From NASA's Planetary Photojournal
This is the south polar cap of Mars on April 17, 2000. In winter and early spring, this entire scene would be covered by frost. In summer, the cap shrinks to its minimum size, as shown here. Even though it is summer, observations made by the Viking orbiters in the 1970s showed that the south polar cap remains cold enough that the polar frost (seen here as white) consists of carbon dioxide. Carbon dioxide freezes at temperatures around -125 degrees C (-193 degrees F). Mid-summer afternoon sunlight illuminates this scene from the upper left from about 11.2 degrees above the horizon. The polar cap from left to right is about 420 km (260 mi) across.
Image Title: South Polar Cap, Summer 2000
Catalog #: PIA02393
Target Name: Mars
Spacecraft: Mars Global Surveyor Orbiter
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Last Modified On: Monday, December 18, 2000