Robert Clark wrote:
I would say that most Mars scientists would guess that Mars Odyssey will provide further evidence for water on Mars rather than less.

Alex writes:
Well, as guesses go, yours is as good as mine. However, guessing is dangerous; scientists should go where the data takes them, not follow pre-conceived notions of what the data "should" reveal.

That said, the Wyatt and McSween paper in Nature is certainly a plausible model, though one should take careful note that they are not claiming to have invalidated the earlier work of the TES team in asserting an andesitic composition (Surface Type 2) for the northern plains. In fact, Wyatt and McSween plainly state that the TES curve fits for the modelled spectra are very close to their own (i.e., the rms values are almost indistinguishable). What Wyatt and McSween did differently, however, was to substitute clays in their end-member spectral set for the high silica glass phases used by the TES team. Since both mineral phases have similar spectral features, this is perfectly valid; however, the underlying assumptions are different, and lead to different conclusions. Wyatt and McSween's path leads to a weathered basalt composition (rather than andesitic, which is hard to reconcile with the apparent lack of subduction and/or plate tectonics). Their distribution mapping shows a rough correlation to Parker's Contact 2, though the correlation is not perfect. Moreover, they acknowledge that the spectral maps are too coarsely resolved from a spatial standpoint, and the pervasive dust and sand mantles (not to mention possible ground ice poleward of 30 degrees) may introduce spurious features into their curve. All in all, however, it is an interesting paper and an issue that should be aided in light of THEMIS's higher resolution.

Alex R. Blackwell
University of Hawaii