In some places, ice was found only an inch (2.5 centimeters) or so under the soil; in other places it lies many feet below the surface.
"It looks like this water is extremely variable and not as coherent or predictable as perhaps thought," Bandfield said.
Just Like Earth
The type of soil above the ice has a huge effect on where the ice layer begins, the research shows.
"If I have a very fluffy, fine-grained top layer of soil it would act as an insulator, and ice can remain closer to the surface than if I had a coarser grained or rockier surface, which will conduct a lot of heat down into the subsurface," Bandfield said.
The two surface materials—rock and dust—vary widely across the ground, giving underground ice a patchy distribution, he said.
Steve Squyres, an astronomy professor at Cornell University in Ithaca, New York, said the patchiness of the ice on Mars is to be expected.
"In fact, if you go to places on Earth where there's permafrost, you find that [the ice] tends to be patchy—it doesn't tend to be the same concentration everywhere because the soil is variable in its porosity ... and I would expect it to be patchy on Mars as well," he said.
"That kind of patchiness cannot be revealed by the [previous images], but it can be revealed by this technique," added Squyres, who worked on previous ice measurements but was not involved with the new study.
The thermal imaging results also corroborate long-term climatic models for Mars that show the planet has been both warmer and colder in the past, similar to how glacial cycles have affected Earth. (Related: "Mars Melt Hints at Solar, Not Human, Cause for Warming, Scientist Says" [February 28, 2007].)
"This isn't necessarily fossil ice from a billion years ago, because it's gone through many climate cycles in the past hundreds of thousands of years," Bandfield said.
"This gives us clues into how the water cycle is working on Mars, and that it's probably quite active today."
In August 2007 NASA will launch Phoenix, a mission designed to sample Martian ground ice directly and analyze its qualities as a possible habitat for microbial life.
The new findings, Bandfield said, will aid Phoenix in that search.
"The spacecraft may find ground ice is shallower and much easier to reach in some spots than in others."
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