Now along comes a team of Japanese space scientists to squash that enticing idea. Led by Junichi Haruyama of the Japan Aerospace Space Exploration Agency (Jaxa), based in Kanagawa, they have analysed data from Jaxa’s lunar orbiter Selene – better known in Japan as Kaguya after a legendary moon princess. Last year the team reported that Selene (which operated from 2007 to 2009) had found rocks made of the mineral anorthosite all over the Moon. This stuff is thought to be formed when meteorites hit the Moon and melt its surface, and the researchers suggested that the Moon might have a thick layer of it several kilometres beneath its surface, created by a massive impact soon after it was formed.
Lunar anorthosite is very pure and bright white, as shown by the lumps of it brought back by the Apollo missions. But here’s the clincher: unlike ice, anorthosite absorbs infrared radiation strongly at a wavelength of 1.25 micrometres, providing a distinctive signature of this mineral. And that absorption was just what was seen by Selene on the inner wall of the Shackleton crater. So it looks as though it isn’t ice.
Haruyama and colleagues don’t rule out the existence of water elsewhere on the Moon, for example hidden away in sub-surface caverns. But they suspect that the amounts might be small. That may still be scientifically interesting, raising questions about how it got there and how it might move around on the surface. Yet without a significant amount of water on the Moon, it is hard to see how any substantial space colony could be established there – the cost of sending up regular water supplies (which would be used not just for drinking but for making hydrogen as fuel) just doesn’t look viable.
That would be a shame, because there’s surely useful science that could be done from a moon base, not least in terms of finding out how this ball of rock formed in the first place (we still don’t really know). It might also literally bring us back down to Earth, forcing us to accept that the universe is a truly inhospitable place, so that we’d better take care to keep our house in order.
Given the history so far, it would be unwise to imagine that this is the last we will hear of water on the Moon. But it would be unwiser still to start planning to build a colony up there.