One Crater on the Moon is Filled with Ice and Gas that Came from a Comet Impact

In the coming years, NASA and other space agencies hope to explore the southern polar region of the Moon. Recent surveys of this region have revealed an environment rich in volatiles – elements that vaporize rapidly due to changes in conditions. In particular, missions like NASA’s Lunar Reconnaissance Orbiter (LRO) and the Lunar CRater Observation and Sensing Satellite (LCROSS) have detected abundant water ice in the permanently-shadowed craters around the South Pole-Aitken Basin.

Where this water came from has remained the subject of much debate, with theories ranging from it being deposited by volcanic activity or solar wind to being delivered by comets. After examining LCROSS data on the Cabeus crater near the Moon’s south pole, a multinational team of researchers from the U.S. and France determined that the water ice and volatiles in the crater were likely delivered by the impactor (a comet) that created it.

The team was led by Dr. Kathleen Mandt of the Johns Hopkins University’s Applied Physics Laboratory (JHUAPL) in Laurel, Maryland. She was joined by colleagues from the JHUAPL, the Aix Marseille Université in Marseille, France, and the Southwest Research Institute (SwRI) in San Antonio, Texas. The paper that describes their research recently appeared in the journal Nature Communications.

For the sake of their study, the team reexamined and modeled data obtained over a decade ago by LRO, the implications of which could inform future missions to the Moon. As SwRI’s Dr. Kurt Retherford, the principal investigator of LAMP, said in a SwRI press release:

“Water is considered an important resource because when you split water molecules, you end up with oxygen and hydrogen, critical components for breathable air and rocket fuel. The rest of the volatiles could be important resources, as well.”

Launching in 2009, the LCROSS and LRO were the first robotic lunar missions launched by NASA in over a decade. Both were designed to gather data that would help inform future missions to the Moon. At the time, NASA was planning on sending crewed missions to the Moon for the first time since the Apollo Era as part of the Constellation program. This program ended that same year and was followed by the Moon to Mars program, which has since morphed into the Artemis Program.

A major aspect of these plans was exploring the southern polar region to find a potential lunar base site. The local presence of water ice in this cratered region made it especially attractive from an In-Situ Resource Utilization (ISRU) standpoint. In October 2009, both the LRO and LCROSS missions explored the Cabeus crater to learn more about the origin and evolution of volatiles observed there.

This consisted of the LRO sending its spent upper stage to impact the surface and eject material from the subsurface into space. This was followed by both orbiters studying the resulting plumes of ejecta for volatiles and how they were integrated into the surface ice. In particular, the LRO examined the material with its Lyman-Alpha Mapping Project (LAMP) instrument, a far-ultraviolet (FUV) imaging spectrograph designed and overseen by the SwRI.

For more than a decade, LAMP has examined the lunar surface for signs of ice and frost in the polar regions. This was no easy task because of the permanently-shadowed nature of the region and the fact that most of the water ice is located beneath the crater floors. By capturing the glowing hydrogen emissions of water – known as the Lyman-alpha line – LAMP provided images that helped scientists characterize the distribution of water and other molecules on the Moon.

These findings helped resolve a long-standing mystery about the Moon that began during the Apollo Era. After examining Moon rocks returned by the Apollo astronauts, researchers noted the presence of water in samples of volcanic glass. This was attributed to contamination at the time, but follow-up studies found that the water came from the Moon. Since then, scientists have been trying to determine where this water came from.

In the end, scientists narrowed it down to three possibilities: volcanic outgassing, comet or micrometeoroid impacts, or surface chemistry initiated by solar wind particles. Each of these possibilities has implications since they also imply when the water was delivered and in what quantities. For their study, the team reexamined the plumes measured by LAMP and compared the elemental composition of the volatiles to that of the potential sources.

In particular, they focused on the abundances of four elements (hydrogen, nitrogen, oxygen, and sulfur) as they relate to carbon. The results indicated that the crater material is not volcanic in origin and is more likely delivered by a comet that impacted the Moon less than a billion years ago. In other words, the water ice and volatiles in the crater were delivered by the very impactor that created it. As co-author Dr. Lizeth Magaña – a recent graduate of the University of Texas at San Antonio-SwRI who recently joined JHUAPL – explained:

“Based on the ratios measured and the composition of comets, such as Rosetta spacecraft measurements of Comet 67P, comets are likely the primary source of these volatiles. Impact gardening, a term referring to impacts that churn and enrich the uppermost regolith of moons and other airless bodies, is perhaps the next most important contributor for seeding the Cabeus crater with volatiles. With all the various sources to consider, ruling out the internal volcanic source with this study helps a lot.”

In addition to resolving the mystery of where the Moon’s water came from, these results will also help NASA planners as they prepare to send astronauts back to the Moon (as well as other space agencies sending astronauts there for the first time). Dr. Kathleen Mandt, a researcher with the JHUAPL and the lead author on their paper, summarized:

“As humans prepare to return to the Moon, we have an unprecedented opportunity to make measurements in Cabeus and other PSRs to characterize the elemental and isotopic composition of lunar volatiles as a function of depth. Studying these potential lunar resources not only supports expanded human exploration, but also helps us to understand the history of the Earth-Moon system.”

Further Reading: SwRI, Nature Communication