dark spot reminiscent of Jupiter's great red spot, but in the 1990s the Hubble Space Telescope showed that this dark spot had vanished.
Both planets have rings, albeit rings so dark they would go unseen by the naked eye of a space traveler flying past them. Uranus's rings were discovered in 1977, when they blocked the light of a star the planet passed in front of. Neptune's rings were found the same way the following decade.
In addition to rings, Uranus has at least eighteen satellites, but none large; the planet is the only giant without a big moon. Neptune's satellite system is only eight moons strong, but the largest falls into the class of big moon. Triton, discovered just two and a half weeks after Neptune itself. Even before Voyager's visit, Triton seemed strange, because it is the only large moon that circles its planet backward, suggesting that it was not born with Neptune but was instead captured by it. Voyager revealed a thin nitrogen atmosphere, an icy surface, complete with a huge polar cap, and geysers shooting material five miles high. Moreover, Triton turned out to be the coldest celestial body any spacecraft has yet measured; -390 degrees Fahrenheit.
The second largest moon is Proteus, which astronomers now realize they had first detected in 1981 when it passed in front of a star; however, not until Voyager saw it did they accept its existence. The third largest Neptunian moon, odd little Nereid, follows the most elliptical path of any known satellite. Discovered in 1949, the moon takes one Earth year to revolve. When nearest Neptune, Nereid lies less than 1 million miles from the planet, but when farthest, the small moon ventures 6 million miles from its master. Nereid's odd orbit may be Triton's fault. Triton probably once roamed around the Sun on its own, but Neptune then snatched it for itself. During this capture, Triton's gravity may have disturbed the other moons, casting some into Neptune and ejecting others, thus leaving Neptune with fewer moons than other giant planets. In addition, Triton may have stretched Nereid's orbit into a long ellipse.
Although Triton is now a satellite, a similar world continues to orbit the Sun on its own: Pluto, outermost of all the Sun's planets, and the last to be found. The astronomer who led the search that would ultimately capture Pluto started his investigations in 1905, a quarter century before the tiny world was actually detected. Percival Lowell, proponent of intelligent life on Mars, thought a ninth planet, Planet X, perturbed Uranus and instigated irregularities that Neptune's existence could not explain. Although Lowell and his assistants searched for many years, he died in 1916 without finding the coveted world. Lowell Observatory resumed the search in 1929, and in 1930 Lowell astronomer Clyde Tombaugh spotted faint Pluto on a photographic plate of the constellation Gemini. But Pluto seemed small, much dimmer than predicted. Lowell had envisioned a world some seven times heavier than Earth, but the real Pluto looked so faint that it might lack the gravitational power to perturb Uranus -- even though Pluto had been found during a hunt Lowell had inspired and just a few degrees from one of two places he had pinpointed. Over the following decades, astronomers estimated that Pluto was smaller than Mars but larger than Mercury, making the far-off world the solar system's second smallest planet.
During the 1970s, however, astronomers learned just how tiny Pluto really is. In 1978, James Christy of the U.S. Naval Observatory in Washington, D.C., discovered a moon circling Pluto. This moon feels Pluto's gravity, which derives its strength from Pluto's mass, so the moon's motion around Pluto betrays Pluto's mass: a mere 1/500 that of the Earth, a sixth that of the Moon. Pluto is so tiny that some astronomers no longer consider it a bona fide planet. It certainly does not affect Uranus significantly. Ironically, the apparent irregularities in Uranus's motion that led Lowell and his observatory to Pluto must have been nothing more than observational error.
Because of Pluto's remoteness, sunlight shines on Pluto with less than a thousandth of its strength on Earth, or a few hundred times brighter than moonlight, so even daytime on Pluto looks fairly dark. Indeed, this darkness partially inspired the planet's name, for Pluto was the god of the underworld, dwelling in darkness. In like fashion, Pluto's moon was christened Charon, after the ferryman who carried souls across the river Styx and into the underworld.
Pluto requires more time than any other planet to orbit the Sun, 248 years. On average, Pluto lies 3.7 billion miles from the Sun, but its orbit is so elliptical that at times Pluto skirts closer to the Sun than Neptune is. Pluto executed its most recent intra-Neptunian excursion between 1979 and 1999; it will not repeat the feat until the twenty-third century. The two planets will never collide, because whenever Pluto equals Neptune's distance from the Sun the two worlds are far apart.
