GUEST COLUMN (continued from front page) of the Lunar poles which are never exposed to the sun may be at temperatures below 120 °K. Such areas might then function as cold traps, collecting and permanently freezing any volatiles which may be present which are solid at these temperatures. This would certainly include water and possibly carbon dioxide as well. Arnold (Ref. 3) estimates that the temperature might be as low as 40 °K. If this is the case, even methane and other low- temperature volatiles might be trapped. The permanently shadowed areas: the bottoms of deep craters, rilles, crevices, etc., are estimated to total as much as 2xl05 km in area. Such areas might be found as far from the poles as 60 ° latitude although the geometry of the situation obviously favors locations nearer the poles. Two sources are usually postulated for Lunar water. One is simply the primordial cloud from which the Moon originally condensed. The prevalence of water in the solar system gives clear evidence that water was a major constituent in the makeup of the planets. It would be remarkable indeed if the Moon had condensed without water. In the case of the rocky inner planets most of this original water was outgassed and a good deal was lost. In the case of Earth and Mars however some was retained. The Moon is small enough that it is probable that most of the primordial water was lost except perhaps at some depth in the interior. This water could occasionally be brought to the surface by vulcanism or major meteoroid strikes. Much of this water would be lost to space. However as it spread around the planet, some would be trapped in the shadowed regions. It is interesting to note that this may still be taking place to some extent. If the rather controversial Lunar Transient Events are indeed volcanic venting then the process is still at work. Another postulated source of water is cometary impact. Such bodies, composed of a high percentage of water and other volatiles would be vaporized by the impact. The vapors would migrate around the Moon (Ref.4) restrained by its gravity until they escaped due to random motion of the molecules heated by the sun, accelerated by decomposition of molecules such as water into its constituent elements by solar ultraviolet. During the time required for this to take place some fraction of the original molecules would be trapped in the shadowed regions. A critical question in regard to this theory is how long the Lunar poles have been where they are. Obviously most of the outgassing of the original water must have occurred early in the Moon’s history. Similarly most of the impact cratering was concentrated in the first one or two billion years. If the Lunar poles were in their present location 4.5 billion years ago and there has been no intervening migration then the cold traps have been in operation for essentially all of the Moon’s history including the Mare-forming events thus leading to the maximum possible accumulation of volatiles. It is generally considered improbable that this is the case. In Ref.3 Arnold postulates that the poles may have become fixed at the present sites some 3 billion years ago. This is well after the major outgassing phase and after the Mare-forming events. Based upon the 3 billion year number, an estimated outgassing rate, and the estimate of the percentage of migration of molecules and cold trapping from Ref. 4, Arnold estimates that the amount of ice accumulated might be about 1 meter in thickness over the shadowed area. This equates to 100 cubic km or about 10'' tons of water. It is clear that, even if these estimates are optimistic by two or three orders of magnitude, the amount of water which may be available dwarfs anything which we can even conceive of importing from Earth or from other space sources. One should not, incidentally, expect that analogs of the shining polar caps of Earth lie hidden in the dark regions of the Moon. The surface of the Moon is constantly being “gardened” by micrometeoroid impact. This stirring and overturning would tend to intermingle the dust and rock. The negative side of this is that each impact will vaporize some water of which some may be lost. On the positive side, a layer of dust would accumulate on the surface in this scenario, providing thermal insulation and protection from later impacts. In fairness, one must end this discussion of possible cold trapped water by pointing out that some scientists contend that there is no such water. Lanzerotti, for example, contends that sputtering by solar wind particles would remove the cold trapped water as rapidly as it is laid down. Another less prominent theory concerning lunar water offers that it could be widespread. Muller (Ref. 5) notes that the Lunar subsurface temperature is below 0 ° C at fairly shallow depths and that a 100 meter overburden of Lunar soil could preserve ice for geologic time. Thus if the Moon so developed that a percentage of the primordial water remained in the interior while the surface and top tens of meters was thoroughly outgassed and desiccated, large amounts of water could be available near the surface in many areas of the Moon. Near the surface, water could only exist as ice, at greater depth the temperature begins to rise again and liquid water could possibly exist at some depth beneath an overburden of dust, rock, and ice. While it would seem difficult to detect liquid water at such a depth without drilling, we have already flown an instrument in Lunar orbit capable of making such a detection. In fact it may have done so. The radar sounder flown on one of the later Apollo missions had the capability of penetrating to considerable depth through very dry rock. It would also penetrate through ice buried in the rock without detecting the difference since the dielectric coefficients are very similar (3 for ice, 4 for regolith). It would not penetrate through liquid water which has a high dielectric coefficient (about 80). In fact the sounder would see a very bright reflection off a water layer and would be unable to see beyond it. The sounder observed exactly this phenomenon in both Mare Crisium and Mare Serenitatis. The reflecting layer came up from deep beneath the mare to within 200 to 1000 m of the surface near the shore. The subsurface water explanation of the observed data is not accepted by the scientific establishment and it may well be that other, more prosaic explanations exist for these observations. However the existence of a liquid water layer is at least a possibility and worthy of further investigation. Significance of Lunar Water The apparent lack of Lunar hydrogen was discussed briefly in an earlier section. This lack has a profound significance in regard to long term utility of the Moon as a support base for space operations. Simply put, hydrogen is the basis for all high energy fuels for rocket engines. It may be used by itself or in various energetic chemical compounds such as methane (CH4) and other hydrocarbons, metal hydrides such as diborane (B2H6), etc. Hydrogen holds this unique position because of its very low atomic weight and highly exothermic reactions with most oxidizers. In fact very few practical fuels exist which do not contain hydrogen. One such is carbon monoxide (CO). Table 1 presents performance data for several propellant combinations for comparison. While the arguments presented in the preceding paragraph are not exhaustive, they clearly indicate that, if propellant manufacturing in large quantity is to take place on the Moon, the presence of hydrogen is most important. If only oxygen can be produced as a result of ore reduction, it will still be significant but clearly of less importance than ability to generate both oxidizer and fuel. The significance of water in the life support system of a Lunar Base is obvious at the simplistic level in that human and most other forms of Earth life depend upon the availability of water. However, in terms of basic life support, the amounts are relatively small on a per-person basis and closure of the water loop in a life support system is one of the easier aspects of life support system design. Thus for simple survival, availability of Lunar water may not be of great significance since it could be imported from Earth except perhaps in the case of very large nonulations. TABLE 1 Propellant Performance Liq. oxygen/carbon monoxide (CO) 275 sec Liq. oxygen/RP-1 (kerosene) 351 sec Liq. oxygen/hydrazine 367 sec Liq. oxygen/hydrogen 492 sec
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