While Bennu was the biggest story of the year to a lot of members of our community, to many others, the big story of the week was the Stratospheric Observatory for Infrared Astronomy’s (SOFIA) detection of fresh evidence for water on the Moon. 

On Monday, NASA hosted a press conference announcing that this infrared-sensitive airborne observatory had detected a molecular transition consistent with water being present in Clavius Crater. Water was detected at the surface in about 1 part in 1000, which means that if you process 1000 thimblefuls of soil, you’d have a bucket of regolith and one thimble of water. A lot of attention was given to this as a lunar resource story, with claims that this water will make it easier for astronauts to survive on the moon.

And, as a lunar resource story, this new water detection has gotten a lot of criticism on Twitter and listservs far and wide.

Put simply, processing regolith to extract that tiny amount of water is energy-intensive and not something we can readily do at the moment. This story also made it sound like we’re dealing with moist soil, with the water just kind of mixed in, and that isn’t the case.

Here is what’s actually going on: There is a long history of making measurements that indicate water either is or could be mixed within the minerals covering much of the lunar surface. These detections have come from neutron detectors on the Lunar Reconnaissance Orbiter (LRO), spectroscopic detections from the Lunar CRater Observation and Sensing Satellite (LCROSS) collision, and from a variety of infrared detections from different missions and telescopes. Up until now, these detections have detected something consistent with the possibility of water but not necessarily water – molecules with hydroxyl, or OH, groups can look like water, and neutron detections just say there is a lot of hydrogen consistent with water.

The SOFIA airborne observatory, however, is one of the rare facilities that can detect a roughly 6-micron spectral line from water, and on a flight to transport the aircraft between two locations, a graduate student, Casey Honnibal, convinced senior researchers to try and detect lunar water at that line. No one really expected the detection was possible, but it turns out Casey, now a postdoc at NASA Goddard, was onto something, and this detection was totally possible. She and the team she was part of observed both Clavius Crater and an area on the equator of the moon and only detected water in the far southern crater.

Now, if you are thinking, “Water shouldn’t exist in sunlit regions on the Moon”, you are absolutely right. This water is mixed in with the minerals in a way that exposes it to the environment of space. It is most likely encapsulated in volcanic glasses or other protective minerals that allow us to see its spectra but that also protects it. This is a common enough phenomenon, and there is a famous diamond that was found on Earth and includes a pocket of water from deep in the planet protected within the crystal structure.

So, to put this in perspective, a grad student pleaded her case and got SOFIA to point at the Moon and look for water. This initial test of her idea was a complete success, showing water in the southern Clavius Crater and not in a region near the equator. This result is consistent with other water detections, and by expanding research into a new molecular transition, gives us even more certainty that what we’re seeing is real. This water is only about one part in 1000, is trapped in minerals, and is thus hard to liberate and not present in large quantities, but it is there. 

I can’t wait to see what Honniball does next as she expands her research into lunar water. For now, I have to be satisfied with her paper in Nature Astronomy.

More Information

NASA press release 

DLR press release 

“Molecular Water Detected on the Sunlit Moon by SOFIA,” C. I. Honniball et al., 2020 Oct. 26, Nature Astronomy