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The obliquity of Mercury: Models and interpretation

Published online by Cambridge University Press:  16 October 2024

Rose-Marie Baland Open the ORCID record for Rose-Marie Baland [Opens in a new window]
Rose-Marie Baland*
Affiliation:
Royal Observatory of Belgium
*
email: [email protected]
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Abstract

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Mercury is locked in an unusual 3:2 spin-orbit resonance and as such is expected to be in a state of equilibrium called Cassini state. In that state, the angle between the spin axis and orbit normal, called obliquity, remains almost constant while the spin axis remains almost in the plane, also called Cassini plane, defined by the normal to the Laplace plane and the normal to the orbital plane. The spin axis and the orbit normal precess together with a period of about 300 kyr. The orientation of the spin axis of Mercury has been estimated using different approaches: (i) Earth-based radar observations, (ii) Messenger images and altimeter data, and (iii) Messenger radio tracking data. The different estimates all tend to confirm that Mercury occupies the Cassini state. The observed obliquity is small and close to 2 arcmin. It indicates a normalized polar moment of inertia of about 0.34. This information, combined with the existence of a liquid iron core, as evidenced by the librations, allows to constrain the interior structure of Mercury. However, the different estimates of the orientation of the spin axis locate the spin axis somewhat behind or ahead of the Cassini plane, and it is difficult to reconcile and interpret them coherently in terms of detailed interior properties. We review recent models for the obliquity and spin orientation of Mercury, which include the effects of complex orbital dynamics, tidal deformations and associated dissipation, and internal couplings related to the presence of fluid and solid cores. We discuss some implications regarding the interpretations of the orientation estimates in term of interior properties.

Keywords

planets and satellites: individual (Mercury)celestial mechanicsmethods: analyticalobliquitylibrationsgeodesydynamicsrotationplanetary interiors
Type
Contributed Paper
Information
Proceedings of the International Astronomical Union , Volume 18 , Symposium S382: Complex Planetary Systems II: Latest Methods for an Interdisciplinary Approach , December 2022 , pp. 1 - 11
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Copyright
© The Author(s), 2024. Published by Cambridge University Press on behalf of International Astronomical Union

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