A new NASA study of its Apollo lunar soils clarifies the Moon’s record of meteorite impacts and timing of water delivery. These findings place upper bounds on how much water meteorites could have supplied later in Earth’s history.
Research has previously shown that meteorites may have been a significant source of Earth’s water as they bombarded our planet early in the solar system’s development. In a paper published Tuesday in the Proceedings to the National Academy of Sciences, researchers led by Tony Gargano, a postdoctoral fellow at NASA’s Johnson Space Center and the Lunar and Planetary Institute (LPI), both in Houston, used a novel method for analyzing the dusty debris that covers the Moon’s surface called regolith. They learned that even under generous assumptions, meteorite delivery since about four billion years ago could only have supplied a small fraction of Earth’s water.
The Moon serves as an ancient archive of the impact history the Earth-Moon system has experienced over billions of years. Where Earth’s dynamic crust and weather erase such records, lunar samples preserve them. The records don’t come without challenge, though. Traditional methods of studying regolith have relied on analyzing metal-loving elements. These elements can get muddied by repeated impacts on the Moon, making it harder to untangle and reconstruct what the original meteoroids contained.
Enter triple oxygen isotopes, high precision “fingerprints” that take advantage of the fact that oxygen, the dominant element by mass in rocks, is unaffected by impact or other external forces. The isotopes offer a clearer understanding of the composition of meteorites that impacted the Earth-Moon system. The oxygen-isotope measurements revealed that at least ~1% by mass of the regolith contained material from carbon-rich meteorites that were partially vaporized when they hit the Moon. Using the known properties of such meteorites allowed the team to calculate the amount of water that would have been carried within.
“The lunar regolith is one of the rare places we can still interpret a time-integrated record of what was hitting Earth’s neighborhood for billions of years,” said Gargano. “The oxygen-isotope fingerprint lets us pull an impactor signal out of a mixture that’s been melted, vaporized, and reworked countless times.”
The findings have implications for our understanding of water sources on Earth and the Moon. When scaled up by roughly 20 times to account for the substantially higher rate of impacts on Earth, the cumulative water shown in the model made up only a small percent of the water in Earth’s oceans. That makes it difficult to reconcile the hypothesis that late delivery of water-rich meteorites was the dominant source of Earth’s water.
“Our results don’t say meteorites delivered no water,” added co-author Justin Simon, a planetary scientist at NASA Johnson’s Astromaterials Research and Exploration Science Division. “They say the Moon’s long-term record makes it very hard for late meteorite delivery to be the dominant source of Earth’s oceans.”
For the Moon, the implied delivery since about 4 billion years ago is tiny on an Earth-ocean scale but is not insignificant for the Moon. The Moon’s accessible water inventory is concentrated in small, permanently shadowed regions at the North and South Poles. These are some of the coldest spots in the solar system and introduce unique opportunities for scientific discovery and potential resources for lunar exploration when NASA lands astronauts on the Moon through Artemis III and beyond.
The samples analyzed for this study came from parts of the Moon near the equator on the side of the Moon facing Earth, where all six Apollo missions landed. The rocks and dust collected more than 50 years ago continue to reveal new insights but are constrained to a small portion of the Moon. Samples delivered through Artemis will open the door for a new generation of discoveries for decades to come.
“I’m part of the next generation of Apollo scientists —people who didn’t fly the missions, but who were trained on the samples and the questions Apollo made possible,” said Gargano. “The value of the Moon is that it gives us ground truth: real, physical material we can measure in the lab and use to anchor what we infer from orbital data and telescopes. I can’t wait to see what the Artemis samples have to teach us and the next generation about our place in the solar system.”
For more information on NASA’s Astromaterials Research and Exploration Science Division, visit:
https://science.nasa.gov/astromaterials
Karen Fox / Molly Wasser
Headquarters, Washington
240-285-5155 / 240-419-1732
karen.c.fox@nasa.gov / molly.l.wasser@nasa.gov
Victoria Segovia
NASA’s Johnson Space Center
281-483-5111
victoria.segovia@nasa.gov
