Formation of telluric planets and the origin of terrestrial water

Simulations of planet formation have failed to reproduce Mars' small mass (compared with Earth) for 20 years. Here I will present a solution to the Mars problem that invokes large-scale migration of Jupiter and Saturn while they were still embedded in the gaseous protoplanetary disk. Jupiter first migrated inward, then "tacked" and migrated back outward when Saturn caught up to it and became trapped in resonance. If this tack occurred when Jupiter was at 1.5 AU then the inner disk of rocky planetesimals and em- bryos is truncated and the masses and orbits of all four terrestrial planet are quantitatively reproduced. As the giant planets migrate back outward they re-populate the asteroid belt from two di erent source populations, matching the structure of the current belt. C-type material is also scattered inward to the terrestrial planet-forming zone, delivering about the right amount of water to Earth on 10-50 Myr timescales.