The Moon's Southern Ice is Relatively Young

Around the Moon’s southern polar region lies the South Pole-Aitken Basin, the single-largest impact basin on the lunar surface. Within this basin, there are numerous permanently shadowed regions (PSRs) that are thought to have trapped water ice over time. These deposits are crucial to future missions like the Artemis Program that will lead to the creation of permanent infrastructure. This water ice will supply crews with a steady source of water for drinking and irrigation and the means for chemically producing oxygen gas and rocket fuel.

For scientists, these PSRs are believed to have emerged when the Moon began migrating away from Earth roughly 2.5 billion years ago. Over time, these regions acted as “cold sinks” and trapped water ice that existed on the lunar surface at the time. However, according to a recent study led by the Planetary Science Institute (PSI), the Moon’s permanently shadowed areas arose less than 2.2 billion years ago and trapped ice even more recently than that. These findings could significantly impact future crewed missions as they indicate that the water ice found in lunar craters could be of more recent origin.

The research was conducted by Senior Scientist Norbert Schorghofer of the Planetary Science Institute and Raluca Rufu of the Planetary Science Directorate at the Southwest Research Institute (SwRI). Their paper, titled “Past Extent of Lunar Permanently Shadowed Areas,” recently appeared in Science Advances. The research was supported by NASA by a grant issued through the Lunar Data Analysis Program (LDAP) and through the Geophysical Exploration of the Dynamics and Evolution of the Solar System (GEODES) node at the Solar System Exploration Research Virtual Institute (SSERVI).

Image showing the distribution of surface ice at the Moon’s south pole (left) and north pole (right), detected by NASA’s Moon Mineralogy Mapper instrument. Credits: NASA

As the Moon began steadily migrating away from Earth, it felt the tidal forces caused by the Earth’s and the Sun’s gravitational pull. For decades, scientists have known that the Moon experienced a major spin axis reorientation at some point, which led to the formation of PSRs around the lunar south pole. Until recently, there was insufficient data to determine precisely when this reorientation occurred. Only a year ago, a French research team developed a coherent history for the evolution of the Earth-Moon distance. As Schorghofer explained in a PSI press release:

“When I heard about their result, I immediately realized it has profound implications for the search of water ice on the Moon. I dropped everything I was doing and began to work out the specifics, with the help of my co-author Raluca Rufu. We calculated the lunar spin axis orientation and the extent of PSRs based on recent advances for the time evolution of the Earth-Moon distance. We have been able to quantify how young the lunar PSRs really are. The average age of PSRs is 1.8 billion years, at most. There are no ancient reservoirs of water ice on the Moon.”

To be clear, the Moon has a long history of hosting water ice on its surface. This water was distributed by comets and meteors roughly 4.1 to 3.8 billion years during the Late Heavy Bombardment or resulted from volcanic outgassing. However, these processes started dying down when permanently shaded areas started appearing 3.4 billion years ago. This means most of the water delivered by impactors or outgassed from the interior could not have been trapped in the polar regions. In short, the water ice we observe today must be more recent.

“These findings change the prediction for where we would expect to find water ice on the Moon, and it dramatically changes estimates for how much water ice there is on the Moon. Ancient water ice reservoirs are no longer expected,” Schorghofer concluded.

Artist impression of LCROSS approaching the Moon. Credit: NASA

This is consistent with what NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS) observed in 2009 while investigating the hydrogen signatures previously observed around the polar regions. After LCROSS deployed an impactor to the Cabeus crater, it detected water in the debris that was kicked up. Since the PSR within the Cabeus crater was less than 1 billion years old, all the water, carbon dioxide, and other volatiles must have collected there more recently. This is rather encouraging since young PSRs could accumulate lots of ice relatively quickly, meaning older ones should have even more.

These findings might also help address the mystery of why Mercury has considerably more ice in its polar regions than the Moon. Previous data has indicated that PSRs on Mercury are much older and may have been capturing water far earlier.

Further Reading: PSI

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