The iron has been oxidized into very fine-grained hematite -- one form of ferric oxide -- which is what gives mars its overall reddish. (This should not be confused with the much coarser-grained hematite identified by Global Surveyor's TES in three small places on the surface. The latter indicates that the rocks in those places really were exposed to a large amount of liquid water in some way during Mars' very early history, and for that reason the biggest such region in the Sinus Meridiani plain is the landing site for the second MER rover.) Several percent of Mars' dust is strongly attracted to magnets on Mars landers, indicating that it's another crystalline form of ferric oxide known as maghemite.

The fine-grained ferric oxides in Mars' soil could have been made when the iron in the soil's tiny grains was oxidized by those faint repeated traces of liquid water that may have been repeatedly produced in Mars' soil by the obliquity cycle. But, in another DPS talk, the Jet Propulsion Laboratory's Albert Yen -- who believes in a Mars whose surface has always been mostly frozen or dry -- suggested that it could have been made by another process that doesn't require water at all.

In fact, he suggested that much of the iron isn't even native to Mars -- it is, instead, the remnants of the continuous rain of tiny micrometeoroids upon Mars' surface throughout its existence. They are on the average several times richer in iron than Mars' rocks are, and when they hit Mars' air, frictional heat could oxidize the iron in them into fine hematite and maghemite without the need for any exposure to liquid water at all.

Moreover, Yen's lab experiments indicate that any grains of metallic iron in the micrometeoroids that survived the entry and landed on Mars intact could then be oxidized simply by the 0.15% trace of free oxygen in Mars' atmosphere. This oxygen is produced by the constant temporary breakdown of a small part of Mars' CO2 into oxygen and carbon monoxide by the Sun's ultraviolet light. That same solar UV light would provide the energy to make the oxygen react efficiently with the metallic iron in the micrometeoroids' remains, once again without a trace of water being necessary.

Yen's theory could explain another puzzle: if you assume that Mars' soil is just weathered from its rocks, where did its extra iron come from? The only other theory so far for that is that the wind patterns at the Viking and Pathfinder sites where the soil has been analyzed sweep away a lot of the lighter dust in the soil produced by the weathering of the local rocks, leaving behind soil with a concentrated amount of denser grains of iron oxides.

But if about 10% of Mars' soil is micrometeoroid remains -- which is entirely feasible -- that by itself could explain its excess of iron. Micrometeoroids are also much richer in magnesium than the rocks analyzed by Mars Pathfinder, which Yen has used to explain why Mars' soil is also richer in that metal.

At any rate, Yen's theory can be easily tested very soon. Micrometeoroids are also pretty rich in nickel -- about 7% of their iron content -- while the chemical processes that went inside planets like Earth and Mars during their initial formation have locked up almost all of their initial supply of nickel in their central iron cores, with their crusts being only a few hundredths of a percent nickel. If the extra iron in the soil at the Pathfinder site over that in its rocks is due to added meteoric iron, those micrometeoroids should also have raised the nickel content of Mars' soil to about 0.1 to 0.6% -- enough for the improved APX spectrometers on the two MER rovers to easily detect it.

Meanwhile, Odyssey continues to steadily count more and more gamma-ray photons from elements in the different parts of Mars' surface, improving the accuracy of its measurements. In the near future, scientists hope to start getting fuzzy maps of several more elements -- including sulfur, calcium and uranium -- and they have just gotten the first decent map of chlorine on Mars' surface, which William Boynton described in his DPS lecture.

Chlorine tends to trickle out of volcanic vents, react with loose particles of soil, and get concentrated in their salts -- which had led scientists to think that it might be concentrated pretty evenly in the soil all across Mars.

But Odyssey's chlorine map shows it to be surprisingly blotchy -- the surface amount varies from 0.3 to 1.1% -- and the pattern isn't closely linked to the distribution of loose soil vs. exposed rocks on Mars' surface. It also doesn't seem to be linked to the distribution of crusty soil that contains higher concentrations of salts -- suggesting that those salts are largely sulfates rather than chlorides.

The area that DOES show the highest concentration of surface chlorine is in the area around the giant shield volcanoes on the Tharsis bulge, which also has that surface layer of loose dust that I mentioned earlier. Since this is also the main area for any remaining volcanic activity on Mars, this makes sense. But -- as with the distribution patterns of other elements on Mars -- we don't really know much about the explanation for Mars' overall chlorine patterns yet.

Click for Part Four

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