There has been discussion on Unmannedspaceflight.com and MarkCarey.com/mars that the material revealed in the trenches dug by the stuck wheel on Spirit have the appearance of the "Magic carpet" material seen early on the MER rover mission.
Like the "Magic Carpet" which was seen when the air bags were retracted, this arose from material been pressed down and dragged away.
If there were thin interstitial ice from atmospheric deposition this ice may have been melted by the squeezing and dragging of the wheel.
cf.
From: Robert Clark - view profile
Date: Wed, Jan 4 2006 1:02 pm
Email: "Robert Clark" <rgregorycl...@yahoo.com>
Groups: sci.astro, alt.sci.planetary, sci.physics
Subject: Could We Make A "Solar Still" On Mars?
http://groups.google.com/group/sci.astro/msg/f76e921c4b865f2e
Bob Clark
"Magic carpet" redux.