Volatile loss reveals how rocky planets grow

The composition of Earth reveals that ≳75% of its building blocks originated from two protoplanets formed via pebble accretion, supplemented by up to ~25% material from planetesimals similar in composition to the asteroid Vesta. In contrast, the work finds Mars strikingly different, assembling from 27 ± 5% pebble-accreted material and 73 ± 5% Vesta-like planetesimals. Volatile depletion is a chemical fingerprint of hybrid accretion, in which both pebble accretion and collisional assembly contribute to terrestrial planet growth. This finding aligns with existing silicon isotope constraints on Mars' accretion, which previously suggested a significant contribution from material similar to Vesta-like planetesimals, as detailed in a 2009 Icarus study. By quantitatively…