National Aeronautics and Space Administration scientists said the findings suggest any liquid water existing beneath that planet's surface -- and any possible organisms in the water -- would be located deeper than scientists had suspected.

"We found that the rocky surface of Mars is not bending under the load of the north polar ice cap," said Roger Phillips of the Southwest Research Institute in Boulder, Colo. "This implies that the planet's interior is more rigid, and thus colder, than we thought before."

Scientists at NASA's Jet Propulsion Laboratory, which manages the Orbiter mission, said radar images show a smooth, flat border between the ice cap and the rocky Martian crust. On Earth, the weight of a similar amount of ice would cause the planet's surface to sag. The fact the Martian surface is not bending means its strong outer shell -- or lithosphere, a combination of its crust and upper mantle -- must be very thick and cold, they said.

The findings are reported in the online version of the journal Science.