Cassini microwave observations provide clues to the origin of Saturn's C ring
Introduction
Saturn's rings are the most massive, extensive and diverse ring system in the Solar System, yet despite decades of ground and spacecraft-based observations (Dougherty et al., 2009, Grossman, 1990, de Pater and Dickel, 1991, van der Tak et al., 1999, Dunn et al., 2002; Dunn et al., 2005, Poulet et al., 2003, Nicholson et al., 2008), their origin and age remains a subject of debate. In particular, the small non-icy material fraction, which is related to their origin through source composition and exposure age (Cuzzi and Estrada, 1998), is poorly understood. At first, astronomers predominantly agreed Saturn's rings were primordial (Harris et al., 1984) until observations from the Voyager spacecraft suggested that dynamical considerations necessitated a short (∼108 yrs) lifetime (Harris et al., 1984, Dones, 1991, Goldreich and Tremaine, 1982). A more recent post-Cassini view returns to an ancient origin (Esposito, 2008), but invokes a much more massive pure-ice primordial ring formed via tidal disruption of a Titan-sized, differentiated satellite (Canup, 2010). Yet another recently revisited scenario (Charnoz et al., 2009a) suggests that the cometary flux was so high during the Late Heavy Bombardment (LHB) that several tens of Mimas masses of cometary material may have been brought into Saturn's Hill sphere, and that a fraction of it could have ended in Saturn's Roche zone. In this ‘destroyed satellite scenario’, it has been recently shown (Charnoz et al., 2009b) that a Mimas-mass moon located 10^5 km from Saturn can be disrupted during a LHB type event (Tsiganis et al., 2005) about 700 Myr after the planet's birth (though possibly even later, see Sec. 5.2). The probability for disruption is model dependent, but can be > 95% (see Table 3, Charnoz et al., 2009b, and references therein). The abundance and character of non-icy material in Saturn's present-day ring system can help distinguish between these origin scenarios by constraining the composition of source material and, for a given initial composition, estimate the rings’ exposure age due to micrometeoroid bombardment (Cuzzi and Estrada, 1998).
The exact composition of Saturn's ring particles remains unclear. Though water ice has long been known as the most prominent component (Cuzzi et al., 1984, Esposito et al., 1984), the single scattering albedos of ring particles are much too low for them to be pure water ice. The rings’ spectrum in the visible and near-IR shows strong absorption at near-UV and blue wavelengths leading to a decrease in reflectivity at wavelengths shorter than 0.6 μm. Yet, little is known about the specific makeup of the non-icy, absorbing material that accounts for this observation, the presence of which apparently gives the rings their slightly non-gray, very pale tan or salmon color (Estrada and Cuzzi, 1996). Several investigators have considered a variety of potential UV absorber candidates such as tholins, PAHs, nanophase iron and nanophase hematite in order to match the observed color. Despite the range of materials considered, all of these studies have generally constrained the non-icy fraction to be less than a few percent by mass (Cuzzi and Estrada, 1998, Cruikshank et al., 2005, Morris et al., 1985, Clark et al., 2008). Furthermore, ring photometry has shown that the optically thin C ring and Cassini Division have darker and less red particles than the optically thick A and B rings (Estrada and Cuzzi, 1996), which indicates the presence of varying amounts of unidentified darkening non-icy material with a spectrally neutral color (Smith et al., 1982, Cuzzi et al., 1984, Dones et al., 1993, Poulet et al., 2003, Nicholson et al., 2008). We will later show that the very small amount of reddening material required to give the rings their apparent color in the visible and near IR wouldn't contribute significantly to the microwave observation (Sec. 5.4), and thus it is the presence of some other non-icy material, besides these reddening materials, that determines the intrinsic thermal emission at the wavelength of our interest.
The Cassini RADAR radiometer (Elachi et al., 2004) provides an ideal window through which to study the non-icy material fraction in Saturn's main rings, as it operates at a wavelength where the absorptivity of water ice is negligible compared to that of most non-icy material, and thus the intrinsic thermal radiation from the ring layer is dominated by the non-icy components. Moreover, whereas visible and near-IR spectra are only sensitive to the top millimeter or less of ring particle surfaces, microwave observations are able to sample the bulk of the ring mass. For solid pure water ice particles, microwave radiation from Saturn at 2.2 cm can penetrate as deep as almost 100 m, while the largest particles in the rings are no larger than 10 m (Marouf et al., 2008).
The C ring lies close to Saturn, situated between the D and B rings, and is the darkest of all the main rings. Since first formed, the rings have been continuously darkened by incoming micrometeoroids. In optically thin rings like the C ring, ring particles are sparse and only a fraction of incoming meteoroids actually hit the rings as opposed to merely passing through them. As the rings become optically thicker, a larger fraction of the meteoroids are absorbed by the ring particles until reaching a level of saturation, where none of the incoming meteoroids can pass through. Saturation is reached for an optical depth τ ≳ 1, while the mass of material to be darkened continues to increase with τ. Thus, the optically thin C ring is easier to darken than the optically thicker A and B rings.
