The long-wavelength infrared spectra taken of the rocks in Mars' southern highlands by MGS and Odyssey show them to be unweathered basalt -- but near-IR spectra taken of the same areas by Earth-based telescopes show them to be covered with a layer of weathered material only a few hundredths of a millimeter thick, so thin that it's transparent in long-wavelength IR. This could be a thin coating of windblown dust, or it could be an extremely thin layer of water-produced weathering on the rocks's surfaces that later flakes off to become more soil.

It may also be that the fact that Mars' air pressure now hovers just at the brink at which liquid water could exist is no accident. Its dying volcanic activity is still probably belching small traces of additional carbon dioxide slowly out of its interior -- and it may be that, whenever this raises its air pressure to the point that liquid water can start to exist on its surface, the water makes more of the minerals in Mars' dust react with carbon dioxide to form slightly more new carbonate minerals, sucking the new CO2 out of the air and lowering the air pressure back to the "triple point" level again. That is, modern Mars may have a natural "pressure regulator" for its atmosphere.

And it's even possible that such periodic super-thin films of liquid water forming on the surfaces of soil grains during the warmest few thousand years of every 100,000-year obliquity cycle might serve as a refuge allowing Martian microbes to remain alive near the surface of today's desolate Mars in its higher-latitude regions, with the microbes remaining in frozen suspended animation as spores during the rest of each cycle. The "Phoenix" mission that has now been selected to land on Mars' northern near-surface ice layer in 2008 and examine it in detail will look for evidence of just this.

(Unfortunately, the parts of Mars' surface most warmed by the summer Sun during both low- and high-obliquity periods are also those exposed most intensely to the Sun's microbe-killing UV radiation -- which is thousands of times more intense than that on Earth, since Mars lacks an ozone layer. However, spores buried in a permafrost layer only a fraction of a meter underground would be completely shielded from this.)

So, thanks to Odyssey and MGS:

(1) We now know far more about the extent to which water exists near the surface of present-day Mars;

(2) We can confirm that significant amounts of it keep moving back and forth across Mars' surface during its climate cycles every 100,000 years (or sometimes shorter periods);

(3) We also know with considerable confidence that even during its most hospitable first few hundred million years, when it had a dense atmosphere, Mars' surface was NOT warm enough for liquid water to exist in large amounts except perhaps under thick ice layers -- and then only in some areas.

But there are still an enormous number of basic unanswered questions. The three American and European landers that will (with luck) touch down on the planet in the next two months -- and which will carry out the first mineralogical analyses ever conducted on its surface -- may provide vital clues to further solve those questions.

Nor should we forget Europe's "Mars Express" orbiter, which will make the first good maps of surface composition from orbit in the near-IR wavelengths to accompany MGS' and Odyssey's longer-wavelength IR maps, and will use a long-wavelength radar sounder to probe up to several kilometers beneath Mars' surface for subsurface layers of both ice and liquid water.

There are, however, questions about Martian geology that have nothing to do with water, and Odyssey also examined them. In my next installment, I'll look into those. Then we'll move on to the rest of the Solar System.

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Spirit Heading To 'Home Plate'
Pasadena CA (JPL) Jan 09, 2006
Last week Spirit completed robotic-arm work on "El Dorado." The rover used all three of its spectrometers plus the microscopic imager for readings over the New Year's weekend.