Origin of the Asteroid Belt and Mars' Small Mass
Résumé
Reproducing the small mass of Mars is a major problem for modern simulations of terrestrial planet accretion (Raymond et al. 2009). Terrestrial planet formation simulations using a planetesimal disk with an outer edge at 1.0 AU have been found to form good Mars analogs (Hansen et al. 2009). However, these initial conditions appear inconsistent with solar system evolution and the asteroid belt. Hydrodynamical simulations show that the evolution of Jupiter and Saturn in a gas-disk generically leads to a two-stage, inward-then-outward migration (Masset & Snellgrove 2001, Morbidelli & Crida 2007, Pierens & Nelson 2008). We present simulations showing that if Jupiter's minimal orbital radius was 1.5 AU, this evolution both truncates the planetesimal disk at 1.0 AU and repopulates the asteroid belt from two distinct parent populations. Our model links the origin of the inner solar system - explaining both the mass of Mars and the properties of the asteroid belt - to a realistic evolution of the giant planets. This scenario represents a paradigm shift in our understanding of the early evolution of the inner solar system. Previously S- and C-type asteroids were thought to have both originated in the 2--3 AU region, with comets forming far away beyond the giant planets. This posed problems in explaining the vast physical differences between S- and C-type asteroids, and the physical similarities between comets and C-type asteroids as shown by Stardust and micrometeorite samples (Brownlee et al. 2006, Gounelle et al. 2008). Our presented scenario finds that S-types likely formed in the 1--3 AU region, with C-types and comets forming in the outer regions of the disk. This provides a much better qualitative explanation of the observed differences and similarities. This work is part of the Helmholtz Alliances "Planetary Evolution and Life", which KJW and AM thank for financial support.