Radial profiles of the Phoebe ring: A vast debris disk around Saturn

Highlights

• We extract a radial brightness profile for the Phoebe ring at visible wavelengths.

• We find that the radial profile changes behavior interior to 110 Saturn radii.

• We attribute this to either Iapetus sweeping up material or orbital instabilities of small grains.

• We find the Phoebe ring’s photometry is dominated by small grains.

• We measure the integrated I/F along the Phoebe ring’s midplane from 80–260 Saturn radii.

Abstract

We present observations at optical wavelengths with the Cassini Spacecraft’s Imaging Science System of the Phoebe ring, a vast debris disk around Saturn that seems to be collisionally generated by its irregular satellites. The analysis reveals a radial profile from 80–260 Saturn radii (R_S) that changes behavior interior to ≈110R_S. We attribute this to either the moon Iapetus...sweeping up small particles, or to orbital instabilities that cause the ring to flare up vertically. Our study yields an integrated I/F at 0.635 µm along Saturn’s shadow in the Phoebe ring’s midplane from 80–250 R_S of $2.7_{-0.3}^{+0.9}\times {10}^{-9}$. We develop an analytical model for the size-dependent secular dynamics of retrograde Phoebe ring grains, and compare this model to the observations. This analysis implies that 1) the “Phoebe” ring is partially sourced by debris from irregular satellites beyond Phoebe’s orbit and 2) the scattered light signal is dominated by small grains (≲20 µm in size). If we assume that the Phoebe ring is generated through steady-state micrometeoroid bombardment, this implies a power-law size distribution with index >4, which is unusually steep among Solar System rings. This suggests either a steep size distribution of ejecta when material is initially released, or a subsequent process that preferentially breaks up large grains.

Introduction

Using the Spitzer infrared space telescope, Verbiscer et al. (2009) discovered a vast dust ring around Saturn, far beyond the bright main rings. This debris disk was dubbed the Phoebe ring after the largest of Saturn’s distant irregular satellites, which seems to be the dominant source for the material. Approximately three dozen known irregular satellites (see Jewitt, Haghighipour, 2007, Nicholson, Cuk, Sheppard, et al., 2008, for reviews) form a swarm of mutually inclined, overlapping orbits—a relic of their capture process (Ćuk, Burns, 2004, Ćuk, Gladman, 2006, Nesvorný, Alvarellos, Dones, et al., 2003, Nesvorný, Vokrouhlický, Morbidelli, 2007, Pollack, Burns, Tauber, 1979). This led to a violent collisional history among these bodies continuing since early times (Bottke et al., 2010). Smaller collisions must be ongoing, both with circumplanetary objects too small to detect observationally, and with interplanetary meteoroids (cf., Cuzzi and Estrada, 1998).

While the disk is diffuse, the debris from these dark irregular satellites (Grav et al., 2015) can have important consequences. Iapetus, the outermost of the large, tidally locked, regular satellites has a leading side approximately ten times darker than its trailing side. Many years before its discovery, Soter (1974) (see also Bell, Cruikshank, Gaffey, 1985, Buratti, Mosher, 1995, Cruikshank, Bell, Gaffey, et al., 1983) hypothesized that inward transfer of such debris through Poynting–Robertson drag might explain Iapetus’ stark hemispheric dichotomy. Burns et al. (1996), and more recently Tosi et al. (2010) and Tamayo et al. (2011), showed that indeed, Iapetus should intercept most of the inspiraling material as it plows through the cloud, and that the longitudinal distribution of dark material on Iapetus can be well explained by dust infall under the action of radiation pressure. Additionally, Denk et al. (2010); Spencer and Denk (2010) showed that runaway ice sublimation and redeposition could accentuate initially subtle albedo differences to match the observed stark contrast.

Furthermore, this process of collisional grinding among the irregular satellites should be ubiquitous among the solar (and perhaps extrasolar) system’s giant planets (Bottke, Nesvorný, Vokrouhlický, et al., 2010, Kennedy, Wyatt, 2011), and this debris should also fall onto the respective outermost regular satellites. Indeed, the uranian regular satellites exhibit hemispherical color dichotomies (Buratti and Mosher, 1991), and Tamayo et al. (2013a) showed that this could similarly be explained through dust infall, though the dynamics are additionally complicated by Uranus’ extreme obliquity (Tamayo et al., 2013b). Bottke et al. (2013) argue the same process has occurred in the jovian system. As the only known debris disk sourced by irregular satellites, the Phoebe ring therefore presents a unique opportunity to learn about generic processes around giant planets, both in our Solar System and beyond.

Tamayo et al. (2014), hereafter THB14, detected the Phoebe ring’s scattered light at optical wavelengths, using the Cassini spacecraft in orbit around Saturn. THB14 combined these optical measurements with the thermal emission data of Verbiscer et al. (2009), finding that Phoebe ring grains have low albedos similar to the dark irregular satellites (Grav et al., 2015). More recently, Hamilton et al. (2015) combined detailed numerical models of dust grains’ size-dependent spatial distributions with new data from the Wide-Field Infrared Survey Explorer (WISE) to extract the particle-size distribution in the disk. They found that the Phoebe ring extends out to at least 270 Saturn radii¹(R_S) and has a steep particle size distribution. However, the Phoebe ring is so faint (normal optical depth $\sim {10}^{-8}$) that scattered light from the planet dominates the signal inside ≈100 Saturn radii (R_S). This is too far out to detect an inner edge swept out by Iapetus, which orbits at ≈59R_S.

In this paper we present results from a new Cassini data set with a substantially higher signal-to-noise ratio than that of THB14. This renders the faint Phoebe ring signature clearly visible in our images, and we are able to additionally extract the Phoebe ring’s radial structure. We begin by presenting our data analysis, and by describing our data reduction methods in Section 2, and our results in Section 3. In Section 4, we then semi-analytically investigate the expected 3-D structure of the Phoebe ring, which should exhibit interesting dynamics closer to Iapetus, where the Sun stops being the dominant perturbation (as it is for grains at large Saturnocentric distances), and Saturn’s oblateness becomes important. In Section 5 we compare our model to the data and we summarize our results in Section 6.