Drip By Drip, Day By Day

Long-held beliefs that the majority of Earth's water arrived during the Moon-forming impact have been challenged in the wake of a study at Rutgers University. Instead, new isotopic evidence suggests that water was largely delivered in smaller amounts during late accretion, reshaping theories about the planet’s early evolution.

Using molybdenum (Mo) isotopes as tracers, a team led by Katherine Bermingham analyzed meteorites and ancient Earth rocks to track the planet’s building blocks. Their findings indicate that Earth's final stages of accretion were dominated by materials from the drier, inner solar system, rather than water-rich meteorites from the outer regions.

The team used negative thermal ionization mass spectrometry (N-TIMS) to measure molybdenum isotopes in meteorites and terrestrial samples, in order to investigate when Earth acquired its water. The results show that Earth’s late-stage material closely matches non-carbonaceous (NC) meteorites, which formed in the inner solar system. In contrast, carbonaceous chondrites (CC), which originated further out and are rich in water, played a smaller role than previously thought.

“Our results suggest that the Moon-forming event was not a major supplier of water, unlike what has been thought previously,” said Bermingham in the team’s press release. Instead, their findings support an alternative model in which Earth’s water accumulated gradually, rather than arriving in a single, massive event.

The researchers introduced a three-step multi-dynamic data acquisition method for N-TIMS to strengthen their conclusions, improving precision over previous studies that relied on static collection techniques. This refined approach allowed for a more detailed comparison between Earth's composition and meteorite groups, further confirming the limited role of outer solar system materials in Earth’s accretion. "Once we gathered the different samples and measured their isotopic compositions, we compared the meteorites signatures with the rock signatures to see if there was a similarity or a difference. And from there, we drew inferences."

The study’s findings align with previous research on ruthenium  and tungsten isotopes, which also point to late-stage accretion being dominated by NC materials. This has wider implications for understanding the formation of terrestrial planets, suggesting that water delivery may have been a more prolonged process than traditionally assumed.

By providing a clearer timeline for Earth's volatile acquisition, the research offers new insights into when the planet became habitable. “We have to figure out from where in our solar system Earth's building blocks – the dust and the gas – came and around when that happened,” Bermingham said. “That’s the information needed to understand when the stage was set for life to begin.”