Retrograde predominance of small saturnian moons reiterates a recent retrograde collisional disruption

TL;DR

This work expands the census of Saturn’s irregular moons by presenting the discovery of 64 new bodies via a CFHT survey (2019–2021), nearly doubling the known sample and enabling finer analysis of orbital distributions. Using time-averaged orbital elements and cross-survey linking (notably with Subaru data), the authors identify substructure within the Inuit and Norse groups, including a Mundilfari subgroup within Norse characterized by an extremely steep size distribution. The analysis supports a recent collisional disruption scenario for Mundilfari, with a differential power-law index of $q=6$ and a disruption timescale of less than $10^8$ years, providing constraints on Saturn’s irregular-moon collisional history and capture processes. Overall, the results refine our understanding of the formation and evolution of Saturn’s irregular satellite system and its complex collisional legacy.

Abstract

We report the discovery and careful orbital determination of 64 new irregular moons of Saturn found in images taken using the Canada-France-Hawaii Telescope from 2019-2021, bringing the total number of saturnian irregulars to 122. By more than doubling the sample of saturnian irregular moon orbits, including pushing to smaller sizes, we can now see finer detail in their orbital distribution. We note the emergence of potential subgroups associated with each of Siarnaq and Kiviuq within the Inuit group. We find that in the inclination range 157-172 degrees the ratio of smaller moons (diameters less than 4 km) to larger moons (diameters greater than 4 km) is significantly larger than that of any other inclination range in the retrogrades. We denote this subset of the Norse group as the Mundilfari subgroup after its largest member. The incredibly steep slope of the Mundilfari subgroup's size distribution, with a differential power law index of q = 6, strengthens the hypothesis in Ashton et al. (2021) that this subgroup was created by a recent catastrophic collision, $<10^8$ yr ago.

Figures (1)

• Figure 1: On-sky plot of the two fields used for this survey relative to Saturn (grey rectangles). All measurements of 5 of the 64 new moons detected are shown (circles) along with the best fit orbit (dashed lines). This sub-sample illustrates the challenges of multi-year linking. (Note that the apparent non-closure of the orbits is due to the projected saturnocentric positions being observed from the moving Earth.)