Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p44a..01m&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P44A-01
Astronomy and Astrophysics
Astronomy
[1221] Geodesy And Gravity / Lunar And Planetary Geodesy And Gravity, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [5450] Planetary Sciences: Solid Surface Planets / Orbital And Rotational Dynamics, [6949] Radio Science / Radar Astronomy
Scientific paper
Profound developments in our understanding of the Earth and Moon have arisen as a direct outcome of rotation studies (e.g., Munk and MacDonald 1960, Lambeck 1980, Wahr 1988, Dickey et al 1994). Measurements of planetary rotation provide powerful probes of planetary interior structure and processes. I will discuss ongoing observations at Mercury, Venus, and Titan, as well as future prospects for other bodies. Observations of radar speckle patterns tied to the rotation of Mercury establish that the planet occupies a Cassini state with obliquity 2.11 +/- 0.1 arcminutes. The measurements show that the planet exhibits librations in longitude that are forced at the 88-day orbital period, as predicted by theory. The amplitude of the oscillations together with spacecraft determinations of the gravitational harmonic coefficient C22 indicates that the mantle of Mercury is decoupled from a core that is at least partially molten. Departures from the exact Cassini state and long-term libration signatures can inform us about core-mantle interactions. New Cassini radar measurements of the spin state of Titan (Stiles et al 2008) can be most readily interpreted as evidence that Titan closely follows a Cassini state. Peale (1969) has shown that, in that state, a clear relationship exists between obliquity and moment of inertia differences. Application of this relationship to Titan in conjunction with measurements of the gravitational harmonic C22 yields a polar moment of inertia that exceeds the 2/5 value for a uniform density sphere, perhaps indicative of a decoupling between core and outer layer, as in Mercury. Libration measurements at Titan require new instrumentation but can provide superb probes of interior structure and dynamical interactions between layers. This also applies to Galilean satellites (Comstock and Bills 2003, Van Hoolst et al 2008). Ongoing measurements of length-of-day variations at Venus are most easily attributed to angular momentum exchange between atmosphere and solid planet. As on Earth a rich spectrum of secular and seasonal trends can be expected in the spin of Venus, with excitations on a wide range of timescales that are diagnostic of diverse geophysical phenomena. Obtaining a well-sampled time history of length-of-day variations will require instrumentation beyond existing Earth-based telescopes. Monitoring of the spin state of Mars can provide information about volatile transport and core properties (Zuber and Smith 1999, Yseboodt et al 2004). The measurements can be obtained in conjunction with seismology packages (e.g. Lognonne et al 2000, Banerdt et al 2004, Dehant et al 2004).
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