Mars images reveal elegant polarity of ice caps

Some portions of the martian south polar residual cap have long, somewhat curved troughs instead of circular pits.

By Richard Stenger
CNN Interactive Staff Writer

March 9, 2000
Web posted at: 11:47 a.m. EST (1647 GMT)

PASADENA, California (CNN) -- Besides showcasing elegant and contrasting patterns around the polar caps of Mars, new images snapped by Mars Global Surveyor offer evidence that the northern and southern extremes have experienced markedly different geologic pasts.

The pictures, released Wednesday, revealed for the first time features usually hidden underneath the top layers of seasonal ice. A camera onboard Surveyor obtained the photos as it orbited the poles during their alternate summers.

The northern residual ice cap, or region that remains frosted in the summer, has a mostly flat and pitted surface that varies in appearance from a cottage cheese to a sponge texture. The southern one, pocked with larger pits, troughs and mesas, often resembles slices of Swiss cheese.

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"It was just so different from anything we have seen on Mars," said Dr. Peter Thomas, an astronomer at Cornell University and lead researcher on the polar cap project.

"We knew there were difference before (between the caps). But it wasn't clear until now if it was a differences in ice or topography," he said.

Thomas said the contrast in topography on the two polar regions indicate that they experienced different climates and geological histories for thousands of years or longer.

Different poles, different ices

The ice on the north polar residual cap is believed to consist primarily of water, based on data from the Mariner 9 and Viking orbiters in the 1970s.

The portion of each martian polar cap that remains frosted through the summer is known as the Residual Polar Cap. The two pictures above show examples of the north (top) and south polar residual caps as they appeared in summer.

Sponge-like surface patterns in the north have many small pits, generally 10 to 20 meters (33-66 feet) wide. They probably form during the summer as water ice sublimes, or changes directly from solid to vapor.

The temperature there usually is near water's freezing point in the summer. And the Viking spacecraft observed water vapor rising from the cap during the hot season.

After the ice sublimes to vapor, wind could push it away from the polar cap, according to Mars Global Surveyor scientists.

In contrast, erosion in the south likely happens through a greater range of physical processes. They include collapse, slumping, slope movement, and probably sublimation, although a different kind than in the north.

The temperature at the south residual cap remains chilly enough to freeze carbon dioxide. Little or no water vapor has been seen coming off the south cap in the summer.

The erosion processes in the south produce layered hills, buttes and mesas with surrounding triangular "aprons," and the large pits and circular depressions that resemble Swiss cheese.

The exact cause of the large pits remains unclear, but it seems that they have collapsed because material underneath them has disappeared, according to Malin Space Science Systems, which operates the camera onboard the Mars Global Surveyor.

The Mars Global Surveyor has orbited the red planet since September 1997. A NASA mission, it is managed by the Jet Propulsion Laboratory in Pasadena, California.

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