The north and south poles on Mars look different from each other, and scientists now think they know why: Circulation patterns in the red planet's thin atmosphere tends to keep all the water in the north, leaving the south pole high and dry.

Mars exploration by unmanned spacecraft has shown the northern hemisphere has a large polar cap made up mostly of frozen water while the southern hemisphere has a much smaller cap made up almost entirely of frozen carbon dioxide.

A new computer model suggests the apparently permanent difference results partly from the much higher elevation in the south -- which is an average of three miles higher than the north.

Mark Richardson of the California Institute of Technology and John Wilson of the Geophysical Fluid Dynamics Laboratory at Princeton University reported in a recent issue of the journal Nature that the difference in elevation shifts the Hadley effect, an atmospheric circulation pattern created when heated air rises from the warmer equator and sinks toward the poles.

"It's the dominant form of atmospheric circulation in the tropics here on Earth," Richardson said, "where you get rising air in the region most strongly heated by the sun that has to be replaced by cooler air at the surfce."

On Earth, however, the circulation is balanced between the northern and southern tropics, causing the gentle trade winds across the relatively flat and even expanse of the ocean.

On Mars, the Hadley effect reaches much farther toward the poles across a dry and dusty surface.

The difference in elevation between the poles also tends to push the thin Martian air more strongly from the high elevations of the south toward the northern lowlands, Richardson said.

The resulting imbalance dumps snow and ice at the north pole of Mars, and also may be responsible for its alternating layers of ice and dust, he said.

"We're used to thinking the southern summer is similar to the northern summer," Richardson said, "but it's not that way on Mars."

David Hinson of Stanford University, one of the leaders of the latest NASA survey of Mars, said Martian weather will be the focus of several unmanned space missions planned in the coming decade.