Combined structural and compositional evolution of planetary rings due to micrometeoroid impacts and ballistic transport

Abstract

We introduce improved numerical techniques for simulating the structural and compositional evolution of planetary rings due to micrometeoroid bombardment and subsequent ballistic transport of impact ejecta. Our current, robust code is capable of modeling structural changes and pollution transport simultaneously over long times on both local and global scales. In this paper, we describe the methodology based on the original structural code of Durisen et al. (1989, Icarus 80, 136-166) and on the pollution transport code of Cuzzi and Estrada (1998, Icarus 132, 1-35). We provide demonstrative simulations to compare with, and extend upon previous work, as well as examples of how ballistic transport can maintain the observed structure in Saturn's rings using available Cassini occultation optical depth data. In particular, we explicitly verify the claim that the inner B (and presumably A) ring edge can be maintained over long periods of time due to an ejecta distribution that is heavily biased in the prograde direction through a balance between the sharpening effects of ballistic transport and the broadening effects of viscosity. We also see that a "ramp"-like feature forms over time just inside that edge. However, it does not remain linear for the duration of the runs presented here unless a less steep ejecta velocity distribution is adopted. We also model the C ring plateaus and find that their outer edges can be maintained at their observed sharpness for long periods due to ballistic transport. We hypothesize that the addition of a significant component of a retrograde-biased ejecta distribution may help explain the linearity of the ramp and is probably essential for maintaining the sharpness of C ring plateau inner edges. This component would arise for the subset of micrometeoroid impacts which are destructive rather than merely cratering. Such a distribution will be introduced in future work.

In situ collection of dust grains falling from Saturn’s rings into its atmosphere

“…The required high ejection speeds are consistent with data from previous theoretical studies (22) and have been observed in laboratory experiments (impact jetting) (41,42). The ejection speed is likely to be larger for smaller ejecta (43,44), implying much higher ejection speeds for nanograins than for micrometer-sized and larger ejecta, which fuel the ballistic transport (10,15).…”

“…The radial density profile of the rings is shaped by various dynamical processes, including viscous spreading, resonances with satellites, intrinsic instabilities, and ballistic transport (5, 9-13) of ejecta released by impacts of interplanetary dust particles on the rings (14). The latter process is primarily responsible for redistributing and mixing the icy and non-icy material across the rings, as well as for the formation of sharp inner A and B ring edges (10,15).…”

Measurement and implications of Saturn’s gravity field and ring mass

“…The ring mass has implications for the age of the rings (59). Traditional age estimates of Saturn's ring-satellite system fall into three categories: dynamical estimates based on the rate of recession of the small satellites from the rings due to gravitational torques, and the back-reaction on the A ring (10,60); structural evolution timescales based on the evolution of unconfined edges such as the inner edges of the A and B rings (11); and compositional timescales based on the assumption that the rings were formed as pure water ice and have subsequently been steadily darkened by the infall of interplanetary debris (9). Most of these timescale estimates depend, directly or indirectly, on the mass of the rings; both dynamical and compositional considerations suggest that low-mass rings are likely to be young and both approaches yield evolutionary ages ~10 8 yrs for the A and B rings, assuming the Voyager measurements of ring masses and interplanetary impact fluxes (59).…”

Astrophysics with New Horizons: Making the Most of a Generational Opportunity

“…Moses 1992;Feuchtgruber et al 1997;Moses et al 2000;Frankland et al 2016;Moses & Poppe 2017), the injection of metallic species into planetary magnetospheres (Christon et al 2015); the spatial and compositional evolution of Saturn's main ring system (e.g. Durisen et al 1989;Cuzzi & Estrada 1998;Estrada et al 2015); and the production of impact ejecta clouds and/or rings from airless bodies, like planetary satellites (e.g. Verbiscer et al 2009;Hedman et al 2009;).…”