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Monday, June 17, 2024 | Back issues
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Sunny Parts of the Moon Hold Water, and Its Shadows May Hide Ice

Two giant leaps for lunar scientists: one research paper published Monday verifies that there are water molecules on the sunlit portions of the moon, and another identifies new permanently shadowed areas of the moon where ice may lurk.

(CN) — Two giant leaps for lunar scientists: one research paper published Monday verifies that there are water molecules on the sunlit portions of the moon, and another identifies new permanently shadowed areas of the moon where ice may lurk.

“This is unlike prior observations that could not distinguish between molecular water and its close cousin, hydroxyl. This observation has finally allowed us to definitively determine that molecular water is present on the sunlit moon,” said NASA postdoctoral fellow Casey Honniball, lead author of the Nature Astronomy study detecting the water, in a conference call with journalists.

Previously, three different spacecraft had verified hydration on the moon’s surface, but hydration doesn’t necessarily mean water: those instruments picked up signatures on the extraordinarily thin three-micron spectral band, signatures that could have belonged to H2O molecules — or to other hydroxyl (OH) compounds, there was no way to tell.

To check if this could have been evidence of lunar water, Honniball climbed aboard a Boeing 747 mounted with the Stratospheric Observatory for Infrared Astronomy (SOFIA), an 8.2-foot diameter telescope maintained by NASA and the German space agency, and took a flight to monitor the moon.

“The flight was 10 hours but we only had 20 minutes’ observing time … before we went and prepared for landing,” Honniball said. “So these observations happened very quickly and took about nine hours of me sitting on a plane to get to do this really exciting observation.”

SOFIA is capable of observing the six-micrometer spectral band, where H2O molecules leave unique signatures compared to other hydroxyl compounds.

The telescope did find water, though not a drop to drink: the moon’s surface conditions are too severe to sustain liquid water, so H2O can only persist in very unusual forms.

“Molecular water, if it’s present, is going to be readily lost to space or bounce quickly away from high temperatures on the moon,” Honniball explained. “The water molecule itself — [this is] what we’re hypothesizing — has been incorporated into impact glass beads, so it is sheltered from the harsh lunar environment, versus being attached to the surface of a grain, where the illumination from the sun could cause the water to energetically release from the surface and get lost to space.”

Honniball said that fast-traveling micrometeorites — tiny meteoroids weighing less than a gram and measuring less than a millimeter — can generate great heat when they impact the surface of the moon, melting lunar material and creating hot vapor clouds. Hydroxyl compounds already exist on the moon’s surface because solar winds deposit hydrogen there, where they interact with the moon’s low levels of oxygen to form hydroxyl.

In the heat of those vapor clouds, preexisting hydroxyls combine to make molecular water, and as the vapor clouds condense — Honniball and her six co-authors theorize — beads of impact glass form, made in part of these water molecules.

Of course, these conclusions come from the findings of just one outing with the telescope, so it’s hard to say firmly just how much water Luna could be hiding.

“Currently we’re only limited to this one observation at one location at one lunar phase,” Honniball said. “We’ve estimated the abundance of the water to be 100-400 parts per million of water, and when we look at the amount of water that we’d really want for using it for a mission, it’s kind of low.”

She says her team has requested an additional 72 hours’ use of the telescope so they can make more detailed observations of the moon at different phases, for instance to determine whether there is only a nominal abundance of water across the moon or if there may be more water molecules present elsewhere.

So the sunny portions of the moon hold water. What of the dark sides of the moon — of which, it turns out, there are very many?

The second Nature Astronomy paper published Monday, led by University of Colorado planetary scientist Paul Hayne, identifies numerous permanently shadowed regions where water ice might be holding out on the moon.

These dark regions are especially prevalent near the moon’s poles because of its very slight axial tilt.

“Unlike the earth, which is tilted something like 23.5 degrees relative to the sun, the moon’s tilt is only 1.5 degrees,” Hayne said in an interview. “If you were to stand at one of the poles of the moon over the course of a lunar day, as the moon rotated, you would observe the sun to skim along the horizon and revolve all the way around you, 360 degrees. Now if you walked down into a crater, at the pole, the sun would still move around the horizon, but now the walls of the crater would block the sun from view. So you would be in an area of permanent shadow.”

Hayne said the moon’s likely been in this configuration for more than a billion years, meaning these shadows are among the longest known.

“Without an atmosphere to even out temperatures, these shadows are actually the coldest known places in the solar system, at temperatures about 30 degrees Kelvin — that’s [30 degrees] above absolute zero,” he said. “Right in our backyard.”

These frigid pockets of lunar perma-shadow are called “cold traps,” and they are numerous. Hayne and his two co-authors counted thousands of micro cold traps, measuring as little as one centimeter in diameter, caused by the unceasing impact of many micrometeorites.

“If you go to the beach, at the end of the day after everyone’s left, there’s millions of footprints everywhere. One footprint destroys the other footprint, and at some point it’s just this mess of little divots — that’s kind of what we’re talking about,” Hayne said. “Get enough craters and it becomes sort of random.”

Previous studies measuring the moon’s cold traps also looked at the small micro cold traps that Hayne and his colleagues focused on. Monday’s findings report a cumulative area of permanently shadowed regions about 20% greater than past research had found. In all, cold traps amount to about 15,000 square miles of permanently shadowed regions on the moon’s surface, or twice the size of New Jersey.

There’s no guarantee that ice will be found in each of these regions, but the research provides a roadmap for future expeditions, showing where astronauts or rovers can go to look for shadow moon ice.

“This discovery dramatically increases the amount of real estate where you could potentially find and extract water,” Hayne said. “The micro cold traps are much more widely distributed within this polar region than the big cold traps are, so instead of having to design astronaut suits and mining equipment able to operate in these extreme, dark, cold conditions … we could operate in sunlit regions and simply stick our tools into these meter-scale, or smaller, cold traps and potentially pull out ice.”

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