Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p22b..03n&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P22B-03
Other
[5417] Planetary Sciences: Solid Surface Planets / Gravitational Fields, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [6281] Planetary Sciences: Solar System Objects / Titan
Scientific paper
For the terrestrial planets, combined analyses of gravity and topography have greatly improved our understanding of these bodies' interiors [1]. The spin state and orientation of a planetary body can also be diagnostic of its internal structure [2]. Recently acquired topography [3], gravity [4] and spin pole constraints [5] now permit these kinds of geophysical analyses at Titan. Titan's degree-two gravity coefficients, but not those of its topography, are in the 10/3 ratio expected for a hydrostatic body. One explanation for this discrepancy is the existence of a floating isostatic ice shell whose thickness varies spatially due to tidal dissipation [6]. Shell thickness variations can result in slow non-synchronous rotation [7]. Furthermore, such variations will affect the gravity, an effect that should be taken into account when using gravity to calculate Titan's moment of inertia [4]. The relationship between the degree-three gravity and topography can be used to place constraints on the thickness and rigidity of the ice shell. Based on the inferred heat fluxes of [6], Titan's ice shell is unlikely to be less than 90% compensated at degree three. The measured degree-three gravity [4] and topography [3] coefficients show a strong correlation (r=0.84). For a completely compensated ice shell, the implied shell thickness is about 350 km, while if the shell is 90% compensated the thickness is 250 km. These shell thickness estimates significantly exceed those based on theoretical models [8,9] and surface topography [6]. One possible explanation for this discrepancy is that there are other sources of degree-three gravity (e.g. bumps on the presumed silicate core) that do not contribute significantly to the surface topography. Further gravity observations will help to resolve this issue. If a satellite's spin and orbit poles remain coplanar as the latter precesses around the invariable pole, the satellite is said to occupy a damped Cassini state and the obliquity (angle between spin and orbit pole) is diagnostic of its moment of inertia [10]. Titan's spin pole is very nearly coplanar with the orbit and invariable poles, suggesting occupation of a Cassini state. Its obliquity of 0.32 degrees [5] corresponds to a normalized moment of inertia of 0.45, much larger than the value of 0.34 derived from gravity [4]. This discrepancy is probably due mostly to decoupling of the ice shell from the interior by an ocean, though excitation of the obliquity by the atmosphere [11] or ocean may also play a role. [1] Wieczorek, M.A., Treatise Geophys. 10, 165-206, 2007. [2] Williams, J.G. et al., JGR 106, 27933-27968, 2001. [3] Zebker, H.A. et al., Science 324, 921-923, 2009. [4] Iess, L. et al., Science 327, 1367-1369, 2010. [5] Stiles, B.W. et al., Astron. J. 135, 1669-1680, 2008. [6] Nimmo, F., B.G. Bills, Icarus 208, 896-904, 2010. [7] Ojakangas, G.W., D.J. Stevenson, Icarus 81, 220-41, 1989. [8] Tobie, G. et al., Nature 440, 61-64, 2006. [9] Sohl, F. et al., JGR 108, 5130, 2003. [10] Bills, B.G., F. Nimmo, Icarus 214, 351-355, 2011. [11] Tokano, T. et al., JGR 116, E05002, 2011.
Bills Bruce G.
Nimmo Francis
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