A spectroscopic study of the surfaces of Saturn's large satellites: HO ice, tholins, and minor constituents

Abstract

We present spectra of Saturn's icy satellites Mimas, Enceladus, Tethys, Dione, Rhea, and Hyperion, 1.0-2.5 µm, with data extending to shorter (Mimas and Enceladus) and longer (Rhea and Dione) wavelengths for certain objects. The spectral resolution (R = λ/ λ) of the data shown here is in the range 800-1000, depending on the specific instrument and configuration used; this is higher than the resolution (R = 225 at 3 µm) afforded by the Visual-Infrared Mapping Spectrometer on the Cassini spacecraft. All of the spectra are dominated by water ice absorption bands and no other features are clearly identified. Spectra of all of these satellites show the characteristic signature of hexagonal H 2 O ice at 1.65 µm. We model the leading hemisphere of Rhea in the wavelength range 0.3-3.6 µm with the Hapke and the Shkuratov radiative transfer codes and discuss the relative merits of the two approaches to fitting the spectrum. In calculations with both codes, the only components used are H 2 O ice, which is the dominant constituent, and a small amount of tholin (Ice Tholin II). Tholin in small quantities (few percent, depending on the mixing mechanism) appears to be an essential component to give the basic red color of the satellite in the region 0.3-1.0 µm. The quantity and mode of mixing of tholin that can produce the intense coloration of Rhea and other icy satellites has bearing on its likely presence in many other icy bodies of the outer Solar System, both of high and low geometric albedos. Using the modeling codes, we also establish detection limits for the ices of CO 2 (a few weight percent, depending on particle size and mixing), CH 4 (same), and NH 4 OH (0.5 weight percent) in our globally averaged spectra of Rhea's leading hemisphere. New laboratory spectral data for NH 4 OH are presented for the purpose of detection on icy bodies. These limits for CO 2 , CH 4 , and NH 4 OH on Rhea are also applicable to the other icy satellites for which spectra are presented here. The reflectance spectrum of Hyperion shows evidence for a broad, unidentified absorption band centered at 1.75 µm.

Surface charging of Saturn's plasma‐absorbing moons

“…[11] All four moons have an icy composition [Cruikshank et al, 2005] and therefore have very low surface conductivity. Surface currents and their effects [e.g., Gurnett, 1972;Borisov and Mall, 2002] will be neglected here.…”

Quantitative compositional analysis of martian mafic regions using the MEx/OMEGA reflectance data 1. Methodology, uncertainties and examples of application

“…Employing such a theory in concert with optical constants of components allows computational exploration of the extensive space of parameters. A widely accepted theoretical model describing such scattering has been developed by Hapke (1981) and has been used to analyze the surface properties of the Moon, asteroids and satellites (e.g., Johnson and Grundy, 2001;Cruikshank et al, 2003Cruikshank et al, , 2005Denevi et al, 2008). Recently, other models (Douté and Schmitt, 1998;Shkuratov et al, 1999) have started to play an important role, which gives the opportunity to extend the analysis of OMEGA spectra beyond the simple recognition of mineral type, by quantifying their abundances.…”

Hapke modeling of Rhea surface properties through Cassini-VIMS spectra

“…The Hapke IMSA model (Hapke, 1993(Hapke, , 2002(Hapke, , 2008Hapke et al 2009) is one of several models developed to study the scattering properties of packed media (Lumme and Bowell, 1981;Drossart 1993;Shkuratov et al, 1999). It is an analytic two stream approximate solution to the radiative transfer equation and it has been applied successfully to perform photometric corrections of imaging data (Hudson and Ostro, 1999;Domingue et al, 2009), to investigate physical properties of regoliths (Mallama et al, 2002;Buratti et al, 2004) and to estimate surface compositions of planetary surfaces (Cruikshank et al, 2001;Cruikshank et al, 2005;Poulet et al, 2002).…”