Most of Earth’s water likely came from the planet’s original building blocks, new research has found. A NASA study analysing lunar soil samples from the Apollo missions has shed fresh light on the history of meteorite impacts in the Earth-Moon system and the role those impacts played in delivering water to Earth.
The study, published Tuesday in the Proceedings of the National Academy of Sciences, was led by Tony Gargano, a postdoctoral fellow at NASA’s Johnson Space Center and the Lunar and Planetary Institute in Houston.
The researchers found that at least about one per cent of lunar soil contains material from carbon-rich meteorites that partially vaporised on impact. By estimating how much water these meteorites carried and adjusting for Earth’s higher impact rate, about 20 times the Moon’s, the study concluded that meteorites supplied only a small portion of Earth’s water.
The team used a new method to study lunar regolith, the fine dust and debris covering the Moon, by measuring triple oxygen isotopes. These isotopes remain stable even during meteorite impacts, making them a reliable “fingerprint” to identify material from incoming meteorites.
The Moon holds a near-complete record of impacts over billions of years. On Earth, tectonic activity and weather erase most of this history. Traditional methods of studying lunar soil focused on metal-rich elements, which can be changed by repeated impacts. Oxygen isotopes, though, give a clearer picture of what meteorites brought to the Moon.
“The lunar regolith is one of the rare places we can still interpret a time-integrated record of what was hitting Earth’s neighbourhood for billions of years,” said Gargano. “The oxygen-isotope fingerprint lets us pull an impactor signal out of a mixture that’s been melted, vaporised, and reworked countless times.”
The findings also matter for the Moon. While the total water delivered by meteorites is tiny compared to Earth, it is important for the Moon, where water is found in permanently shadowed regions near the poles. These icy areas are some of the coldest places in the solar system and will be key for scientific studies and future NASA Artemis missions.
The samples studied were collected near the equator on the Moon’s Earth-facing side, where all six Apollo missions landed more than 50 years ago. Though limited, they continue to reveal new discoveries. Future lunar samples brought back by Artemis missions are expected to give scientists a deeper understanding of water delivery, the Moon’s history, and the early solar system.
