Water in the Lunar Desert

The environment on the moon is pretty boring.  Rocks, dust, and craters as far as the eye can see in all directions.  Untouched for billions of years, save for meteors and a few recent visits by a blue neighbour.  In 2009, the cold, dry surface of the moon was found to harbour trace amounts of water.  Now, less than a decade later, the first map of lunar water has been produced.

A new map showing water quantities inn the lunar soil. Credit: Milliken lab / Brown University

The map was produced with data taken by NASA’s Moon Mineralogy Mapper, which flew aboard India’s Chandrayaan-1 spacecraft, the craft that discovered the water in 2009, along with a similar molecule known as Hydroxyl.  Before we take a look at what the map tells us, it is important to note that the amount of water found here is less than the dryest desert on Earth.  It is important, but contextually a very small amount, which explains why it took so long to find.

Instantly we notice some interesting things about the map, aside from the fact it shows the Apollo missing lading sites.  The water is barely present in equatorial latitudes, but becomes more and more concentrated near the poles.  Why is this?

Well if we look at the things that do interact with the moon from afar, we can find a clue.  It’s not the human touch, and meteor impacts are a bit more spread out.  It’s actually the invisible effects of the Solar wind, that has constantly bombarded the moon since it’s formation.  Solar wind particles can form hydroxyl and water molecules when they settle on the surface.  Although the moon no longer has a dipolar magnetic field, the solar wind still settles near the poles.  I’m not exactly sure why, though my guess would be that they hit the ground as they spread out around the Moon, not unlike wind moving over a car as it drives.

The other thing to notice is that there is some concentration around crater edges and ancient lunar volcanoes.  This suggests that there is water present in the deeper crust that was brought to the surface by volcanic activity and by deep impacts.

There are a couple of caveats with this map.  The first is that it relies on the reflection of light from the moon.  There are many craters on the moon that receive no light from the Sun, and are constantly in shadow.  There would be no data for those regions, though many scientists believe these dark areas are the most likely places to find existing ice water.

The second is that the map only gives a depth of about a milimeter, meaning the deeper soil could have different concentations and distributions.  Though I would hypothesize it to be the same in the deeper regolith, as each layer would contain a similar distribution of water as it’s covered over by the next layer from meteorite activity.

The third and final caveat is that, as the same study found, the concentration of water changes throughout the day by as much as 200 parts per million.  This shows that the mechanism that gives rise to water on the surface is active today, and very dynamic.

One thing is clear – there is plenty of water to be found on the moon, and we have barely scratched the surface (pun intended) of how much is left to find.

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