The Effect of Tides and an Inner Core on the Forced 88 day Libration of Mercury
(2012) Lunar and Planetary Science Conference — Location: The Woodlands, Texas (19.March.2012)
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- Abstract
- The rotation speed of Mercury is not constant but shows small periodic variations. These longitudinal librations are due to the torque exerted by the sun on the aspheri- cal shape of Mercury. It was realized by Peale [3] that observation of the main libration at 88 days would allow determining whether the core of Mercury is (at least par- tially) liquid, because that libration is about a factor of two larger for a liquid core than for a solid core. Margot et al. [2], using Earth-based radar observations to esti- mate the librations, demonstrated that the core is indeed highly likely partially liquid. Besides giving information on the core state, an ac- curate determination of the libration will also allow con- straining the mantle density and the size, density, and composition of the core. However, in order to be able to make accurate inferences on the interior structure of Mercury, all relevant effects on the libration have to be known more precisely than the observational precision. It is usually assumed that the core is spherically sym- metric and that its rotation is not coupled to the rotation of the mantle on the short period of libration. The core, however, can be aspherical and can be coupled to the mantle by various physical mechanisms. Electromag- netic coupling, topographic coupling, viscous coupling [4] and inertial coupling [6] between the liquid core and the mantle have been shown to be sufficiently small so that their influence can be neglected given the current and future spacecraft precision on the libration. Gravita- tional coupling between the mantle and a solid inner core is thought to have a similarly small influence on the li- bration amplitude [4], although the influence of the inner core could lead to a noticeable difference in the libration on a timescale of several years [10]. Here we investigate the influence of tides on the li- brations of Mercury, which have up to now not been ac- curately quantified. We moreover rediscuss and modify previous analyses [4, 10] of the effect of a solid inner core on Mercury’s libration. Numerical results are pre- sented for a wide range of recent models of the interior structure of Mercury [7].
- Affiliations
Royal Observatory of Belgium Reference Systems and Planetology
Citations
Van Hoolst, T., Rivoldini, A., Baland, R.-M., & Yseboodt, M. (2012). The Effect of Tides and an Inner Core on the Forced 88 day Libration of Mercury. Lunar and Planetary Science Conference, The Woodlands, Texas. https://hdl.handle.net/2078.5/186569