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Article Dans Une Revue Icarus Année : 2013

The role of tidal torques on the evolution of the system of Saturn's co-orbital satellites Janus and Epimetheus

Gérard Caudal
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Résumé

This paper discusses the effect of tides raised by Saturn on its co-orbital satellites Janus and Epimetheus, and its consequences on the long-term evolution of the co-orbital horseshoe pattern of those moons. This tidal effect is found to produce a loosening of the co-orbital lock of Janus and Epimetheus. The increase of the difference D between their semi-major axes under this process is estimated as 2.77 km/Myear, regardless of whether the moons are composed of monolithic or fractured ice. On the other hand, assuming that the outer edge of Saturn's A ring is permanently maintained by Janus and Epimetheus at their 7:6 resonance, Lissauer et al. (1985) have shown that the torques exerted on Janus and Epimetheus due to resonances with Saturn's rings would produce a tightening of the co-orbital lock. The rate of decrease of orbital difference D under that latter process depends upon the precise relative radial location of the A-ring outer edge as compared with Janus' lower orbit semi-major axis, which is quantified here by means of a parameter po characterizing the efficiency of the effect of Janus' ring resonance. Under the combined effects of those two processes, depending on the value of po relative to a critical value po≈19%, the system will evolve toward either a transition from horseshoe to tadpole orbit, or to a destruction of the co-orbital lock, after a few tens of Myears. Most recent observations of the A ring's outer edge by Spitale and Porco (2009) tend to indicate that the future evolution will be a net tightening of the co-orbital system as proposed by Lissauer et al. (1985), until a tadpole situation will be reached after about 15 Myears. From a study of the past history of the co-orbital system, it is shown that such a scenario is compatible with a capture from free orbits to co-orbital horseshoe pattern some 25 Myears ago.

Dates et versions

hal-00786325 , version 1 (08-02-2013)

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Citer

Gérard Caudal. The role of tidal torques on the evolution of the system of Saturn's co-orbital satellites Janus and Epimetheus. Icarus, 2013, 223 (2), pp.733-740. ⟨10.1016/j.icarus.2013.01.021⟩. ⟨hal-00786325⟩
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