Hi there! Today I will tell you about the detection of a ringlet in the rings of Saturn, by Matthew Hedman and Brian Carter, at the University of Idaho (USA). This ringlet presents an interesting dynamics, this is why it caught my attention. Such a discovery is made possible thanks to the Cassini-Huygens space mission, which orbits Saturn since 2004.

The mission Cassini-Huygens

Cassini-Huygens is a joint mission of the NASA, the ESA, and the Italian Space Agency ISA. It consists of a spacecraft, Cassini, which orbits Saturn since 2004, and a probe, Huygens, which landed and died on Titan in January 2005.
This mission Cassini-Huygens is one of the most ambitious ever made, this is why it required an American-European collaboration. It has given us, and is still giving, invaluable information on the system of Saturn. For instance, it permitted the expected discovery of a global subsurface ocean for Titan, and a more surprising one for Enceladus,. Mimas may also have one, from the measurements of its rotation, and that would be even more surprising. Before Cassini-Huygens, we thought that the system of Saturn was a kind of old, frozen and boring world, while it is actually pretty recent, and even the mid-sized icy satellites may present complex interiors. As a consequence, this pushed some of our colleagues to propose new scenarios of formation of the satellites of Saturn, either as droplets composed of ring material which would have migrated outward, or as remnants of larger progenitors, which were impacted.
These are just examples, and I cannot give an exhaustive list of discoveries due to Cassini-Huygens. We have now images of the surface of Titan, we have in situ measurements of its winds, we know the satellites and the planet Saturn much better… Let us focus on the rings.

Rings of Saturn facts

The rings of Saturn are known since Galileo Galilei, and the evolution of Earth telescopes made possible the discoveries of structures in them. The most famous of them is the Cassini Division, which is a 4,000 km wide gap between the two densest of Saturn’s rings, i.e. the A and the B rings. To have a quick overview:

• 186,000 km: Orbit of Mimas, the closest of the major satellites of Saturn
• 141,800 km: Orbit of Pandora
• 140,200 km: The F ring (pretty faint)
• 139,500 km: Orbit of Prometheus
• 139,350 km: The new ringlet
• 122,000 to 137,000 km: The A ring (dense)
• 133,600 km: the Encke gap, i.e. a lack of material in the A ring
• 117,500 to 122,000 km: The Cassini Division (still some material, but pretty few)
• 92,000 to 117,500 km: The B ring (the densest one)
• 74,600 to 92,000 km: The C Ring (faint)
• 67,000 to 74,500 km: The D Ring (faint)
• 58,200 km: Radius of Saturn, where its atmospheric pressure reaches 1 bar

It is known whether a ring is faint or dense from its optical density, which is then associated with an estimated surface density of the ring, seen as a flat structure. Here, I have mixed the main structures of the rings with fainter ones, which are more relevant in this study. I have particularly emphasized the new ringlet, which discovery is presented in this study.

Beside this, you can notice the presence of some small satellites embedded in the rings. I mention Prometheus and Pandora since they are close to the new ringlet, but there are actually more, e.g. Janus, Epimetheus, Atlas, Pan,…

Discovering this new ringlet

A ringlet is a kind of narrow ring of dusty material, i.e. small particles, their radius being something between the centimeter and the meter. Discovering a new ring is challenging because it is very faint. Here, it was discovered on images of the Narrow Angle Camera (NAC) of the Imaging Science Subsystem of the Cassini spacecraft. To make its presence obvious, it is necessary to use images which are not saturated, to remove the background luminosity, and to equalize the response of the different pixels constituting the image (flat-fielding). For this study, the authors used mostly images taken between 2012 and 2014, but some in 2006 as well.

An interesting dynamics

The authors find that this ring is an ellipse with a small eccentricity (0.0012), which precesses. This means that this ellipse is not fixed, but moves around Saturn, while the particles constituting the ringlet move much faster, on the ellipse. Their orbital period is pretty the same as Prometheus’, i.e. some 15 hours, but surprisingly the precession period of the ellipse is longer, i.e. 133 days, against 130 for Prometheus, and is very close to the one of the F Ring.
This is pretty unexpected for the following reason: in an ideal (keplerian) problem, i.e. a point-mass planet around which orbits a particle, the orbit does not precess. The precession is due to departures from this problem, mostly the polar flattening of Saturn, but also the gravitational perturbation of the other satellites. It can be easily shown that, if you get closer to Saturn, you precess faster. Here, the ringlet precesses slower than Prometheus while its orbit is inside. The authors have an elegant explanation, in showing convincingly that the collisions between the particles can synchronize the precession of this ringlet with the one of the F ring, providing that this ringlet is faint enough. I admit that I had not heard of this mechanism before, but the authors convince me. I would have a priori suspected the gravitational interaction of Pandora, but its precession is even slower than the measured one. There is at least one another example of synchronization of the precessions in the system of Saturn: Titan is so massive that it forces the precession of the orbit of Rhea.
The authors also mention the possibility that the particles of this new ringlet are affected by a co-orbital mean-motion resonance with Prometheus. I choose to focus on the synchronization of the precession with the F Ring, since I consider this is the most exciting result of the study. This would be the first accurate measurement of this collision-assisted synchronization, and we can expect in the future many other examples of this mechanism.

Some links

• The webpage of Matt Hedman
• The study, accepted for publication in Icarus, and made freely available on arXiV by the authors, many thanks for sharing!
• The space mission Cassini-Huygens

That’s all folks! Please, don’t hesitate to leave a comment!