The disappearance of NASA's Mars Polar Lander spacecraft last December crushed the hopes of many planetary scientists eager to get on intimate terms with the mysterious icy caps of the Red Planet. But all is not lost. The latest results from the orbiting Mars Global Surveyor spacecraft contain spectacular views of the planet's polar regions, revealing hitherto unknown differences between the north and south poles of Mars.

Like the sea ice around our own Arctic and Antarctic ice caps, the polar ice of Mars expands in the winter and shrinks in the summer. It is now clear that the ice caps are composed largely of 'dry ice' -- solidified carbon dioxide, which 'sublimates' straight from solid to gas into the thin Martian atmosphere when the ice caps shrink

There is some water ice in these polar caps. But no one knows how much -- only that it could be vital for understanding Mars's past history. The dry river-valley-like features carved into the planet's surface testify that it once had flowing water. Some of this may still remain on the planet, locked away in frozen form.

This was one of the issues that the Mars Polar Lander was due to address, by analysing samples of the polar ice, when it touched down on the southern ice cap on 3 December 1999. But communication was lost before touchdown, and NASA has now pronounced the mission 'dead'.

In contrast, the Mars Global Surveyor has been beaming back spectacular close-up images of the Martian surface since it entered planetary orbit in 1997 -- Images that Peter Thomas of Cornell University, Ithaca, New York, and co-workers have scoured for differences between the planet's polar ice caps.

Even a superficial comparison with Earth illustrates that the two Martian caps need not be similar. Our North Pole is situated on permanent sea ice, whereas the southern ice cap sits on solid rock -- the continent of Antarctica. The situation on Mars is rather similar, Thomas's group has found.

The north polar cap seems to consist of vast fields of rough, pitted ice that simply sit on the rocky plains beneath. The southern cap, on the other hand, is less homogeneous. Six kilometres higher than the north polar region, it has been carved into elaborate shapes by some kind of erosion process. In some places there are roughly circular depressions in the ice with walls several metres high. Elsewhere the ice is striated into smooth, fingerprint-like grooves, reminiscent of desert sand dunes.

In Nature1, Thomas's team speculates about the sequence of events that creates these features. They think that the deposition of additional ice in the winter causes the existing ice layer to sag and collapse. Around escarpments nine metres or more high, the sides of the depressions may crumble, littering the edges with icy debris. The shapes of surface features are then more finely sculpted, they propose, by the sublimation of upper layers of ice -- something that does not occur on the terrestrial ice caps.

Regardless of how these strange structures are created at the Martian south pole, they make one thing clear. The southern ice cap is no mere frosty veneer (as is the northern ice cap), but rather a substantial geological feature in its own right, like a mountain glacier or an ice sheet on Earth. The permanent part of this ice cap may have gradually built up over thousands of years, as each year a little more of the seasonal ice is retained. That this does not seem to happen in the north is a sign that the climate of Mars may have long been different in the two hemispheres.