Evolution of lunar polar ice stability

The polar regions of the Moon and Mercury both have permanently shadowed environments, potentially capable of harboring ice (cold traps). While cold traps are likely to have been stable for nearly 4 Gyr on Mercury, this has not 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 77 degrees. At this time, lunar polar temperatures were much warmer and cold traps did not exist. Since that era, lunar obliquity has secularly decreased, creating environments over approximately the last 1-2 Gyr where ice could be stable (assuming near current recession rates). We argue that the paucity of ice in the present lunar cold traps is evidence that no cometary impact has occurred in the past billion years that is similar to the one(s) which are thought to have delivered volatiles to Mercury's poles. However, the present ice distribution may be compatible with a cometary impact if it occurred not in today's lunar thermal environment, but in 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, warmer, temperatures. 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 stability and mobility to examine likely fossil remains of past ice delivery (e.g. a comet impact) that could be observed on the present Moon. We attempt to quantify when in the Moon's outward evolution areas first became stable for ice deposition and when ice mobility would have ceased. (c) 2014 Elsevier Inc. All rights reserved.