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3 questions after the discovery of water molecules on the sunlit moon

In 2018, astronomers directly confirmed for the first time that water, in the form of ice, is on the moon’s surface. Aptly named water ice resides in the coldest, darkest parts of our planet’s satellite, like the shadow-shrouded craters that dot its polar regions, the deepest parts of which never see sunlight.

But new research published Monday verified a suspicion that researchers had long been unable to confirm. A team of scientists who studied a slice of the moon aboard NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) — considered to be the world’s largest flying observatory — detected the first evidence that water molecules can exist on the unforgiving landscape of the sunlit lunar surface. That means that those molecules could be found across more parts of the moon than scientists previously imagined.

“Now we can really begin to try and understand the cycle of water on the moon,” Casey Honniball, the study’s lead author, said.

The discovery of molecular water on an illuminated part of the moon came as a surprise to Honniball and her team, who weren’t sure if their time on the observatory would reveal anything useful. Honniball said she’s “pretty sure” she screamed on the phone with her thesis advisor at the time when she realized what they had found.

More research is needed to answer the myriad questions raised by their discovery, and to determine what it could mean for NASA’s Artemis program, which aims to put “the first woman and next man on the moon” by 2024. But this latest revelation undoubtedly brings us another step closer to understanding the many mysteries of our closest celestial neighbor.

Here’s a look at three of those major questions, and the kind of research scientists will be pursuing next to answer them.

What does the lunar “water cycle” look like?

“Molecular water” doesn’t refer to any type of water we may picture here on Earth. It describes individual water molecules that are too spread out on the lunar surface to form either liquid water or ice.

The moon is continuously being hit with solar radiation at “full force,” Honniball said, and lacks a thick atmosphere like the one on our planet to protect it.

Regardless of how it looks to us from our perspective, half of the moon is always illuminated by the sun, in the same way that half of Earth experiences day while the other half experiences night. It takes the moon about 28 Earth days to both orbit our planet and complete a full rotation on its axis.

That means that if you stood at the same spot on the moon for a full cycle, both the lunar day and the lunar night would last approximately 14 Earth days. During that time, the moon’s surface goes through “vast temperature swings.” At the moon’s equator, daytime temperatures can reach 250 degrees Fahrenheit, while the night can get as cold as negative 208 degrees Fahrenheit.

“Prior to this, we didn’t really think that a water molecule could survive on the surface because the lunar surface has a really harsh environment,” Honniball said. “We thought if any water was going to be there, it would immediately be lost to space or migrate to the polar regions.”

It’s not yet clear exactly how those molecules arrived in the first place. Small rock particles known as “micrometeorites” are constantly bombarding the moon’s surface, and it’s possible that they deposit tiny amounts of water on impact.

A press release on Honniball’s discovery from NASA describes another potential process where solar wind brings hydrogen to the surface, which reacts with “oxygen-bearing minerals” in the lunar soil to create hydroxyl, which is represented by the chemical formula OH because it’s made up of one oxygen atom bonded to one hydrogen atom. Radiation from those micrometeorites could be responsible for “transforming” hydroxyl into water molecules, or H2O.

But the revelation that molecular water can exist on the sunlit lunar surface suggests that something is protecting those molecules. Micrometeorites create a scorching heat that melts some of the lunar material they hit on impact, Honniball explained. She suspects that any water involved in that process could possibly be incorporated into that melted material, and housed in a tiny, glass-like bead when it cools.

That, they theorize, is what protects the water “from being lost to space or from migrating around,” Honniball said.

Honniball and her fellow researchers can’t yet be sure how or where molecular water is stored — it’s also possible that the molecules are protected in the empty spaces between grains of lunar soil. But she said that given the moon’s hostile conditions, they “like the impact glass theory better.”

Does H2O move around the moon’s surface?

Previous missions have “found evidence of hydration,” in the moon’s “sunnier regions.” But because both hydroxyl — which Honniball noted is chemically closer to drain cleaner — and molecular water feature the same oxygen and hydrogen bond, it was impossible for equipment like NASA’s Moon Mineralogy Mapper to tell which of the two was being observed.

The flying observatory upon which Honniball and her team conducted their research is, however, capable of observing the wavelength specific to water molecules, something that no spacecraft is equipped to do. It’s also impossible from ground-based observatories because water vapor in the Earth’s atmosphere obscures our ability to see similar particles.

SOFIA circumvents those limitations because it can reach altitudes of up to 8.5 miles, higher than 99 percent of the water vapor in our atmosphere. Its sharp telescope, known as the Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST), is typically used to observe distant galaxies, nebulas, black holes and other far-off celestial objects. That instrument allowed Honniball’s team to finally put an end to the “water or hydroxyl?” debate.

Aboard SOFIA, the researchers looked at just a fraction of the moon, about a kilometer in width from 55 degrees to 75 degrees latitude, located in the massive Clavius crater.


Above, a slow flyover of Clavius crater, looking south. Video courtesy of NASA’s Scientific Visualization Studio. 

The team didn’t find an abundance of water — according to NASA, the Sahara desert is 100 times wetter than what they observed. Honniball’s paper concludes that the distribution of water across that small range “is a result of local geology” and may not be equally dispersed across the entire moon.

Moving forward, Honniball’s biggest question is whether molecular water is migrating around the moon’s surface. To figure that out, she and her colleagues plan to conduct observations on SOFIA over the course of multiple lunar phases.

“[Those observations] will tell us a lot about how water is formed on the moon, how it’s stored, [and] if it’s a potential source of water ice to the lunar poles,” Honniball said.

How widespread is molecular water?

Based on the new observation, the abundance of molecular water on sunny parts of the moon is currently believed to be “kind of low. With more observations, we might find locations that concentrate water, like volcanic deposits,” Honniball said.

But another report published on Monday indicates that far more shadowy areas on the moon’s surface might be able to accumulate water ice than researchers once thought. The researchers involved suggest that water ice “trapped at the lunar poles may be more widely distributed and accessible as a resource for future missions.”

The presence of water on the moon is “an absolute game-changer” for both future exploration and paving the way toward a sustainable human presence there, said Jack Burns, a professor in the department of astrophysical and planetary sciences at the University of Colorado Boulder, via email.

Burns, who also serves as the director and principal investigator of the NASA-funded Network for Exploration and Space Science, said that if astronauts could tap into the water that exists on the moon, they could potentially use it for drinking, growing crops and shielding themselves against space radiation. He noted they could even convert it into rocket fuel, which is made up of liquid oxygen and hydrogen — and it would be much cheaper to make, Burns explained, due to the moon’s lower gravity.

He added that learning to search for and potentially mine water for human use on the moon “will also point us toward Mars.”

“A mission to Mars is at least 2.5 years in duration because of the greater distance, so we will need to ‘live off the land’ from the start,” Burns said. “Lessons learned from [the moon] will be crucial in making human voyages to Mars possible.”