Scientists have confirmed, for the first time, the presence of water on the sunlit surface of the Moon, a discovery which indicates that water molecules may be distributed across the lunar surface, and not limited to the cold, shadowed places as previously thought.

Using NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA), the researchers, including those from the University of Hawaii in the US, detected water molecules (H2O) in Clavius Crater — one of the largest craters visible from the Earth, located in the Moon’s southern hemisphere.

While earlier studies of the Moon’s surface, including those conducted during the Indian Space Research Organisation’s (ISRO) Chandrayaan-1 mission, detected some form of hydrogen, the NASA scientists said these were unable to distinguish between water and its close chemical relative, hydroxyl (OH).

Highlights of the findings:

• Data from the current study, published in the journal Nature Astronomy, revealed that the Clavius Crater region has water 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. As a comparison, the researchers said 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.

• The scientists believe several forces could be at play in the delivery or creation of this water. One possibility they said could be from micrometeorites raining down on the lunar surface, carrying small amounts of water, which may 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 water then gets stored – making it possible to accumulate – also raises some intriguing questions. The water could be trapped into tiny bead like 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 bead like structures.

• 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.

How SOFIA played a big role in this discovery?

• SOFIA is a joint project of NASA and the German Aerospace Center.

• SOFIA’s 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 NASA’s Lunar Crater Observation and Sensing Satellite, confirmed ice in permanently shadowed craters around the Moon’s poles. Meanwhile, several spacecrafts – including the Cassini mission and Deep Impact comet mission, as well as Chandrayaan-1 mission – and NASA’s 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.

• 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 per cent of the water vapour in Earth’s 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 to water molecules, at 6.1 microns, and discovered a relatively surprising concentration in sunny Clavius Crater.

• SOFIA’s follow-up flights will look for water in additional sunlit locations and during different lunar phases to learn more about how the water is produced, stored, and moved across the Moon. The data will add to the work of future Moon missions, such as NASA’s Volatiles Investigating Polar Exploration Rover (VIPER), to create the first water resource maps of the Moon for future human space exploration.

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