Fine-Scale Structures In Saturn'S Rings: Waves, Wakes And Ghosts
Creators: name: BAILLIE, Kevin
Publisher: name: Zenodo
Year: 2011
Subjects: Saturn, Rings, Disk, Satellites, Resonances, Waves, Wakes

description: The Cassini mission provided wonderful tools to explore Saturn, its satellites and its rings system. The UVIS instrument allowed stellar occultation observations of structures in the rings with the best resolution available (around 10 meters depending on geometry and navigation), bringing our understanding of the physics of the rings to the next level. In particular, we have been able to observe, dissect, model and test the interactions between the satellites and the rings.

We first looked at kilometer-wide structures generated by resonances with satellites orbiting outside the main rings. The observation of structures in the C ring and their association with a few new resonances allowed us to estimate some constraints on the physical characteristics of the rings. However, most of our observed structures could not be explained with simple resonances with external satellites and some other mechanism has to be involved. We located four density waves associated with the Mimas 4:1, the Atlas 2:1, the Mimas 6:2 and the Pandora 4:2 Inner Lindblad Resonances and one bending wave excited by the Titan -1:0 Inner Vertical Resonance. We could estimate a range of surface mass density from $0.22 ~(\pm 0.03)~$ to $1.42 ~(\pm 0.21)~\mathrm{g~cm^{-2}}$ and mass extinction coefficient from $0.13~(\pm 0.03)$ to $0.28~(\pm 0.06)~\mathrm{cm^{2}~g^{-1}}$. These mass extinction coefficient values are higher than those found in the A ring (0.01 -- 0.02 $\mathrm{cm^{2}~g^{-1}}$) and in the Cassini Division (0.07 -- 0.12 $\mathrm{cm^{2}~g^{-1}}$ from Colwell et al. (2009), implying smaller particle sizes in the C ring. We can therefore imagine that the particles composing these different rings have either different origins or that their size distributions are not primordial and have evolved differently. We also estimate the mass of the C ring to be between $3.7~(\pm 0.9)\times 10^{16} ~\mathrm{kg}$ and $7.9~(\pm 2.0)\times 10^{16} ~\mathrm{kg}$, equivalent to a moon of $28.0 ~(\pm 2.3)$ km to $36.2 ~(\pm 3.0)$ km radius (a little larger than Pan or Atlas) with a density comparable to the two moons (400 $\mathrm{kg~m^{-3}}$). From the wave damping length and the ring viscosity, we also estimate the vertical thickness of the C ring to be between $1.9 ~(\pm 0.4)$ m and $5.6 ~(\pm 1.4)$ m, which is consistent with the vertical thickness of the Cassini Division (2 -- 20 m) from Tiscareno et al. (2007) and Colwell et al. (2009). Conducting similar analysis in the A, B rings and in the Cassini Division, we were able to estimate consistent masses with previous works for the these rings.

We then investigated possible interactions between the rings and potential embedded satellites. Looking for satellite footprints, we estimated the possibility that some observed features in the Huygens Ringlet could be wakes of an embedded moon in the Huygens gap. We discredited the idea that these structures could actually be satellite wakes by estimating the possible position of such a satellite.

Finally, we observed a whole population of narrow and clear holes in the C ring and the Cassini Division. Modeling these holes as depletion zones opened by the interaction of a moonlet inside the disk material (this signature is called a "propeller"), we could estimate a distribution of the meter-sized to house-sized objects in these rings. Similar objects, though an order of magnitude larger, have been visually identified in the A ring. In the C ring, we have signatures of boulders which sizes are estimated between $\rcmin$ and $\rcmax$ m, whereas similar measures in the Cassini Division provide moonlet sizes between $\rcdmin$ and $\rcdmax$ m. Using numerical simulations for the propeller formation, we estimate that our observed moonlets belong to a population of bigger particles than the one we thought was composing the rings: Zebker et al. (1985) described the ring particles population as following a power-law size distribution with cumulative index around 1.75 in the Cassini Division and 2.1 in the C ring. We believe propeller boulders follow a power-law with a cumulative index of 0.6 in the C ring and 0.8 in the Cassini Division.

The question of whether these boulders are young, ephemeral and accreted inside the Roche limit or long-lived and maybe formed outisde by fragmentation of a larger body before migrating inward in the disk, remains a mystery. Accretion and fragmentation process are not yet well constrained and we can hope that Cassini extended mission will still provide a lot of information about it.