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Margot J.-L.,University of California at Los Angeles | Peale S.J.,University of California at Santa Barbara | Solomon S.C.,Carnegie Institution of Washington | Solomon S.C.,Lamont Doherty Earth Observatory | And 7 more authors.
Journal of Geophysical Research: Oceans | Year: 2012

Earth-based radar observations of the spin state of Mercury at 35 epochs between 2002 and 2012 reveal that its spin axis is tilted by (2.04 ± 0.08) arc min with respect to the orbit normal. The direction of the tilt suggests that Mercury is in or near a Cassini state. Observed rotation rate variations clearly exhibit an 88-day libration pattern which is due to solar gravitational torques acting on the asymmetrically shaped planet. The amplitude of the forced libration, (38.5 ± 1.6) arc sec, corresponds to a longitudinal displacement of ∼450 m at the equator. Combining these measurements of the spin properties with second-degree gravitational harmonics (Smith et al., 2012) provides an estimate of the polar moment of inertia of Mercury C/MR2 = 0.346 ± 0.014, where M and R are Mercury's mass and radius. The fraction of the moment that corresponds to the outer librating shell, which can be used to estimate the size of the core, is C m/C = 0.431 ± 0.025. © 2012. American Geophysical Union. All Rights Reserved.

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