Scientists have become increasingly convinced in recent years that Mars, which now appears mostly dry on the surface, was once warm and wet -- the sort of place where life might have got a toehold and possibly even flourished.

But looming questions have formed around this expectation: Where did the water come from, and for how long was the Red Planet wet?

A new study points a finger at one of the most obvious features on Mars, a hulking, elevated region known as the Tharsis rise that may have released tremendous amounts of lava, along with water and carbon dioxide that combined to possibly create a habitable planet. The research also narrows the range of time, under this scenario, that Mars would have been wet.

"Imagine that Mars is a beach ball and that the Tharsis mass load is your fist. As your fist pushes into the beach ball, there is bulge created on the opposite side of the, and a depression or trough surrounds your fist."

Reporting in the March 15 issue of the journal Science, a group of 11 scientists used computer models to analyze gravity and topography data from the Mars Global Surveyor, which is currently orbiting Mars. The results suggest that the volcanic activity, thought to have occurred more than 3.5 billion years ago, left certain prominent features of the Martian landscape still visible today.

Among the most prominent is the Tharsis rise, whose immensity has puzzled researchers for three decades. This natural monument to Mars' early years sits roughly 6 miles (10 kilometers) above the surrounding terrain and covers an area of 11.6 million square miles (30 million square kilometers). That's more than three times the size of the United States on a planet that is only about half the size of Earth.

Around much of the Tharsis rise is a low-lying area called the Tharsis trough. And opposite the Tharsis rise, on the other side of Mars, is another smaller area of raised elevation called Arabia bulge.

The features have a common origin, said Roger J. Phillips of Washington University in St. Louis.

Phillips said the Tharsis rise is the result of 300 hundred million cubic kilometers of lava -- enough to cover Mars 1.2 miles (2 kilometers) deep if it had been spread around the planet -- that got bunched up under and above the surface. The estimate is far greater than what scientists have expected from past studies.

This "mass load" of material created the other features, by the simple virtue of its weight.

"Imagine that Mars is a beach ball and that the Tharsis mass load is your fist," Phillips explained. "As your fist pushes into the beach ball, there is bulge created on the opposite side of the ball (the Arabia bulge), and a depression or trough surrounds your fist (the Tharsis trough)."

Phillips and his colleagues say Tharsis may be a one-of-a-kind phenomenon among the four inner, rocky planets.