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The great tilt : Mars takes on a new face

As announced in Nature in its March 17th issue (online on the 2nd of March), the planet Mars tilted by 20 to 25 degrees, 3 to 3.5 billion years ago. And the cause was a vast volcanic structure, the largest in the solar system. Because of its extraordinary mass, the Tharsis volcanic dome caused a rotation of the crust and mantle of Mars with respect to its core. The far-reaching implications of this great tilt exposed in the article profoundly strengthens the links between internal dynamics, magnetic field, volcanic activity, tectonism and climate evolution. A new face for the planet Mars during the first billion years of its history, at a time when life could have appeared, stems from this study
The existence of this great tilt gives us a single solution for three mysteries. We finally understand why the rivers formed where we see their dry beds today, why some buried water ice considered until now anomalous formed far from the poles of Mars, and why the Tharsis bulge is centred today on the equator. The great tilt had other consequences : contrary to what was generally thought, rivers and volcanic activity coexisted for a long period up to 3.5 billion years ago with an atmosphere which was cold but denser than today’s. These are the conclusions of a team of French and American planetologists led by Sylvain Bouley (GEOPS/Université Paris Sud/CNRS ; GET/Université de Toulouse & Institut de Recherche pour le Développement ; LPL/University of Arizona ; LMD/UMPC).

The great tilt of the planet Mars is demonstrated by the combined work of geomorphologists, geophysicists and climatologists. It’s not the rotation axis of Mars which shifted (a phenomenon called variation of obliquity) but the external layers (mantle and crust) which turned with respect to the internal core. It’s as if Paris shifted to the north pole or if we turned the flesh of an apricot around its stone ! The cause of the tilt ? The giant volcanic Tharsis dome. Its growth began during the Noachian period (more than 3.7 billion years ago). At that time, volcanic activity was localized in the northern hemisphere at about 20° north latitude. Volcanic activity continued at Tharsis for the whole Hesperian period (3.7 to 3.2 billion years ago) forming a plateau more than 5000 km in diameter and 12 km thick on average. The mass of this enormous volcanic plateau, about a billion of billion tons, or 1/70th of the mass of the Moon, is such that it caused the crust and mantle of Mars to pivot. The Tharsis dome shifted to the equator, corresponding to a new equilibrium position.

Before the tilt, the poles of Mars were therefore different. This different orientation has been demonstrated both by surface observations and geophysical modelling. In 2010, Isamu Matsuyama (LPL/University of Arizona) showed that if we remove the Tharsis bulge from the planet Mars, it becomes oriented differently with respect to its axis. In this new study, Sylvain Bouley (Université Paris-Sud/GEOPS) and David Baratoux (Université de Toulouse/IRD/IFAN) show for the first time that the rivers were distributed along a southern tropical band of a Mars rotating around poles shifted about 20° with respect to the actual poles. These poles are consistent with those independently calculated by Isamu Matsuyama. This remarkable correlation is supported by the observations of other groups of scientists who had already observed traces of glacier melting and retreat, plus proof of subsurface ice, in the ancient pole regions. A tilt like this is not trivial for the face of a planet. Indeed, the pre-tilt form of Mars was different from today’s. The geoid and topography of Mars in this configuration were recalculated by Isamu Matsumaya so as to reveal the new face for primitive Mars.

Une nouvelle chronologie pour Mars.

This work profoundly changes the generally accepted scenario, which advocates that the Tharsis bulge mainly formed before 3.7 billion years ago, and preceded the rivers since it controlled their flow direction by its load on the elastic outer shell of the planet (the elastic lithosphere). Using the topography before the tilt, Sylvain Bouley, Antoine Séjourné and François Costard (Université Paris-Sud/GEOPS) have shown that despite different relief, with or without Tharsis, the rivers mainly flow in both cases from the cratered highlands of the southern hemisphere to the low plains of the northern hemisphere. This observation therefore shows that the rivers can be entirely contemporaneous with formation of the Tharsis dome.
The geoid and topography of Mars before the great tilt also allow study of the early climate of the planet. Using climate models, François Forget et Martin Turbet (LMD/UMPC) demonstrate, with a cold climate and an atmosphere denser than it is today, accumulation of ices around 25° South, in regions corresponding to the sources of now dry river valleys.

This study upsets our picture of the surface of Mars as it must have been 4 billion years ago, and modifies the timing of events profoundly. According to the new scenario, the period of liquid water stability permitting the formation of fluvial valleys is contemporaneous with and, most likely, a consequence of the volcanic activity of the Tharsis dome. The great tilt that Tharsis provoked happened after fluvial activity ended (3.5 billion years ago) and then gave Mars the face we know today. From now on, when we study the earliest days of Mars, we must learn to think with this new geography.

Late Tharsis formation and implications for early Mars, Sylvain Bouley, David Baratoux, Isamu Matsuyama, Francois Forget, Antoine Séjourné, Martin Turbet & Francois Costard. Nature, 2 mars 2016.