History of the Lunar Polar Cryosphere

Introduction: Since the discovery of ice in cold, permanently shadowed polar craters on Mercury[1], debate has reopened over the presence of ice in similar environments on the Moon. Though the Moon has similar shadowed craters currently, this has not always been the case. Here we examine a 4.5 billion year history of insolation in the lunar polar environment and the resulting surface and subsurface temperatures. We present thermal calculations for lunar polar subsurface locations to depths of 1 km in order to examine relative stability of water ice over lunar orbital history. The lunar orbit and equator planes both precess in response to torques from the Earth and Sun. The lunar spin axis responds to the changing orbit as determined by its Cassini state. Cassini states are configurations in which the obliquity is adjusted so that the spin pole precesses about the orbit pole in the same period as the orbit pole precesses about the invariable (ecliptic) pole. Such a state is the expected outcome of tidal dis-sipation within the Moon. Tidal dissipation within the Earth drives the lunar orbit outward, which in turn influences the rates of orbit and equator plane precession. The Moon's original , low obliquity Cassini state ceased to exist at about half its current semimajor axis. Thereafter, the lunar spin pole reoriented into a new, higher obliquity Cas-sini state which then evolved into the current low obliquity. During the transition, there was a brief period of even higher obliquity values (up to 77 o). The duration of this transition is not well constrained, as it depends on the dissipation rate within the Moon at that time, but was likely of order 10 4-10 5 years [2,3]. We model the subsurface thermal response to surface radiative forcing for a calculated lunar spin pole history. We examine cases within and surrounding an idealized, currently shadowed, near polar crater that received direct illumination in earlier orbital epochs. We show depths at which temperatures may have been low enough for ice to remain, if it were present, and comment on its mobility. Orbital History: Two slowly varying angles, inclination and obliquity, control the subsolar latitude on the Moon. The inclination is the angular separation between the Moon's orbit plane and the ecliptic. The obliquity is the angular separation between the Moon's spin pole and orbit pole. Inclination variations are a direct response to torques from the Sun (which makes …