History of the Fractional Pollution of Saturn’s Rings from Cassini UVIS

Abstract
Over the history of Saturn’s rings, they have been bombarded by exogenous meteoritic material which has altered their composition.  Utilizing our Markov-chain based model for the bombardment of the rings, we can characterize the history of the bombardment of the rings and track the fractional pollution over time.  However, this model does not tell us what the current state of the fractional pollution is, nor does it tell us what the composition of the meteoritic pollutant is.  For that, we can utilize spectroscopic observations from the UVIS instrument that was onboard the Cassini Spacecraft.
Utilizing the FUV dataset collected by UVIS, we calculate the current state of the fractional pollution of Saturn’s A, B and C rings by computing best-fits to the observed spectra using Hapke’s model for bidirectional reflectance of an intimate mixture of regolith grains (ice grains plus exogenous grains).  Further, we can test different pollutants to determine which pollutant spectra fits best to the observations.  Our candidate pollutant is cometary material reflectance as measured by the Rosetta spacecraft [5], which we compare against amorphous carbon.
Comparing our bombardment model with the fractional pollution calculated from UVIS, allows us to estimate the age of Saturn’s rings, and to gain insight into the history of the rings.

1. Bombardment Model Updates
We update our bombardment model [1] to include the latest estimates for the exogenous mass flux into the Saturn system [2].  We also compare our results to those presented by Estrada et al. at EPSC/DPS 2019 [6].

2. Spectroscopy
To determine the current fractional pollution of the rings, we perform a non-linear least-squares fit analysis using a Hapke [3] model for bidirectional reflectance of an intimate mixture of regolith of water-ice and pollutant grains.
As Poppe 2016 [7] modeled, we assume that the exogenous material entering the Saturn system originates primarily from the Kuiper belt.  As a proxy for Kuiper belt material spectra, we use the phase dependent spectra of comet 67P/Churyumov–Gerasimenko from the Rosetta Alice instrument [4,5] as our pollutant ring material.  
By computing a best-fit Hapke model, using these to material spectra, we can estimate the fractional pollution present in the Rings.

3. Summary
Our best fit spectra for the A, B and C rings yield a fractional pollution which is then compared to the updated bombardment model simulation in order to understand the history of the rings and their age.

References
[1] Elliott, J.P., Esposito, L.W.: Regolith depth growth on an icy body orbiting Saturn and evolution of bidirectional reflectance due to surface composition changes, Icarus, Vol. 212.1, pp. 268-274, 2011.
[2] Altobelli, N. et al.: Exogenous dust at Saturn observed by CASSINI-CDA, Cassini Science Symposium, 12-17 August 2018, Boulder Colorado, USA, 2018.
[3] Hapke, B.: Theory of reflectance and emittance spectroscopy. Cambridge university press, 2012.
[4] Stern, S. A., et al.: First extreme and far ultraviolet spectrum of a Comet Nucleus: Results from 67P/Churyumov-Gerasimenko. Icarus, Vol. 256, pp. 117-119, 2015.
[5] Feaga, L. M., et al.: Far-UV phase dependence and surface characteristics of comet 67P/Churyumov-Gerasimenko as observed with Rosetta Alice. A&A, Volume 583, 2015
[6] Estrada P. R., et al.: Evolution of Saturn’s Rings due to Combined ViscousSpreading and Micrometeoroid Bombardment. EPSC-DPS Joint Meeting, Geneva, 2019.
[7] Poppe A. R.: An improved model for interplanetary dust fluxes in the outer Solar System. Icarus, Vol 264, pp 369-386, 2016.