Daily global maps, created with images from NASA's Mars Global Surveyor spacecraft, provide a moving picture of Martian weather during 1999-2000 similar to the familiar satellite weather maps we see of Earth.

This picture captures two dust storms, each large enough to cover Arizona or New Mexico. One is located near the lower left, the other at the lower right. Taken on April 8, 2001 (mid-southern winter), this is a mosaic of six MOC daily global images centered around Hellas Planitia in the martian southern hemisphere. Hellas Planitia is the dominant elliptical feature just below the center of the picture. The bright, nearly white surfaces along the lower (southern) edge of the picture are covered by wintertime frost. The strong temperature difference between the winter frost and the warmer air just off the edge of this polar cap generates winds that -- at this time of year -- are often strong enough to lift dust into large, reddish-brown, billowy clouds. Photo: NASA/JPL/Malin Space Science Systems

The Martian weather maps were presented at the American Geophysical Union meeting in Boston by Dr. Andrew Ingersoll, professor of planetary science at the California Institute of Technology, Pasadena, and an interdisciplinary scientist with Mars Global Surveyor and member of the imaging team.

In one day, Global Surveyor's camera covers the planet in 12 or 13 overlapping colored strips, which scientists seamed together to make a global movie. Scientists use these movies to study the advance and retreat of the polar frost as well as the motion of dust storms and ice clouds to understand the changing weather on Mars.

"Dust plays the same role in Martian weather that water plays on Earth," Ingersoll said. "Dust heats the atmosphere by absorbing sunlight and starts small storms that every few years merge into major weather events. Some years are dusty and some are not. We blame our wet and dry years on El Nino, but no such scapegoat exists for Mars. That is the mystery. Without oceans or another large heat reservoir that sloshes back and forth, every year on Mars should be the same as every other."

The Martian year just past, which started when Mars Global Surveyor entered its mapping orbit in March 1999, was a low dust year. Countless dust devils and hundreds of local dust storms scoured the surface but only a few grew to regional scale, and none made it into the global category. In contrast, the Viking landers experienced two great global dust storms the year after they touched down in 1976, Ingersoll explained. "The Global Surveyor movies are better than anything Viking could have produced. A second or third year of data would allow us to document this mysterious interannual variability and understand why global dust storms are so unpredictable."

As during the Viking era, most of the dust storms occurred when Mars was closest to the Sun, which happens during the northern fall season. The largest storms started in the north and spread across the equator at speeds up to about 32 kilometers per hour (20 mph). During the Viking years the large dust storms started in the south, where it was springtime. Also, more storms started near the edge of the polar caps than during the Viking era.

"Apparently, Mars is poised at a critical point where natural fluctuations in the daily weather are just barely able to trigger a global dust storm," Ingersoll said. "It is unlikely that Mars was put together in such an unstable state. More likely, it maintains itself there. One way is if the supply of dust is limited. It gets scoured away by the global winds and hides behind boulders and other obstacles. Gradually the dust devils and local storms bring it out into the open, at which point the planet is poised again for another global dust storm."

Mars Global Surveyor is managed by the Jet Propulsion Laboratory for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena.