On Earth such depressions occur beneath ice caps because the weight of the ice pushes the underlying crust down into the planet's mantle, like a sleeper's head depressing a pillow.
"On Mars this is not taking place," Plaut said. "This tells us that the upper crust and mantle of Mars is very stiff—much stiffer than the Earth."
The amount of water in the ice cap is more than most scientists had expected, but not by a huge amount.
And even when combined with the amount believed to reside at the planet's north pole, it's still only a small fraction of the water that scientists believe once existed on the red planet.
There are only two places where the rest can be: buried underground or lost to space through the thin Martian atmosphere. (Related: "Mars's Water Could Be Below Surface, Experts Say" [January 25, 2007].)
"Those are still avenues that are being investigated," Plaut said.
Ray Arvidson, a professor at Washington University in St. Louis, Missouri, and a member of the Mars rover team, says that the new find is simply another part of scientists' expanding efforts to trace the missing Martian water.
Other orbiting instruments are looking for ice in the Martian soil, while the Mars rovers and additional instruments are mapping out deposits of water-altered minerals on the Martian surface.
Years ago, Arvidson said, Mars researchers could only speculate about water based on ancient river channels and canyons. But now it's possible to look for it directly.
"The more we look, the more water we're seeing," Arvidson said. "It's really exciting."
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