Probing the origin of the dark material on Iapetus

TL;DR

This study combines spectral analysis and dynamical modeling to investigate the origin of the dark material on Iapetus, suggesting Phoebe as the main source and considering multiple contributing sources.

Contribution

It applies the G-mode spectral classification to Cassini/VIMS data and evaluates dust transfer mechanisms, proposing a multi-source origin for Iapetus' dark material.

Findings

Phoebe identified as the main source of dark material.

Mass transfer via Poynting-Robertson drag is efficient for outer ring particles.

Multiple sources, including extinct satellites, contributed to the dark material.

Abstract

Among the icy satellites of Saturn, Iapetus shows a striking dichotomy between its leading and trailing hemispheres, the former being significantly darker than the latter. Thanks to the VIMS imaging spectrometer on-board Cassini, it is now possible to investigate the spectral features of the satellites in Saturn system within a wider spectral range and with an enhanced accuracy than with previously available data. In this work, we present an application of the G-mode method to the high resolution, visible and near infrared data of Phoebe, Iapetus and Hyperion collected by Cassini/VIMS, to search for compositional correlations. We also present the results of a dynamical study on the efficiency of Iapetus in capturing dust grains travelling inward in Saturn system to evaluate the viability of Poynting-Robertson drag as the physical mechanism transferring the dark material to the satellite. The results of spectroscopic classification are used jointly with the ones of the dynamical study to describe a plausible physical scenario for the origin of Iapetus' dichotomy. Our work shows that mass transfer from the outer Saturnian system is an efficient mechanism, particularly for the range of sizes hypothesised for the particles composing the newly discovered outer ring around Saturn. Both spectral and dynamical data indicate Phoebe as the main source of the dark material. However, we suggest a multi-source scenario where now extinct prograde satellites and the disruptive impacts that generated the putative collisional families played a significant role in supplying the original amount of dark material.