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MARS may be getting hotter, and transforming its atmosphere in the
process.

Images taken by the Mars Global Surveyor satellite over two successive
Martian summers suggest that the carbon dioxide ice cap at the planet鈥檚 south
pole is shrinking. If this is part of a long-term trend, Martian global warming
could evaporate the whole ice cap in a few thousand years.

This would release enough carbon dioxide to give Mars an atmosphere one-tenth
the density of the Earth鈥檚, and would transform the prospects for explorers.
Instead of working in a near vacuum wearing a space suit, they鈥檇 be able to run
around with an oxygen mask and a heavy coat. 鈥淚t鈥檚 what the terraforming people
were always talking about,鈥� says Mike Caplinger of Malin Space Science Systems
in San Diego, who analysed the Global Surveyor images, although he admits it鈥檚
difficult to make long-term predictions based on only two observations.

David Smith of the NASA Goddard Space Flight Center thinks the shrinkage may
be part of a climate cycle like Earth鈥檚 El Ni帽o Southern Oscillation, and
that the ice cap might soon spread again. Recent dust storms that raised Martian
temperatures by about 10 掳C could also have contributed to the ice cap鈥檚
retreat.

The size of the Martian ice caps is known to fluctuate with the seasons, as
carbon dioxide freezes out of the atmosphere in autumn and winter and evaporates
in spring and summer. Mars Global Surveyor took high-resolution photos showing
how much of the permanent southern polar cap remained after the seasonal ice had
evaporated. They show a scattering of broad pits 30 to 100 metres wide with
steep walls several metres high, like holes in a slice of Swiss cheese laid out
on a flat surface. The walls of many of the pits retreated 1 to 3 metres between
one Martian summer and the next, and Caplinger could find no evidence that the
ice was being re-deposited elsewhere.

NASA is expected to extend the project for at least another Martian year to
see if the trends continue.