Physics
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusmgp33a..03m&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #GP33A-03
Physics
1507 Core Processes (8115), 5450 Orbital And Rotational Dynamics, 6235 Mercury
Scientific paper
As Mercury follows its eccentric orbit (e ~0.206) with near-zero obliquity, it experiences periodically reversing torques due to the gravitational influence of the Sun on the asymmetric figure of planet. The torques affect the spin angular momentum and cause small deviations of the spin frequency from its resonant value of 3/2 times the mean orbital frequency. The resulting oscillations in longitude are called forced librations because the forcing and rotational response occur with a period dictated by the orbital motion (P ~88 days). The measurement of the forced librations can provide important information about the state and size of the core of Mercury, as demonstrated by Peale (1976). In order to determine the libration amplitude, we implemented a new Earth-based radar technique to measure planetary spins based on a description by Holin (1988,1992). Since May 2002, we have accumulated about a dozen measurements of the spin rate with a fractional uncertainty of 1 part in 105. Because the spin rate deviations due to the librations are predicted to be of order 2 arcseconds per day for a solid planet, corresponding to 1 part in 104 of the spin rate, our measurements can be used to measure the librational response of the planet. A libration amplitude much larger than that expected for a solid planet would indicate that the mantle is decoupled from the core and would suggest a liquid outer core. This situation would strengthen the possibility of a dynamo in a thin shell at Mercury.
Holin Igor V.
Jurgens Raymond F.
Margot Joelle
Peale Stan
Slade Martin A.
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