Calling the pictures the "best-ever weather-related images" from Mars, Michael Meyer, program scientist for the Mars Surveyor 2001 mission said today that the global surveyor is revealing a great deal about seasonal changes on Mars.

There is plenty of dramatic weather on the planet now as spring comes to its southern hemisphere.

A group of images released today by NASA shows dust devils, wind-swept sand dunes and melting southern polar ice caps. The pictures have excited some scientists who say the pictures will help them to decipher time scales of geologic change on Mars. This in turn will teach them more about the age and evolution of Mars' surface.

Pictures taken above mid-latitude plains show towering dust devils that swirl across the Martian landscape in the early afternoons. Some of these swirling storms rise 5 miles (8 kilometers) above the Martian surface and may carry several tens of tons of fine red dust, estimates Peter Thomas, a member of the Mars Global Surveyor imaging team from Cornell University. The images of the dust devils aren't much to look at. All that is visible is the shadows cast by the whirlwinds, which appear as tiny dark triangles on the martian landscape.

The most notable of the photos released today show melting frost on polar sand dunes. The frost-covered dunes look pock-marked with black splotches that look almost like shrubs popping out of the frost. What the black circles are, scientists guess, are areas where the frost has melted away from the dark sand underneath. Once a small clearing has opened up in the ice, it continues to expand outward. The exposed dark sand absorbs more heat from the sun than the surrounding frost, and the areas thaw outward. That accelerates the melting of surrounding frost and ice. Over time the expanding dark spots resemble bushes growing atop the frost.

Other photos show dunes that change with winds over time. Many of the dunes on Mars are about the same size as sand dunes on Earth.

The Mars Global Surveyor will continue to send back images during its mission to map the surface of Mars. The surveyor's five scientific instruments are designed to generated a detailed portrait of the seasonal changes on Mars during a full Martian year, a year that lasts about two Earth years.