Spectral cube images from Chandrayaan-1
There’s a lot of internet chatter about the recent discovery of water on the Moon – or rather, the discovery of LOTS of water on the Moon. Most of the reactions have focused on the press embargo that was (once again) thrown to the wayside by some naughty journalists, or been rather muted at best. Water on the Moon is not such a big deal: just from looking at the Moon you can tell that plenty of stuff has smashed into it in the past; something was bound to have left some water behind at some stage.
But having listened to the NASA briefing this afternoon, I’m actually finding this quite an interesting discovery. Essentially the news is that the spectral signatures of water and hydroxyl (OH) have been spotted all over the Moon’s surface, including the hot Sun-facing dayside, by three independent missions, Cassini, Deep Impact (oh sorry, EPOXI – when did they rename that?) and Chandrayaan-1. The water is not actually liquid and visibly present, but there’s enough of it that it could potentially be extracted (cue the sci-fi enthusiasts and billionnaires to plan lunar colonies).
Water absorption feature in the spectra from Cassini and Chandrayaan-1
As the Moon is presumed to have lost all its volatile materials, including water, in its formation process, the question remains as to how the stuff actually got there. Could continued pumelling by water-rich comets and meteors have produced such a global distribution of surface water? The water and hydroxyl signals show a daily cycle of loss and replenishment – a Moon day of course lasting about an Earth month.
One scenario for in-situ formation of the water molecules is the interaction between solar wind particles, which include hydrogen atoms, and oxygen molecules in the lunar surface. If water can be formed on the Moon by such a process, it could well be present on other oxygen-rich bodies in the inner solar system that don’t have an atmosphere (or I guess a magnetic field) to shield the body from the solar wind particles. That includes Mercury and a bunch of asteroids.
The process behind this discovery is also interesting from a technical point of view, and drives home once again the importance of calibration. The first signs of water and hydroxyl leading to this discovery were spotted back in 1999 in calibration data for the Visual and Infrared Mapping Spectrometer (VIMS) on board Cassini. But not until last summer (2008!) was the instrument sufficiently well calibrated that scientists could be sure that the water signal wasn’t originating in the telescope or instrument itself.
In science, working on the calibration of an instrument, like writing software, is one of those jobs that is looked down on by “real” scientists and generally not considered worthy enough to be funded and supported at the level it should be. But an instrument is only ever as good as its data are characterised, and too many excellent instruments have been let down by poor calibration. I’ve no idea why the VIMS calibration took so long, I’ll read the papers to see if the authors say more about it, but it’s nevertheless an important message to remember.
This discovery is hot off the press and where the research goes from here is unclear. There will certainly be follow-up observations to get more information on the distribution of water molecules on the Moon, the variability in abundance, and the processes that govern their formation and destruction. Whatever direction the work takes, it’s fair to say that these results have rejuvenated the lunar research field, and exciting findings are set to follow.
Nice work to space newcomer India too, for making such an important conribution with its very first space mission, Chandrayaan-1.
Images: ISRO/NASA/JPL-Caltech/Brown Univ. (top), NASA/ISRO/JPL-Caltech/USGS/Brown Univ. (bottom)
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