A great article. Moomaw is still the best space writer working today.
The only part I disagree with is the certainty with which he presents the views of Phil Christensen that because Christensen's instruments TES and THEMIS do not see large scale carbonates on Mars, they can't be there.
This argument would have greater force if these instruments were actually tested in the field. It's been proven that for similar intruments, they can be insensitive to even large outcrops of carbonate.
There is in fact good *observational* evidence that either carbonate or sulfate is widespread on the Martian surface, both of which would require extensive interaction with liquid water to form at this scale.

What are these hard, white surfaces that are wide spread at both the Pathfinder site and the Viking lander sites and therefore quite likely all over Mars? I've yet to see any other explanation for these than that they are either indurated carbonate or sulfate such as gypsum:

Soil Characteristics Revealed by Sojourner.

"During its traverse to Yogi, the rover stirred the soil and exposed material from several centimeters in depth. During one of the turns to deploy Sojourner's Alpha Proton X-Ray Spectrometer (inset and white arrow), the wheels dug particularly deeply and exposed white material. Spectra of this white material show it is virtually identical to Scooby Doo, and such white material may underly much of the site."
New Views of the Red Planet
July 22, 1997
http://www.planetary.org/html/Images/mars/hot-top-mars072297.html

Three Classes of Martian Rocks.

New Views of the Red Planet
July 22, 1997
http://www.planetary.org/html/Images/mars/hot-top-mars072297.html

Bob Clark