New evidence of water on the moon

Astronomers this week made the startling discovery of water on the sunlit side of the moon! It’s not the first water we know about up there - there’s ice in the cold, shaded craters at the poles, and incredibly tiny amounts of water gas above the surface - but nobody expected it to survive in the heat of the sun. Speaking with Phil Sansom, NASA’s Casey Honniball explained how she’s made something of a scientific splash with the discovery of what they say are the unmistakable infrared signals of trace water...

Casey - We used a camera that can see infrared light. It flies at 45,000 feet in the atmosphere, which is above commercial airliners.

Phil - What's the point? Why not just have a camera here on earth?

Casey - The Earth's atmosphere completely blocks any light from the 6 micron fingerprint that we're specifically looking for to detect the water molecule.

Phil - So when you say it's a 6 micron fingerprint, do you mean some radiation that has a wavelength of 6 microns that only water emits out?

Casey - That is correct.

Phil - Why hasn't anyone looked for this before?

Casey - You know, I asked myself this question all the time. I think the reason we never looked at it before is because we thought the moon was supposed to be dry. It wasn't until about a decade ago that we found there was some hydrogen-bearing species and it could have been in the form of the water molecule or its close cousin hydroxyl, which is just a hydrogen atom and an oxygen atom. But we didn't really believe that the water molecule itself could be on the sunlit surface of the moon.

Phil - Why not? Why couldn't it be on the sunlit side?

Casey - The lunar environment itself is very harsh. And so we thought that any water that would be present would be lost to space, or it would migrate away to these cold polar regions on the moon.

Phil - I mean, what was your reaction then when you actually saw the signal that means, yes, there's water there?

Casey - I was quite surprised and excited and shocked. I'm pretty sure I screamed at my advisor on the phone!

Phil - What's the explanation then? Because you've just told me that people didn't think it was possible to have water existing on this sunlit side.

Casey - What we believe is happening is that somehow the water is being shielded from the harsh lunar environment. And our current ideas are that the water is stored inside impact glasses. An impact glass is formed when a micrometeorite impacts the surface of the moon. It melts part of the lunar soil, and it can either form molecular water or it can deliver some water, which will then be stored into this melt, which when the melt cools, it creates this impact glass.

Phil - Is it sort of water trapped in tiny glass crystals?

Casey - Yeah, exactly.

Phil - Could it also be there as ice sheltered somehow? Because there's another paper that was released at the same time as a pair with yours, that was saying that based on some scientists modelling, they think that ice can exist in tiny little cold traps inside the moon.

Casey - I like to think about it as our two papers present two different types of reservoirs for water. The water that we are seeing could not be due to it being stored in a micro cold trap. And that's because these micro cold traps are so cold and they're so dark, they don't emit any light at the 6 microns that we're looking at.

Phil - Oh, okay. So you've got your water that you've sensed, which you think is in these tiny glass crystals, and you've got this other paper, which is doing some, some modelling, which thinks that you can have little bits of ice. Between all this prospect of water, how much water are we talking about?

Casey - From our observations, what we estimate is on the surface is about 100 to 400 parts per million of water. And to kind of put that into a little bit of perspective, the sand in the Sahara desert has a hundred times more water present than on the lunar surface.

Phil - Oh my God. When you talk about water on the moon, I thought you meant more than that!

Casey - Yeah. Unfortunately we're not talking about puddles!