NASA and DLR’s Stratospheric Observatory for Infrared Astronomy—a telescope-on-an-airplane called SOFIA for short—has detected yet more evidence of water on the Moon, this time in the form of H2O molecules possibly trapped within pieces of glass that form when meteorites strike the Moon’s surface. These particular findings, announced on October 26, 2020, focus on an area within the 230-km-wide Clavius crater which is near the Moon’s south pole but not in permanent shadow. This indicates that even sunlit areas of the Moon can contain water—although to what extent and whether it’s in a form that could be retrieved and utilized by future surface missions remains to be seen.

The amount of the water detected is sparse by terrestrial standards—perhaps a soda can’s volume within a cubic meter of lunar soil. But combine this with previous discoveries of water in various forms and our Moon continues to appear to be a much wetter place than was once thought… in fact, this particular observation was made during SOFIA’s first time even looking at the Moon!

From a NASA news release on Oct. 20:

SOFIA has detected water molecules (H2O) in Clavius Crater, one of the largest craters visible from Earth, located in the Moon’s southern hemisphere. Previous observations of the Moon’s surface detected some form of hydrogen, but were unable to distinguish between water and its close chemical relative, hydroxyl (OH). Data from this location reveal water in concentrations of 100 to 412 parts per million – roughly equivalent to a 12-ounce bottle of water – trapped in a cubic meter of soil spread across the lunar surface. The results are published in the latest issue of Nature Astronomy.

SOFIA’s results build on years of previous research examining the presence of water on the Moon. When the Apollo astronauts first returned from the Moon in 1969, it was thought to be completely dry. Orbital and impactor missions over the past 20 years, such as NASA’s Lunar Crater Observation and Sensing Satellite, confirmed ice in permanently shadowed craters around the Moon’s poles. Meanwhile, several spacecraft – including the Cassini mission and Deep Impact comet mission, as well as the Indian Space Research Organization’s Chandrayaan-1 mission – and NASA’s ground-based Infrared Telescope Facility, looked broadly across the lunar surface and found evidence of hydration in sunnier regions. Yet those missions were unable to definitively distinguish the form in which it was present – either H2O or OH.

“Prior to the SOFIA observations, we knew there was some kind of hydration,” said Casey Honniball, the lead author who published the results from her graduate thesis work at the University of Hawaii at Mānoa in Honolulu. “But we didn’t know how much, if any, was actually water molecules – like we drink every day – or something more like drain cleaner.”

“Prior to the SOFIA observations, we knew there was some kind of hydration. But we didn’t know how much, if any, was actually water molecules – like we drink every day – or something more like drain cleaner.”

— Casey Honniball, postdoctoral fellow at NASA’s Goddard Space Flight Center in Greenbelt, Maryland and lead author

Several forces could be at play in the delivery or creation of this water. Micrometeorites raining down on the lunar surface, carrying small amounts of water, could deposit the water on the lunar surface upon impact. Another possibility is there could be a two-step process whereby the Sun’s solar wind delivers hydrogen to the lunar surface and causes a chemical reaction with oxygen-bearing minerals in the soil to create hydroxyl. Meanwhile, radiation from the bombardment of micrometeorites could be transforming that hydroxyl into water.

How the water then gets stored – making it possible to accumulate – also raises some intriguing questions. The water could be trapped into tiny beadlike structures in the soil that form out of the high heat created by micrometeorite impacts.

Another possibility is that the water could be hidden between grains of lunar soil and sheltered from the sunlight – potentially making it a bit more accessible than water trapped in beadlike structures.

“It was, in fact, the first time SOFIA has looked at the Moon, and we weren’t even completely sure if we would get reliable data, but questions about the Moon’s water compelled us to try,” said Naseem Rangwala, SOFIA’s project scientist at NASA’s Ames Research Center in California’s Silicon Valley. “It’s incredible that this discovery came out of what was essentially a test, and now that we know we can do this, we’re planning more flights to do more observations.”

Read the full news release here, and learn more about the joint NASA/DLR SOFIA observatory in the video below: