A new study published in the journal Nature Geoscience indicates that water may be more prevalent on the lunar surface than previously thought.
If the Moon has enough water, and if it’s reasonably convenient to access, future explorers might be able to use it as drinking water or to convert it into hydrogen and oxygen for rocket fuel or oxygen to breathe. Image credit: NASA’s Goddard Space Flight Center.
“Water on the Moon is of intense interest for many reasons,” said co-author Dr. Michael Poston, a researcher with the Southwest Research Institute.
“Water has been the focus of many lunar missions, largely because it is a critical resource for a Moon habitat. When you split water molecules, you end up with oxygen and hydrogen, critical components for breathable air and rocket fuel.”
“We find that it doesn’t matter what time of day or which latitude we look at, the signal indicating water always seems to be present,” said lead author Dr. Joshua Bandfield, a senior research scientist with the Space Science Institute.
“The presence of water doesn’t appear to depend on the composition of the surface, and the water sticks around.”
The new results contradict some earlier studies, which had suggested that more water was detected at the Moon’s polar latitudes and that the strength of the water signal waxes and wanes according to the lunar day (29.5 Earth days).
Taking these together, some scientists proposed that water molecules can ‘hop’ across the lunar surface until they enter ‘cold traps’ in the dark reaches of craters near the north and south poles.
In planetary science, a cold trap is a region that’s so cold, the water vapor and other volatiles which come into contact with the surface will remain stable for an extended period of time, perhaps up to several billion years.
The debates continue because of the subtleties of how the detection has been achieved so far. The main evidence has come from remote-sensing instruments that measured the strength of sunlight reflected off the lunar surface.
When water is present, instruments like these pick up a spectral fingerprint at wavelengths near 3 micrometers, which lies beyond visible light and in the realm of infrared radiation.
But the surface of the Moon also can get hot enough to ‘glow,’ or emit its own light, in the infrared region of the spectrum.
The challenge is to disentangle this mixture of reflected and emitted light. To tease the two apart, researchers need to have very accurate temperature information.
The study authors came up with a new way to incorporate temperature information, creating a detailed model from measurements made by the Diviner instrument on NASA’s Lunar Reconnaissance Orbiter (LRO).
They applied this temperature model to data gathered earlier by the Moon Mineralogy Mapper spectrometer onboard India’s Chandrayaan-1 spacecraft.
The new finding of widespread and relatively immobile water suggests that it may be present primarily as hydroxyl (OH), a more reactive relative of H2O that is made of one oxygen atom and one hydrogen atom.
OH doesn’t stay on its own for long, preferring to attack molecules or attach itself chemically to them. OH would therefore have to be extracted from minerals in order to be used.
The research also suggests that any H2O present on the Moon isn’t loosely attached to the surface.
“The next step is to determine whether it’s water, OH, or a mixture of the two — and where it came from,” Dr. Poston said.
“Is it from external sources, delivered by comet or asteroid impacts? Is it from internal processes on the Moon itself, such as ancient volcanism? Or could it be an ongoing process of the solar wind reacting with lunar materials to create OH or H2O?”
“Some of these scientific problems are very, very difficult, and it’s only by drawing on multiple resources from different missions that we are able to hone in on an answer,” said LRO project scientist Dr. John Keller, from NASA’s Goddard Space Flight Center.
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Joshua L. Bandfield et al. Widespread distribution of OH/H2O on the lunar surface inferred from spectral data. Nature Geoscience, published online February 12, 2018; doi: 10.1038/s41561-018-0065-0
