SOFIA Detects Molecular Water on Sunlit Surface of Moon

SOFIA Detects Molecular Water on Sunlit Surface of Moon

Using data gathered by the Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST) onboard the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA), astronomers have detected water molecules (H2O) in sunny Clavius Crater, one of the largest craters visible from Earth, located in the Moon’s southern hemisphere. The SOFIA data reveal water in concentrations of 100-412 parts per million — roughly equivalent to 355 ml (12 ounce) of water — trapped in 1 m3 of soil spread across the lunar surface.

This illustration highlights Clavius Crater on the Moon with an illustration depicting water trapped in the lunar soil there, along with an image of SOFIA. Image credit: NASA.

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 NASA’s Lunar Crater Observation and Sensing Satellite, confirmed ice in permanently shadowed craters around the Moon’s poles.

Meanwhile, NASA’s ground-based Infrared Telescope Facility, the agency’s Cassini and Deep Impact missions, as well as India’s Chandrayaan-1 mission 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 its close chemical relative, hydroxyl (OH).

“We had indications that water might be present on the sunlit side of the Moon,” said Dr. Paul Hertz, director of the Astrophysics Division in the Science Mission Directorate at NASA Headquarters.

“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.”

“Prior to the SOFIA observations, we knew there was some kind of hydration,” said Dr. Casey Honniball, a postdoctoral researcher at NASA’s Goddard Space Flight Center.

“But we didn’t know how much, if any, was actually water molecules or something more like drain cleaner.”

SOFIA was able to pick up the specific wavelength unique to water molecules, at 6.1 microns, and discovered a relatively surprising concentration in Clavius Crater.

“Without a thick atmosphere, water on the sunlit lunar surface should just be lost to space,” Dr. Honniball said.

“Yet somehow we’re seeing it. Something is generating the water, and something must be trapping it there.”

According to the team, 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 Sun’s 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 lunar water then gets stored 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.

“Water is a valuable resource, for both scientific purposes and for use by our explorers,” said Dr. Jacob Bleacher, chief exploration scientist for NASA’s Human Exploration and Operations Mission Directorate.

“If we can use the resources at the Moon, then we can carry less water and more equipment to help enable new scientific discoveries.”

The discovery is described in a paper published today in the journal Nature Astronomy.

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C.I. Honniball et al. Molecular water detected on the sunlit Moon by SOFIA. Nat Astron, published online October 26, 2020; doi: 10.1038/s41550-020-01222-x

This article is based on a press-release provided by the National Aeronautics and Space Administration.

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