Devolatilization during pebble accretion: A hybrid pathway in forming terrestrial planets

Earth and Mars are both depleted in moderately volatile elements with sublimation temperatures between 400 K and 1300 K, such as F, Na and Li. A similar depletion pattern is observed in the spectra of both white dwarfs and Sun-like stars suspected of recently having accreted rocky planetary materials. Here we model the sublimative loss of volatiles (devolatilization) during pebble accretion, while simultaneously considering growth by collisions with volatile-predepleted planetesimals. We find that bulk silicate Earth composition is well-explained by a mixture of at least 70% contribution from two protoplanets that grew by pebble accretion together with up to 30% contribution from planetesimals with composition resembling the asteroid Vesta. Bulk silicate Mars, in contrast, is best described by 20-40% contribution from pebble accretion and 60-80% contribution from compositionally Vesta-like planetesimals. Our results lend support to a hybrid formation scenario where terrestrial planets around the Sun and other stars grow by a combination of rapid pebble accretion and a prolonged period of planetesimal accretion and giant impacts.