How Earth's Moon Formed: New Oxygen Isotope Measurements Change Model

Earth’s moon likely formed in a giant impact event approximately 4.5 billion years ago. A Mars-sized object known as Theia (“mother of the Moon”) crossed into Earth’s orbit, crashing into the young Earth. The debris left over from that collision eventually coalesced and formed the Moon. If this giant impact theory is correct, there should be evidence recorded in both terrestrial and lunar rocks.

A new analysis of lunar samples, collected during the Apollo 12, 15, and 17 missions, by DCO’s Ed Young (University of California Los Angeles, USA) and colleagues backs up the giant impact theory, and is published today in Science [1]. However, unlike some previous studies, which suggest Theia struck Earth with a glancing blow, the new work implies a high-energy, high-angular-momentum impact.

According to the glancing blow giant impact theory, the moon should be composed primarily of impact debris and mostly Theia material. Modern Earth, on the other hand, should contain far less Theia. Since different planetary bodies have distinct 17O/16O and 18O/16O ratios, scientists can infer different planetary origins by taking precision measurements of the ratio of oxygen isotopes in rock samples.

In the study, Young et al. used mass spectrometry to measure oxygen isotopes in lunar and terrestrial samples. The samples from Earth included San Carlos (Arizona, USA) mantle xenolith olivines and mantle xenolith spinels, Mauna Loa (Hawaii, USA) basalt and olivine, anorthosite from the Bushveld complex (Transvaal Basin, South Africa), and metamorphic garnet from Gore Mountain (New York, USA). These samples originated in Earth’s mantle and hold key information about Earth’s past.

The team’s results disagree with some of the most recent work on oxygen isotopic differences between Earth and the Moon, which suggested a difference in 17O/16O of 12 parts per million [2]. Young et al.’s data, showing no difference in 17O/16O between Earth and Moon, could not exist if the Moon formed through a glancing blow impact. Instead, Theia must have struck Earth with much higher momentum, mixing material from both planetary bodies thoroughly during the impact.

Tungsten isotope ratios, on the other hand, are not identical between Earth and the Moon. This observation suggests that after the Moon formed, Earth received additional material from primitive planetary bodies and chondritic planetesimals. The new oxygen isotope data also constrain the identity of these late-veneer impactors, suggesting that they may have been water rich.

“The new Panorama high mass-resolution instrument, developed with funding from the Deep Carbon Observatory, allows us to verify that our samples are contaminant free prior to analysis, something that was not possible without the development of this new generation of mass spectrometer,” said Young.

See also a news feature in the same issue of Science by Eric Hand, titled "Rare isotopes offer clues to the chemistry of the planet", which features the work of several DCO scientists including Young's team.

Images: Top: Cross-polar transmitted light image of an Apollo 17 lunar sample (76335). Credit: Paul Warren, UCLA. Lower right: Issaku Kohl making oxygen isotope ratio measurements of extraterrestrial materials. Credit: Kathleen Micham, UCLA.

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