Lunar Water

A crucial factor in deciding when humans return to the Moon will be the presence of water. Water is heavy and therefore very expensive to carry from the surface of the Earth into space, but man can't survive without it. However, if you can find a supply of it at your destination you can make a significant reduction on launch costs (or put the payload saved to other uses). In addition, the water can be broken down via electrolysis into hydrogen and oxygen, both of which can be used as rocket propellant.

Ordinarily, water molecules cannot exist on the lunar surface because they would be dissociated into hydrogen and oxygen atoms by solar ultraviolet radiation. These atoms would then immediately escape the lunar surface and be lost to space. However, measurements carried out by first the Clementine and then the Lunar Prospector probes in the 1990s suggested indirect evidence for water-ice deposits in the permanently shadowed regions of the lunar poles. (Because of the awkward viewing angle, the lunar poles cannot be seen very well from the Earth.) The evidence is indirect because, by definition, the permanently shadowed regions are in total darkness and cannot be imaged by conventional sensors.

The Clementine evidence was derived from radar observations which have a unique signature when reflected off ice. The Lunar Prospector mission followed this up by analysing the polar regions with a neutron spectrometer; the number of medium-speed and "slow" neutrons detected is consistent with the presence of hydrogen in the lunar surface. The results indicated a hydrogen concentration of 500 parts per million, from which a water concentration of 0.5 percent is inferred. (It is not unreasonable to assume that the hydrogen detected is locked up with oxygen to form water.)

How much ice does this concentration translate to in total? A conservative estimate is at least 10 million tonnes.

The remaining question is whether this 0.5 percent is localised in the form of discrete deposits of pure ice hidden in individual craters, or distributed evenly across the polar regolith as a permafrost. The word "permafrost" conjures up images of the lunar surface glittering like a pavement on a winter's morning, but the reality is that even a diluted concentration of 0.5 percent is still drier than any terrestrial desert.

Obviously, localised deposits would be more accessible for future expeditions to exploit (and this is where the available evidence seems to be pointing), but how do we find out for sure?

Check out the Clementine and Lunar Prospector web sites for further information regarding these missions and the search for lunar ice.

Postscript: Mercurian Ice

Remarkably, the arguments for lunar water have been strengthened by radar observations which strongly suggest the presence of ice at the polar regions of the planet Mercury - supposedly the hottest and driest planet in the solar system. These deposits appear to lie hidden inside craters that are permanently shaded from the Sun. The existence of lunar and Mercurian ice now seems almost beyond doubt, but it does raise the question of how it got there in the first place. The best answer appears to be the retention of water vapour molecules from cometary impacts. Over millions of years, a significant fraction of these molecules could have been fortunate enough to settle in cold regions that never receive any sunlight.

When will we learn the truth about Mercury and its mysterious ice deposits? To date, Mariner 10 is the only probe to have visited Mercury, but 55% of the planet is still unmapped and will probably remain so for the near future. A return mission was proposed as part of NASA's low-cost Discovery program, but it lost out to more glamorous missions such as Deep Impact, a probe which will fire a large copper projectile into a comet.

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