Pale Blue Dot - Carl Sagan [51]
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 composition. 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 composed 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).
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