The early geological history of Mars is re-interpreted in light of new, topography-based evidence published online in Nature this week. Mars is home to the largest complex of volcanic rocks in the Solar System - the Tharsis region - which started forming more than 3.7 billion years ago and resulted in a prominent bulge on the surface of the planet. This event also induced a reorientation of Mars with respect to its spin axis (true polar wander), which is responsible for the present-day equatorial position of the volcanic region.
It has been suggested previously that the bulk of the Tharsis region had formed by the end of the Noachian period (about 4.1 to 3.7 billion years ago), and influenced the orientation of valley networks on the planet. Sylvain Bouley and colleagues performed simulations that allowed them to reconstruct the topography of Mars before the formation of the Tharsis volcanic region. They show that the observed directions of valley networks do not require the presence of the Tharsis volcanic load and that tropical precipitation and valley incision (creation) are likely to have taken place at the same time as the Tharsis bulge formation, with prolonged volcanic activity during the Hesperian period, about 3.7 to 3 billion years ago.
The authors conclude that rainfall or snowfall occurred during the formation of the Tharsis bulge, and propose that their topographic map provides a new framework within which to examine the first billion years of the geological history of Mars.