The Moon's missing water

02 July 2012
Posted by Dominic Ford.

Craters around the Moon's south pole may lack the rich reservoirs of water ice and other volatiles that have been seen close to its north pole.  Writing in the journal Nature, MIT researcher Maria Zuber and her colleagues used the Lunar Reconnaissance Orbiter (LRO) to study a large crater called Shackleton, close to the lunar south pole. This far south, the Sun remains so low in the sky that Shackleton's floor has been in permanent shadow for at least the past two billion years. Acting like a deep freezer, it is hoped such craters could provide a haven where material from comets which collided with the Moon billions of years ago is preserved in a nearly pristine state for analysis. Early results from the Moon's north pole have shown promising evidence of rich ice deposits, but the south pole has presented more mixed picture.

Shackleton CraterThe crater was surveyed using a technique similar to radar, but using an infrared laser. As LRO flies over the lunar surface, it pulses the laser at the rock below it. By timing how long it takes the pulses to be reflected back, LRO can measure the altitude of the lunar surface to within a few centimetres. The advantage of using infrared waves is that the detector can not only the measure distance, but also the roughness of the terrain.  Smooth surfaces generate clean reflections, but roughness generates nanosecond differences in the arrival times of reflections from different parts of the surface.

Using LRO's maps, individual areas of the lunar surface can be dated by counting the numbers of craters present. Smooth terrain is likely to have formed relatively recently, such that there hasn't been time for micrometeorites to roughen it with craters. Because there is no weathering on the Moon to wear old craters away, the very oldest surfaces can, by contrast, be very heavily cratered. The floor of Shackleton is just such a place, believed to be over three billion years old.

But this doesn't fit with the idea that Shackleton has rich deposits of ancient ice. This frost would be expected to have covered up old craters with a new, young, icy surface. Instead, the floor of the crater is just as rough as the surface around its rim.

Some hope for a thin layer of frost stems from the fact that the crater's floor is relatively bright compared to its surroundings -- frost is whiter than the grey rock of the Moon's surface. But Zuber points out that solar wind radiation tends to darken the colour of rock, and that the observed colour different may well be explained simply by a lack of exposure to solar wind radiation.

The presence or absence of these deposits will make a big difference for any future human explorers on the Moon who plan to visit for more than a few days, by providing a large and readily available reservoir of drinking water. But based on the present evidence, it looks like the lunar north pole would be a better site for a lunar base than the south pole.

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