Did the Moon’s Water Come from Earth?

A recent study published in Nature Astronomy examines how processes within the Earth’s magnetic field could be contributing to the formation of water on the surface of the Moon. This study was led by the University of Hawai’i (UH) and comes during an increased interest in finding water ice across the lunar surface, which has previously been confirmed to exist within the permanently shadowed regions (PSRs) of the lunar north and south poles due to the Moon’s small axial tilt of only 1.5 degrees compared to the Earth’s 23.5 degrees. Additionally, better understanding the lunar surface water content could also help scientists gain better insights into the Moon’s formation and evolution, which is currently hypothesized to have formed from a Mars-sized object colliding with the Earth approximately 4.5 billion years ago, or approximately 100 million years after the Earth formed.

The Earth’s magnetic field, which is produced from the planet’s spinning liquid outer core, is responsible for life being able to both exist and thrive on our small, blue world. It does this by shielding us from harmful solar radiation and space weather that could strip away our atmosphere and cause catastrophic damage across the surface, making it inhospitable for life. Unlike our spherically shaped atmosphere, the magnetic field is warped and shaped by the solar wind, which includes a long tail on the night side of the planet comprised of two parts, the plasma sheet and the even farther out magnetotail. While the solar wind has long been attributed to producing water ice on the Moon, it is the plasma sheet and magnetotail that is the focus of this most recent study, as the team analyzed data from when the Moon passes through the Earth’s magnetotail during its month-long orbit.

The various aspects of the Earth’s magnetic field, including the plasma sheet and magnetotail, which were the focus of this study. (Credit: NASA/Goddard/Aaron Kaase)

“This provides a natural laboratory for studying the formation processes of lunar surface water,” said Dr. Shuai Li, who is an assistant researcher in the UH M?noa School of Ocean and Earth Science and Technology, and lead author of the study. “When the Moon is outside of the magnetotail, the lunar surface is bombarded with solar wind. Inside the magnetotail, there are almost no solar wind protons and water formation was expected to drop to nearly zero.” 

For this study, Dr. Li and his team analyzed satellite data obtained between 2008 and 2009 from NASA’s Moon Mineralogy Mapper (also called M3), which was onboard the Indian Space Research Organisation’s Chandrayaan-1 spacecraft. Through this, they discovered large amounts of lunar surface water on the Moon’s nearside occurring at various times as the Moon passes through the Earth’s magnetotail. The reason for the abundance of lunar surface water, despite the Moon not being bombarded by solar wind during its passage through the magnetotail, was due to high energy electrons emanating from the plasma sheet of the magnetic field. The team noted the amount water increases in the Moon’s mid-latitudes as it enters and exits the magnetotail but does not change while the Moon is traversing the center of the magnetotail. It’s important to note the Moon always has one side facing the Earth due to it being tidally locked with our planet.

Map displaying water content across the lunar surface, which was the focus of this study as researchers examined how the Earth’s magnetic field contributes to water on the Moon. As the data indicates, lunar water is primarily concentrated near the lunar poles. (Credit: Li, et al., 2023)

“To my surprise, the remote sensing observations showed that the water formation in Earth’s magnetotail is almost identical to the time when the Moon was outside of the Earth’s magnetotail,” said Dr. Li. “This indicates that, in the magnetotail, there may be additional formation processes or new sources of water not directly associated with the implantation of solar wind protons. In particular, radiation by high energy electrons exhibits similar effects as the solar wind protons.”

This study builds on a 2018 study and 2020 study, both led by Dr. Li, regarding the discovery of water ice in the lunar polar regions and how the lunar surface is rusting due to oxygen within the Earth’s magnetotail, respectively. Going forward, Dr. Li hopes to collaborate with NASA’s Artemis program on a lunar mission designed to track the connection between the Earth’s plasma environment and the amount of surface water content at the lunar poles.

As noted, the current hypothesis is our Moon formed approximately 4.5 billion years ago when a Mars-sized object collided with the Earth. Eventually, it formed into a sphere and spent the next several billion years getting pummeled by impacts and solar radiation, to include the formation of water ice at the poles, also known as the permanently shadowed regions.

What new discoveries will researchers make about formation and evolution of water on the Moon in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!

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