Majority of Earth’s water was here before the moon-forming impact

The Earth-Moon system is thought to have been formed by a collision between two large planetary objects with unique isotopic compositions. But the explanation for why the Earth and Moon don’t have unique isotopic characteristics themselves, like most planets in our Solar System, has eluded scientists for years… until now.

A new study from Richard C. Greenwood and colleagues sought to determine the origins of our Moon, which is thought to have formed after a collision between a proto-Earth and a solid impactor. The results were published by the American Association for the Advancement of Science (AAAS).

Through analysis of a vast collection of lunar and terrestrial samples, the researchers found support for the theory of a collision with extremely high energy. This impact was so high energy that it resulted in a nearly complete mixing of materials between proto-Earth and its impactor. Furthermore, their findings also suggest that the majority of water on Earth was delivered before the Moon-forming impact, rather than later, as is often proposed.

The researchers analyzed the oxygen isotopic compositions of an extensive collection of lunar and terrestrial samples. Their results showed a 3- to 4-parts per million (ppm) difference between the oxygen isotopic concentrations of the lunar rocks and the terrestrial basalts. However, no significant difference was found between the lunar samples and terrestrial olivine, which is a common mineral in Earth’s subsurface.

These findings are consistent with high-energy impact simulations, the researchers say, which suggest a near-complete mixing. The study’s authors suggest that the 3- to 4-ppm difference they discovered could be explained by a “late veneer,” referring to input of stony meteorite material to Earth in an impact event that occurred after the Moon-forming impact.

Also of great significance, the authors say their findings further imply that a large percentage of the Earth’s water was present earlier than the major impact event that formed the Moon. They believe that no more than 5-30% of water was contributed to Earth from the late veneer process. Overall, this new research has advanced our knowledge about the formation of the Earth-Moon system, and how one of the most important resources for our survival (water) came to be on this planet.

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By Connor Ertz, Earth.com Staff Writer