object vaporizes upon impact, creating a temporary thin water vapor atmosphere on the Moon. Coldtrapping on the dark side of the Moon tends to thermalize many molecules before they are photodissociated by solar UV and X-rays and sputtered to escape velocity by solar wind particles. A fraction of these thermalized molecules, however, esscape this fate by being permanently coldtrapped in the perpetually shadowed lunar polar craters. Arnold estimates the buildup in 2 x 109 years to be 1013-1014 kg. Subsequently, careful calculation by Lanzerotti et al. [8] indicated that the present input of water ice contributed by incoming comets and Apollo objects is exceeded by loss mechanisms: meteorite impact vaporization cosmic ray flux and sputtering by solar wind ions deflected by the Earth’s geomagnetic tail. The other sources of ices proposed by Arnold—outgassing associated with Transient Lunar Phenomena (TLPs) and solar wind Fe reduction in the regolith—are presumed minor in comparison. Although the loss rates are still debated [9], if Lanzerotti et al. are correct, the Moon would have no polar ices from present cometary flux. However, did the present day impact rate always prevail? Several hypotheses may be advanced to suggest this was not the case. Suppose that the cometary flux in the early solar system was vastly larger, following the theory of cometary formation proposed by Whipple [10], If enough ice was formed, perhaps some primordial ice remains. Unfortunately, retrocalculations of tidal effects [11] show changes in the Moon’s obliquity since early Archaean times, although the precise calculation depends critically on the tidal lag angle, 6. Some idea of when the Moon’s obliquity changed enough to evaporate any possible primordial ices can be gleaned from lunar orbital parameters derived from a study of tidal sandstone laminae [12]. The value of the Earth-Moon distance of 54.53 Earth radii 6.8 X108 years ago obtained in this study infers a <5=2.5° when compared to MacDonald’s retrocalculations [11] and an obliquity change of 1.5° since then. If <5 remained constant (it depends critically on the elasticity of the Earth) the obliquity of the Moon was 11° only 1.2 billion years ago. Even if the MacDonald model of tidal braking is oversimplified because it does not take into account the detailed shape of the Earth’s surface, unless there are large errors, the obliquity of the Moon was too large at a point in time certainly after any great primordial cometary bombardments postulated in the early Archaean (pre-Nectarian on the Moon) had ended. The result is, unless we assume significant burial by impact ejecta, all the primordial ices deposited in the lunar paleopole coldtraps have since been exposed to direct sunlight and rapidly vanished. Yet another mechanism is possible to secure present-day polar ices. Recently the Nemesis Hypothesis has been advanced to explain periodic extinctions (for background see [13] and [14]). These mechanisms require a brown dwarf companion star periodically perturb the Oort Comet Cloud causing a rain of comets into the inner solar system. Not only the Earth would be affected; the Moon would also suffer this episodic cometary bombardment. As the most recent one ended, according to this Hypothesis, only 1.3 X 107 years ago, cometary ices might yet remain in the lunar polar craters against the loss mechanisms. In addition, we do not know the contribution to the water-ice input from past TLP when so little is known of it. TLPs have been observed at observatories [15] and much more frequently by amateur astronomers [16, 17]. A summary of Apollo lunar atmosphere experiments [18] show little, if any, contributions from TLPs now, and so we will disregard it, but only provisionally, because we have no certainty of their fundamental causes or what they were like in the past.
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