Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p31b1404s&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P31B-1404
Physics
5418 Heat Flow, 5422 Ices, 5450 Orbital And Rotational Dynamics (1221), 5462 Polar Regions, 6250 Moon (1221)
Scientific paper
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 plane precesses in response to torques from the Earth and Sun. 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 pole. Such a state is the expected outcome of tidal dissipation within the Moon. Tidal dissipation within the Earth drives the lunar orbit outward, which in turn influences the rate of orbit plane precession. The Moon's original low obliquity Cassini state ceased to exist at a semimajor axis of about 34 Earth radii. Thereafter, the lunar spin pole reoriented into a new, higher obliquity Cassini state which slowly evolved into the Moon's current low obliquity [Ward, 1975; Siegler et al, 2007]. During the transition, there was a brief period of even higher obliquity values (~70 degrees). 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 104-105 years. Though it is clear the lunar surface environment would be thermally unstable for ice during this transition, the same is not necessarily true for the polar subsurface. Furthermore, the period before this transition may have been thermally suitable for collecting volatiles in the near surface. It is therefore a worthwhile study to explore where early near surface ice might have relocated in response to orbital forcing. Here we relate a modeled subsurface thermal response to surface temperature forcing for a calculated lunar spin pole history. We examine the 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 would have provided a safe haven for ice if it were present and comment on its mobility. One of the motivations of this study is to understand the contrast in polar volatile inventories between the Moon and Mercury. Radar observations and thermal models support the conclusion that permanently shadowed polar craters of Mercury contain abundant near surface water ice. Thermal modeling shows that the Moon should also currently have near polar environments capable of preserving surface ice [Vasavada et al, 1999]. However, there is little conclusive evidence for surface lunar ice [Campbell et al, 2006]. A plausible explanation for this is that both bodies once had ice, but differing obliquity histories made lunar surface ice unstable and mobile, while mercurian ice remained unchanged. The present obliquity of Mercury is small, and has likely always been so. In contrast, the Moon experienced this period of very high obliquity, during which presently shadowed polar regions would have been fully illuminated.
Bills Bruce
Paige David
Siegler Matthew
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