August 7
The identification and study of five meteorites on the surface of Mars by NASA's twin rovers Spirit and Opportunity has presented a fresh mystery about the possible presence of surface water in the past.

To date between them the rovers have on average identified a meteorite on the Martian surface every eight months since landing in January 2004. It seems Mars is littered with grounded space rocks.

"Meteorites fall constantly on Mars," said Hap McSween of the University of Tennessee. "The weathering rate on Mars is so slow that they persist for long periods of time. It's akin to the situation in Antarctica where we find meteorites by the hundreds every year."

Like rocks lying on an Antarctic snow field, the iron meteorites on Mars are in stark contrast to their surroundings and are easily spotted.

Although the meteorites are alien to Mars, they interact with the Martian environment, so evidence of Mars' supposed water-rich past should be written on their surfaces.

If any of the iron meteorites so far discovered had come into contact with liquid water, ice or water vapor, it should have adopted the same rusty hue as the rest of the surface.

Sand dunes move around on Mars and fine dust settles out of the atmosphere, so there are certainly mechanisms for burying meteorites and then re-exposing them. In many places they might sit on the surface for many millions of years without being buried.

Stone meteorites hold their cards close to their chest and would keep any evidence of alteration by water well protected in their interiors. It may be that scientists have already seen a stone meteorite on Mars and just not realized it.
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