In Mars' 4.6-billion-year history, perhaps no more pivotal event took place than the formation of the Tharsis riseor at least so say a team of scientists from Washington University in St. Louis in today's issue of Science. This bulging elevation (red and white in image), which reaches 10 kilometers above the surface and stretches for 30 million square miles, dominates the Red Planet's western hemisphere. (The name is in fact derived from the biblical Tharsis, a land at the western extremity of the known world.) And when it was created by an enormous load of volcanic material in Mars' earliest Noachian epoch, it apparently caused global changes to both the planet's landscape and climate.

Roger J. Phillips and his colleagues compared gravity and topography fields from the Mars Global Surveyor spacecraft to a model of how those fields would behave were a Tharsis volcanic load dumped on top of an otherwise spherical shell. Not only did the model accurately predict Mars' broad-scale gravity, but also certain features, like the trough around Tharsis and the Arabia bulge on the planet's other side. "Imagine that Mars is a beach ball and that the Tharsis load is your fist," Phillips says, describing the model. "As your fist pushes into the beach ball, there is a bulge created on the opposite side of the ball, and a depression or trough surrounds your fist."

They also suggest that the development of Tharsis influenced the location and direction of the Martian valley networks and outflow channelsfeatures long taken as evidence of running water on Mars at one time. Additionally, the scientists say that Tharsis' volcanism may have released enough water and carbon dioxide to create a greenhouse effect, which in turn might have warmed the planet enough to allow for running water in the first place.