* Astronomy

Title: Theory of the rotation of Janus and Epimetheus
Authors: B. Noyelles

The Saturnian coorbital satellites Janus and Epimetheus present a unique dynamical configuration in the Solar System, because of high-amplitude horseshoe orbits, due to a mass ratio of order unity. As a consequence, they swap their orbits every 4 years, while their orbital periods is about 0.695 days. Recently, Tiscareno et al.(2009) got observational information on the shapes and the rotational states of these satellites. In particular, they detected an offset in the expected equilibrium position of Janus, and a large libration of Epimetheus. We here propose to give a 3-dimensional theory of the rotation of these satellites in using these observed data, and to compare it to the observed rotations. We consider the two satellites as triaxial rigid bodies, and we perform numerical integrations of the system in assuming the free librations as damped. The periods of the three free librations we get, associated with the 3 dimensions, are respectively 1.267, 2.179 and 2.098 days for Janus, and 0.747, 1.804 and 5.542 days for Epimetheus. The proximity of 0.747 days to the orbital period causes a high sensitivity of the librations of Epimetheus to the moments of inertia. Our theory explains the amplitude of the librations of Janus and the error bars of the librations of Epimetheus, but not an observed offset in the orientation of Janus.

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Title: The Rotation of Janus and Epimetheus
Authors: Matthew S. Tiscareno, Peter C. Thomas, Joseph A. Burns
(Version v2)

Epimetheus, a small moon of Saturn, has a rotational libration (an oscillation about synchronous rotation) of 5.9 ± 1.2 degrees, placing Epimetheus in the company of Earth's Moon and Mars' Phobos as the only natural satellites for which forced rotational libration has been detected. The forced libration is caused by the satellite's slightly eccentric orbit and non-spherical shape.
Detection of a moon's forced libration allows us to probe its interior by comparing the measured amplitude to that predicted by a shape model assuming constant density. A discrepancy between the two would indicate internal density asymmetries. For Epimetheus, the uncertainties in the shape model are large enough to account for the measured libration amplitude. For Janus, on the other hand, although we cannot rule out synchronous rotation, a permanent offset of several degrees between Janus' minimum moment of inertia (long axis) and the equilibrium sub-Saturn point may indicate that Janus does have modest internal density asymmetries.
The rotation states of Janus and Epimetheus experience a perturbation every four years, as the two moons "swap" orbits. The sudden change in the orbital periods produces a free libration about synchronous rotation that is subsequently damped by internal friction. We calculate that this free libration is small in amplitude (<0.1 degree) and decays quickly (a few weeks, at most), and is thus below the current limits for detection using Cassini images.

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A large crater on Saturn's tiny moon Janus is visible in this Cassini spacecraft image.
The image was taken in visible light by the Cassini spacecraft's narrow-angle camera on March 5, 2009, when it was approximately 1.1 million kilometres  from Janus and at a Sun-Janus-spacecraft, or phase, angle of 53 degrees.


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Credit: NASA/JPL/Space Science Institute

The lit terrain seen is on the leading hemisphere of Janus (179 kilometres across).
Resolution in the original image was 6 kilometres per pixel.
North on Janus is up and rotated 7 degrees to the left.

This image of Janus was captured by the Cassini spaceprobe during a close flyby of the moon on June 30, 2008.
The distance to the moon was approximately 33,000 km.


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Credit:     NASA/JPL/Space Science Institute