Past observations have suggested that there is water on the moon. New observations from the telescope conclude that these findings contain water.
The spacecraft saw evidence of water ice in craters with permanent shadow at the lunar poles (SN: 5/9/16), as well as evidence of water molecules on the sunlit surface (SN: 23/09/09). But water sightings in sunlit regions depended on the detection of infrared light at a wavelength that other hydroxyl compounds, which contain hydrogen and oxygen, could also emit.
Now, the Stratospheric Observatory for Infrared Astronomy, or SOFIA, has detected a unique infrared signal for water near the lunar south pole, researchers reported online on Oct. 26 in Nature Astronomy. "This is the first unequivocal detection of molecular water on the sunlit moon," says study co-author Casey Honniball, a lunar scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "This shows that water is not only in the permanently shaded regions. That there are other places on the moon that we could find."
These observations could inform future missions to the moon that will explore lunar water as a potential resource for human visitors (SN: 16/12/19).
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SOFIA, operated by NASA and the German Aerospace Center, is a 2.5-meter telescope mounted on a jumbo jet for clear sky views (SN: 17/02/16). During a flight in August 2018, the telescope detected 6-micrometer infrared light emanating from a region near the moon’s southern Clavius crater. This wavelength of light is generated by the vibrations of water molecules heated by sunlight, but not by other hydroxyl-containing compounds, which consists of an oxygen atom attached to a hydrogen atom.
“I thought it was really bright” confirming the presence of water on the moon with observations at this wavelength, says Jessica Sunshine, a planetary scientist at the University of Maryland in College Park. Sunshine was involved in past observations that detected signs of water on the moon, but did not participate in the new study.
Based on the brightness of the observed infrared light, Honniball's team calculated a water concentration of about 100 to 400 parts per million around the Clavius crater. That’s less than half a liter of water per ton of lunar soil. This focus was on what the researchers expected, based on observations of past spacecraft.
These water molecules are not frozen in ice, like water from regions of the moon with permanent shadow. It’s not liquid either, Sunshine says. "There are no moon puddles." Instead, water molecules are thought to be bound together within another material on the lunar surface.
“The only way to be watching the water on the moon (lit by the sun) is if it’s protected from this harsh environment,” Honniball says. These water molecules could be enclosed in forged glass by micrometeorite impacts or embedded between soil grains that protect water from solar radiation.
Water could form on the moon itself, from hydrogen ions in the continuous stream of sun-charged particles that reacted with oxygen on the surface (SN: 10/6/14). Or if the water is stored in an impact vessel, it could be delivered to the moon by micrometeorites.