It should be noted that many of the structures we see in the C ring lack a satisfactory explanation. Apart from having a profound darkening effect on the rings, micrometeoroid bombardment and transport of their impact ejecta has been shown to explain many aspects of C ring structure (see Estrada et al., 2015), but much of the structure, especially the plateaus, remain enigmatic. A great deal of the mystery of the C ring revolves around the measured ring opacity (see Sec. 5) which can be associated with the particle size distribution. A complete radial profile of the C ring opacity is lacking, but observations suggest that the particle size distribution in the C ring differs across the ring and within different ring regions (e.g., Marouf et al., 2008, Cuzzi et al., 2009; Colwell et al., 2011, 2012) which further complicates efforts to explain the observed structure. Though our analyses in this paper are not meant to address the specifics of C ring structure, we do advance a compelling model to explain the anomalously low opacity (and thus structure) in the middle C ring (Sec. 5.2).
Despite the complexities associated with the particle size distribution across the C ring, the C ring has a lower optical depth and smaller particle albedo than the A and B rings, so one would expect the C ring to have the highest abundance of non-icy material and hence be the ideal place for studying the rings’ contamination history. It had been noted as early as Voyager that the presence of some then as-of-yet unknown non-icy component is required to account for the regional and local color variations in ring color (e.g., Smith et al., 1982); however, because previous radio-wave observations have been limited by resolution and sensitivity, no detailed C ring non-icy material fraction has been reported until now. In this work, we determine the non-icy fraction and attempt to constrain candidate materials from the brightness temperature profile of the C ring obtained from analysis of Cassini RADAR observations with resolutions as high as ∼2000 km. The observed flux is composed of scattered Saturn radiation and intrinsic thermal emission, the latter being directly related to the non-icy material fraction.
In Section 2 we give an overview of Cassini Radiometry observations of the rings and describe the calibration process and the resulting brightness temperature map. In Section 3, we present the method we use to model the microwave scattering and emission in the rings. We also list the ring parameters that determine the simulated brightness temperature. In Section 4, we compare simulated brightness temperatures from the model with Cassini observations, vary the ring properties, and search for the best-fit parameters. In section 5, we discuss the implication of our results and suggest possible ring origin scenarios that could explain our findings. In Section 6, we summarize our conclusions on the C ring particle porosity, the radially varied non-icy material fraction profile and their implications for the rings’ origin and age.
Section snippets
Observations & deconvolution
The Cassini RADAR instrument (Elachi et al., 2004) scanned Saturn and its rings at 2.2 cm wavelength on twelve occasions during its prime mission between Dec 2004 and Oct 2008. There are two categories of radiometric observations of Saturn's rings: low-resolution map scans from a distance of ∼20 RS (where RS = 60,330 km is Saturn's radius) and high-resolution “spoke scans” (not related to the fuzzy features seen in the B ring) from a distance of ∼5 – 8 RS. The map scan “footprints” (Fig. 1 upper
Brightness temperature model
The observed brightness temperature Tb is composed of four components: (1) directly transmitted Saturn radiation (which exists in occultation data only); (2) scattered Saturn radiation; (3) intrinsic thermal emission; and (4) cosmic microwave background (CMB) contributions.
To simulate the observed brightness we use the Monte-Carlo-based photon counting code SimRings (Dunn et al., 2002). SimRings is able to deal with multiple scattering within the ring layer (where the light source is from
Results
For a particle size distribution with our nominal choice of power law index q = 3.15, small particles dominate the scattering process in the C ring. Most of these small particles have a size parameter smaller than the bound beyond which non-sphericity becomes important. Furthermore, considering that the C ring particles are relatively widely separated due to the low optical depth there, they can be treated as independent spheres. Therefore, Mie scattering theory is expected to be a good
Interpretation & discussion
The radial variability of the derived non-icy material fraction is surprising and warrants discussion. We expect that after their formation, the rings have been continuously polluted and darkened by extrinsic meteoroid bombardment. The nominal one-sided incident micrometeoroid flux on a flat plate (i.e., flux at “infinity”) is given by before being affected by gravitational focusing (Morfill et al., 1983, Cuzzi and Estrada, 1998, Estrada et al., 2015). For the
Conclusions
We have calibrated and analyzed the Cassini Radiometry observations of Saturn's rings. The high brightness temperature in the C ring as seen where the ring obscures the planet suggests a scattering phase function that is much more forward-directed than a Mie phase function. We find that we can obtain a consistent scattering phase function by adding 70–75% porosity to the C ring particles. As a consistency check, we have simulated back scattering in the C ring with these highly porous particles
Acknowledgments
We thank NASA Outer Planets Research Program (Grant Number NNX14AR27G) for funding this work. We thank Luke Dones for providing an estimate for the timescale of catastrophic breakup of Saturnian fragments. We thank Joseph Burns, Matthew Tiscareno and Michael Evans for helpful conversations. Finally we gratefully acknowledge those who designed, developed and operate the Cassini/Huygens mission, which is a joint endeavor of NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI)
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Saturn's C ring and Cassini division: Particle sizes from Cassini UVIS, VIMS, and RSS occultations
2020, IcarusCitation Excerpt :Inverse opacities from the power-law parameters assuming a nominal ρ = 1 g/cm3 particle density are also shown in Table 2. Zhang et al. (2016) used Cassini microwave radiometry to probe the fraction of non-icy material in Saturn's rings. By fitting brightness temperature measurements of Saturn's ring ansa to several ring models they determined that in the inner and outer C ring, particles are 1–2% silicates with porosities of ~75% (ρ ~ 0.24 g/cm3).