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Water on the Moon?
The prospect of the existence of water on the moon is a well debated topic. But if scientists are able to find enough evidence, it could lead to a whole new sphere of possibilities. On August 9, 1976, Luna 24 launched toward the Moon on a Proton rocket, and nine days later landed safely in the southern part of the unexplored Mare Crisium. 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 a detection level about 10 times above the threshold. 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 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. Since then four other spacecraft have been used to produce data confirming this signal. NASA is fully supportive of the validity of these results. 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: 1. Water vapor delivered to the lunar atmosphere by impacting comets and meteoroids. A fraction of these molecules will propagate in the Moon's atmosphere to permanently shadowed craters that are cold enough to make the molecules stick to these surfaces over geologically long time-scales. 2. Protons delivered by the solar wind and implanted in the lunar soil to react with oxygen (45% of the soil's content) and form water, or at least hydroxyl. 3. Water vapor from the lunar interior. In other words, we know at one time water vapor was reaching the surface in volcanic vents. Why could not some of this vapor simply have seeped up to the soil near the surface? 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. 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. 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. The article has been extracted from a report by by Arlin Crotts in the Space Review Journal. Arlin Crotts is a Professor of Astronomy at Columbia University. He is an astrophysicist working in lunar science and several other fields.
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