Despite most of us thinking of the Moon as a rather cold and desolate place, the nomenclature of lunar surface features conjures a rather difference image - that of a body awash with seas (Mare) and oceans (Oceani). These dark and relatively homogeneous regions, mainly found on the near side, are of course not water oceans, but volcanic plains. Nevertheless, the search for water in our Solar System continues unabated. Not only is water an essential ingredient for life as we know it, but it is an important resource for future human exploration - providing drinking water, oxygen and fuel components. This is why the detection of water is a bonus prize in the Google Lunar X PRIZE(GLXP) competition.
Most of our understanding of the Solar System, and indeed the Universe, comes from remote sensing - from data acquired from afar. And so the Moon is unique in being the only planetary body, apart from the Earth, for which we have samples from known locations (we know their "provenance"). The rock and regolith (soil) samples returned by the Apollo and Luna missions vastly improved our understanding of our closest neighbour, but they still only give us samples from a handful of sites - this is why more in-situ analyses and sample returns are needed to fully answer the key science questions at the Moon.
The returned lunar samples show evidence of a very dry world - without the water-bearing minerals that we see on Earth - but recently we have discovered that this is not the whole story. Radar observations from the Clementine spacecraft were the first to suggest that water ice might exist in the bottom of craters at high latitude. These data showed a highly scattered radar return - often a signature of multiple scattering in ice, but possibly also arising from a rather rough surface. It had for some time been theorised that water ice could exist in permanently shadowed polar craters; close to the poles, the Sun never rises very high, the solar flux on the surface is low, and it is possible for some craters to remain free from sunlight for millions of years. But this evidence was not conclusive.
The next piece of the puzzle came from the neutron spectrometer on-board Lunar Prospector. Although not capable of detecting water directly, it did detect regions of high hydrogen concentration at both poles, within the top ~40 cm of the surface. Again, there are alternative explanations for the results, but the synergy with the radar data was quite compelling.
More direct evidence came from the Moon Mineralogy Mapper (M3) instrument on-board the Chandrayaan-1 spacecraft. This instrument was an imaging visible and near-infrared spectrometer. This means that it recorded sunlight reflected from the lunar surface and tried to determine mineralogy from the way in which this light is modified, leaving its spectral fingerprint behind. In particular, M3 had a spectral range that included the fingerprint of the hydrogen-oxygen bonds found in water. Importantly, M3 obtains data from only the first few millimetres of the surface.
The most recent data supporting the case for lunar water come from the LCROSS mission. This mission consisted of two parts, a modified rocket stage that was designed to crash into the lunar surface, and an instrumented spacecraft following behind that would image the impact site and hopefully fly through any debris kicked up on impact. The LCROSS impact was targeted at a shadowed crater at the south pole and the impact and observations were very successful. Two results (so far!) support the case for water on the Moon - the near-infrared spectrometer produced data that were only matched by the signature of water, and the ultra-violet spectrometer saw an emission from hydroxyl (the same O-H water by-product seen by M3).
The important thing about the LCROSS results is that they imply water in a significant quantity - approximately 100 kg from a 20 m diameter crater. This is not just a thin layer bound to surface rocks, but enough to perhaps extract and utilise.
So how does this discovery affect the GLXP teams aiming for the water detection prize? The rules say:
"The Water Detection Bonus Prize will be awarded to the first team that provides scientifically conclusive proof of the presence of naturally occurring water on the Moon. The detection of water must be made from a vehicle that has landed on the surface of the Moon and must be featured in a peer-reviewed paper to the satisfaction of the Google Lunar X PRIZE Judging Panel."
This presumably means H2O, rather than a hydrated mineral form. The LCROSS impact crater was expected to be around 20 m wide and 4 m deep. We don't know exactly from where the observed water originated, but this at least places some bounds. There are still important questions to be answered, from both a scientific perspective (where did this water come from? how long has it been there?) and a technical one (in what form is it? can it be easily extracted?). But it raises the stakes for the GLXP water prize.
Putting aside the technical difficulties of landing close to, getting inside, and surviving the lack of power and warmth inside such a crater, what are the implications of this discovery to the GLXP teams? In truth, it is hard to say. We still don't know enough about the location and form of the water before the LCROSS spacecraft impacted. Clearly to maximise the chances of success, any lander would need to be able to get below the surface, but the data we have now do not tell us whether we need a mole, deep drill, or simply a scoop.
To conclude, the discovery of significant amounts of water in shadowed polar craters is exciting, but not a game-changer for the GLXP teams. One probably still has to go to a permanently shadowed polar crater, but the chances of success are considerably higher now - the question is, are they high enough to justify the additional effort and risk?
- The Wet Side of the Moon, New York Times op-ed by William Marshall
- Ice on the Moon, a slightly outdated, but useful summary of the search for ice on the Moon