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Similar to both the Earth�s ocean basins and continents and the moon�s mare and highlands, Mars also displays a dichotomy in global composition with two distinct geologic terrains.
Evidence Of Carbonates May Already Exist
by Bruce Moomaw
Cameron Park - April 11, 2000 - Some scientists have proposed that the Sun's ultraviolet light breaks them back down into CO2 on the surface, so that all surviving carbonates must be buried underground -- but more recent studies suggest that other Martian processes would remake them as fast as UV light destroyed them.

Kelsey proposed that Mars is rich in sulfates, "which will easily become sulfuric acid under current Martian atmospheric conditions" -- and this acid may have dissolved away all surface carbonate deposits, leaving behind only layers of hematite like that in the Sinus Meridiani.

But two scientists at the Conference reported, on the basis of lab tests, a different possibility: the carbonates may actually be there in large amounts, but the TES instrument may be much less sensitive to them than had been thought!

Laurel E. Kirkland used a similar instrument to survey carbonate-rich Mormon Mesa in Nevada, and found that -- apparently because of the minerals' rough surfaces -- "If the 1971 IRIS [on the Mariner 9 probe], and the 1996 TES, or the proposed 2001 THEMIS [on the next Mars Surveyor Orbiter] were flown over Mormon Mesa, none would detect the carbonate-rich soil or massive calcrete present."

And S. Fonti of Italy concluded from separate tests that carbonate minerals at Mars' cold surface temperatures, even when smooth-surfaced, give off only very weak emission bands: "We can say that much care has to be taken when trying to assess an upper limit for the carbonate component in the Martian regolith, and that the detection of such a component by an orbiting spectrometer could be extremely difficult."

He concludes that some possible Earth-based mid-infrared detections of Mars carbonates may be of warmer dust floating in the Martian atmosphere.

E.A. Cloutis reported that his tests showed that a shorter-wavelength near-infrared spectrometer would clearly detect carbonates as it could analyze sunlight reflected directly off the Martian surface.

Moreover, some Earth-based near-IR spectra -- and even, according to Lawrence Soderblom, the very distant near-IR spectra of Mars taken by the Deep Space-1 spacecraft -- may already show them.

Fortunately, the 2003 European Mars Express probe does carry a near-IR spectrometer with a resolution of only a few hundred meters -- but I do wonder whether a higher-resolution THEMIS-type near-IR multispectral camera might be a worthwhile addition to a near-future US Mars probe.

Meanwhile, S. Erard reported evidence that the European ISO astronomy satellite may in fact have detected carbonates in its distant thermal-IR spectra of Mars, and that some "features similar to those attributed to carbonates are actually present in previous data sets [from Mars orbiters], although little attention has been paid to them."

And J.E. Moersch reported that his own thermal-IR airborne maps of Death Valley showed that, although the current TES instrument could not detect the Valley's plentiful carbonates, THEMIS could clearly do so. We'll simply have to wait and see.

Christensen and S.W. Ruff did report, though, that the TES has definitely ruled out the popular idea that one famous Martian surface feature -- the "White Rock" area seen on the floor of a Martian crater -- is a deposit of carbonates or some other salt.

Its spectra seem to be merely those of a patch of cemented Martian dust like many others seen on Mars, and in fact it isn't even that light-colored -- it just appears so in photos by contrast with the dark soil surrounding it.

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