Silicates Detected on Saturnian Moon Iapetus

Silicates on Iapetus from Cassini's Composite Infrared Spectrometer

Authors:

Young et al

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.

Saturn's Circumplanetary Disk Estimated From Cratering on Iapetus

Constraints on planetesimal disk mass from the cratering record and equatorial ridge on Iapetus

Authors:

Rivera-Valentin et al

Abstract:

Iapetus, the outermost regular satellite of Saturn, has a drastic albedo dichotomy and an equatorial circumferential ridge that reaches heights of 20 km and widths of 70 km. This moon is thought to have formed concurrently with Saturn, and so would have experienced an intense bombardment after its formation. The ridge, which has been inferred to be one of the most ancient features on Iapetus' surface, could reasonably be expected to have been eroded by impacts; however, it has retained long continuous sections and a nearly pristine triangular shape with ridge slopes reaching ∼ 40∘. We use these observations, along with crater counts on Iapetus' surface, to constrain the total bombardment mass experienced by the satellite since its formation. The ridge morphology and the crater population recorded on Iapetus indicate it received less than 20% of the bombardment predicted by the classic Nice model for early Solar System evolution. Under the recently proposed scenarios of planetsimal-driven migration of the young outer planets including more realistic disk conditions, our results would imply a planetesimal disk mass of MD∼12−34M⊕.

How did Iapetus get its Equitorial Ridge?

Topographic Constraints on the Origin of the Equatorial Ridge on Iapetus

Authors:

Lopez Garcia et al

Abstract:

Saturn's moon Iapetus has an equatorial ridge system, which may be as high as 20 km, that may have formed by endogenic forces, such as tectonic and convective forces, or exogenic processes such as debris infall. We use high-resolution topographic data to conduct a topographic analysis of the ridge, which suggests a predominantly triangular morphology, with some ridge face slopes reaching 40 degrees, allowing for an exogenic formation mechanism.