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Half of Mars May Have Ice
Ixlr8 sends in a BBC story suggesting that up to half of Mars may have ice at varying depths below the surface. Quoting: "Up until now, scientists had been able to search for water deposits using a spectrometer fixed to the orbiting Mars Odyssey spacecraft. However, only readings that are accurate to within several hundred kilometers can be obtained. By comparing seasonal changes in thermal infrared patterns, detected by the same Odyssey spacecraft, [scientists] can make readings accurate to within just hundreds of meters."
Try Space.com on the same story (http://www.space.com/scienceastronomy/070502_mars _ice.html). for a slightly more meaty version. Evidently using data from a new bird with higher resolution combined with assumptions on effectiveness of soil insulation.
http://www.sciencedaily.com/releases/2007/05/07050 2143733.htm
Don't just stand there, get that other dog!
I read (some of) the Nature article. They are not using the spectrometer data at all - except by way of saying "there really is ice there". What they are doing is measuring, using infrared cameras only, how quickly the surface changes temperature due to seasonal changes. Water (ice) has a huge specific heat capacity, so it changes temperature more slowly. The more ice mixed in with the dust/rock, the more slowly it changes temperature.
To get depth, they note that the surface temperature changes first, then the temperature change slowly trickles down to deeper layers. So stuff that changes temperature really fast has no matter near the surface, stuff that changes slow has some water near the surface, and stuff that changes initially very fast but then stops and lets everything else catch up and surpass it has ice but its buried deep.
Naturally it's more complicated than this. They used computer simulations to help figure out what their observations probably meant in terms of amount of ice and depth of ice, but that's the gist it (as far as I can tell).