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tmosphere 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 opaque tholin haze。 It would bee a world very like Titan。 Conversely； if you moved Titan into orbit about Neptune； almost all its atmosphere would freeze out as snows and ices； the tholin would fall out and not be replaced； the air would clear； and the surface would bee visible in ordinary light。 It would bee a world very like Triton。
These two worlds are not identical。 The interior of Titan seems to contain much more ice than that of Triton； and much less rock。 Titan's diameter is almost twice that of Triton。 Still； if placed at the same distance from the Sun they would look like sisters。 Alan Stern of the Southwest Research Institute suggests that they are two members of a vast collection of small worlds rich in nitrogen and methane that formed in the early Solar System。 Pluto； yet to be visited by a spacecraft； appears to be another member of this group。 Many more may await discovery beyond Pluto。 The thin atmospheres and icy surfaces of all these worlds are being irradiated—by cosmic rays； if nothing