Lunar Polar Ice and the Obliquity History of the Moon M.A. Siegler (1), B.G. Bills 2 (2) and D.A. Paige (1) (1)UCLA Dept. of Earth and Planetary Sciences, Los Angeles, CA, 90095, (2)NASA Jet Propulsion Laboratory, Pasadena, CA, 91109 (siegler@ucla.edu) Abstract Water ice is currently stable from sublimation loss in shadowed environments near the lunar poles. However, most current temperature environments are generally too cold to allow efficient diffusive migra- tion into the subsurface by that would protect water from non-sublimation loss. This has not always the case. Higher past lunar obliquities caused currently shadowed polar regions to have warmer thermal envi- ronments. These past environments may have been both cold enough to be able to capture surface ice, but warm enough to drive it into the subsurface. 1. Lunar Orbit History Roughly halfway through its outward migration (due to tidal interaction with the Earth) the Moon was tilted (currently 1.54 o ) up to 83 o with respect to the ecliptic. During this time, all polar craters would ful- ly illuminated at some point in the year and have been far too warm to preserve water ice [1]. This extreme change in insolation is a result of a spin-orbit configuration, within which the Moon cur- rently resides, known as a Cassini state. A Cassini state results from dissipation within the satellite and drives the spin axis of the satellite to precess at the same angular rate as its orbit. As spin precession is controlled by the satellite moments of inertia and orbit precession by its semimajor axis, the satellite is driven into an obliquity that will cause the spin and orbit angular precession rates to synchronize [2]. According to our model, when the lunar semima- jor axis measured roughly 30 Earth radii (RE, cur- rently 60.2) it transitioned between two stable Cassini states, reaching very high obliquities (~77 o ) [1,3]. Since that time (roughly 2.5-3.5 Bya) the obliquity has slowly decrease (to the current 6.7 o ), causing each currently shadowed crater to go through a pe- riod of partial illumination. In addition to the variation of obliquity, the incli- nation and precession of the lunar orbit also varied [14]. This caused dramatic variation in the illumina- tion environment of the early Moon. Recent reanaly- sis [1] of these orbital models combines these orbital parameters to create a history of the axial tilt (maxi- mum yearly sun angle) as seen in Figure 1. Figure 1: Axial tilt (with respect to the ecliptic) history of the Moon. The large increase at approximately 30 RE semimajor axis is a result of a transition between two Cassini States. The current tilt is roughly 1.54 o . 2. Thermal Model Next, we examine the thermal environments re- sulting from the slow orbital evolution since the Cas- sini State transition. Past work [1] modeled the ef- fects of this evolution at a single location (Shackleton crater, 89.7°S, 111°E). Temperatures in this prototyp- ical crater were found to exceed 380K during the peak of the transition, likely erasing any ice existing in the subsurface before this time. Temperatures were not found to cool enough to allow ice deposition (<150K) until roughly 35RE (Figure 2). After about 45RE, temperatures dropped below 90K, too cold to allow ice to be mobile in the subsurface. Figure 2: Thermal history of an example lunar polar crater (Shackleton crater, 89.7°S, 111°E). The grey bar marks rough limits of temperatures where ice would be stable on the sur- face, but mobile enough to diffuse downward before being lost. EPSC Abstracts Vol. 6, EPSC-DPS2011-1615, 2011 EPSC-DPS Joint Meeting 2011 c  Author(s) 2011