Mars was very good on last night's walk despite the moon. Its red color is usually attributed to iron oxide and this has traditionally suggested water.
This morning the BBC was interviewing Albert Yen of JPL who notes that a watery surface on Mars is very unlikely. The surface lacks carbonate deposits that would have resulted from the carbon dioxide in the atmosphere and water and the Martian surface has widespread deposits of olivine and other minerals that break down in the presence of water.
Yen proposes a source of dry rust - iron rich meteorites. Actually the idea is very neat. It is commonly assumed that about 50 metric tons of meteors fall onto Mars yearly. Over a billion or so years you get a layer that is few centimeters thick. Iron can lose an electron with a sufficiently energetic photon (UV range. Now if that electron is captured by the oxygen in the atmosphere before it returns to the iron, the iron atom is oxidized. Neat.
It will be interesting to see if this holds up - the hypothesis is testable. A bit of searching shows a recent paper from Dr Yen (pdf file) describing an instrument to characterize the reactive nature of the Martian surface.
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