The Sciences

Water on the Moon...? Yup. It's real.

Bad AstronomyBy Phil PlaitSep 24, 2009 9:42 PM


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Yesterday, I put up a post about the possibility of water on the Moon. I've now read the scientific papers from the researchers themselves, and it looks like this is for real. So yup. Water on the frakking Moon. Wow. This image shows a typical region on the Moon where the Indian spacecraft Chandrayaan-1 found water and its component molecule, hydroxyl:

Holy Haleakala. There's a lot going on here, so let me break this up into bite-sized pieces for you. First, the detection. Three different spacecraft -- Cassini, Deep Impact, and Chandrayaan-1 -- all see the signature of water (the first two swung by the Moon on the way to Points Elsewhere, and Chandrayaan-1 orbited the Moon). Water is composed of one oxygen and two hydrogen atoms. It can be formed when hydrogen combines with a hydroxyl molecule (OH^-). When a water or hydroxyl molecule absorbs energy, the component atoms oscillate, like ping pong balls glued to the ends of a Slinky. Their favorite flavor of energy is near infrared light, specifically at 2.8 microns, roughly three to four times the wavelength of light our eyes can see. There's lots of that kind of light from the Sun, which would normally just bounce off the Moon and be seen as reflected light by the spacecraft. But the water and hydroxyl molecules love to eat that 2.8 micron light, so they absorb it, and the spacecraft don't see as much as expected. When you take a spectrum of the Moon, you see a big dip at that wavelength (imagine shopping for cereal at the grocery store, and there's a gap on the shelf where Cap'n Crunch should be; you would surmise that's a popular cereal and is being absorbed by consumers. It's pretty much the same thing). That's exactly what happened. The spectra published by the scientists clearly shows a nifty little dip in the Moon's spectrum at 2.8 microns. Nothing else we know of makes such a dip, and the constituent components of water are easy to find. If it smells like water and quacks like water, it's water. Second, the amount. All the spacecraft showed that there is something around 0.1 - 1% water in the lunar surface by weight. That's a nice amount. It's not huge, so we're not talking ice skating in Clavius or anything like that, but it's enough to make mining it (if you can call it that) possible, if not easy. I still think water trapped as ice in deep polar craters is a better bet, but we'll see. That's spitballing on my part. Third, the source. Where does this stuff come from? We know that comets can hit the Moon and deposit water, and over the past couple of billion years may have dumped as much as ten trillion liters of water on the surface. Pummeling by the solar wind and micrometeorites over the eons will mix that with the surface dust, giving (theoretically) the right amount of water seen. However, there's more: the amount of water detected changed over the month-long lunar day. There was more at local morning, and less at noon a week later. This seems to imply strongly that the Sun is doing something here. Suppositions are that the solar wind, which is thick with hydrogen, slams into the lunar rock, freeing oxygen, which then combine to form water. Sunlight tends to break this water up, but as I understand it mixing later on in the night time could form water molecules. So we see the most water in the morning and the abundance drops by noon. This image, from Deep Impact, shows that correlation. The left part shows the temperature of the Moon: hottest on the right where it's local noon, and coolest on the left, near sunrise. The right image shows the water levels: lowest where it's hot, highest where it's cold. So it's really clear the Sun is affecting the water levels.

Fourth, why this is surprising. Rocks brought back from the Moon (notably from Apollo) are really deficient in water. In fact, they were incredibly dry. So finding water, especially in levels like 0.1 - 1%, is shocking. An important finding is that the water levels appear to be only on the surface. Radar and such can penetrate much farther and can map hydrogen, which has been seen near the poles but not lower latitudes where these new data show the water. However, radar isn't sensitive to a thin surface layer, so can easily miss this water. So previous findings totally missed this water due to circumstances, even though the water was right there. We just needed to have the right instruments at the right time in the right place, and once we did, bam! Water. So there you go: water on the Moon, all over the place, from the cratered highlands to the smoother darker plains, in amounts up to a liter per cubic meter of material (a quart per cubic yard). Scientifically it's a big surprise, and that's always cool and fun. Practically, it means that eventually, lunar colonies may be able to extract water from the lunar surface. There's a big push right now to figure out how to mine the Moon's dust to extract oxygen and other necessary materials for a colony, and now it looks like they may get a bonus out of it. How will this affect our plans to return to the Moon? I'm not sure. But I can guarantee that a lot of folks who have been having fun playing with ideas to mine the Moon are already sharpening their pencils. The Moon is the nearest astronomical object in the sky, and arguably the best studied. Yet it it can hide the very thing we look for most, right in plain sight... or in this case, in the plain sites. Obviously, there's a lot more to learn, but we have to go there to find out. I say we do it.

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