Water find adds to big picture
LAST week NASA made the tantalising comment that it was about to release information about a significant discovery about the moon.
On Monday we learned what it was: water had been detected on the sunlit side of the moon. It was already known that there was water ice in the polar regions, but now it has been detected well away from the lunar poles.
There is quite a history of thoughts and observations relating to water on the moon. The Greek philosopher Plutarch considered the presence of water there.
Even now we still use the names of “seas’’ on the moon called mare, the Latin word for “sea’’. Possibly the most famous of these is Mare Tranquillitatis, meaning Sea of Tranquillity. However, these dark areas, clearly visible to the unaided eye, are not seas, but large areas of volcanic lava.
The famous physical chemist Harold Urey wrote about the possibility of lunar water, though some of his assertions were incorrect and he was met with opposition.
In his 1967 paper simply titled Water on the Moon, he noted that the subject had been received “mostly with great scepticism”, mentioning that among the negative comments he received, there had even been a suggestion that the liquid could be vodka, in order to make fun of his ideas.
In the 1990s two spacecraft, Clementine and Lunar Prospector, both detected evidence for lunar water in the polar regions, and in 2009 the
LCROSS spacecraft flew through a plume of material excavated by the deliberate collision with the moon of a Centaur rocket stage, and detected the presence of water.
Those three missions provided evidence of water only in the polar regions.
In those parts of the moon, there are permanently shadowed regions where water, if present, will never be exposed to sunlight and will exist undisturbed as water ice.
Another mission in 2009 carried a detector that showed evidence of water well away from the poles.
However, the authors of the current research point out that that detection was not necessarily water, it could have been hydroxyl molecules (with the formula OH instead of H2O). So the detection announced this week is the first unambiguous detection of water molecules in sunlit areas away from the poles.
To do this, the research team, led by Casey Honniball of the NASA Goddard Space Flight Centre, observed the moon in infra-red light at a wavelength of six microns, much longer than the eye can see. A micron is a thousandth of a millimetre. Visible light has a wavelength range between
about 0.4 and 0.7 microns. The wavelength of six microns is very significant, because it is a wavelength of radiation emitted by H2O molecules.
The problem is that our atmosphere absorbs a lot of infra-red light, especially in certain wavelength ranges. This is especially true around a wavelength of six microns. So the researchers had to observe from a great altitude, above most of the atmosphere, in order to have a clear view.
Boeing 747 aircraft have been in the news quite a bit this year because of their retirement from service. However, still very much in use is a specially equipped one called SOFIA — the Stratospheric Observatory For Infra-red Astronomy, which is operated by NASA and the German Aerospace Centre.
Using SOFIA, the researchers found water emissions in the region of the large crater called Clavius, which was named after Christopher Clavius (1538-1612), a German astronomer and mathematician.
Of course, the question arises: How could water exist in the sunlit areas of the moon? The researchers think that it may be trapped inside glasses, or in tiny locations between grains of material.
Knowing more about the distribution of water on the moon is important. It helps us to piece together the overall history of our solar system, but it also has practical value for future human visits to the moon. Water is needed for human consumption, and it can also be used to manufacture rocket fuel.
It is fortunate that SOFIA survived suggestions in 2014, and again early this year, that it be withdrawn from service as a cost-cutting measure.