Water on the Moon
by Arlin Crotts
|“Extraordinary claims require extraordinary evidence” is the statement often heard in response to any result implying lunar water. To what extent is water on the Moon really an extraordinary claim?|
What it brought back was very special. The core sample was found by scientists M. Akhmanova, B. Dement’yev, and M. Markov of the Vernadsky Institute of Geochemistry and Analytic Chemistry to contain about 0.1% water by mass, as seen by absorption in infrared spectroscopy (at about 3 microns wavelength), at a detection level about 10 times above the threshold. The trend was for the water signal to increase looking deeper below the lunar surface. The original title of their paper in the February 1978 Russian-language journal Geokhimiia translates to “Water in the regolith of Mare Crisium (Luna-24)?” and in the English-language version of the journal “Possible Water in Luna 24 Regolith from the Sea of Crises”—but the abstract claims a detection of water fairly definitively. The authors point out that the sample shows no tendency to absorb water from the air, but they were not willing to stake their reputations on an absolute statement that terrestrial contamination was completely avoided. Nonetheless, they claim to have taken every possible precaution and stress that this result must be followed up. The three Soviet lunar sample return missions (Luna 16, 20, and 24) from 1970 to 1976 brought back a total of 327 grams of lunar soil. The six Apollo lunar landing missions in 1969–1972 returned 381,700 grams of soil and rock. By 1978 it was widely held that the Apollo samples contained virtually no water, typically measuring in parts per billion, orders of magnitude less than the 1,000,000 parts per billion seen by Luna 24. The Soviet findings were completely ignored. (According to my searches, no other author has cited their work.)
Recently scientists learned that, for a major subclass of Apollo samples, the no-water result was seriously mistaken. Alberto Saal at Brown University and his collaborators showed, in the journal Nature in 2008 and other work in 2009, that volcanic glasses from magma originating deep in the lunar interior contain water at levels up to 70 parts per million. Furthermore, they calculate that most of this water was lost in the venting process, so probably started out at levels approaching one part per thousand, nearly at the level seen by Luna 24. Furthermore, the tendency has been for lunar scientists to think that the same processes that drove off most of the water from the Moon drove off other volatile, light elements. For this reason it is also surprising that Saal and collaborators are finding up to 0.09% sulfur (probably as sulfur dioxide), even before one accounts for loss processes. One could say, shockingly, that at least parts of the lunar interior are rich in sulfur, and hardly more depleted in water than many minerals on Earth, such as basalts along mid-ocean ridges. Few scientists have found cause to dismiss to the Saal et al. compositional measurements. They seem sound. Other researchers (such as Francis McCubbin and coworkers) have found supporting results.
In 1996 to 1999, Faith Vilas and collaborators employed methods akin to those used on the Luna 24 samples, and identical to those used to search for water on asteroids, to investigate whether hydration might be indicated on the Moon’s surface near the poles. By this time, investigators had come to accept that water might be found in the very coldest, permanently-shadowed craters near the lunar poles, but Vilas and collaborators found a hydration signal over vast areas of the polar region, even areas that might be heated above 100°C during the lunar day. This was a surprise, and one could argue that other interpretations were possible, but water seemed the simplest explanation. Apparently, these authors could not convince any journal to publish this result for at least a decade. In 2008 (in a not-widely-read Japanese journal), this team was able to publish a more subdued version of their original findings.
On the one hand, the primordial glancing collision theorized to have created the Moon from the temporarily disrupted Earth 50 million years after its first formation likely heated the debris to thousands of degrees above absolute zero, driving off most of the volatiles. On the other, it is possible to find loopholes in this statement, even if the model is largely true. “Extraordinary claims require extraordinary evidence” is the statement often heard in response to any result implying lunar water. This is despite our knowledge of water present on nearly every other large body in the Solar System. To what extent is water on the Moon really an extraordinary claim?
Extraordinary evidence was provided by the Moon Mineralogy Mapper (M-cubed), a hyperspectral imaging instrument funded by NASA for India’s Chandrayaan-1 lunar orbiter. M-cubed made use of the same 3-micron signature utilized for Luna 24. Since then four other spacecraft have been used to produce data confirming this signal (in addition to some of the first indications in 1998 from the Lunar Prospector epithermal neutron experiment). NASA is fully supportive of the validity of these results. These are data that everyone seems prepared to believe, although I will say that in addition to all of the other, earlier sources, I have seen additional, convincing evidence. (Ironically, much of this evidence is associated with sites chosen for later Apollo landings that were then cancelled.) One must be willing to relax the “extraordinary” requirement only slightly in order to see the patterns, however. Hopefully, now we can have a reasonable conversation about the nature of water on the Moon.
There are several possible sources of lunar hydration, but three primary hypotheses:
|We have three basic hypotheses, all predicting different surface and depth distributions, in ways that will likely prove critical for harvesting this water for use by humans and rocket engines.|
Water from comets will collect in these special cold traps near the poles, but the hydration signal is found over the whole polar region. This is more like what is expected from the third option, because the soil just below the surface near the poles is very cold. Water molecules become motionless in the soil—maybe not as long as molecules in the permanent cold-trap craters, but they do not need to. Water vapor from the interior will tend to get caught at least several meters down, where an individual molecule will need to unstick and re-stick many times before reaching the vacuum of the surface. Regarding the solar wind-implanted protons, not until the recent M-cubed results did scientists consider the possibility that molecules might stick to surfaces in the polar regions outside the cold-trap craters, surfaces that are heated to high temperatures every month. It is still not clear that this idea will work. Water vapor from the interior interacting before reaching the surface, however, is always insulated from these hot surface temperatures.
As I write this, the LCROSS spacecraft have taken their fatal plunge into the lunar polar crater Cabeus as planned, and excavated a much smaller ejecta plume than many had anticipated. Given the weakness of this signal, it will probably be some time before we know what the water composition of this material might be. If it is high, in the range of a percent or more, this bodes well for a model in which water permeates the soil to significant depth, as indicated by the internal-origin option. Of course water vapor seeping to the surface might be patchy, as it is for volcanic processes on Earth. If we see little water, I am not sure what it tells us. We may have just hit the wrong patch.
LCROSS has come and gone. There are vanishingly few spacecraft in the works that will address the problem of the origin of water and its distribution in depth and across the lunar surface. We have three basic hypotheses, all predicting different surface and depth distributions, in ways that will likely prove critical for harvesting this water for use by humans and rocket engines (as liquid oxygen and liquid hydrogen): scraping up solar-wind implanted protons over the surface at large will require radically different equipment than plowing the surfaces in permanently-shadowed craters. If the water comes from the interior, we probably need to drill down a few meters to reach it, where it is probably in the form of ice that can be heated and extracted. In any case we need to look at the surface and down perhaps 15 meters (probably with ground-penetrating radar) in order to locate the best places to mine water. A dry hole might easily cost a billion dollars. We also cannot pollute the neighborhood with our own rocket exhaust before we find out where and how to access this water, not to mention studying its scientific implications. Currently our exploration plans, requiring robots before we send humans, are nowhere near such a capability. We have not been thinking in these terms. Now that we know there is water on the Moon, the solar system may now be open to us. Hopefully now all what will limit us is the equipment that we can send there, not our inability to accept the evidence before us that water exists on the Moon.