Abstract
A giant collision is believed to be at the origin of the Pluto–Charon system. As a result, the initial orbit and spins after impact may have substantially differed from those observed today. More precisely, the distance at periapse may have been shorter, subsequently expanding to its current separation by tides raised simultaneously on the two bodies. Here we provide a general 3D model to study the tidal evolution of a binary composed of two triaxial bodies orbiting a central star. We apply this model to the Pluto–Charon binary, and notice some interesting constraints on the initial system. We observe that when the eccentricity evolves to high values, the presence of the Sun prevents Charon from escaping because of Lidov-Kozai cycles. However, for a high initial obliquity for Pluto or a spin-orbit capture of Charon’s rotation, the binary eccentricity is damped very efficiently. As a result, the system can maintain a moderate eccentricity throughout its evolution, even for strong tidal dissipation on Pluto.
Highlights
Tidal torques raised on Charon by Pluto are even stronger, and so the satellite is assumed to be synchronous with Pluto, which corresponds to a final equilibrium situation (e.g., Farinella et al 1979; Cheng et al 2014)
We observe that the main features already present for the simulations with orbit #1 persist: (1) for large A values the eccentricity is quickly damped to zero, while for small A values it can increase to very high values; (2) the semi-major axis always evolves to the present value, except for small A values; (3) for small A values Lidov-Kozai cycles occur and stabilize the system; (4) the spin of Charon quickly evolves into spin-orbit resonances; and (5) the rotation ratio of Pluto (Ω0{n) initially increases, decreases towards the synchronous value
In this paper we revisit the tidal evolution of the Pluto–Charon binary
Summary
In 1978, a regular series of astrometric observations of Pluto revealed that the images were consistently elongated, denouncing the presence of Pluto’s moon, Charon (Christy & Harrington 1978). Most previous studies on the past orbital evolution of the Pluto–Charon system (Farinella et al 1979; Lin 1981; Mignard 1981; Dobrovolskis et al 1997; Cheng et al 2014) assume that both spin axes are normal to the binary orbital plane (2D model), and limit the evolution to the rotations. This is the expected outcome of tidal evolution, after a large collision the obliquity of Pluto can take any value (e.g., Dones & Tremaine 1993; Kokubo & Ida 2007; Canup 2011).
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