Abstract
In this review paper I address the current knowledge of the formation of Mars, focusing on its primary constituents, its formation time scale and its small mass compared to Earth and Venus. I argue that the small mass of Mars requires the terrestrial planets to have formed from a narrow annulus of material, rather than a disc extending to Jupiter. The truncation of the outer edge of the disc was most likely the result of giant planet migration, which kept Mars’ mass small. From cosmochemical constraints it is argued that Mars formed in a couple of million years and is essentially a planetary embryo that never grew to a full-fledged planet. This is in agreement with the latest dynamical models. Most of Mars’ building blocks consists of material that formed in the 2 AU to 3 AU region, and is thus more water-rich than that accreted by Earth and Venus. The putative Mars could have consisted of 0.1 % to 0.2 % by mass of water.
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Acknowledgements
The author is grateful to Germany’s Helmholtz Alliance for partially funding this research through their ‘Planetary Evolution and Life’ programme while still based in France. I thank Kevin Walsh and Alessandro Morbidelli for their input, David O’Brien and Tilman Spohn for constructive reviews, and Kevin Walsh and David O’Brien again for providing me with data from their numerical simulations.