In 2010, Isamu Matsuyama (University of Arizona) had already used a geophysical model to show that, if the Tharsis dome is removed from Mars, the planet takes on a different orientation with respect to its axis. So before this tilt, the poles of Mars were not in the same place as they are today. As a result, the Tharsis dome shifted to the equator, corresponding to its new equilibrium position. This mass was so huge that it caused Mars' crust and mantle to swivel around. Volcanic activity continued for several hundred million years, forming a plateau exceeding 5,000 km in diameter, with a thickness of about 12 km and a mass of a billion billion tons (1/70th the mass of the Moon). The tilt was caused by the gigantic Tharsis volcanic dome, which first started to form over 3.7 billion years ago at a latitude of around 20°N. This phenomenon had been predicted theoretically but never demonstrated. It wasn't the rotation axis of Mars that shifted (a process known as variation of obliquity) but rather the outer layers (mantle and crust) that rotated with respect to the inner core, rather like turning the flesh of an apricot around its stone. Some 3 to 3.5 billion years ago, the planet underwent a huge tilt, which has now been identified thanks to the combined work of geomorphologists, geophysicists and climatologists. Mars hasn't always looked like it does today. These findings are published on 2 March 2016 in the journal Nature by a mainly French team including researchers from Géosciences Paris Sud (CNRS/Université Paris-Sud), Géosciences Environnement Toulouse (CNRS/Université Toulouse III - Paul Sabatier/IRD) and the Laboratoire de Météorologie Dynamique (CNRS/École polytechnique/UPMC/ENS), together with a researcher from the Lunar and Planetary Laboratory (University of Arizona, US). It also provides a solution to three puzzles: we now know why rivers formed where they are observed today why underground reservoirs of water ice, until now considered anomalous, are located far from the poles of Mars and why the Tharsis dome is today situated on the equator. The discovery of this huge shift changes our vision of Mars during the first billion years of its history, at a time when life may have emerged. Because of its extraordinary mass, it caused the outer layers of Mars (its crust and mantle) to rotate around its core. This was caused by a massive volcanic structure, the Tharsis volcanic dome, which is the largest in the Solar System. The surface of the planet Mars tilted by 20 to 25 degrees 3 to 3.5 billion years ago.
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