On Earth, crustal tectonics (or "continental drift") recycles these minerals by dragging them back down into Earth's hot interior every few hundred million years, so that they are broken down and the CO2 released again through our volcanoes -- but Mars is too small to have crustal tectonics, and so most of its early CO2 would have vanished forever, reducing the air pressure and lowering greenhouse warming until water could virtually always exist on Mars' surface only as ice or vapor.

And indeed most of the 15 Mars meteorites identified on Earth contain significant amounts of carbonates (in which the possible microfossil evidence in the famous ALH84001 rock is found).

But planetologists have been going nuts trying to confirm any carbonate deposits on Mars' surface today.

Earth-based telescopic searches have been ambiguous -- and the thermal IR spectrometer ("TES") on Mars Global Surveyor, which was expected to find such deposits, has so far not detected a confirmable trace.

Where are the carbonates? If they don't exist in large amounts, does this mean that Mars never had much liquid water anywhere near its surface after all -- or could there be some other explanation? (For instance, there may be some surface process that breaks surface carbonates back down -- but allows them to build up in near-surface deposits formed by subsurface liquid water.)

Allen Treiman thinks the explanation is that the surface carbonate minerals are indeed there, but that the TES is simply much less sensitive to them than had been thought because their surfaces are rough.

The result was an interesting fight between him and Steven Ruff, who said in his own talk that the TES has proven that the famous "White Rock" -- a 12 by 15-km patch of light-colored material on the floor of a Martian crater, which has been thought by many to be a deposit of carbonates or other water-deposited salts -- is nothing of the kind.

Ruff claimed that the TES' infrared spectra of the Rock show conclusively that it is basically the same regular Martian soil as the dark-colored dunes that surround it (although it has caked into a lighter-colored and firmer form).

He added that it's actually no lighter in color than many other regular areas of Martian soil and looks light only by comparison with those surrounding dunes; and that the spectrometer's temperature measurements show that it is regular soil rather than harder rocky material.

Treiman responded that any individual spectrum by TES has been shown by his tests to be hopelessly insensitive to carbonates.

Ruff replied that his conclusions are based on combining a whole series of TES spectra, thus increasing their sensitivity.

Treiman responded that since the spacecraft is sailing over the surface of Mars, such a series of spectra never really cover the same spot -- but Ruff replied to that by saying that the TES took enough spectra of the White Rock that one can accurately say by combining them that the Rock contains no carbonates.

This intellectual ping-pong match ended inconclusively. However, astrogeologist Jack Farmer later told me that his own tests lead him to a centrist position: he agrees with Treiman that the TES is disastrously insensitive to carbonate deposits because they're usually small and are thus washed out of its blurry wide-angle vision, but he thinks (unlike Treiman) that the sharper-resolution "THEMIS" multispectral IR camera on the upcoming 2001 Mars Surveyor Orbiter will definitely be able to detect them.

Apparently we'll have to wait and see.