HR: 13:55h
AN: G33A-02 INVITED     [Abstracts]
TI: Earth-based Measurements of Planetary Rotational States
AU: * Margot, J
EM: jlm@astro.cornell.edu
AF: Cornell University, Dept of Astronomy 304 Space Sciences, Ithaca, NY 14853 United States
AU: Peale, S J
AF: UCSB, Dept of Physics, Santa Barbara, CA 93106 United States
AU: Jurgens, R F
AF: JPL, 4800 Oak Grove Dr., Pasadena, CA 91109 United States
AU: Slade, M A
AF: JPL, 4800 Oak Grove Dr., Pasadena, CA 91109 United States
AU: Holin, I V
AF: SRI, c/o L. Ksanfomality, Moscow, 000 Russian Federation
AB: The rotational response of planetary bodies to a variety of forcings provides fundamental insights into their interior structure and rheology. High-precision rotation measurements of the solid planets can be obtained with an Earth-based radar technique originally proposed by Green (1962, 1968) and also studied by Holin (1988, 1992). We have devised a practical implementation of this technique using the radio telescopes at Goldstone, Green Bank, and Arecibo. Initial data have been obtained for Mercury, Venus, and Mars. Our observations of Mercury provide instantaneous spin measurements with a fractional precision of 10$^{-5}$, a level that is sufficient to characterize the amplitude of the 88-day librations in longitude with 10% uncertainties. A dozen measurements obtained over the past two years strikingly trace the phase of the expected libration signature derived from the torque equation, yielding considerable confidence that the librations have been unambiguously detected. Least-squares fits to the data accumulated to date indicate a libration amplitude of $\sim$60 arcseconds. Peale (1976) showed that the measurement of the amplitude of those librations, the obliquity, and the $C_{20}$ and $C_{22}$ gravitational harmonic coefficients can provide important information about the state and size of the core of Mercury. One can use the Mariner 10 determination of the gravitational harmonics (Anderson et al., 1987) to estimate the libration amplitude. If the core was solid and coupled to the mantle, the amplitude of the 88-day librations would be within 50% of 20 arcseconds. Our measurements reveal librations with an amplitude that is three times larger than that, indicating that an outer shell only participates in the librations. The mantle of Mercury must therefore be decoupled from a core that is liquid, with profound implications for the thermal evolution and magnetic field generation of the planet. The observational evidence for a liquid core at Mercury strengthens the hypothesis that a dynamo may generate the magnetic field, even though a remanent crustal field can still explain the Mariner 10 magnetic field data (Stephenson 1976, Aharonson et al., 2004). Our measurements cannot provide a precise constraint on the size of the core until improved values of the gravitational harmonic coefficients are obtained by the MESSENGER and BepiColombo spacecraft.
DE: 5430 Interiors (8147)
DE: 5440 Magnetic fields and magnetism
DE: 5450 Orbital and rotational dynamics
DE: 6235 Mercury
DE: 6949 Radar astronomy
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting