Cold, dark and only briefly tipped toward the Sun, the poles of Mars are the planet's deep freezes. These areas are covered in giant white caps that advance and retreat with the passing seasons. Much of the planet's water is locked in the polar ice, along with vast deposits of solid carbon dioxide—dry ice. Summers on Mars are warm enough to free some of the poles' frozen carbon dioxide. But the water ice is a permanent fixture—and hostile as the poles are, they may be the best place for a human base on Mars.

Astronomers have been looking at Mars' polar caps for more than three centuries. By the early 1700s, one observer had resolved the Red Planet's lowest latitudes clearly enough to describe them as "white stains/7 But it was only in 1781 that the British observer Sir William Herschel (1738-1822) suggested that Mars 7 polar caps, like Earth's, might be made of frozen water.

Herschel was partly correct. Polar temperatures on Mars are far too cold to melt water ice even in summer— yet the ice caps shrink dramatically in this season. The caps do contain water ice, but also large quantities of more volatile frozen carbon dioxide (CO2), which sublimates— changes from solid to gas—as temperatures rise.

Summer in Mars' southern hemisphere comes when the planet is closest to the Sun. This proximity means that the southern cap shrinks far more than its northern counterpart. But Mars moves fastest at this time of year, so the south pole speeds through its warmest weather. The short-lived thaw, and the fact that the polar water ice does not melt at any time of the year, means that the southern cap never vanishes completely.

Winter is the longest season on southern Mars. As temperatures fall, CO2 freezes out of the atmosphere and falls as snow that enlarges the polar cap. Similar effects also change the size of the northern polar cap. Each cap is much larger in winter than in summer.

Other changes take place over much longer time scales. Around and under the polar ice. Mars' surface is built up in smooth layers that are each some 100 feet thick. They are crater-free, so they must have been laid down recently—in astronomical terms, at least. Mars is a very dusty planet. Over the course of about 50,000 years, global winds blow huge clouds of volcanic ash and dust from the equatorial regions to the poles. As these deposits build up, they are mixed with frozen water and CO2. If there is then a relatively long warm spell on the planet—caused by a change in axis tilt or orbital eccentricity—the CO2 sublimates, leaving the dust behind. This causes the new stratum to slump into a terrace with a sheer drop. Over a million years, the cycle of deposit and collapse has built a series of 100-foot-high steps on the edges of the polar caps.

For all of their instabilities, the polar caps are still water reservoirs, and if we ever establish a base on Mars, it might have to be in these regions.