Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p44b..01m&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P44B-01
Astronomy and Astrophysics
Astronomy
[5417] Planetary Sciences: Solid Surface Planets / Gravitational Fields, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [5450] Planetary Sciences: Solid Surface Planets / Orbital And Rotational Dynamics, [6235] Planetary Sciences: Solar System Objects / Mercury
Scientific paper
Over the past nine years we have used the Green Bank Telescope (GBT) in conjunction with the Goldstone Solar System Radar (GSSR) to characterize the spin state and interior of Mercury. We implemented a technique [1,2] that provides instantaneous spin rate measurements with 10-5 fractional precision and spin orientation measurements at the arcsecond level. On the basis of measurements at 21 distinct epochs between 2002 and 2006, we found observational evidence that Mercury closely follows a Cassini state and that it exhibits forced librations in longitude [3], as predicted by theory [4,5]. A long-period (˜12 year) libration signature may be present in the data. Since 2006 we have secured measurements at 11 additional epochs (out of 23 attempts). We are in the process of re-analyzing the entire set of observations with the goals of (1) refining the determination of the obliquity and of the libration amplitude, two parameters that are critical in the determination of the core size; (2) confirming the presence or absence of a long-period libration component, with the prospect of elucidating the excitation mechanism; (3) quantifying deviations of the pole from the strict Cassini state, which could inform us about dissipation due to solid-body tides and core-mantle interactions. Our first goal is particularly important now that MESSENGER is securing measurements of the low-degree gravitational harmonics with an expected precision of better than 1%. The core-size error budget indicates that the precision of the ground-based estimates of obliquity and librations will ultimately dictate the quality of the core size determination, as well as the attendant inferences regarding the interior structure, thermal evolution, and magnetic field generation of the planet. [1] Green, in Radar Astronomy, McGraw-Hill, 1968. [2] Holin, Radiophys. Quant. Elec. 31, 1988. [3] Margot et al, Science 316, 2007. [4] Peale, Nature 262, 1976. [5] Peale, in Mercury, U. of Arizona Press, 1988.
Margot Joelle
Padovan S.
Peale Stanton J.
Solomon Stanley C.
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