The farther away from the Sun you are, the colder it gets. The temperature decreases pretty rapidly so that at the distance of Pluto, the temperature has fallen from 230 K near the Earth, to about 40 K. At these temperatures, nitrogen gas becomes a liquid (70 K) although helium is still a gas (liquid helium = 4 K). The features seen on the surface by the Hubble Space Telescope in the above full-planet projection could be as exotic as an ocean of liquid nitrogen with continents of solid water ice which would appear as bright regions because of water ices high reflectivity.
Copyright 1997 Dr. Sten Odenwald
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