Silicates on Iapetus from Cassini's Composite Infrared Spectrometer

TL;DR

This study uses Cassini's infrared data to identify silicate minerals on Iapetus, providing the first spectral evidence of silicates on this icy moon's dark terrain, which has implications for its composition and origin.

Contribution

The paper presents the first detection of silicate spectral features on Iapetus using Cassini's CIRS data, advancing understanding of its surface composition.

Findings

Detected emissivity feature at ~855 cm-1 indicating silicates

Identified a possible doublet at 660 and 690 cm-1

Muscovite spectrum at 125K matches observed features

Abstract

We present the first spectral features obtained from Cassini's Composite Infrared Spectrometer (CIRS) for any icy moon. The spectral region covered by CIRS focal planes (FP) 3 and 4 is rich in emissivity features, but previous studies at these wavelengths have been limited by low signal to noise ratios (S/Rs) for individual spectra. Our approach is to average CIRS FP3 spectra to increase the S/R and use emissivity spectra to constrain the composition of the dark material on Iapetus. We find an emissivity feature at ~855 cm-1 and a possible doublet at 660 and 690 cm-1 that do not correspond to any known instrument artifacts. We attribute the 855 cm-1 feature to fine-grained silicates, similar to those found in dust on Mars and in meteorites, which are nearly featureless at shorter wavelengths. Silicates on the dark terrains of Saturn's icy moons have been suspected for decades, but there have been no definitive detections until now. Serpentines reported in the literature at ambient temperature and pressure have features near 855 and 660 cm-1. However, peaks can shift depending on temperature and pressure, so measurements at Iapetus-like conditions are necessary for more positive feature identifications. As a first investigation, we measured muscovite at 125K in a vacuum and found that this spectrum does match the emissivity feature near 855 cm-1 and the location of the doublet. Further measurements are needed to robustly identify a specific silicate, which would provide clues regarding the origin and implications of the dark material.