For decades, scientists have suspected that water ice exists on Earth’s Moon. Water is also known to be widely distributed – not just present in the permanently shadowed areas of our natural satellite, but in the sunlit parts too, as confirmed by NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) in 2020, which noted molecular water trapped in regolith grains. Granted, the amount of water is small – there’s actually 100 times more water in the Sahara Desert than there is in lunar soil. But that doesn’t make its presence any less significant. The fact it’s there at all is important enough.
The big question is how it came to be there. Could the water have been carried by comets and asteroids? Was it shared by Earth? Scientists have spent years trying to figure out this mystery, but now it seems as if they could be closer than ever. Shaui Li is a planetary scientist at the University of Hawaii at Mānoa. He became interested in working out where water on the Moon could have come from when he was in graduate school in 2011. This was at a time when data from the Moon Mineralogy Mapper imaging spectrometer on board India’s first mission to the Moon, Chandrayaan-1, was well-calibrated and available.
“Whether and how much water exists on the lunar surface and where it has come from is a question that has been asked many times over in the lunar community,” he tells All About Space. “The debate continued even after the return of Apollo lunar samples because most of them were exposed to the atmosphere, which led scientists to suspect that the observed water in the Apollo samples was sourced from Earth. At the time I started graduate school, the Moon Mineralogy Mapper data was just available, and that dataset was the only one scientists could use to study lunar surface water.” It’s this information, gathered between 2008 and 2009, which has been leading Li to an exciting theory, because it includes data collected when the Moon traversed through Earth’s magnetotail – an area which, he says, effectively forms a natural laboratory for studying the formation processes of lunar surface water.