d counterpoint characteristic of each world。
On Earth the magnetic and geographical poles are quite close together。 On Uranus the magnetic axis and the axis of rotation are tilted away from each other by some 60 degrees。 No one yet understands why： Some have suggested that we are catching Uranus in a reversal of its north and south magnetic poles； as periodically happens on Earth。 Others propose that this too is the consequence of that mighty； ancient collision that knocked the planet over。 But we do not know。
Uranus is emitting much more ultraviolet light than it's receiving from the Sun； probably generated by charged particles leaking out of the magnetosphere and striking its upper atmosphere。 From a vantage point in the Uranus system； the spacecraft examined a bright star winking on and off as the rings of Uranus passed by。 New faint dust bands were found。 From the perspective of Earth； the spacecraft passed behind Uranus； so the radio signals it was transmitting back home passed tangentially through the Uranian atmosphere； probing it—to below its methane clouds。 A vast and deep ocean； perhaps 8；000 kilometers thick； of super…heated liquid water floating in the air is inferred by some。
Among the principal glories of the Uranus encounter were the pictures。 With Voyager's two television cameras； we discovered ten new moons； determined the length of the day in the clouds of Uranus （about 17 hours）； and studied about a dozen rings。 The most spectacular pictures were those returned from the five larger； previously known moons of Uranus； especially the smallest of them； Kuiper's Miranda。 Its surface is a tumult of fault valleys； parallel ridges； sheer cliffs； low mountains； impact craters； and frozen floods of once…molten surface material。 This turmoiled landscape is unexpected for a small； cold； icy world so distant from the Sun。 Perhaps the surface was melted and reworked in some long…gone epoch when a gravitational resonance between Uranus； Miranda； and Ariel pumped energy from the nearby planet into Miranda's interior。 Or perhaps we are seeing the results of the primordial collision that is thought to have knocked Uranus over。 Or； just conceivably； maybe Miranda was once utterly destroyed； dismembered； blasted into smithereens by a wild careening world； with many collision fragments still left in Miranda's orbit。 The shards and remnants； slowly colliding； gravitationally attracting one another； may have re…aggregated into just such a jumbled； patchy； unfinished world as Miranda is today。
For me； there's something eerie about the pictures of dusky Miranda； because I can remember so well when it ‘vas only a faint point of light almost lost in the glare of Uranus； discovered through great difficulty by dint of the astronomer's skills and patience。 In only half a lifetime it has gone from an undiscovered world to a destination whose ancient and idiosyncratic secrets have been at least partially revealed。
CHAPTER 9 AN AMERICAN SHIP AT THE FRONTIERS OF THE SOLAR SYSTEM
。 。 。 by the shore of Triton's Lake 。 。 。
I will clear my breast of secrets。
—EURIPIDES； ION （CA。 413 B。C。）
Neptune was the final port of call in Voyager 2's grand tour of the Solar System。 Usually； it is thought of as the penultimate planet； with Pluto the outermost。 But because of Pluto's stretched…out； elliptical orbit； Neptune has lately been the outermost planet； and will remain so until 1999。 Typical temperatures in its upper clouds are about …240°C； because it is so far from the warming rays of the Sun。 It would be colder still except for the heat welling up from its interior。 Neptune glides along the hem of interstellar night。 It is so far away that； in its sky； the Sun appears as little more than an extremely bright star。
How far？ So far away that it has yet to plete a single trip around the Sun； a Neptunian year； since its discovery in 1846。* It's so far away that it cannot be seen with the naked eye。 It's so far away that it takes light—faster than which nothing can go—more than five hours to get from Neptune to Earth。
* It takes so long to circuit the Sun because its orbit is so vast； 23 billion miles around； and because the force of the Sun's gravity—which keeps it from flying out into interstellar space—is at that distant paratively feeble； less than a thousandth what it is in the Earth's vicinity。
When Voyager 2 raced through the Neptune system in 1989； its cameras； spectrometers； particle and field detectors； and other instruments were feverishly examining the planet； its moons； and its rings。 The planet itself； like its cousins Jupiter； Saturn； and Uranus； is a giant。 Every planet is an Earthlike world at heart—but the four gas giants wear elaborate； cumbersome disguises。 Jupiter and Saturn are great gas worlds with relatively small rocky and icy cores。 But Uranus and Neptune are fundamentally rock and ice worlds swaddled in dense atmospheres that hide them from view。
Neptune is four times bigger than the Earth。 When we look down on its cool； austere blueness； again we are seeing only atmosphere and clouds—no solid surface。 Again； the atmosphere is made mainly of hydrogen and helium； with a little methane and traces of other hydrocarbons。 There may also be some nitrogen。 The bright clouds； which seem to be methane crystals； float above thick； deeper clouds of unknown position。 From the motion of the clouds we discovered fierce winds； approaching the local speed of sound。 A Great Dark Spot was found； curiously at almost the same latitude as the Great Red Spot on Jupiter。 The azure color seems appropriate for a planet named after the god of the sea。
