Where is the Moon's water?

If there is water on the Moon, will it be on the surface or will it be within the ground?
This is a spectacular high-Sun view of the Mare Tranquillitatis pit crater revealing boulders on an otherwise smooth floor. This image from LRO's NAC is 400 meters (1,312 feet) wide, north is up. Image credit: NASA/Goddard/Arizona State University

This is a spectacular high-Sun view of the Mare Tranquillitatis pit crater revealing boulders on an otherwise smooth floor. This image from LRO's NAC is 400 meters (1,312 feet) wide, north is up. Image credit: NASA/Goddard/Arizona State University

There is water on the moon! As we’ve outlined before, it’s somewhat tricky to keep water on the surface of the moon, because the combination of heat and particles from the Sun, a lack of an atmosphere, and no magnetic field means that it’s pretty hard to keep water that’s exposed to sunlight from evaporating away into space.

What that means is that if you want to have water persist anywhere on the Moon, it has to be sheltered from the Sun somehow. An easy way for this to happen is at the poles, where some craters are deep enough that the Sun’s rays never reach into the bottom of the crater. Places like this are called “cold traps”, because it can trap material in a solid form that would otherwise escape if it weren’t so cold.

Near the south pole of the Moon in particular, we found frost in some of these deep dark places. This frost makes the surface more reflective than it would be if there were only rock sitting around in those craters - so the coldest places also wind up being more reflective if you’re bouncing light off of ice. This particular study is careful to note that we’re not seeing frozen pond-style pools of water, but more like the frost that builds on the outer edges of leaves in fall.

But craters aren’t the only places that water ice could hide - we have a sneaking suspicion that the Moon also has tunnels woven under its surface. The Moon had a surprisingly long era of volcanic activity in its younger years, and where there are lava flows, you can wind up with lava tunnels. We are pretty sure that the moon has these. We see them most easily as they collapse, because then you get a snake-like pattern of collapsed ground, twisting its way across the surface as a series of giant potholes.

These images from NASA's LRO spacecraft show all of the known mare pits and highland pits. Each image is 222 meters (about 728 feet) wide. Image credits: NASA/GSFC/Arizona State University

These images from NASA's LRO spacecraft show all of the known mare pits and highland pits. Each image is 222 meters (about 728 feet) wide. Image credits: NASA/GSFC/Arizona State University

Every so often, there’s a more isolated cave-in, giving us a glimpse into a sublunar cavern - a deep shadow cast into the depths catches the eye and the imagination. If water had accumulated in these hidden tunnels, they would also be relatively protected from evaporation. However, it’s one thing to have a plausible place for water, and another to find it for sure in those places. Lava tunnels are an extremely appealing place for water, though - because they’re also an appealing place to put a human base on the Moon. While we don’t have to worry about humans evaporating, any shelter from intense heat and cold helps us as well. So if there were also water down there, they’d be a great place to put an inhabited base.

You can definitely also wind up with watery molecules bound up in the rocks themselves. A recent study suggests that instead of having lots of water ice hanging around, the Moon may have a lot of hydroxyl, which is one hydrogen and one oxygen bound to each other, rather than the two hydrogens and one oxygen that make up your standard water molecule. Hydroxyl binds easily to other things, so it can wind up binding itself to minerals in the earth - you can extract it and create water, but it’s more energy intensive than just having water lying around.

So the true answer is that there’s going to be a mixture of places where water will be found - on the surface in sheltered places, possibly in underground tunnels, and some not-quite water bound up in minerals!


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Is There Water On The Moon?

A NASA spacecraft explores the moon's permanent shadows. Image credit: NASA's Goddard Space Flight Center

A NASA spacecraft explores the moon's permanent shadows. Image credit: NASA's Goddard Space Flight Center

Originally posted on Forbes!

There is! Water’s existence on the Moon is not an easy thing to arrange, because water evaporates very easily. On our relatively balmy Earth, our evaporating water is mostly caught and suspended in the atmosphere, which in turn is protected from the Sun and other cosmic hazards by the Earth’s magnetic field.

The Moon, having no magnetic field, and also having no atmosphere, has no protection for its water, and no way of catching any of the water which evaporates under the heat of the light from the Sun. This combination of missing ingredients means that we can all reasonably expect that any water on the surface on the Moon would not last long before evaporating. Once the water evaporates and turns into a gas, it can rapidly be stripped away from the Moon, lost to interplanetary space.

The Moon is a jagged and weird place, and so the only shelter the Moon can offer from the Sun’s rays is simply that of shadow. Most of the Moon is illuminated by the Sun at some point in the month -- when the side facing the Earth is dark, it’s the far side that’s bearing the brunt of the Sun’s roasting. But the jaggedness and weirdness of the moon means that there are some very dark shadowy places at the poles of the Moon - craters which are never angled in such a way that the Sun’s light can reach their depths. And, in the same way that the snow patch underneath a parked car doesn’t melt very rapidly, any water ice in the depths of one of these permanently shadowed craters could also stay put for much longer than any exposed water ice would ever manage.

NASA's Moon Mineralogy Mapper, an instrument on the Indian Space Research Organization's Chandrayaan-1 mission, took this image of Earth's moon. Blue shows the signature of water, green shows the brightness of the surface as measured by reflected infrared radiation from the sun and red shows an iron-bearing mineral called pyroxene. Image credit: ISRO/NASA/JPL-Caltech/Brown Univ./USGS

NASA's Moon Mineralogy Mapper, an instrument on the Indian Space Research Organization's Chandrayaan-1 mission, took this image of Earth's moon. Blue shows the signature of water, green shows the brightness of the surface as measured by reflected infrared radiation from the sun and red shows an iron-bearing mineral called pyroxene. Image credit: ISRO/NASA/JPL-Caltech/Brown Univ./USGS

However, it wasn’t until 2008, when the Indian satellite Chandrayaan-1 was placed into orbit around the Moon that the presence of water in these deep craters was 100% confirmed. In 2009, multiple instruments onboard Chandrayaan-1 recorded signatures of significant amounts of water at the poles of the Moon.  Since those first reports, other satellites (including the LCROSS lunar impactor) have confirmed the initial results, and the Lunar Reconaissance Orbiter continues to map out the depths of these craters at the poles of the Moon even today.

How did that water get there? That’s much harder to determine. But it’s estimated that the water arrived through several pathways. The easiest, given the cratered nature of the moon, is that it arrived with an impacting object, like a comet or water-laden asteroid. It’s hard to say if that would donate enough water, so there may yet be other ways that water arrived on the surface of the Moon, or perhaps is formed through interactions with the solar wind.

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