Alan Stern (Southwest Research Institute), Marc Buie (Lowell Observatory), NASA and ESA

Since its discovery in 1930, Pluto has remained one of the more mysterious bodies in the Solar System. In 1996, the Hubble Space Telescope observed Pluto, taking snapshots as it rotated through its 6.4-day period. The results, quite surprisingly, showed that the surface of Pluto is complex with more large-scale contrast than any planet except Earth. The images show about a dozen distinctive albedo features, or regions, that had not been seen before. Among these include a ragged northern polar cap bisected by a dark strip, a bright linear marking, and other dark and bright spots on the surface. The images also confirm the presence of polar caps, which had been inferred since the 1980s.

Pluto's Distance and surface composition

Because Pluto is so far from the Sun (5.8 billion kilometers or 3.7 billion miles), its surface is composed of a complex distribution of frosts. These frosts migrate seasonally across the surface, just as they do on Earth. However, the seasons on Pluto are not caused by its tilt as on Earth, but result from the distance of Pluto from the Sun. Pluto's average distance is 39.44 AU, but it ranges from 29.7 to 49.3 AU (1 AU is defined to be the average distance between the Earth and the Sun and equals 149 million kilometers or 93 million miles). Because of this variation in distance, when Pluto is further from the Sun nitrogen, carbon monoxide, and methane gases partially freeze on the surface. During the warm season, when Pluto is closer to the Sun, these ices sublimate (go directly from solid to gas) enhancing Pluto's atmosphere.

Ellen Cohen