Missing evidence for giant impacts during terrestrial planet formation

According to the standard theory of terrestrial planet formation, the giant impact era extends over tens of millions of years and encompasses dozens of impacts between planetary embryos ranging in size from Earth’s Moon to potentially two half-Earth bodies. These collisions are expected to occur at or exceeding escape velocity and create enormous quantities of ejected planetary debris. Due to the strong scattering present in the planet-forming terrestrial disk, this debris is scattered onto stable orbits in what is now the main asteroid belt.

Using a large suite of terrestrial planet formation N-body simulations modified so that modeled giant impacts generate ejecta according to scaling laws developed from impact simulations (Leinhardt & Stewart, 2011), we estimate the efficiency of this giant-impact-ejecta-to-asteroid-belt process. We find that on average about a percent of all planetary ejecta ends up in the asteroid belt. In a typical “Grand Tack” terrestrial planet formation simulation this translates to nearly an entire modern asteroid belt’s mass of material. This is entirely inconsisent with the understanding that most of the modern asteroid belt is related to primitive chondritic like material with only a small fraction <5% (excluding Vesta) related to possibly differentiated inner solar system material.

In order to be consistent with the record of debris in the asteroid belt, planet formation must have either been significantly less violent — consistent with a pebble accretion model for planet formation — or debris much more friable or even non-existent, if it were mostly vapor, for instance. The pebble accretion model for planet formation is the pre-dominant mechanism for the formation of the giant planets and consistent with the formation of super-Earth planets as well. It seems that it could also be the primary growth mechanism for even small worlds. Regardless, we have shown the main asteroid belt is a Lagerstätten of the solar system containing leftover planetary debris from the era of planet formation.