The American space agencys Mars Global Surveyor orbiter captured the images as part of its ongoing monitoring of the seasonal advance and retreat of polar frosts on the planet s surface.

The four images render wide-angle views of four distinct craters -- two each in Mars' northern and southern hemispheres. They are located the middle polar latitudes of each hemisphere.

The images (upper left and right) show Lomonosov and an unnamed crater 10 degrees north of it as they appeared in August and October during the northern spring. Frost that had accumulated during the hemispheres six-month winter has been shrinking since May. In the case of the unknown crater, just 30 miles (48 kilometers) across, portions of the frost may well persist through the summer on its cool, shady floor -- much as it was seen to do in Viking orbiter images acquired in the 1970s.

In the southern hemisphere, it is now autumn on Mars, with frost appearing as early as August in Barnard Crater (lower left). By mid October, the frost line had moved farther north, and the white stuff appeared in images of Lowell Crater (lower right) as well.

In each of the four images, north is toward the top. In the two northern hemisphere images, the Sun illuminates the scene from the lower left. In the southern hemisphere, the sun does so from the upper left.

NASAs Global Surveyor has been orbiting Mars since September 1997. It will be joined late next year by another NASA satellite, the 2001 Mars Odyssey, scheduled for launch on April 7, 2001.