Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.u21a0005m&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #U21A-0005
Astronomy and Astrophysics
Astronomy
5417 Gravitational Fields (1221), 5430 Interiors (8147), 5450 Orbital And Rotational Dynamics (1221), 5455 Origin And Evolution, 6235 Mercury
Scientific paper
The IAU Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites (WGCCRE) has published orientation models for Mercury since 1980. The IAU-recommended spin axis orientation (Seidelmann et al, CMDA 98, 2007) is based on assumptions made in 1980 and does not reflect current knowledge. It differs from the measured spin axis orientation (Margot et al, Science 316, 2007) by ~0.04 degrees, an unacceptably large offset for precision work. The resulting positional errors on the surface are of order ~1,500~m, about two orders of magnitude larger than the resolution achievable by MESSENGER cameras. I present an updated orientation model with a formulation for incorporating the forced librations in longitude. An accurate model is crucial for mission planning and data analysis, as well as for characterizing the interior properties of the planet (Peale, Nature 262, 1976). Twenty-one radar measurements obtained from 2002 to 2006 at a wide range of geometries yield the orientation of the spin axis (α=281.0097°, δ=61.4143°) and a robust obliquity value of 2.11 ± 0.1 arcminutes (Margot et al, Science 316, 2007), precisely within the range of theoretical expectations (Peale et al, MAPS 37, 2002). The spin axis uncertainty contours fall on the locus of possible Cassini state positions defined by the orbit pole and the Laplace pole, indicating that Mercury is in Cassini state 1. The occupancy of the Cassini state and the obliquity measurement place important constraints on the moments of inertia and on the interior structure of the planet. This work was supported by the NASA Planetary Astronomy program 07-PAST07-0020.
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