Water on the moon needs to be extra accessible than we thought
Looked at more closely
The first study reports the detection of water molecules on lunar surfaces that were exposed to sunlight near the 231-kilometer-long Clavius Crater, thanks to observations by NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA) and the German Aerospace Center. It has long been believed that water has the best chance of staying stable in regions of the moon such as large craters that are permanently covered in shadow. According to the researchers, such regions and the water they contain would be protected from temperature disturbances caused by sun rays.
As it turns out, there is water in broad daylight. "This is the first time that we can say with certainty that the water molecule is present on the lunar surface," says Casey Honniball, researcher at NASA's Goddard Space Flight Center and lead author of the SOFIA study.
The SOFIA observations suggest water molecules built into the structure of glass beads, which allows the molecules to withstand solar radiation. The amount of water contained in these glass beads is comparable to 12 ounces spread over a cubic meter of earth and spread over the surface of the moon. "We expect the water abundance to increase as we approach the poles," says Honniball. "But what we observed at SOFIA is the opposite" – the pearls were found at a latitude closer to the equator, although this is probably not a global phenomenon.
SOFIA is an aerial observatory built from a modified 747 that flies high through the atmosphere so its 9-foot telescope can observe objects in space with minimal disruption from Earth's water-laden atmosphere. This is particularly useful for observation in infrared wavelengths, and in this case, has helped researchers differentiate molecular water from hydroxyl compounds on the moon.
The glassy water features on the moon were previously found in a study on lunar mineralogy from 1969 (thanks to observations from a balloon observatory). However, these observations have not been reported and published. "Maybe they didn't realize the great discovery they actually made," says Honniball.
The amount of water contained in the glass beads is somewhat small to be useful to humans, but it is possible that the concentration is much higher in other areas (the SOFIA study only focused on one area of the Moon).
More importantly, the results show the possibility of a "moon water cycle" that could replenish the water reserves on the moon, which for a world that has long been believed to be dry and dead is hardly understandable. "It's a new area that we haven't explored in depth before," says Clive Neal, a planetary geologist at Notre Dame University who was not involved in either study.
The smallest shadows
The second study, however, could be more relevant to NASA's immediate lunar exploration plans. The new findings suggest that the moon's water ice reserves are maintained in so-called “microcold traps” with a diameter of just one centimeter or less. New 3D models created using thermal infrared and optical images from NASA's Lunar Reconnaissance Orbiter show that temperatures in these micro-traps are low enough to keep the water ice intact. They may be responsible for accommodating 10 to 20% of the water stored in all of the moon's permanent shadows in a total of around 40,000 square kilometers, mainly in regions closer to the poles.
"Instead of just a handful of large cold traps in 'craters by name', there is an entire galaxy of tiny cold traps spread across the polar region," said Paul Hayne, planetary scientist at the University of Colorado at Boulder, lead author of the study. “Micro cold traps are much more accessible than larger, permanently shaded regions. Instead of designing missions that venture deep into the shadows, astronauts and rovers could stay in sunlight while drawing water from micro-cold traps. “There could be hundreds of millions or even billions of these places scattered across the lunar surface.
More data creates more puzzles
The studies are not perfect. There is still no clear explanation of how these water-bearing glasses were formed. Honniball says they likely came from meteorites that either created the water on impact or delivered it as it is. Or they could be the result of ancient volcanic activity. Neal points out that the SOFIA study cannot provide a full picture of why the distribution of glass occurs as a function of latitude or how it might change over a full lunar cycle. Direct observation is needed to confirm what both studies suggest and to answer the questions they raised.
We may not have to wait long for this type of data. In the run-up to the Artemis missions, which will bring astronauts back to the surface of the moon, NASA is planning to start a series of robotic missions that will also help characterize the water ice content on the moon. The most famous of these missions is VIPER, a rover slated to launch in 2022 to look for underground water ice.
Given the new evidence, NASA might decide to change the goal of VIPER a bit to also study surface water, and take a closer look at all the glass features under the sun, or look at how well the microcold traps might work to maintain the water ice. Other NASA payloads, as well as missions from other countries, are likely to examine the contents of the surface water more closely. Neal suggests that a lunar exosphere monitoring system would be very useful in unraveling the history of water on the moon and how a possible lunar water cycle leads to stable (or unstable) water on the surface.
"The more we look at the moon, the less we seem to understand," says Neal. "Now we have a few more reasons to study it again. We need to get to the surface, take samples, and set up monitoring stations to get final data for studying this type of cycle."