Siegler, Matthew - Evolution of Lunar ice stability

Abstract: 
The polar regions of the Moon and Mercury have similar permanently shadowed environments, potentially capable of harboring ice. However, this has not always been the case for the Moon. Roughly 3±1 Gya, when the Moon is believed to have resided at approximately half of its current semimajor axis, lunar obliquities have been calculated to have reached as high as 77o (Goldreich et al. 1969; Ward, 1975; Wisdom and Touma, 1994; Siegler et al., 2011) This is due to a dissipation driven spin orbit coupling known as a Cassini State. Combined with the modeled orbital inclination for this time period, this left the lunar poles with a maximum solar illumination angle (here termed solar declination) of approximately 83o (Siegler et al. 2011). Lunar polar cold traps did not exist. Since that era lunar obliquity has secularly decreased, creating environments over approximately the last 1-1.5 Gyr (assuming near current recession rates) where water ice, if delivered to the Moon, should be stable. 
     In analogy to Mercury, where evidence points to nearly pure ice deposits likely deposited by a large cometary impact within the last several 10’s of Mys (Crider and Killen, 2005), we would expect similar thermal environments on the evolving Moon to also retain relatively pure water ice for 10’s to 100’s of Mys. Though evidence points to a lack of Mercury-like pure ice deposits on the Moon (Campbell et al, 2006; More refs), this analogy makes it difficult to explain how all ice from of any similar impact over the past 1.5 Gyr could be lost. Essentially, to explain the paucity of ice in locations where it would be stable in the current thermal environment, one must claim that no comet similar to the one(s) which struck Mercury (assumed in the past few 10’s on Myr) has struck the Moon in several hundred Myrs or longer. 
     One hypothesis to explain this discrepancy might be that such a cometary impact occurred not in today’s lunar thermal environment, but a past one. If ice were delivered during a past epoch, the distribution of ground ice would be dictated not by present day temperatures, but rather by these ancient temperatures. This ancient ice, buried and mixed into the regolith by impact gardening. In this paper, we attempt to recreate the thermal environments for past lunar orbital configurations to characterize the history of lunar environments capable of harboring ice. We will develop models of ice mobility and degradation to examine likely fossil remains of past ice delivery (e.g. a comet impact) that could be observed on the present moon. We then compare this to interpreted geographical distribution of lunar ground ice from existing data sets.