NASA’s SOFIA Discovers Water on Sunlit Surface of Moon

Image of Moon and illustration depicting water trapped in lunar soil and NASAs Stratospheric Observatory for Infrared Astronomy
This illustration highlights the Moons Clavius Crater with an illustration depicting water trapped in the lunar soil there, along with an image of NASAs Stratospheric Observatory for Infrared Astronomy (SOFIA) that found sunlit lunar water.
Credits: NASA

NASAs Stratospheric Observatory for Infrared Astronomy (SOFIA) has confirmed, for the first time, water on the sunlit surface of the Moon. This discovery indicates that water may be distributed across the lunar surface, and not limited to cold, shadowed places.

SOFIA has detected water molecules (H2O) in Clavius Crater, one of the largest craters visible from Earth, located in the Moons southern hemisphere. Previous observations of the Moons surface detected some form of hydrogen, but were unable to distinguish between water and its close chemical relative, hydroxyl (OH). Data from this location revealwater 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.

We had indications that H2O the familiar water we know might be present on the sunlit side of the Moon, said Paul Hertz, director of the Astrophysics Division in the Science Mission Directorate at NASA Headquarters in Washington. Now we know it is there. This discovery challenges our understanding of the lunar surface and raises intriguing questions about resources relevant for deep space exploration.

As a comparison, the Sahara desert has 100 times the amount of water than what SOFIA detected in the lunar soil. Despite the small amounts, the discovery raises new questions about how water is created and how it persists on the harsh, airless lunar surface.

Water is a precious resource in deep space and a key ingredient of life as we know it. Whether the water SOFIA found is easily accessible for use as a resource remains to be determined. Under NASAs Artemis program, the agency is eager to learn all it can about the presence of water on the Moon in advance of sending the first woman and next man to the lunar surface in 2024 and establishing a sustainable human presence there by the end of the decade.

SOFIAs 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 NASAs Lunar Crater Observation and Sensing Satellite, confirmed ice in permanently shadowed craters around the Moons poles. Meanwhile, several spacecraft including the Cassini mission and Deep Impact comet mission, as well as the Indian Space Research Organizations Chandrayaan-1 mission and NASAs 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 Mnoa in Honolulu. But we didnt know how much, if any, was actually water molecules like we drink every day or something more like drain cleaner.

SOFIA Discovers Water on a Sunlit Surface of the Moon
Scientists using NASAs telescope on an airplane, the Stratospheric Observatory for Infrared Astronomy, discovered water on a sunlit surface of the Moon for the first time. SOFIA is a modified Boeing 747SP aircraft that allows astronomers to study the solar system and beyond in ways that are not possible with ground-based telescopes. Molecular water, H2O, was found in Clavius Crater, one of the largest craters visible from Earth in the Moons southern hemisphere. This discovery indicates that water may be distributed across the lunar surface, and not limited to cold, shadowed places.
Credits: NASA/Ames Research Center

SOFIA offered a new means of looking at the Moon. Flying at altitudes of up to 45,000 feet, this modified Boeing 747SP jetliner with a 106-inch diameter telescope reaches above 99% of the water vapor in Earths atmosphere to get a clearer view of the infrared universe. Using its Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST), SOFIA was able to pick up the specific wavelength unique towater molecules, at 6.1 microns, and discovered a relatively surprising concentration in sunny Clavius Crater.

Without a thick atmosphere, water on the sunlit lunar surface should just be lost to space, said Honniball, who is now a postdoctoral fellow at NASAs Goddard Space Flight Center in Greenbelt, Maryland. Yet somehow were seeing it. Something is generating the water, and something must be trapping it there.

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 Suns 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.

For a mission designed to look at distant, dim objects such as black holes, star clusters, and galaxies, SOFIAs spotlight on Earths nearest and brightest neighbor was a departure from business as usual. The telescope operators typically use a guide camera to track stars, keeping the telescope locked steadily on its observing target. But the Moon is so close and bright that it fills the guide cameras entire field of view. With no stars visible, it was unclear if the telescope could reliably track the Moon. To determine this, in August 2018, the operators decided to try a test observation.

It was, in fact, the first time SOFIA has looked at the Moon, and we werent even completely sure if we would get reliable data, but questions about the Moons water compelled us to try, said Naseem Rangwala, SOFIAs project scientist at NASA’s Ames Research Center in California’s Silicon Valley. Its incredible that this discovery came out of what was essentially a test, and now that we know we can do this, were planning more flights to do more observations.

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