When NASA lands the first woman and next man on the moon in 2024 as part of the Artemis program, it will be aiming for the lunar south pole region. It’s an environment of extremes, full of craters, incredibly cold temperatures, and areas of full sunlight or complete darkness.
Some of the craters in this region never see sunlight because of the angle. If you were standing at the south pole, the sun would skim the rims of craters as it hovered on the horizon and shone sideways on the surface.
Temperatures dip to negative 388 degrees Fahrenheit in these craters, according to measurements from NASA’s Lunar Reconnaissance Orbiter. Typically, that means frost would perpetually trap water in the soil.
Yet a new study published in the journal Geophysical Research Letters finds that water is escaping the uppermost layer of moon. Surprisingly, that top layer is thinner than the width of a red blood cell, according to the study.
“People think of some areas in these polar craters as trapping water and that’s it,” said William M. Farrell, study author and plasma physicist at NASA’s Goddard Space Flight Center. “But there are solar wind particles and meteoroids hitting the surface, and they can drive reactions that typically occur at warmer surface temperatures. That’s something that’s not been emphasized.”
The moon doesn’t have a protective atmosphere like Earth. It’s at the mercy of the sun’s solar wind, a stream of charged particles that can hit the moon and spray up water molecules.
Meteoroids also continuously hit the moon’s surface, further disturbing the tiny frost and soil particles. Because there is so little gravity on the moon, the particles can travel as far as 19 miles away from their point of origin.
“So every time you have one of these impacts, a very thin layer of ice grains is spread across the surface, exposed to the heat of the Sun and to the space environment, and eventually sublimated or lost to other environmental processes,” said Dana Hurley, study co-author and planetary scientist at the Johns Hopkins University Applied Physics Laboratory.
But similar in the way that water is being lost from the moon, water could also be added when ice-covered comets collide with the moon. The researchers are also trying to determine the water dispersal on the moon and see if it’s trapped in the top layer or if it reaches down to the crust.
The observations from the Lunar Reconnaissance Orbiter about the craters reveals that their top layers are younger than the rest of the moon because of this activity caused by outside objects.
“We can’t think of these craters as icy dead spots,” Farrell said.
Having this insight before returning humans to the moon may mean that they don’t have to visit these harsh craters as a water source and could still rely on parts of the lunar south pole that are illuminated by sunlight.
“This research is telling us that meteoroids are doing some of the work for us and transporting material from the coldest places to some of the boundary regions where astronauts can access it with a solar-powered rover,” Hurley said. “It’s also telling us that what we need to do is get on the surface of one of these regions and get some firsthand data about what’s happening.”
Future observations by robotic or manned missions could answer more questions about water on the moon.
“We suspected there was water at the poles and learned for sure from LCROSS, but we now have evidence that there’s water at mid latitudes,” Farrell said. “We also have evidence that there’s water coming from micro-meteoroid impacts, and we have measurements of frost. But the question is, how are all these water sources related?”