A unified model for hydrogen in the Earth and Moon: No one expects the Theia contribution

Steven J. Desch, Katharine L. Robinson

Research output: Contribution to journalArticlepeer-review

7 Scopus citations


The Moon is thought to have formed after a planetary embryo, known as Theia, collided with the proto-Earth 4.5 billion years ago. This so-called Giant Impact was the last major event during Earth's accretion, and its effects on the composition of the Earth and the newly forming Moon would be measureable today. Recent work on lunar samples has revealed that the Moon's water was not lost as a result of this giant impact. Instead, the Moon appears to contain multiple hydrogen reservoirs with diverse deuterium-to-hydrogen (D/H) ratios. For the first time, we incorporate hydrogen isotopic measurements of lunar samples to help constrain the composition of Theia. We show that the Moon incorporated very low-D/H (δD ≈ -750‰) materials that only could have derived from solar nebula H2 ingassed into the magma ocean of a large (∼0.4 ME) planetary embryo that was largely devoid of chondritic water. We infer Theia was a very large body comparable in size to the proto-Earth, and was composed almost entirely of enstatite chondrite-like material. These conclusions limit the type of impact to a “merger” model of similarly-sized bodies, or possibly a “hit-and-run” model, and they rule out models that mix isotopes too effectively.

Original languageEnglish (US)
Article number125546
JournalChemie der Erde
Issue number4
StatePublished - Dec 2019


  • D/H measurements
  • Enstatite chondrites
  • Moon, formation
  • Planet formation

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology


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