title: Late Tharsis formation and implications for early Mars.
abstract: The Tharsis region is the largest volcanic complex on Mars and in the Solar System. Young lava flows cover its surface (from the Amazonian period, less than 3 billion years ago) but its growth started during the Noachian era (more than 3.7 billion years ago). Its position has induced a reorientation of the planet with respect to its spin axis (true polar wander, TPW), which is responsible for the present equatorial position of the volcanic province. It has been suggested that the Tharsis load on the lithosphere influenced the orientation of the Noachian/Early Hesperian (more than 3.5 billion years ago) valley networks and therefore that most of the topography of Tharsis was completed before fluvial incision. Here we calculate the rotational figure of Mars (that is, its equilibrium shape) and its surface topography before Tharsis formed, when the spin axis of the planet was controlled by the difference in elevation between the northern and southern hemispheres (hemispheric dichotomy). We show that the observed directions of valley networks are also consistent with topographic gradients in this configuration and thus do not require the presence of the Tharsis load. Furthermore, the distribution of the valleys along a small circle tilted with respect to the equator is found to correspond to a southern-hemisphere latitudinal band in the pre-TPW geographical frame. Preferential accumulation of ice or water in a south tropical band is predicted by climate model simulations of early Mars applied to the pre-TPW topography. A late growth of Tharsis, contemporaneous with valley incision, has several implications for the early geological history of Mars, including the existence of glacial environments near the locations of the pre-TPW poles of rotation, and a possible link between volcanic outgassing from Tharsis and the stability of liquid water at the surface of Mars.
authors.affiliation: GEOPS-Géosciences Paris Sud, Université Paris-Sud, CNRS, Université Paris-Saclay, Rue du Belvédère, Bâtiment 504-509, 91405 Orsay, France.|Institut de Mécanique Céleste et de Calcul des Ephémérides, UMR8028, 77 Avenue Denfert Rochereau, 75014 Paris, France.
authors.forename: Sylvain
authors.initials: S
authors.lastname: Bouley
authors.affiliation: Geosciences Environnement Toulouse, Université de Toulouse III UMR 5563, 14 Avenue Edouard Belin, 31400 Toulouse, France.|Institut de Recherche pour le Développement et Institut Fondamental d'Afrique Noire, Dakar, Sénégal.
authors.forename: David
authors.initials: D
authors.lastname: Baratoux
authors.affiliation: Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA.
authors.forename: Isamu
authors.initials: I
authors.lastname: Matsuyama
authors.affiliation: Laboratoire de Météorologie Dynamique, Institut Pierre Simon Laplace, CNRS, Université Pierre et Marie Curie, Paris, France.
authors.forename: Francois
authors.initials: F
authors.lastname: Forget
authors.affiliation: GEOPS-Géosciences Paris Sud, Université Paris-Sud, CNRS, Université Paris-Saclay, Rue du Belvédère, Bâtiment 504-509, 91405 Orsay, France.
authors.forename: Antoine
authors.initials: A
authors.lastname: Séjourné
authors.affiliation: Laboratoire de Météorologie Dynamique, Institut Pierre Simon Laplace, CNRS, Université Pierre et Marie Curie, Paris, France.
authors.forename: Martin
authors.initials: M
authors.lastname: Turbet
authors.affiliation: GEOPS-Géosciences Paris Sud, Université Paris-Sud, CNRS, Université Paris-Saclay, Rue du Belvédère, Bâtiment 504-509, 91405 Orsay, France.
authors.forename: Francois
authors.initials: F
authors.lastname: Costard