Observations by spacecraft a decade ago had also suggested a more widespread distribution of water on the moon. Those measurements focused on a shorter, three-micron wavelength that was more ambiguous, unable to differentiate between a water molecule, which consists of two hydrogen atoms and one oxygen atom or hydroxyl, which has one hydrogen atom and one oxygen atom.
“Hydroxyl is actually the active ingredient in drain cleaners,” said Casey I. Honniball, a postdoctoral researcher at NASA’s Goddard Spaceflight Center in Greenbelt, Md., and lead author of the study that used SOFIA. “Hypothetically, if drain cleaner were on the moon, we could not tell the difference between the drain cleaner and water using the three-micron wavelength.”
The six-micron emissions are a “distinct chemical fingerprint” for water, Dr. Honniball said.
These observations cannot be performed from Earth’s surface because there is too much water in the lower atmosphere. Also, no lunar spacecraft, present or planned, has an instrument to examine this particular wavelength.
But SOFIA can. The aircraft, NASA’s Stratospheric Observatory for Infrared Astronomy, is a 747 with a sliding door that opens to allow a 106-inch, 17-ton telescope to peer into the night sky. But the observatory, a collaboration between NASA and the German Aerospace Center that has been in operation since 2010, is expensive, and the Obama and Trump administrations both sought to end the program. Each time, Congress has restored financing and SOFIA has continued flying.
Monday’s reported findings would not have been possible without it. At an altitude of some 45,000 feet, SOFIA rises above 99.9 percent of the water vapor in the atmosphere, said Naseem Rangwala, SOFIA’s project scientist.
The SOFIA results are in rough agreement with the earlier measurements and do not change the estimate of the amount of water on the moon. The concentration at Clavius is low — “roughly equivalent to a 12 ounce bottle of water within a cubic meter,” Dr. Honniball said.
“To be clear, it’s not puddles of water, but instead water molecules that are so spread apart that they do not form ice or liquid water,” Dr. Honniball said.