Saturn's F ring is intermittently shepherded by Prometheus

Abstract

One of the stranger planetary rings is Saturn's narrow, clumpy F ring, lying just outside the main rings, in a region disturbed by chaotic orbital dynamics. We show that the F ring has a stable "true core" that dominates its mass and is confined into discontinuous short arcs of particles larger than a few millimeters in radius. The more obvious micron-size particles seen in images, outlining and obscuring the true core, contribute only a small fraction of its mass. We found that these arcs of large particles orbit Saturn in a specific corotational resonance with the nearby 100-kilometer diameter ringmoon Prometheus, which stabilizes the F ring material and allows it to persist within the disturbed region for decades or longer. Toward the end of the observing period, a small chaotic glitch in the orbit of Prometheus temporarily disrupted the confinement, but the arcs seem to be able to adapt.

Fig. 1.. Angular locations of F ring detections and nondetections.

(Bottom) Filled symbols show Cassini RSS F ring detection longitudes, and open symbols show nondetection longitudes, all regressed to a common time, t^∗ (see section S4). The vertical axis shows the inertial orbital longitudes of the j detections $g_{\mathit{Fj}}^{\ast }$ at t^∗, covering 360°. The horizontal axis (denoted Δg_jm in section S1) is the folded (modulo m = 110) difference between each detection longitude and the longitude of Prometheus periapse at t^∗. The plotted horizontal axis actually repeats two folded “cycles” of Δg_jm = 0, each of angular width 360°/m = 3.27°, for visibility. The nondetections seem randomly distributed in both coordinates. The dark blue circles align preferentially with the (more stable) longitude of Prometheus’ periapse (blue vertical lines at Δg_jm = 0) and avoid the (less stable) longitude of its apoapse (red vertical lines one-half cycle away). The cyan squares—three lying near the “forbidden” red vertical lines—are the last four detections of the mission, which we argue should be ignored because they had been corrupted by a recent encounter with Prometheus on a recently altered orbit (section S4). (Top) We simply bin the detections (blue-cyan) and nondetections (gray) to better illustrate the clustering of the detections. The nondetection panel shows the mean and SD of the counts per bin. D, detection; ND, nondetection.

Fig. 2.. Two additional tests of the CER hypothesis.

(Top) A brute force test, in which the longitudes of the arcs and Prometheus (and the periapse of Prometheus) are integrated and the mean anomaly of Prometheus recorded when an encounter occurs with each arc. In the CER hypothesis, Prometheus should be at or near its periapse (mean anomaly = 0) when it encounters an arc, furthest from the arc and perturbing it only weakly. This seems to be consistent with the data, on average, although there seems to be a trend (unexplained) with inertial longitude. The final four arc detections of the mission are shown in green; we have argued that by this time, Prometheus had glitched to a new orbit, so their larger divergence is not problematic for the hypothesis. (Bottom) Resonance variable ϕ (eq. S19) for the arcs and Prometheus as a function of arc detection time. The final four arc detections of the mission are circled in green; as above, the large divergence of three of them is not problematic for the hypothesis.