A Possible Sky-High Signal of Sun-Warmed Water The first new whiff of lunar water emerged from data gathered by NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA). This modified Boeing 747SP jet provides its 2.7-meter telescope a view above 99 percent of the atmosphere’s obscuring water vapor—a unique capability that allows agile observations in infrared without the use of space-based facilities.
In late August 2018 a team led by Casey Honniball, a NASA Postdoctoral Program fellow at the agency’s Goddard Space Flight Center and a researcher at the University of Hawaii at Manoa, used infrared instruments onboard SOFIA to study the sunlit lunar surface. The observations, which spanned a mere 10 minutes, focused on a region at high southern latitudes near the moon’s large crater Clavius, and they revealed a strong infrared emission at a wavelength of six microns (µm) from the crater and the surrounding landscape. Warmed by the sun, something on the lunar surface was reemitting the absorbed radiation just as molecular water—plain H2O—would. A Glass Half-Full
Using SOFIA is a new and unique approach for lunar science, Honniball says, but it is not the first time Earth-bound observations have revealed a six-micron emission from the moon. Balloon-borne observations by astronomers G. R. Hunt and J. W. Salisbury showed the spectral feature, she says. But Hunt and Salisbury made no mention of this in their paper on that research, published in 1969. Instead they focused on characterizing minerals on the lunar surface. “Maybe they just didn’t know they made a huge discovery,” Honniball speculates. Honniball and her colleagues have already received additional time on SOFIA for follow-up observations. “We hope to map a majority of the moon to characterize the behavior of water,” she says. “Does it vary across the lunar surface with lunar time of day and latitude? This will help us understand its sources and where it resides.”
“We are unaware of any other material reasonable for the Moon that exhibits a single spectral feature at 6 µm other than H2O,” Honniball and her fellow researchers report in their new paper. The authors suggest that the putative water is most likely stored in naturally occurring volcanic glass or sandwiched between microscopic grains of rock dust. Either scenario could provide shielding from the extreme temperatures and near-vacuum conditions on the moon’s surface, allowing the water to persist. As to how it got there in the first place, no one is certain, but the leading explanation is that the water could have formed from free oxygen and hydrogen liberated from lunar rocks by micrometeorite impacts.
NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) is shown airborne with the sliding door over its 17-metric-ton infrared telescope wide open. Credit: NASA and Jim Ross
“Currently we do not have a good idea if the water we see with SOFIA is in amounts that make melting the glass worth it,” Honniball says. “However, if we find abundances are high enough, this may be a more feasible option than mining water ice in permanently shadowed regions, which are extreme environments and hard to work in.” And that, in turn, could tell the world just how useful this newfound water might someday prove to be. Extraction will be straightforward if the water exists predominantly on the surfaces of rock grains: one will just need to scoop up lunar soil and subject it to moderate heating. If, however, the water is locked in glass, the material must be melted to release the water for collection—a much more energy-hungry process.
Regardless of what substance is behind SOFIA’s signal, Schmitt notes that basic chemistry should allow moisture to be wrung from even bone-dry lunar material. “Heating of hydrogen-bearing regolith to several hundred degrees would result in some of the hydrogen reacting with oxygen in silicates to produce water almost anywhere on the moon,” he says. Jack Schmitt, a geologist who, as a member of the Apollo 17 crew, remains the only professional scientist to have walked on the moon, says the SOFIA measurement may not be revealing true molecular water but something more fragile and transient. “The question that I would ask,” Schmitt says, “is if the SOFIA data may be related to the possible weak bonding of solar wind hydrogen with oxygen at the surface of grains of silicate glasses and minerals in the regolith rather than being actual molecular water?” One product of such reactions could be hydroxyl, a molecule just one hydrogen atom short of water. Honniball, however, says the six-micron emission seen by SOFIA is not consistent with hydroxyl.
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