spacetoday.net: Pluto colder than expected

Posted: Wed, Jan 4, 2006, 7:14 AM ET (1214 GMT)
Pluto-Charon illustration (CfA)

The surface of the distant planet Pluto is about ten degrees colder than its moon, evidence of different compositions for the two objects. Millimeter-wave observations of the infrared emissions from Pluto and Charon, made by the Submillimeter Array observatory atop Mauna Kea, Hawaii, found that Pluto's surface has a temperature of 43 kelvins, about ten degrees colder than the surface of Charon, which closely matches the expected blackbody temperature at that distance from the Sun. Scientists said the difference is due to the different compositions of the two worlds: Pluto's surface features nitrogen ice, which sublimates to form a thin atmosphere that helps cool the surface, while Charon's surface is composed primarily of water ice, which requires much warmer temperatures to sublimate.