Water discovered on the sunlit surface of the Moon

Wednesday, 28 October, 2020

Water discovered on the sunlit surface of the Moon

NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) aircraft telescope has confirmed the presence of water on the sunlit surface of the Moon — a breakthrough indicating that water may be distributed across the lunar surface and not limited to cold, shadowed places. The discovery, described in the journal Nature Astronomy, builds 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 spacecraft as well as NASA’s ground-based Infrared Telescope Facility have 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).

“Prior to the SOFIA observations, we knew there was some kind of hydration,” said study lead author Casey Honniball, a postdoctoral fellow at NASA’s Goddard Space Flight Center. “But we didn’t know how much, if any, was actually water molecules — like we drink every day — or something more like drain cleaner.”

SOFIA offered a new means of looking at the Moon. Flying at altitudes of almost 14,000 m, this modified Boeing 747SP jetliner with a 2.7 m-diameter telescope reaches above 99% of the water vapour in Earth’s atmosphere to get a clearer view of the infrared universe. Using the faint object infrared camera for the SOFIA telescope (FORCAST), the instrument was able to pick up the specific wavelength unique to water molecules, at 6.1 µm, and discovered a relatively surprising concentration of H2O molecules in the sunny Clavius crater — one of the largest craters visible from Earth, located in the Moon’s southern hemisphere.

Data from the crater location reveal water in concentrations of 100 to 412 ppm — roughly equivalent to a small bottle of water — trapped in a cubic metre of soil spread across the lunar surface. As a comparison, the Sahara Desert has 100 times the amount of water than what SOFIA detected in the lunar soil. Yet despite the small amounts, the discovery raises new questions about how water is created and how it persists on the harsh, airless lunar surface.

“Without a thick atmosphere, water on the sunlit lunar surface should just be lost to space,” said Honniball. “Yet somehow we’re seeing it. Something is generating the water, and something must be trapping it there.”

Video credit: NASA/Ames Research Center.

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 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.

A separate study in the same issue of Nature Astronomy meanwhile suggests that water could be trapped in permanently shadowed areas — known as cold traps — where temperatures stay below freezing, across more of the Moon than currently expected. Using theoretical models and data from NASA’s Lunar Reconnaissance Orbiter, Paul Hayne and colleagues assessed a whole range of possible sizes for cold traps, from 1 cm to 1 km in diameter. They found that small-scale ‘micro’ cold traps are hundreds to thousands of times more numerous than larger cold traps, and they can be found at both poles. The authors suggest that approximately 40,000 m2 of the lunar surface has the capacity to trap water, which could be accessible as a resource for future missions.

“Water is a valuable resource, for both scientific purposes and for use by our explorers,” said 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.”

Follow-up flights from SOFIA 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 to create the first water resource maps of the Moon for future human space exploration. NASA 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.

Image highlights the Moon’s Clavius Crater, with an illustration depicting water trapped in the lunar soil there. Image credit: NASA/Daniel Rutter.

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