The result is fuelling the debate about whether Mars was ever hospitable to life. The mineral that gives the planet its colour is a reddish iron oxide. Until now, astronomers thought that it probably formed in a chain of chemical reactions as iron in rocks dissolved into pools and rivers on the warm young planet.
The iron oxidised, precipitated, and was then blown all over the planet. But Albert Yen of NASA's Jet Propulsion Laboratory in Pasadena, California, began to doubt this after the Mars Pathfinder mission reached the Red Planet in 1997. The mission revealed that there is more iron and magnesium in Martian topsoil than within its rocks. This suggests the minerals actually came from the small, metal-rich meteors and dust particles that constantly fall onto Mars, says Yen. Calculations suggest they deposit 5 centimetres of surface layer every billion years.
If that's the case, Mars might not have been so wet after all. To test whether this topsoil would have needed water to oxidise and turn red, Yen exposed metallic iron to ultraviolet light, simulating sunlight, in a chamber containing gases similar to the Martian atmosphere at temperatures as low as -60 ¡C. Red iron oxides started to form within a week. No water was necessary, Yen told this week's meeting of the American Astronomical Society's planetary science division in Monterey, California. Yen does not claim water never flowed on Mars - the planet's networks of dry valleys and channels are good evidence that it did, he says. But flowing water seems to have played only a small role in weathering the surface. "There is something of a paradox about Mars," agrees Joshua Bandfield of Arizona State University in Tempe. His team recently showed that the planet has no large deposits of carbonates, which should have formed if giant pools of water had persisted on the surface (New Scientist, 30 August, p 12).
Bandfield suggests that liquid water must have occasionally burst out of the ground, carving channels and gullies, but that it quickly froze again in the frigid Martian climate. Although the finding makes the evolution of life on Mars seem unlikely, Bandfield insists it can't be ruled out: "There appears to be quite a bit more living going on in the Amazon rainforest than in the dry valleys of Antarctica. But if the question is whether or not life exists in either climate, the answer is yes to both." Planetary scientists are hopeful that NASA's rovers will shed more light on Mars's history when they arrive at the planet next January. Yen says they should resolve the question of whether there is enough meteor dust to explain Mars's colour. "I'm sticking my neck out here and making a prediction- I believe that the Mars rovers will find nickel in the soil," he told New Scientist. Nickel is abundant in many meteors but rare in Martian rock.
New Scientist issue: 6th September 2003
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