Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p33d..08b&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P33D-08
Other
[5417] Planetary Sciences: Solid Surface Planets / Gravitational Fields, [5422] Planetary Sciences: Solid Surface Planets / Ices, [5462] Planetary Sciences: Solid Surface Planets / Polar Regions
Scientific paper
The lunar orbital inclination is small (5.1 deg) and the obliquity, or angular separation between spin and orbit poles, is also currently small (6.7 deg). As a result some polar crater floors are permanently shadowed. However, the situation was not always so conducive to the preservation of water, or other volatiles, in the shallow sub-surface of the lunar polar regions. The orbital inclination has varied some, during the orbital evolution, and the obliquity has been quite dynamic. Ward (1975) first identified a past episode of very high obliquity, which occurred during a transition between Cassini states 1 and 2. We examine several scenarios for that transition, and show that many important aspects of the transition depend sensitively upon the lunar degree two gravity field at that time. Unfortunately, past variations in lunar gravity are only poorly constrained. Tidal variations are easily modeled, but do not explain the present value. Previous studies of this problem have included increasing tidal and rotational deformation of the Moon when it was closer to Earth. In addition, we also include the influence of changing obliquity upon the tidal deformation. As the obliquity influences the gravity field and the gravity field also influences the obliquity, there is an interesting feedback loop. The present degree two gravity field is far from that predicted by tidal and rotational contributions. We consider several simple models of its variation with Earth-Moon distance: constant value equal to present value, hydrostatic tidal value plus (constant bias, linear bias, and quadratic bias). All have the present gravity at the present distance. In addition, we also consider these cases with an obliquity dependence. These 7 models yield a considerable range of Earth-Moon distances and peak obliquity values encountered during the Cassini state transition. We will discuss the implication of these various scenarios for retention of polar volatiles. More generically, we conclude that the epoch, duration, and peak obliquity during transition are all sensitively dependent upon poorly constrained values for lunar gravity coefficients and dissipation within the Moon.
Bills Bruce G.
Moore William B.
Siegler Matthew A.
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