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Coward, Anonymous writes:

Our solar system is thought to have begun billions of years ago as a protoplanetary disk. Over time, gravity caused this matter to clump together and eventually form planets. There is debate over the process and its duration, with previous studies suggesting several ten million years until the initially dry Earth was formed. Water was delivered at the end of this period.

A new article in Science Advances (open) describes Iron isotope evidence for very rapid accretion and differentiation of the proto-Earth.

Moreover, new planet formation models based on the rapid accretion of pebbles onto asteroidal seeds suggest that Earth's main accretion phase may have been completed within the ~5–million year lifetime of the protoplanetary disk.

The authors find that the ratio of iron isotopes ⁵⁴Fe to ⁵⁶Fe in most meteorites differs from the Earth, and deduce that the planet formed rapidly with little material from the outer solar system, where those meteorites originated. The terrestrial iron isotope ratio does match that of a rare class of meteorites known as CI chondrites.

The only epoch in the history of the solar system when the CI-like material is readily available within the terrestrial planet–forming region is during the lifetime of the protoplanetary disk. This period represents the time when the in-falling envelope material of CI composition is channeled through the disk to fuel the growth of the proto-Sun and is estimated to have lasted approximately 4.8 ± 0.3 million years (Ma).

This conclusion has implications for where the Earth's water and oxygen came from:

An initially more reduced proto-Earth relaxes these constraints and only requires that Earth oxidized (i.e., acquired most of its mantle iron budget) by the accretion of CI-like dust. Water is the key ingredient for oxidation, and as such, our results are consistent with the accretion of a component of Earth's water and other volatile elements during the protoplanetary disk's lifetime. This may be achieved via the direct accretion of water adsorbed to dust or reflects the fact that the snowline will be inside of Earth's orbit toward the end of the protoplanetary disk's lifetime, allowing direct accretion of ice during this stage.

Regarding the Moon:

Critically, the rapid timescales proposed here can be reconciled with Earth's mantle ¹⁸²W isotope composition if the Moon-forming impact occurred at least 40 Ma after the main accretion and differentiation of the proto-Earth.

Original Submission