The Water on the Moon May Trace Back to Early Earth — and Comets

Despite all appearances, the moon is a world with water. Strewn throughout the soil, water molecules are scattered all around the moon’s surface. Where they originated has long stumped scientists, but a new paper published in PNAS has looked into the origins of the moon’s hydration, suggesting that it may have come from a complex combination of sources.

“This is a major step forward in unraveling where lunar water comes from,” said Maxwell Thiemens, a study author and a researcher at Vrije Universiteit Brussel (VUB) in a press release. “Our data suggest that the moon inherited water tracing back to Earth’s formation, followed by later contributions from comets, delivering the water reservoirs we see today.”

Water Molecules on the Moon

Astronomers once thought that the moon was a world without water. But recent moon missions and moon analyses indicate that water molecules are spread throughout the moon’s surface, frozen and unfrozen, mixed into the soil, stashed inside volcanic glasses and mineral grains, and concentrated in minuscule granules.

In 2008, for instance, scientists identified signs of water in soil samples from the Apollo missions. In 2018, they confirmed that ice is found in the shaded portions of the moon, including its craters, where temperatures drop to around negative 410 degrees Fahrenheit, and in 2020, they confirmed that ice is found in the sunlit portions of the moon, too. Though the amount is modest — NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) suggests that the moon’s surface is 100 times drier than the Sahara Desert — the moon is much wetter than originally thought.

But where did this water originate? One theory suggests that comets, the cosmic snowballs that orbit the sun, have crashed into the moon, delivering their water to its surface. Another theory is that solar winds, including flows of hydrogen from the sun, have interacted with oxygen at the moon’s surface, transforming into water.

To test these theories, the study authors analyzed the waters from several soil samples from the Apollo missions. Looking to the isotopes in the samples, they surmised that the water on the moon may have originated first from materials a lot like those on Earth, and second from materials like those on comets.

Read More: Scientists Are Still Pondering These Mysteries of the Moon

Sourcing Isotopic Signatures

Assessing the samples’ isotopic compositions through stepwise heating, the study authors found oxygen isotopes that are seen on enstatite chondrites, meteorites that are supposedly made out of the same material that formed Earth and provided it with water. Heating the samples to 122 degrees, 302 degrees, and 1,832 degrees Fahrenheit, they also found oxygen isotopes that are seen on comets.

According to the study authors, these findings suggest that the water on the moon may have come from a combination of sources: first, from the matter that may have made Earth, and second, from the matter that makes comets.

In addition to these indications that the water on the moon came from a combination of Earth-like and comet-like sources, the analysis also found isotopic signatures that challenge solar wind’s contributions. These isotopic signs thus undermine the theory that the moon’s water was made on the moon, through interactions with solar wind.

The study authors say that their findings offer important insights into the formation of the moon, and clarify the connection between the saturated surface of Earth and the arid surface of its satellite. They also say that the findings may impact future moon missions, and may inform plans for sustaining a permanent presence on the moon.

“The data not only enhance our understanding of the moon’s past but also pave the way for future space exploration and resource utilization,” said Thiemens in a press release. “These findings should redefine how we think about water as a resource for long-term lunar habitation.”

Read More: There's No Wind or Rain On The Moon, But There Are Extreme Temperatures

Article Sources:

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• NASA. Moon Water and Ices

• PNAS. Triple Oxygen Isotopes of Lunar Water Unveil Indigenous and Cometary Heritage

• VUB Press. VUB Study Reveals Origins of Lunar Water and Its Connection to Earth's Early History

• Science. Earth’s Water May Have Been Inherited From Material Similar to Enstatite Chondrite Meteorites

Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.