Cassini CAPS identification of pickup ion compositions at Rhea

TL;DR

This study uses Cassini data to identify pickup ion compositions at Rhea, revealing CO2+ and negatively charged carbon-based ions, which shed light on the moon's exosphere and surface composition.

Contribution

First detection and analysis of nongyrotropic CO2+ and negative carbon-based ions at Rhea, providing new insights into its exospheric composition and surface-sputtering processes.

Findings

Detected CO2+ ions with comparable densities during Rhea encounters.

Identified negatively charged ions consistent with carbon-based compounds.

Suggested exogenic origin of negative ions, indicating surface composition insights.

Abstract

Saturn's largest icy moon, Rhea, hosts a tenuous surface-sputtered exosphere composed primarily of molecular oxygen and carbon dioxide. In this Letter, we examine Cassini Plasma Spectrometer velocity space distributions near Rhea and confirm that Cassini detected nongyrotropic fluxes of outflowing CO$_2^+$ during both the R1 and R1.5 encounters. Accounting for this nongyrotropy, we show that these possess comparable alongtrack densities of $\sim$2$\times$10$^{-3}$ cm$^{-3}$. Negatively charged pickup ions, also detected during R1, are surprisingly shown as consistent with mass 26$\pm$3 u which we suggest are carbon-based compounds, such as CN$^-$, C$_2$H$^-$, C$_2^-$, or HCO$^-$, sputtered from carbonaceous material on the moons surface. These negative ions are calculated to possess alongtrack densities of $\sim$5$\times$10$^{-4}$ cm$^{-3}$ and are suggested to derive from exogenic compounds, a finding consistent with the existence of Rhea's dynamic CO$_2$ exosphere and surprisingly low O$_2$ sputtering yields. These pickup ions provide important context for understanding the exospheric and surface-ice composition of Rhea and of other icy moons which exhibit similar characteristics.