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Title:
Planetary Interior Structure Revealed by Spin Dynamics
Authors:
Margot, J.; Peale, S. J.; Jurgens, R. F.; Slade, M. A.; Holin, I. V.
Affiliation:
AA(California Institute of Technology, MC 150-21, Pasadena, CA 91125 United States ; ), AB(UC, Santa Barbara, Dept. of Physics, Santa Barbara, CA 93106 United States ; ), AC(Jet Propulsion Laboratory/Caltech, MC 238-420, Pasadena, CA 91109 United States ; ), AD(Jet Propulsion Laboratory/Caltech, MC 238-420, Pasadena, CA 91109 United States ; ), AE(Space Research Institute, Fryazino, Moscow, M 00000 Russian Federation ; )
Publication:
American Geophysical Union, Fall Meeting 2002, abstract #P22D-08
Publication Date:
12/2002
Origin:
AGU
AGU Keywords:
1227 Planetary geodesy and gravity (5420, 5714, 6019), 5430 Interiors (8147), 5450 Orbital and rotational dynamics, 6235 Mercury, 6949 Radar astronomy
Bibliographic Code:
2002AGUFM.P22D..08M

Abstract

The spin state of a planet depends on the distribution of mass within the interior, gradual and discrete changes in its moments of inertia, dissipation mechanisms at the surface and below, and external torques. Detailed measurements of the spin dynamics can therefore reveal much about planetary interior structure, interactions at the core-mantle and atmosphere-surface boundaries, and mass redistribution events. Studies of the spin precession, polar wobble, and length of day variations have been used to determine Earth's moments of inertia and rigidity and to study the effects of atmospheric angular momentum changes, post-glacial rebound, and large earthquakes. In planetary investigations the spin measurements are particularly important because other means of constraining interior properties require in-situ or orbiting sensors (e.g. seismometers, magnetometers, and Doppler tracking of spacecraft). Here we describe the successful implementation of a new Earth-based radar technique (Holin, 1992) that provides spin state measurements with unprecedented accuracy. Our first observations were designed to characterize Mercury's core. Peale (1976) showed that the measurement of four quantities (the obliquity of the planet, the amplitude of its longitude librations, and the second-degree gravitational harmonics) are sufficient to determine the size and state of Mercury's core. The existence of a molten core would place strong constraints on the thermal and rotational histories of the planet, with profound implications for the composition and rotation state of the planet at the time of formation. A solid core would have a fundamental impact on theories of planetary magnetic field generation. We observed Mercury with the Goldstone radar and the Green Bank Telescope in May-June 2002. We illuminated the planet with a monochromatic signal, recorded the scattered power at the two antennas, and cross-correlated the echoes in the time domain. We obtained strong correlations which directly constrain the instantaneous spin rate and orientation. Our measurements provide the first experimental proof that Mercury is in a Cassini state, a three-order of magnitude improvement in the knowledge of the spin orientation, a measurement of the obliquity which places new constraints on the moments of inertia, and an upper-limit to the amplitude of the longitude librations which constrains interior properties. The IAU-recommended values for the spin orientation of Mercury have not changed since the Mariner days (Davies et al., 1980). The new spin solution can be used to improve the geodetic control of the Mariner 10 images, a task that was pioneered and perfected by Merton G. Davies (1917-2001).
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