Another nice article by Bruce on the MER rover missions:

First Stop Gusev
by Bruce Moomaw
Sacramento (SPX) Jun 07, 2004
http://www.spacedaily.com/news/mars-mers-04zzzzw.html

However in it he presents the view that the near surface of Gusev, representing geologically recent material, may never have been subjected to extensive water exposure:

"Not only is Gusev's main plain covered by windblown dry basalt-lava sand - whose high content of the mineral olivine proves that it can never have been exposed to liquid water for other than extremely brief periods - but all the rocks Spirit has analyzed seem to be made of the same never-wet basalt.
"Some of these basalt rocks have a thin water-weathered crust, suggesting that they have been repeatedly exposed to thin films of liquid water over the eons during Mars' endless hundred-thousand-year "obliquity cycles" of increasing and decreasing axial tilt, which have dramatic effects on the planet's seasonal weather even today.
"But we are still talking about only very small amounts of liquid water, not the kind of extensive surface or near-surface liquid water which scientists hope to find evidence that Mars had during its earliest days, and which the two rovers were in fact designed to look for (and which Spirit's twin "Opportunity" has in fact found on the opposite side of the planet)."

However, I believe there is evidence to support the view there has been recent, extensive near surface water there.

The reasons I discuss here:

From: Robert Clark (rgregoryclark@yahoo.com)
Subject: Mystery material in the Bonneville crater on Mars.
Newsgroups: sci.astro, alt.sci.planetary, sci.geo.geology, sci.geo.mineralogy
Date: 2004-03-20 05:50:12 PST
http://groups.google.com/groups?th=4005c8b6bfcc019a

In short the material seen on one of the sides of the Bonneville crater was similar to a mystery material seen on Humphrey and Mazatzal rocks. This mystery material is interpreted to be either a high-silica glass or a basalt weathering product, perhaps even clay.
This would be analogous to mystery material seen widespread from orbit by MGS in the northern hemisphere of Mars that has been interpreted as either andesitic rock or liquid water altered basalt. Either of these interpretations suggest extensive liquid water in the northern hemisphere of Mars. If the mystery material seen in Bonneville really is representative of this mystery material seen from orbit that would suggest extensive liquid water near surface and probably recent in Gusev.
This is supported by the announcement of large amounts of sulfate, upwards of 15%, in a trench dug by Spirit:

More Signs of Water Found on Mars.
By AAP 06/08/04 9:55 PM PT "We have found more evidence of salts, more evidence for the action of water -- much more compelling evidence than we have found anywhere else at Gusev,' Squyres told a televised press conference at NASA's Jet Propulsion Laboratory."
http://www.macnewsworld.com/story/34331.html

This sulfate is within centimeters of the surface and is therefore probably due to recent liquid water, near surface. Steve Squyres suggests this may be due to small amounts of water percolating from subsurface, but the large amounts of sulfate, 15%, argues for a large amount of liquid water.

Note also the presence of olivine does not rule out the possibility of liquid water near surface as has been proven by Opportunity at the Meridiani landing site.

A question is why is the mystery material so limited in extent in Gusev? An image showing this material on the side of Bonneville is below. It may be the material is usually underground but was thrown up by impact with the Humphrey and Mazatzal rocks and uncovered in the Bonneville crater.
Note also that the andesite or altered basalt seen from orbit, called Surface Type 2 by NASA scientists, is largely confined to the northern hemisphere. If this is the same as the mystery material in Gusev, than its limited extent may simply because Gusev in the southern hemisphere is outside the largest areal extent of this material. There has also been seen from orbit areas on Mars that have a combination of both this Surface Type 2 and Surface Type 1, which is interpreted as common basalt.
It would be interesting to find out if Gusev is one of these transition regions.

Bob Clark

Spirit, Sol 84, uncalibrated color image by Daniel Crotty.