"Mars is a cold, dry desert, but our camera has shown it is far from being a stagnant place," said Dr. Michael Malin, principal investigator for the Mars Global Surveyor camera at Malin Space Science Systems in San Diego.

"Over the past few months, we have captured a unique record of seasonal and meteorological events, which demonstrates that Mars is active and dynamic today," Malin added.

The spacecraft's camera monitors the planet's weather on a daily basis from orbit, just like weather satellites on Earth. Today, Mars is a much more dynamic place than the planet the Viking spacecraft saw in the late 1970s.

The weather has been particularly active during the past two months, as spring arrived in the southern hemisphere and autumn approached in the north.

"Storm clouds have been brewing over the north polar ice cap all through the month of July, and soon, ever-increasing portions of the north polar cap will be plunged into wintertime darkness," Malin said.

"As the season changes rapidly, clouds will cover much of the northern plains and it might begin to snow as the polar cap expands," he added.

In other regions of Mars, dust devils are the prevailing weather story. Dust devils result from spinning vortices of air that arise when the ground is heated and general wind flow is light.

On Earth they are relatively small features, but on Mars, dust devils are thought by some to be a major transporter of the fine, pinkish dust that gives the sky its unearthly brownish color, as seen by the Mars Pathfinder and Viking landers. Dust devils may also help initiate the seasonal raising of dust over wide areas of Mars.

In mid-May, swirling columns of dust as high as five miles (eight kilometers) were observed in northern Amazonis Planitia. Dust devils in this area, northwest of the large Tharsis volcanoes, appear to be common; they were also seen by the Viking missions of the 1970s. The average dust devil is slow-moving and may carry several tons of dust within its height of 1.2 miles (two kilometers).

Each lasts for a few hours at most during the hottest part of the Martian day in the late afternoon, Malin said. Although the winds in these vortices are sufficient to raise dust, they have much less power than tornadoes on Earth, which develop under very different meteorological circumstances.

Global Surveyor's camera has also returned tantalizing evidence of recent shifting sands in dune fields first seen in Mariner 9 pictures of Mars from the early 1970s.

Scientists are interested in dune fields isolated within large impact craters because their dark color suggests that the dust which covers much of the rest of the planet does not accumulate on their sandy surfaces.

"This indicates that the dunes must be moving and that over time we may be able to see changes that will allow us to measure the rates of wind erosion on Mars," Malin explained.

Sand dunes also are giving Mars scientists some new insights as to how Mars' seasonal polar ice caps retreat at the end of each winter as seasonal warming occurs.

The most dramatic views show patches of dark sand poking through fields of carbon dioxide frost. First seen in 1998 in the north polar region, the same features have been seen this year on dunes near the south pole.

"These pictures look like aerial photographs of dunes on Earth," Malin said. "They are so unusual in this context that we thought for a while that we were looking at a process that involves small 'explosions,' but the new images showed that wind was responsible for the streaks we were seeing."

The dark spots on frost-covered dunes continue to grow and spread as spring approaches until, eventually, the entire dune field is frost-free.