Pebble accretion for Earth’s composition and water delivery

  • CART Astrophysics Seminar Series

March 5, 2026 2:00 PM - March 5, 2026 3:00 PM
PAIS 3205

Host:
Diana Dragomir
Presenter:
Susmita Garai (UNM EPS)
Two leading models of planet formation invoke either stochastic collisions among km-sized planetesimals or pebble accretion of sub-mm–cm solids regulated by gas drag. While collision-based models typically require tens to hundreds of millions of years to assemble terrestrial planets, pebble accretion can produce Mars to Jupiter-mass bodies within the lifetime of the protoplanetary disk. Although widely accepted for the rapid growth of giant planet cores, the role of pebble accretion in forming terrestrial planets, including Earth, remains debated. Here, we investigate the consequences of pebble accretion for Earth’s origin. We show that no combination of known chondritic meteorites reproduces Earth’s major element composition, whereas a mixture of chondritic components: metal grains, chondrules, and refractory inclusions, matches Earth’s Fe, Ni, Si, Mg, Ca, Al, and O within uncertainties. Accretion of such pebbles naturally yields sufficient mass inside 1 AU to account for the terrestrial planets. The best-fitting pebble mixture also reproduces Earth’s moderately volatile depletion pattern, siderophile partitioning between mantle and core, and the mantle Hf-W anomaly, requiring dominant pebble accretion followed by a late-stage impact(s). Laboratory experiments further demonstrate that hydrogen reduction of iron oxide during pebble accretion generates substantial water in Earth’s primitive atmosphere, while explaining volatile loss and the mantle’s Fe/Mg ratio. These results explain key geochemical observations that collisional models do not readily account for. We conclude that pebble accretion was the dominant process in building a ~0.6-0.7 Earth-mass proto-Earth and a nearby ~0.3-0.4 Earth-mass impactor (Theia), with late collisions completing assembly. This framework also implies that water-rich exoplanets may inherit substantial water inventories during formation, with important implications for planetary habitability.

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