  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.
