Martian Caves Could Harbor Life
http://www.scienceagogo.com/news/20070218233457data_trunc_sys.shtml

Martian Caves Could Harbor Life

Scientists poring over images from Mars Odyssey believe they have
identified what could be the entrances to seven large caves. Termed
"skylight" entrances because they drop vertically from the surface of
the planet, the entrances range in diameter from 100 to 300 meters.
Because they sit almost "flat" on the surface of the planet, skylight
entrances can be studied extensively from orbit, something not possible
with a more vertically oriented entrance.

It appears that the sub-surface regions the entrances open into may be
relatively deep - at least 70 to 100 meters - according to thermal
imaging, which can only provide a minimum depth. One lucky observation,
however, caught sunlight illuminating the floor of one of the caves at a
depth of 130 meters. The observations were presented at the Lunar and
Planetary Science Conference in Houston.

The caves are of great interest to scientists as they may be the only
natural structures on Mars capable of providing protection to life. As
well as offering shielding from cosmic rays, UV radiation, dust storms,
micro-meteorites and solar flares, the caves also appear to have a much
less harsh temperature cycle than the surface of the planet. Thermal
infrared data suggest that temperatures in the caves varies relatively
little between night and day (Fig. A: An optical image of the cave named
"Annie", Fig. B: An afternoon thermal image showing the cave to be much
cooler than the surface, Fig. C: A night image showing the cave to be
much warmer than the surface).

All the caves - dubbed Dena, Chloe, Wendy, Annie, Abbey, Nikki and
Jeanne - are located high up on Arsia Mons, a 20,000 meter high volcano
that is second only to Olympus Mons in size. The astro-biologists in the
research team believe that such a high altitude may preclude life in
these seven caves, but they speculate that other caves at lower
altitudes might be more conducive to biological processes. The team has
already begun looking for skylight entrances at lower altitudes.

As well as harboring native Martian life, such caves may also offer
future human explorers a relatively safe environment to dwell in while
they explore the planet.

Source: Lunar and Planetary Science Conference