Between 1985 and 1990, astronomers watched Pluto and its moon repeatedly eclipse each other. The duration of these eclipses established the diameters of Pluto and Charon. Pluto is slightly smaller than Triton, and Charon is about half Pluto's size, giving Pluto and Charon the largest moon-to-planet size ratio in the solar system. For comparison, the Moon is a quarter of the Earth's diameter. Charon is so large and so close to Pluto that from the planet's surface the moon would look seven times wider than the Moon does from Earth. Just as the same side of the Moon always faces the Earth, so the same side of Charon always faces Pluto. In addition, the same side of Pluto always faces Charon, so the moon never rises or sets; it simply hovers over the same part of Pluto, and a Plutonian would see the moon forever lodged at the same attitude above the horizon. Because one side of Pluto faces away from Charon, that hemisphere never sees the moon at all.
Like Neptune's Triton, Pluto has a nitrogen atmosphere. It was first detected in 1988, when Pluto passed in front of a star whose light faded gradually rather than abruptly, signaling a gaseous envelope around Pluto. The atmosphere's composition was inferred in 1992, when astronomers discovered that Pluto's surface had nitrogen ice, some of which must vaporize and become gas. Thus, far-off Pluto is the only other planet whose main gas is the same as Earth's.
Pluto and Triton have similar densities, indicating similar compositions -- about 75 percent rock and 25 percent ice. This rock-rich composition suggests that both Pluto and Triton formed far from any planet, where it was cold. At low temperatures, oxygen binds with carbon to make carbon monoxide (CO), leaving little oxygen to join with hydrogen and form water (H2O); so Pluto and Triton were born with relatively little water ice. In contrast, had they formed around a giant planet, they would have bathed in the newborn giant's warm glow. Warmth converts carbon monoxide into methane (CH4) and liberates the oxygen, which combines with hydrogen to create water. Thus, most satellites of the giant planets, which formed around their planets, possess greater proportions of water ice than Pluto and Triton do. Although Pluto and Triton had similar origins, fate intervened: Triton was captured by Neptune, while Pluto remained free.
The solar system does not end with Neptune and Pluto. Beyond their orbits lie trillions of comets, ice-bearing bodies that hibernate in the extreme cold of the outermost solar system. If one falls toward the Sun, however, sunlight warms the comet and vaporizes some of its ice, sparking a beautiful tail. Contrary to the impression it creates, the tail does not necessarily trail the comet. Instead, the tail points away from the Sun, because the solar wind blows cometary gas and dust away.
Most comets that astronomers can see travel along highly elliptical orbits around the Sun. For example. Comet Halley (rhymes with "valley") darts from beyond Neptune to within Venus's orbit. Comets pass Earth all the time, but only about once a decade does a great comet appear -- so bright and beautiful that it captivates the public. A comet's greatness depends on several factors, including its size (the bigger, the better); how close it gets to the Sun (the closer, the better, because the more the ice vaporizes); and how close the comet comes to Earth (the closer, the better). Sometimes the same comet on different passes can be both great and not so great. Halley's Comet was great in 1910, when it came so close that the Earth passed through its tail, but was nothing special in 1986, when it passed far from Earth. Its next return, in 2061, should be better.
The 1990s blessed Earth with two great comets. After twenty years of great-comet drought, Comet Hyakutake skirted by Earth in 1996. It was small but came so close to Earth that in dark skies it looked spectacular. During 1997, a comet passed far from Earth but was so large-20 miles across, versus Halley's 5 x 5 x 10 miles-that it amazed even people who lived among bright city lights. This was the great Comet Hale-Bopp, which remained visible to the unaided eye longer than any other comet of the twentieth century.
With one exception, all great comets follow long orbits around the Sun, taking over 200 years to revolve. For example, Comet Hyakutake will return in 14,000 years and Hale-Bopp in 2,400 years. The one exception to the rule is Halley, which swings around the Sun every 76 years, similar to a human lifetime; indeed, Mark Twain was born during one passage and died during the next. Short-period comets other than Halley fail to impress, having swung by the Sun so many times that they have exhausted much of their tail-generating ice. Several asteroids pursue highly elliptical, cometike orbits and may be burned-out comets.
Despite their dramatically different appearances, asteroids and comets are really two sides of the same coin; both are objects left over from the formation of the solar system, objects that did not get swallowed by the Sun or planets. Unlike asteroids, comets bear ice, because comets arose in the cold of the outer solar system. Yet both asteroids and comets are fossils of the solar system's birth. Similar objects smashed together to form the Earth and other planets.
