Bright clumps in the D68 ringlet near the end of the Cassini Mission

Abstract

The D68 ringlet is the innermost narrow feature in Saturn's rings. Prior to 2014, the brightness of this ringlet did not vary much with longitude, but sometime in 2014 or 2015 a series of bright clumps appeared within D68. These clumps were up to four times brighter than the typical ringlet, occured within a span of ∼ 120 • in corotating longitude, and moved at an average rate of 1751.7 • /day during the last year of the Cassini mission. The slow evolution and relative motions of these clumps suggest that they are composed of particles with a narrow (sub-kilometer) spread in semi-major axis. The clumps therefore probably consist of fine material released by collisions among larger (up to 20 meters wide) objects orbiting close to D68. The event that triggered the formation of these bright clumps is still unclear, but it could have some connection to the material observed when the Cassini spacecraft passed between the planet and the rings.

Modeling Saturn's D68 clumps as a co-orbital satellite system

“…A similar symmetric mode in a system of four bodies would require the outer two bodies to converge at a faster rate than the middle two bodies. The observed drift rates, however, show the opposite trend, with the middle two clumps drifting at a faster rate than the outer clumps (Hedman 2019). If, however, the dust around four source bodies was stirred up by an object that passed nearby, it is certainly possible this object could have missed other source bodies in the D68 ringlet.…”

“…It is certainly possible for there to be only four non-stationary clumps and for this to be a transient phenomenon. In fact, Hedman (2019) identified slow changes in the clumps' azimuthal separations over time that could be evidence for libration. It is unlikely, however, for the clumps to be on the edge of a libration cycle, due to how azimuthally compact the whole configuration is.…”

“…We first considered the observed configuration with four masses separated by angles of 29 • , 32 • , and 26 • because these are the observed separations (Hedman 2019).…”

“…In 2014-15, four bright clumps formed and remained relatively evenly spaced with small longitudinal variations about mean separations of 26 • , 32 • , and 29 • (Hedman 2019). Hedman (2019) investigated these clumps in depth and designated them T (trailing), M (middle), L (leading), and LL (leading leading). The most likely explanation for the sudden increase in brightness in the four clump regions of the ringlet is that fine material was released by collisions into or among larger objects located near or within D68.…”

Changes in a Dusty Ringlet in the Cassini Division after 2010

“…Furthermore, several of these dusty rings have been observed to change significantly over timescales of years to decades. Most often these changes include the formation of bright clumps of material (French et al 2012;Hedman et al 2013;Hedman 2019), but in other cases they involve larger scale structural changes that can be attributed to variations in periodic perturbing forces or discrete disturbances spanning broad ring regions (Chancia et al 2019;Hedman and Showalter 2016). This paper describes a new type of timevariable phenomena in dusty rings, where the radial profile and orbit shape of a narrow ringlet appears to have suddenly changed.…”