Among Mars's many intriguing qualities is its asymmetry. For instance, whereas its Northern Hemisphere is relatively smooth, the Southern Hemisphere contains copious craters. Now a report published today in Nature suggests that height differences between Martian surfaces in the two hemispheres can explain why more water persists on the planet's North Pole than down south.

Mark Richardson of the California Institute of Technology and John Wilson of the National Oceanic and Atmospheric Administration (NOAA) modeled so-called Hadley circulation in the Red planet's atmosphere. In this process, which works in much the same way as it does on earth, atmospheric gases warmed at the equator rise up and migrate toward the poles, where they sink before traveling back to the equator. These circulating gases carry water vapor and dust picked up from the planet's surface. The scientists determined that the thin Martian air rises and falls more vigorously in the Southern Hemisphere than it does north of the equator. As a result, there is an overall movement of water vapor from south to north.

The eccentricity of Mars's orbit around the Sun had previously been thought to contribute to the planet's asymmetric atmospheric circulation. But the new results show that it is instead the height difference between Martian landforms--its South Pole is approximately six kilometers higher than its North Pole--that causes the disparate circulation patterns. These findings, the researchers conclude, reveal why water ice is more stable at the North Pole. According to Peter Gierasch of Cornell University, the results represent "a new constraint on the interplay between the surface of Mars and its atmosphere, one that should help us understand the planet much better."