The dark, cold and wet side of the Moon

Craters such as this one in the Moon’s south pole region live in permanent darkness, making them, NASA thought, perfect environments for preserving water for eons.

It has had to think again, however. It turns out that even here, despite temperatures that dip to –233 degrees Celsius and can presumably keep frost locked in soil virtually forever, water is slowly escaping the super thin uppermost layer of the Moon’s surface. 

“People think of some areas in these polar craters as trapping water and that’s it,” says NASA plasma physicist William Farrell. 

“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 emphasised.”

Of course, Farrell and colleagues note in a paper in the journal Geophysical Research Letters, it’s possible that as well as seeping out, water is also coming in. Icy comets that crash into the Moon, plus the solar wind, could be replenishing it as part of a global water cycle.

Either way, the topmost layer of polar crater floors is getting reworked over thousands of years, according to their calculations. And that means the faint patches of frost that scientists have detected at the poles could be just 2000 years old, instead of millions or billions of years old as some might expect

“We can’t think of these craters as icy dead spots,” Farrell notes.