compared to ~500 Torr for water ice, the water sublimes around 3 X 108 times faster, leaving the less volatile sodium and other compounds behind. The vapor from the more volatile water and carbon dioxide cometary ices would be partly recondensed in a shadowed area elsewhere and partly destroyed by photodissociation and solar wind proton sputtering. The same processes would act on the less volatile cometary solid residue, but they would have a smaller effective surface and would shield lower level molecules. The expanding unshielded low pressure vapor component would be by comparison rapidly photodissociated unless it recondensed; upon sunrise the process would begin anew unless it were permanently coldtrapped in permanently dark polar craters. The continued production of vapor from the less volatile cometary recondensate residue occurs much more slowly from photodissociation from solar UV and X-ray photons and sputtering by solar wind ions. This rate can be estimated. Considering sodium compounds, allowing the fraction of sodium in the original comet was around 3%, after water and carbon dioxide effervescence the fractional amount f rises to around 10%. Following Morgan et al. [32] we can estimate the local vapor production rate (for sodium, or example): 0p is the local proton flux, /a is the atomic percent element (~/) and Yp the average yield per solar wind proton. The r subscript applies to the solar UV and X-ray photons above the dissociation potential of the compound x=hvx. £ is the zenith angle of the sun. For sodium, this gives us an expected sputtering of ~1.5 X 106 Na stoms cm"2s-1 and, near the poles for photodissociation, ~1X1O8 Na atoms cm-2s-1, at C=89.5°. One can compare this to the calculated production of sodium vapor from sodium- bearing lunar minerals in the regolith from sputtering and photodissociation. Using the values of Morgan et al. for the regolith,/= 0.0033. Thus, all other things being equal, in the temporary lunar penumbral zones near the terminator on the lunar equator, for
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