It's long been known that the climate on Mars, with its carbon dioxide-based atmosphere, is hostile toward human life. And with two papers published in the current issue of Science, scientists are gaining a better understanding of just how that climate changes through time. In fact, it seems to fluctuate more than was previously thought.

In the first report, Michael C. Malin of Malin Space Science Systems and colleagues examined high-resolution images of the red planet's south polar cap taken in 1999 and 2001 by the Mars Orbiter Camera onboard the Mars Global Surveyor. Comparisons of the irregular pits, ridges and mounds on the cap uncovered significant erosion of the top layers of carbon dioxide frost. The researchers calculated that 25 to 50 percent of the measured features retreated by one to three meters over the course of the past Martian year (or 687 days).

Using a second instrument onboard the Mars Global Surveyor, scientists from the NASA/Goddard Space Flight Center and the Massachusetts Institute of Technology measured elevation changes in the carbon dioxide snow deposits on the planet's north and south poles. "We have measure when and where the carbon dioxide resides during the winter over the entire surface of Mars," co-author Maria Zuber explains, "which will allow more accurate models of Martian weather to be developed." They found that the biggest elevation changes across seasons occurred at the poles but the bulk of snow accumulation and evaporation takes place at lower latitudes around 75 degrees. The researchers also noted off-season snow accumulation and evaporation on the planet for the first time.

The new analyses, David A. Paige of the University of California at Los Angeles writes in an accompanying commentary, "show that Mars's present-day global carbon cycle may be even more dynamic and interesting than previously thought." Indeed, according to Malin, "the Mars environment we see today may not be what it was a few hundred years ago, and may not be what will exist a few hundred years in the future."