The Role of Secondary Volatiles For all the above reasons, one cannot negate the existence of lunar ices a priori. How then is it possible to discern polar ices, if the dark craters are inscrutable from the Earth? A gamma ray spectrometer aboard a lunar probe would be the best solution. Such a probe was used by the Apollo missions to map the geochemistry of the equatoral zones of the Moon [19] and of recent proposals the most promising is by Canough [20], to use a surplus Apollo gamma ray spectrometer on a relatively inexpensive polar probe. But, in the absence of committed lunar missions it would be desirable to attempt some indirect Earth-based experiment. An indirect Earth-based method is possible if we consider the roles of secondary volatiles other than H2O and CO2 ices. ‘Secondary volatiles’ here means compounds less volatile than H2O and CO2 ices, and sodium compounds are chosen above all cometary secondary volatiles because of the ease of spectroscopic detection. Resonant scattering of sunlight in the D-line doublet (5889.5 and 5895.5 Angstroms) produce emission lines, which, because of low pressure, are wholly within the solar sodium absorption D lines unless shifted from radial velocity. This is near the maximum intensity of solar radiation and easily detected from ground-based telescopes. Ionized sodium has been detected in numerous cometary spectra [21, 22, 23, 24, 25] and was even responsible for the yellow tinge in Skylab astronaut drawings of the comet Kohoutek [26], The parent molecules of this Na+ are not unambiguously identified but are likey NaCl, Na2O [27] and, I add, NaOH. That sodium compounds are less volatile than the carbon dioxide and water ices is further borne out by the weakness of doublet emission beyond 0.4 AU heliocentric distance [28]. The sodium in cometary materials has been estimated as between 3-10% [29] or as low as 0.1% by weight based on carbonaceous chondrite meteors [30,31]. Regarding the carbonaceous chondrites as volatile-depleted relative to silicate compounds in comparison with a ‘pristine’ cometary nucleus, one can use 3% as a safe magnitude, so for Arnold’s 1014 kg. we have 3 X 1012 kg of Na by weight or 7.8 X 1037 atoms. These secondary volatiles would not only reside inside permanently shadowed lunar craters with other cometary ices, but also in zones where the sunlight shines only occasionally, mostly hidden by topography, and always at a low sun angle. Cometary debris would be coldtrapped there too while the area would be in shadow, but only the less volatile comet compounds (including the sodium compounds) would endure the intermittent sunlight for any reasonable time. I call these areas of intermittent, low angle sunlight near the poles ‘lunar penumbral zones’ and it is expected that these areas would be enriched in less volatile cometary compounds, compared to other visible areas on the Moon, if cometary ices are also trapped in the permanently dark lunar polar craters (see Fig. 1). To quantify this process, suppose, ideally, we have a cometary impact. As with the case of hypervelocity impacts, the comet is vaporized but recondenses in the dark poles, and other places on the moon where the sun at that time is not shining. As the sun shines on these cometary recondensates, the rate of loss of water and carbon dioxide ices compared to sodium compounds is dominated by thermal vaporization. For example, if the parent sodium compound is NaCl: where m is the molecular mass, N the number of molecules and p(T) the vapor pressure at T Since the vapor pressure of NaCl in lunar sunlight is ~3 X 10~6 Torr
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