Recent NASA images from Mars indicate that a significant amount of water may be stored near the surface of the Red Planet in a sort of Martian permafrost.
Pictures taken by the Mars Global Surveyor spacecraft, which began mapping the planet in 1999, show regions of smoothly undulating hills interspersed with bumpier ground.
In Thursday's issue of the journal Nature, geologists interpret the smooth hills as mounds of dust frozen solid by ice. The bumpy areas between them appear to be places where the ice has disappeared and the powdery red soil has blown away.
The geologists saw this kind of surface in photos from Mars' mid-latitudes, far from the polar ice caps. That location indicates that the frozen ground may have been created during a Martian "ice age" about 100,000 years ago.
Scientists are keenly interested in whether there is water on Mars because liquid water is considered an essential chemical for the development of life.
"This is an exciting result," said Bruce Jakosky, a planetary scientist at the University of Colorado in Boulder. "It suggests that the crust of Mars can be a substantial reservoir or a sink for water."
There is no evidence that the water ever exists as a liquid during these cycles of permafrost formation and destruction, said Brown University professor John Mustard, one of the study's authors.
During cold periods, water vapor infiltrates the porous soil and freezes solid, he said. During warmer times the ice goes straight from a solid to a gas.
Mustard estimated that as much as 15,000 cubic miles of water may be frozen in the Martian soil — enough to cover the whole planet to a depth of 16 inches.
Planetary scientists also made a case for water on Jupiter's moon Callisto in Thursday's Nature.
A study of the moon's thermal features by researchers from Spain's Universidad Complutense indicates that liquid water could exist beneath its icy crust. Some previous studies had concluded that water on Callisto would freeze solid.