The North polar cap has a relatively flat, pitted surface that resembles cottage cheese, while the South polar cap has larger pits, troughs and flat mesas that give it a holey Swiss-cheese appearance

"Looking like pieces of sliced and broken Swiss cheese, the upper layer of the Martian South polar residual cap has been eroded, leaving flat-topped mesas into which are set circular depressions," said Dr. Peter Thomas of Cornell University, Ithaca, NY, and lead author of the paper.

"Nothing like this has ever been seen anywhere on Mars except within the South polar cap, leading to some speculation that these landforms may have something to do with the carbon dioxide thought to be frozen in the South polar region."

In a paper to be published March 9, 2000, in the journal Nature, members of the Mars Global Surveyor imaging team have described some of the newly discovered differences in polar terrain

"The unusual shapes of the landforms on the North and South polar caps suggest that these regions have had different climates and histories for thousands or perhaps even millions of years," said Thomas.

"We are discovering them for the first time because Mars Global Surveyor is working to provide high-resolution views of the tremendously diverse terrain on Mars over all Martian seasons."

"These landforms may be telling us what the South polar cap is made of," says Dr. Andrew Ingersoll of the California Institute of Technology, Pasadena, CA and one of the authors of the paper.

Some portions of the martian south polar residual cap have long, somewhat curved troughs instead of circular pits. These appear to form in a layer of material that may be different than that in which "swiss cheese" circles and pits form, and none of these features has any analog in the north polar cap or elsewhere on Mars. This picture shows the "fingerprint" terrain as a series of long, narrow depressions considered to have formed by collapse and widening by sublimation of ice. Unlike the north polar cap, the south polar region stays cold enough in summer to retain frozen carbon dioxide. Viking Orbiter observations during the late 1970s showed that very little water vapor comes off the south polar cap during summer, indicating that any frozen water that might be there remains solid throughout the year.

"The North residual cap -- the part that survives the summer -- is made of water ice. The South residual cap seems to be made of frozen carbon dioxide, otherwise known as dry ice, but we don't know if this is a veneer a few meters thick or a solid block that extends down 2 or 3 kilometers. These images may help us decide."

The North polar cap is covered mainly by pits, cracks, small bumps and knobs that give it a cottage-cheese look. The pits that have developed on the surface are spaced close together relative to the very different depressions in the South polar cap. These pits probably developed slowly over successive spring and summer seasons

"The polar images demonstrate again that understanding Mars' complicated history requires studying many areas in detail, just as understanding the Earth does," Thomas said

"If we discovered that both polar caps are mostly water, it would leave a mystery about why there is so little carbon dioxide on Mars. Earth has a lot of carbon dioxide, but creatures living in the ocean have turned it into limestone rocks. Without oceans or life, Mars should have a lot more carbon dioxide on its surface than we seem to be finding," explained Ingersoll