Stamp of recent water ice

CONES POKING out from the surface of Mars could be evidence for recent water ice on the red planet this time just beneath some of the most parched regions of its rocky terrain according to a report in the journal Geophysical Research letters.

High-resolution images of an area the size of Canada taken by the Mars Orbiter Camera reveal what appear to be rootless cones. These geological formations are found on Earth where molten lava has flowed over waterlogged ground.

The images show vast fields of objects, which measure 20 - 300 m across at the base. But unlike ordinary martian volcanoes, the cones are not associated with rock fissures.

The location of the cones, in Mars' equatorial region, is "rather disturbing", says Peter Lanagan of the Lunar and Planetary Laboratory at the University of Arizona in Tucson who carried out the work. It suggests water was present "in an area of Mars where we don't expect to see water", he says. Most distribution models predict that water occurred around Mars' poles.

Drawing geological analogies between Earth and Mars "can be a dangerous game", cautions John Mustard, who studies martian climate at Brown University in Providence, Rhode Island. But the shape and location of the cones make the analogy probable, he concedes, indicating that there might well have been water around the equator.

Rootless cones are so called because they are not produced by lava erupting through cracks in surface rock. On Earth, they form when hot lava flows over damp ground causing the water beneath to boil. The water flashes to steam very quickly fuelling an explosive release of lava upwards, which builds a cone.

The cones on Mars occur in an area that was flooded by lava within the past 10 million years and where there are channels hinting at the presence of water before then. This suggests that water was present when the lava flowed across.

"Although 10 million years ago sounds like a long time," says Lanagan, "for people studying Mars that's recent." Water was thought to have evaporated from equatorial Mars hundreds of millions of years ago.

The big question, says Mustard, "is whether that reservoir of water is still there". Martian researchers may not have to wait for too long for confirmation. NASA's Mars Odyessy mission en route to Mars is equipped with a gamma-ray spectroscope that will look for the signature of hydrogen atoms in water ice.

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