Isotopically distinct terrestrial planets via local accretion

TL;DR

This study evaluates the depleted disc model for terrestrial planet formation, demonstrating it naturally produces Earth and Mars with distinct isotopic compositions due to limited material mixing and the presence of an isotopic gradient.

Contribution

It introduces and tests the depleted disc model, showing it can explain isotopic differences between Earth and Mars without extensive material mixing.

Findings

Terrestrial planets accrete mostly locally with distinct feeding zones.

An isotopic gradient in the disc explains compositional differences.

Limited mixing in the inner Solar System preserves isotopic gradients.

Abstract

Combining isotopic constraints from meteorite data with dynamical models of planet formation proves to be advantageous in identifying the best model for terrestrial planet formation. Prior studies have shown that the probability of reproducing the distinct isotopic compositions of the Earth and Mars for both classical and Grand Tack models is very low. In the framework of the Grand Tack model, for Mars to be isotopically different from the Earth, it had to form under very specific conditions. Here, we subjected a fairly new and unexplored model--the depleted disc model--to the test. It presupposes that the region in the inner protoplanetary disc from Mars' orbit and beyond is depleted in mass such that Mars is left with insufficient material to grow to a larger size. Our aim is to test the whether the distinct isotopic compositions of the Earth and Mars are a natural outcome of this model. We found that the terrestrial planets accrete material mostly locally and have feeding zones that are sufficiently distinct. The Earth and Mars, and by extension, Venus, can have distinct isotopic compositions if there is an isotopic gradient in the terrestrial planet region of the protoplanetary disc. Our results suggest that the material in the inner Solar System most likely did not undergo substantial mixing that homogenised the potential isotopic gradient, in contrast to the Grand Tack model where the feeding zones of the terrestrial planets are nearly identical due to the mixing of material by Jupiter's migration.