I was just re-reading some of the pre-landing papers and presentations about Gusev. Boy, angular chunks of unweathered basalt and the same dry dust seen elsewhere is sure not what people were hoping to find!

What I'd almost forgotten about with all the focus on data from the ground, though, is that Gusev shows elevated hydrogen in the Odyssey data. One author said 35% H20 by weight in the top meter, and while I'm not sure that's exactly right (I think that's from a more optimistic model), it sure is much more "blue" than the Viking 1 or MPF sites. The Viking 2 site showed a fair amount of hydrogen but I guess the theory there is that the top few cm are dry, and there is lots of icy dirt below. At Gusev's equatorial location, water ice shouldn't be stable in the top meter of soil, right?

I think the lake advocates figured the hydrogen would be in the form of hydrated minerals, clays, evaporites, whatever, but we're not seeing those.

So if Gusev is just full of lava and/or ash, what explains the hydrogen signature?

By the way, a document well worth reading is this "Summary of Testable Hypotheses at Gusev" from the Jan 2003 landing site workshop. So far as I can tell, the "all volcanic" set of predictions is in the lead....

http://marsoweb.nas.nasa.gov/landingsites/mer2003/doc/arcadia_03/talks/Cabrol/cabrol.pdf