In 2024, NASA plans to put fresh boot prints on the moon for the first time in more than 50 years. Unlike the Apollo missions, which spent at most a few days on the lunar surface, this time the agency wants the crew to stick around. Establishing a sustained human presence on the moon will mean learning how to live off the land, a tall order in such a hostile and desolate environment. NASA has plans for the astronauts to build shelters out of moon dirt and to use urine as fertilizer. But a big unknown is how much water they’ll have to work with.
They’ll need water for life support, and—broken down into its constituent elements, hydrogen and oxygen—to make rocket fuel. NASA has known for years that the moon harbors a substantial amount of water ice at its poles. The problem is that a lot of the ice detected at the pole is located at the bottom of large craters where the combination of extremely low temperatures, rugged terrain, and the lack of sunlight will create all sorts of problems for the robotic explorers that will be tasked with finding and harvesting it. But two papers published in Nature Astronomy today suggest that there may be far more water on the moon than previously thought and that it isn’t limited to the large permanently shadowed regions of the moon’s south pole.
In late 2018, a team of researchers led by Casey Honniball, a postdoctoral researcher at NASA’s Goddard Flight Center, used a 747 jumbo jet that the agency had converted into an astronomical observatory to search for water on the moon. The high-flying observatory, called Sofia, or the Stratospheric Observatory for Infrared Astronomy, is equipped with unique instruments to remotely survey the lunar surface at infrared wavelengths that can reveal the presence of water. Honniball focused her observations on Clavius, the second-largest crater on the near side of the moon, located in its southern hemisphere. She discovered water molecules spread across the crater, even in its sunlit portions.
“Molecular water was not believed to be able to survive on the lunar surface,” Honniball told reporters during a press conference on Friday evening. Prior to her observations, scientists thought that water could only endure on the surface when it was caught in cold traps, permanently shadowed regions of the moon where temperatures never rise above -200 degrees Fahrenheit. They thought that any molecular water—that is, H~2~O in small enough concentrations that it can’t be considered a solid, a liquid or a gas—that isn’t in the shadows would either be destroyed by radiation or heated up so that it bounced across the surface until it eventually reached a cold trap and froze as water ice. “Now we definitely know that molecular water is present on the moon,” she said.
Honniball’s data shows that water can survive outside the shadowed regions, albeit in a different form. Instead of ice, she says the data suggests that the molecules are trapped in glassy regolith, which shields them from the harsh lunar environment. “The temperature of the lunar area we observed was at about 25 degrees Celsius [77 degrees Fahrenheit], and no form of water is stable in a vacuum at those temperatures,” says Paul Lucey, a planetary scientist at the University of Hawaii and a coauthor of the paper. “The water has to be trapped somehow in order for us to see it.” The big question is how the water came to be trapped in these grains in the first place.
One theory is that water hitched a ride to the moon as protons in the solar wind. When these protons interacted with the oxygen-rich lunar regolith, they formed hydroxyl, which is just water that’s missing one of its hydrogen atoms. Data from several spacecraft have shown that hydroxyl is all over the lunar surface, but they weren’t equipped with the types of instruments that are needed to tell the difference between hydroxyl and water. “We didn’t plan to look for it, so there wasn’t wasn’t really an instrument designed to look for bound water on the moon,” says Matt Siegler, a researcher at the Planetary Science Institute, who wasn’t involved with the research. “It’s one thing to see hydroxyl, but actual water molecules is another thing altogether.”
But where there’s a lot of hydrogen and oxygen, there’s a good chance that there might be water, too. All that’s needed to turn it into water is energy. When a meteorite strikes the moon, the intense heat causes hydroxyl molecules to combine into water. It also melts the regolith, turning it into glass that traps the water molecules. Or, another theory suggests that the water may already be present on the meteorite and get trapped in the newly-formed glass during the impact.
No lunar orbiters have had the equipment needed to tell the difference between hydroxyl and water, but NASA’s Sofia observatory has instruments that were able to observe in just the right part of the electromagnetic spectrum to detect traces of water. “Earth’s atmosphere has a lot of water vapor, so we needed to get above as much of the atmosphere as we possibly could to gather some signal,” said Honniball. “It turned out that the only instrument currently operating that can make this sort of measurement was the Sofia airplane.”
Honniball’s discovery is good news for NASA, which wants to harvest the constituent elements of water—oxygen and hydrogen—for useful things on the moon like breathable air or rocket fuel. While this could also be accomplished with hydroxyl, it’s harder to extract it from lunar regolith because of its tight bond with the grains. And even though the water is there, it’s unclear if there’s enough of it to be useful. “When we look at the amount of water, it’s kind of low,” Honniball said. Her data shows it to be present at 100 to 400 parts per million in the regolith. “We would want more to use it for a mission,” she said. “So what we really want to know is: Are there locations that have more water?”
But even if there’s not enough molecular water on the surface to be useful to future astronauts, a second paper published today suggests that there may be far more water ice locked up in shadowy pockets on the lunar surface than previously thought. In 2018, data from India’s Chandrayaan lunar orbiter confirmed the existence of water ice in large shadow-covered craters at the moon’s south pole. Now, a team of researchers led by Paul Hayne, a planetary scientist at the University of Colorado Boulder, has found that there are likely a lot of smaller cold traps all over the surface that would be much easier for robots and crew to access than the cratered regions at the pole.
Using a computer model built from data supplied by NASA’s Lunar Reconnaissance Orbiter, the team discovered that there are probably tens of billions of “micro cold traps” on the moon. These are permanent shadows created by tiny craters and surface variations that might only be a few centimeters across, but nevertheless keep the temperatures low enough to capture ice. Their models suggest that they would approximately double the total area where frozen water could exist on the moon. “We found that the number of cold traps at the scales we can’t see actually dominate on the moon by a lot,” says Hayne. “They are so abundant that if you were just standing on the surface looking down you would see hundreds of these quarter-sized cold traps just in the few meters around you.”
Hayne says these pockets have previously been ignored simply because scientists couldn’t see them. The thermal infrared camera on the Lunar Reconnaissance Orbiter can only spot the ones that are about 250 square meters or larger. Hayne and his colleagues used that data to create models of all the cold traps on the lunar surface, including the ones too small to be detected by the orbiter. Once they built the models, to verify their predictions, they validated them against the larger cold traps that are already known. But the existence of the smaller predicted traps still needs to be proven with data from the surface. And even if these little pockets can be found, that doesn’t mean they will have filled up with ice.
Starting next year, NASA will begin launching uncrewed missions to lunar orbit and the surface of the moon, and many of these robotic explorers will be equipped with instruments that will scout for water. One of them will carry an instrument developed by Hayne and his colleagues at the University of Colorado, allowing the team to gather the first in-situ evidence of these micro cold traps and to determine whether they contain water. Earlier this year, NASA selected Astrobotic, a Pittsburgh-based space robotics company, to develop a small autonomous rover that is specifically designed to scout for ice around the south pole.
We’ve learned a lot about our moon in the 50 years since the last humans left the surface, but the research unveiled today underscores how much there is left to discover.
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