Mars provides a dictionary definition for the phrase 'polar differences'. A vast ice cap of water ice and snow dominates the planet's north pole. Yet around the south pole, a comparatively tiny cap appears to be composed mostly of frozen carbon dioxide, popularly known as dry ice.

A new study may explain why.

Fresh clues have been found in a computer model of broad circulation patterns that change with the seasons and appear to control how and where water is transported on the Red Planet and why snowstorms are largely limited to the north. Further, the research suggests that the atmospheric circulation may be controlled to some extent by the fact that Mars' southern hemisphere is generally higher than the surface elevations in the north hemisphere.

Mars has an extremely thin atmosphere that is mostly carbon dioxide, with a little water vapor, some dust, and a few other gases thrown in.

Despite significant differences in composition and density, the air above Mars behaves similarly to Earth's atmosphere -- rising when heated, sinking when cooled. Air tends to rise near the equator, where it is heated the most, and sink near the poles. Combined with a twist provided by each planet's rotation, this basic scheme is behind all weather, from Martian dust storms to terrestrial hurricanes and Arctic snowstorms.

As on Earth, the circulation pattern on Mars changes seasonally because of the planet's tilted axis. During summer in the north of Mars -- when the northern hemisphere is tilted toward the Sun -- a more vigorous circulation pattern exists in the North. The reverse happens when it is summer in the south.

Yet on average, the study found, air on Mars tends to migrate northward at high altitudes and return to the south nearer the surface.

"On Earth, you have a net transfer of heat and water away from the equator," Wilson explained in a telephone interview. "On Mars, we speculate that heat and water, on average, are transported across the equator from south to north."

This heat and water moves north at high altitudes. Near the north pole, it cools. Clouds of water ice or snow form, Wilson said, and some of it falls to the surface, just as it would on Earth.

Peter Gierasch of Cornell University, in an analysis of the study also to be published by Nature, writes that the research convincingly shows the transport mechanism at work, even if its causes are not fully understood.

"Richardson and Wilson have pointed out a new constraint on the interplay between the surface of Mars and its atmosphere, one that should help us to understand the planet much better," Gierasch said.

The idea that surface elevation differences may control the global climate of Mars is more speculative.

For unknown reasons, the southern hemisphere of Mars is, on average, about 3 miles (4-5 kilometers) higher than the north. If Mars had seas, most of the southern hemisphere would be above sea level, while northern regions would be largely submerged.

The difference appears to be due to variations in the thickness of the planet's crust, or outermost layer, Wilson said. Data returned from the orbiting Mars Global Surveyor spacecraft will be key to understanding this structure of the planet, he said.

But clues have emerged as to how these elevation differences might effect climate.

It may be, Wilson said, that higher elevations in the south cause air to be warmed at higher altitudes there, creating a more vigorous low level inflow into the southern hemisphere and high level outflow back to the northern hemisphere when it is winter there.

Or, he said, the rising slope across the equator could simply force warming air to move up the slope.

On Earth, the equator runs through a whole lot of ocean, where the elevation is constant. The presence of a rising slope across the equator of Mars would enable heated air to naturally travel "up slope" during southern hemisphere summer, Wilson said. But during northern summer, motion across the equator may be inhibited by the requirement that the warmed air travel downslope into the northern hemisphere.

The researchers are still looking into which mechanism might be at play.