Short- and long-period comets originate from different regions of the outer solar system. The first region, just past the orbit of Neptune, supplies most short-period comets and is often called the Kuiper belt, after prominent American astronomer Gerard Kuiper, who proposed its existence in 1951. However, an obscure Irish astronomer, Kenneth Edgeworth, had earlier suggested the same idea, so this zone might better be called the Edgeworth-Kuiper belt. Because the Edgeworth-Kuiper belt is flat, short-period comets usually travel near the same plane as the planets. In 1992, astronomers began detecting objects in the Edgeworth-Kuiper belt. Pluto and Triton may simply be the largest Edgeworth-Kuiper belt members.
Edgeworth-Kuiper belt members can migrate inward, toward the Sun. In 1977, American astronomer Charles Kowal discovered the first example, between the orbits of Saturn and Uranus. Kowal called it Chiron, after the son of Saturn and grandson of Uranus, a name that fit the object's odd location. The name turned out to be even more appropriate, however, because Chiron was a centaur, half man and half horse, and Chiron the celestial object is part asteroid and part comet. Chiron was first catalogued an asteroid, but none had ever been seen so far from the Sun, and in 1988 astronomers saw Chiron brighten as its ice began to vaporize-the same behavior comets exhibit. Yet Chiron is much larger than other comets. It is about 120 miles across, six times larger than Hale-Bopp and nearly twenty times bigger than Halley. In 1992 astronomers discovered another object like Chiron; it spends most of its time between the orbits of Saturn and Neptune. Since then, other "centaurs" have emerged, icy bodies that cut across the orbits of the outer planets, likely refugees from the Edgeworth-Kuiper belt.
Far beyond the Edgeworth-Kuiper belt lies another cometary reservoir, the remote Oort cloud, which stretches about two light-years from the Sun, halfway to the nearest star. Because of its much greater distance, astronomers have never observed objects lodged in the Oort cloud. It houses the long-period comets, such as Hyakutake and Hale-Bopp. Unlike short-period comets, these comets come from all directions, above and below the plane of the solar system, because unlike the flat Edgeworth-Kuiper belt, the Oort cloud is roughly spherical.
Whatever its origin, a speeding comet sprinkles space with cast-off dust and ice. Asteroids also splatter material, when other asteroids hit them, and this asteroidal material can be larger and tougher than cometary debris. If the Earth runs into a bit of interplanetary flotsam, called a meteoroid, friction with the atmosphere burns it up in a streak of light called a meteor or shooting star. Countless meteoroids strike the Earth's atmosphere each day, but most are small and never survive their fiery passage to the ground. Larger meteoroids produce bright meteors called tireballs. Those large and tough enough to reach the surface originate primarily from asteroids; after landing, they are called meteorites.
Because comets crisscross the solar system, so do the meteoroid streams they spawn, which follow the comets' orbits. When the Earth encounters one of these streams, large numbers of meteoroids smash into the atmosphere and cause a meteor shower. Debris from Halley's Comet, for example, triggers two meteor showers, the Eta Aquarids in May and the Orionids in October. Meteoroids in a stream travel on nearly parallel paths through space, so all meteors in a particular shower appear to radiate from a single point, just as parallel railroad tracks appear to converge in the distance. A meteor shower is normally named for its radiant: the Orionids, for instance, appear to emanate from the constellation Orion. Still, Orionid meteors streak across other constellations; it's just that the path of any Orionid meteor, if traced back, will pass over Orion.
The best meteor showers are the Perseids in August and the Geminids in December, which produce about sixty meteors an hour, or one a minute. The Perseids have been seen for nearly two thousand years. They arise from Comet Swift-Tuttle, which revolves around the Sun every 130 years and last passed us in 1992, replenishing the supply of meteoroids near the Earth. The Geminids are more recent, having first appeared in the 1800s, but they may disappear in the twenty-first century as the stream drifts away from Earth. The most unpredictable meteor shower strikes in November. A product of the unspectacular Comet Tempel-Tuttle, the Leonids normally display only a dozen meteors an hour, but in some years -- such as 1799, 1833, 1866, and 1966 -- they have erupted into titanic meteor storms that shot thousands of meteors across the sky in a single hour. Potential outbursts occur every 33 years, the orbital period of Comet Tempel-Tuttle.
Spectacular though meteor storms are, the meteoroids that generate them are the most minuscule of the many objects that circle the Sun. Yet they, along with asteroids and comets, are remnants of the solar system's birth: a meteor flash is really a miniature reenactment of Genesis, the process that formed the Earth and the other planets circling the Sun.
But the Sun is only one star in a galaxy full of stars, many of which may also have solar systems full of planets, asteroids, comets, meteoroids-and life. To these other stars we now turn.
Copyright © 1999 by Ken Croswell