“Sharper views show ground ice is patchy and variable”

link

Reanalyzing data from Themis has been able to produce a more finely resolved map of water ice in the Mars surface, providing details down to a few hundred meters, and shows that its depth depends on the nature of the surface (deeper under rocks, shallower under dust). It apparently also provides evidence for a water cycle between the subsurface ice and atmospheric vapor.

The technique requires the same surface to be iprobed at different times, so that , as quoted from the article:
"The new approach uses thermal imaging as a thermometer to measure how fast the ground changes temperature during local spring, summer and fall. The dense, icy layer retains heat better than the looser soil above it, so where the icy layer is closer to the surface, the surface temperature changes more slowly than where the icy layer is buried deeper."

At present only a couple of quite small areas have been mapped, but in principle I guess this could be applied more completely.
I thought it was very nifty.

(http://www.nasa.gov/mission_pages/odyssey/odyssey-20070502.html)