Computer Model Explains Variations in Mars’ Poles
The north and south poles on Mars look very different from each other, and scientists now think they know why: Circulation patterns in the Red Planet’s very thin atmosphere tend to keep all the water in the north, leaving the south pole high and dry.
The exploration of Mars by unmanned spacecraft has shown that 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, also known as dry ice.
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 surface.”
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 huge difference in elevation between the north and south 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 may be responsible for its alternating layers of ice and dust, he said.
“On Earth, we’re used to thinking the southern summer is similar to the northern summer,” Richardson said, “but it’s not that way on Mars.”
Mars is tilted on its axis at about the same degree as Earth, causing a change in seasons as it orbits the sun.
Unlike Earth, however, Mars has a large variation in orbit that puts it about 20% closer to the sun when it is summer in the south, making the season much more intense than summer in the northern hemisphere.
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.
He said the model developed by Richardson and Wilson makes useful comparisons between actual measurements taken by the Mars Global Surveyor spacecraft now orbiting the planet and computer simulations.
“It seems quite plausible to me,” Hinson said. “I think it’s an important step in understanding how climate change on Mars might be occurring.”
