Water resources will be valuable to lunar visitors and inhabitants alike, and researchers have now studied Moon craters to learn more about the water ice that lurks within them.
Water will be of use in myriad ways for explorers on the Moon: It can be used for drinking, of course, but it can also be split apart to yield oxygen (O) and hydrogen (H), which are useful as propellants. And not lugging it from Earth is a huge savings in terms of cargo space and spent fuel, said Ariel Deutsch, a geoscientist at Brown University. “There are big benefits if you’re able to access resources in situ.”
The Moon orbits nearly upright, unlike Earth, and portions of deep craters near its poles are never exposed to sunlight. In these permanently shadowed regions, temperatures can plummet to roughly 30 kelvins (−243°C), rendering these spots among the coldest in the solar system. They’re the perfect spot for water ice to lurk.
Over the Moon’s roughly 4.5-billion-year history, water ice has been transported to its surface in multiple ways, including volcanism, impacts, and the solar wind.
Volcanism and impacts are two water-yielding processes that were particularly active long ago. Volatiles like water from deep within the Moon’s interior can be outgassed through volcanic eruptions, and comets and asteroids that collide with the Moon can deliver water.
A more ongoing process is interactions between the solar wind—energetic particles emanating from the Sun—and the lunar surface, which can form OH molecules and, eventually, H2O molecules.
To better understand lunar water resources, Deutsch and her colleagues studied 82 small craters near the Moon’s north and south poles that contained water ice. They first estimated the craters’ ages. (The ice within a crater can’t be any older than the crater itself, the team assumed.) Getting a handle on age was important, said Deutsch, because different mechanisms that delivered water to the Moon’s surface dominated at different times. “If you’re able to learn about when the volatiles were being delivered, maybe that tells you a little about how they came to the surface.”
The traditional way of estimating a crater’s age is to look at how many other craters are on top of it. It’s the same idea as leaving a piece of paper out in the rain and counting droplets, said Deutsch. “The more drops, the older it is.” But that technique doesn’t work well for craters that are smaller than roughly 15 kilometers in diameter. Therefore, Deutsch and her colleagues turned to a different method to analyze their sample.
The Roughness of Rocky Debris
The researchers quantified the surface roughness of the Moon within and around each crater. These observations, made using topographic data from the Lunar Orbiter Laser Altimeter aboard NASA’s Lunar Reconnaissance Orbiter, reflect the presence of large boulders tens or hundreds of meters in diameter. Such rocky debris would be scattered by a crater-forming impact and then would slowly wear down over time as it was jostled by moonquakes and exposed to thermal expansion and contraction stresses, Deutsch and her collaborators surmised. Younger craters should therefore be rougher than their older brethren, the scientists concluded.
The team verified this trend using a data set of over 400 age-dated lunar craters. The researchers then turned to estimating the ages of their 82 water ice–bearing craters. Deutsch and her collaborators found that craters in their sample tended to be rougher, and therefore younger, than the oldest craters on the Moon. Several of the small ice-bearing craters may have formed as recently as about a billion years ago. That’s far too young for their water ice to have been delivered by volcanism, said Deutsch.
Surprisingly, the roughest craters on the Moon did not host water ice, the team found. These craters might not be stable environments for ice, said Deutsch. That’s because fresh craters, which tend to be relatively deep, have steep walls that more efficiently scatter solar energy into their interior.
Another explanation for the lack of water ice in the youngest craters hinges on its rate of delivery. If some ice-destroying process is operating continuously on the Moon, ice might survive only when it’s delivered in significant quantities, said Deutsch. “Maybe we haven’t had a strong enough delivery event in the most recent past to outpace the erosive events.”
These results were published last month in Geophysical Research Letters.
Going forward, Deutsch and her colleagues plan to study whether the Moon’s water ice resources are accumulating or diminishing. Thermal data, imagery, and neutron measurements obtained remotely will all be useful for understanding the abundance and distribution of water ice, said Deutsch, but it’ll ultimately be necessary to revisit our nearest celestial neighbor.
David Kring, a planetary scientist at the Lunar and Planetary Institute in Houston, Texas, not involved in the research, agreed. “We’re desperately in need of some type of robotic or human exploration capability that can ground truth the orbital and Earth-based detections.”
NASA’s Volatiles Investigating Polar Exploration Rover (VIPER) mission will do just that, said Kring. VIPER, which could land on the Moon as early as December 2022, will explore the lunar south pole and its water ice.
—Katherine Kornei (@KatherineKornei), Science Writer