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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