Surrounding this dimly lit； chilly； stormy； remote world is—here also—a system of rings； each posed of innumerable orbiting objects ranging in size from the fine particles in cigarette smoke to small trucks。 Like the rings of the other Jovian planets； those of Neptune seem to be evanescent—it is calculated that gravity and solar radiation will disrupt them in much less than the age of the Solar System。 If they are destroyed quickly； we must see them only because they were made recently。 But how can rings be made？
The biggest moon in the Neptune system is called Triton。* Nearly six of our days are required for it to orbit Neptune； which—alone among big moons in the Solar System—it does in the opposite direction to which its planet spins （clockwise if we say Neptune rotates counterclockwise）。 Triton has a nitrogen…rich atmosphere； somewhat similar to Titan's； but； because the air and haze are much thinner； we can see its surface。 The landscapes are varied and splendid。 This is a world of ices—nitrogen ice； methane ice； probably underlain by more familiar water ice and rock。 There are impact basins； which seem to have been flooded with liquid before refreezing （so there once were lakes on Triton）； impact craters； long crisscrossing valleys； vast plains covered by freshly fallen nitrogen snow； puckered terrain that resembles the skin of a cantaloupe； and more or less parallel； long； dark streaks that seem to have been blown by the wind and then deposited on the icy surface despite how sparse Triton's atmosphere is （about 1/10；000 the thickness of the Earth's）。
* Robert Goddard； the inventor of the modern liquid…fueled rocket； envisioned a time when expeditions to the stars would be outfitted on and launched from Triton。 This was in a 1927 afterthought to a 1918 handwritten manuscript called 〃The Last Migration。〃 Considered much too daring for publication； it was deposited in a friend's safe。 The cover page bears a warning： 〃The＇se＇ notes should be read thoroughly only by an optimist。〃
All the craters on Triton are pristine—as if stamped out by some vast milling device。 There are no slumped walls or muted relief。 Even with the periodic falling and evaporation of snow； it seems that nothing has eroded the surface of Triton in billions of years。 So the craters that were gouged out during the formation of Triton must have all been filled in and covered over by some early global resurfacing event。 Triton orbits Neptune in the opposite direction to Neptune's rotation—unlike the situation with the Earth and its moon； and with most of the large moons in the Solar System。 If Triton had formed out of the same spinning disk that made Neptune； it ought to be going around Neptune in the same direction that Neptune rotates。 So Triton was not made from the original local nebula around Neptune； but arose somewhere else—perhaps far beyond Pluto—and was by chance gravitationally captured when it passed too close to Neptune。 This event should have raised enormous solid…body tides in Triton； melting the surface and sweeping away all the past topography。
In some places the surface is as bright and white as freshly fallen Antarctic snows （and may offer a skiing experience unrivaled in all the Solar System）。 Elsewhere there's a tint； ranging from pink to brown。 One possible explanation： Freshly fallen snows of nitrogen； methane； and other hydrocarbons are irradiated by solar ultraviolet light and by electrons trapped in the magnetic field of Neptune； through which Triton plows。 We know that such irradiation will convert the snows （like the corresponding gases） to plex； dark； reddish organic sediments； ice tholins—nothing alive； but here too posed of some of the molecules implicated in the origin of life on Earth four billion years ago。
In local winter； layers of ice and snow build up on the surface。 （Our winters； mercifully； are only 4 percent as long。） Through the spring； they are slowly transformed； more and more reddish organic molecules accumulating。 By summertime； the ice and snow have evaporated； the gases so released migrate halfway across the planet to the winter hemisphere and there cover the surface with ice and snow again。 But the reddish organic molecules do not vaporize and are not transported—a lag deposit； they are next winter covered over by new snows； which are in turn irradiated； and by the following summer the accumulation is thicker。 As time goes on； substantial amounts of organic matter are built up on the surface of Triton； which may account for its delicate color markings。
The streaks begin in small； dark source regions； perhaps when the warmth of spring and summer heats subsurface volatile snows。 As they vaporize； gas es gushing out as in a geyser； blowing off less…volatile surface snows and dark organics。 Prevailing low…speed winds carry away the dark organics； which slowly sediment out of the thin air； are deposited on the ground； and generate the appearance of the streaks。 This； at least； is one reconstruction of recent Tritonian history。
Triton may have large； seasonal polar caps of smooth nitrogen ice underlying layers of dark organic materials。 Nitrogen snows seem recently to have fallen at the equator。 Snowfalls； geysers； windblown organic dust； and high…altitude hazes were entirely unexpected on a world with so thin an atmosphere。
Why is the air so thin？ Because Triton is so far from the Sun。 Were you somehow to pick this world up and move it into orbit around Saturn； the nitrogen and methane ices would quickly evaporate； a much denser atmosphere of gaseous nitrogen and methane would form； and radiation would generate an