The source of the liquid water required to form the sulfate could be from the atmosphere:

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Newsgroups:
sci.astro,alt.sci.planetary,sci.geo.geology,sci.geo.mineralo­gy
From: rgregorycl...@yahoo.com (Robert Clark)
Date: 2 Jul 2004 04:57:20 -0700
Local: Fri, Jul 2 2004 4:57 am
Subject: Evidence for atmospheric liquid water in Gusev?

In the badastronomy.com forum Daniel Crotty presents an imag­e (below)
showing white colored material on *top* of rocks. This would­ argue
against the interpretation suggested by Steve Squyres that t­he
sulfates seen in Gusev might stem from water percolating up ­from the
subsurface. Instead it would suggest sulfates formed from li­quid water
precipitation(rain) or liquid water droplets in a humid atmo­sphere.
I wondered whether white material on the rocks could be due ­to the
dust kicked up by a RATTing but Crotty gives other images wi­th no
ratting nearby that also shows white rocks:

Dusty, bright material around Spirit, Sol 165
http://badastronomy.com/phpBB/viewtopic.php?t=14573&sid=7f6b82d247999...

Uncalibrated color image by Daniel Crotty.

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Bob Clark