Where did Earth's water come from?

The origin of Earth’s water is highly debated. The accretion of the terrestrial planets in the relatively warm inner solar system supports the idea that the terrestrial planets began their lives dry and acquired their volatiles by delivery of water-rich material from the icy outer solar system during the later stages of their accretionary growth. However, the presence of organics and hydrous silicate phases in primitive inner solar system materials (as represented by ordinary (OC) and Rumurutu (RC) chondrite meteorites) suggests that water was present in the inner solar system during the main accretionary phase of the terrestrial planets. But can that water be retained during heating and incipient melting of planetary building blocks? 

How and when is water lost during heating of planetary building blocks?

Heat produced by decay of short-lived radionuclides drove thermal metamorphism, melting, and formation of magma oceans in planetesimals that accreted within the first few million years of solar system formation. To examine the efficiency and timing of water loss during thermal processing, we have analyzed water concentrations of nominally anhydrous minerals (NAMs) and quenched melts in meteorites that represent various stages of thermal processing, from moderate thermal metamorphism, to incipient melting (as represented by primitive achondrites; Peterson et al. 2023 GCA; Peterson et al. 2024 GCA), to the magma ocean stage (as represented by achondrites; Newcombe et al. 2023 Nature; Peterson et al. 2023 EPSL).

We measured water in meteorites called ‘ungrouped achondrites’ that are thought to represent the melted portions of planetesimals from both the inner and outer solar system. We found that the minerals and quenched silicate melts in these meteorites were all surprisingly dry, containing among the lowest water concentrations measured in extraterrestrial materials to date (the minerals contained <2  H2O), regardless of their origin in the inner or (water-ice-rich) outer solar system (Newcombe et al. 2023, Nature). This result suggests that the melted portions of planetesimals degassed their water efficiently, such that water delivery to the terrestrial planets must have come from unmelted material. 

Publications about water in achondrite (melted) meteorites

Peterson, L. D., Newcombe, M. E., Alexander, C. M. D., Wang, J., Klein, F., Bekaert, D. V., & Nielsen, S. G. (2023). The H content of aubrites: An evaluation of bulk versus in situ methods for quantifying water in meteorites. Earth and Planetary Science Letters, 620, 118341.

Newcombe, M. E., Nielsen S. G., *Peterson L. D., Wang J., Alexander C. M. O'D., Sarafian A. R., Shimizu K., Nittler L. R., Irving A. J. (2023) Degassing of early-formed planetesimals restricted water delivery to Earth. Nature, https://doi.org/10.1038/s41586-023-05721-5

Is a magma ocean required for water loss from planetesimals?

We can test whether water loss from early-formed planetesimals occurred before or during the magma ocean stage by examining the volatile contents of primitive achondrite meteorites that represent planetary bodies that experienced incipient melting but did not have a magma ocean, such as the ureilites, acapulcoites and lodranites. My wonderful student Dr. Liam Peterson measured water in NAMs from ureilites (Peterson et al. 2023 GCA) and acapulcoite and lodranite primitive achondrites (Peterson et al. 2024 GCA). Liam's measurements suggest that the ureilite parent body contained ~2 – 20 µg/g H2O and the acapulcoite-lodranite parent body contained <38 µg/g H2O, indicating that water loss from these parent bodies occurred prior to incipient melting and was efficient (e.g., we estimate that >99.8 % of accretionary H was lost from the ureilite parent body).

Publications about water in primitive achondrite (partially melted) meteorites

Peterson, L. D., Newcombe, M. E., Alexander, C. M., Wang, J., Sarafian, A. R., Bischoff, A., Nielsen, S. G. (2023). The H2O content of the ureilite parent body. Geochimica et Cosmochimica Acta, 340, 141-157. http://dx.doi.org/10.1016/j.gca.2022.10.036 10.1016/j.gca.2022.10.036

Peterson, L. D., Newcombe, M. E., Alexander, C. M. D., Wang, J., & Nielsen, S. G. (2024). The H-poor nature of incompletely melted planetesimals: The view from acapulcoites and lodranites. Geochimica et Cosmochimica Acta. https://doi.org/10.1016/j.gca.2024.02.002