Cassini CAPS Identification of Pickup Ion Compositions at Rhea

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 CO2+ during both the R1 and R1.5 encounters. Accounting for this nongyrotropy, we show that these possess comparable along‐track densities of ∼2 × 10−3 cm−3. Negatively charged pickup ions, also detected during R1, are surprisingly shown as consistent with mass 26 ± 3 u which we suggest are carbon‐based compounds, such as CN−, C2H−, C2−, or HCO−, sputtered from carbonaceous material on the moon's surface. The negative ions are calculated to possess along‐track densities of ∼5 × 10−4 cm−3 and are suggested to derive from exogenic compounds, a finding consistent with the existence of Rhea's dynamic CO2 exosphere and surprisingly low O2 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.

Detection of Negative Pickup Ions at Saturn's Moon Dione

“…These authors also discuss a possible detection of O − pickup ions at a slightly earlier time during this flyby but were unable to make a conclusive identification. In principle, our estimated mass for the Dione D2 negative ion feature could be consistent with the carbon‐bearing negative ion species suggested by Desai et al (2018). However, given the known presence of a CO 2 –O 2 exosphere and the consistency of our mass estimate with oxygen, O − pickup ions from an exospheric source would appear to be the more likely candidate for the negative pickup ion feature that we observe at Dione.…”

“…However, we note that it is also possible that this may indicate the presence of several, but less abundant, negative ion species in addition to O − , with energy peaks that are effectively hidden by the limited energy resolution of CAPS-ELS. We note that the negative pickup ions observed at Rhea were also found to have a nongyrotropic distribution (Desai et al, 2018).…”

“…As the count rate remains relatively steady across the time range when the feature is seen, this implies that the O − source rate is also relatively constant throughout the source region (e.g., in the vicinity of Dione's surface). The relatively high proportion of negative ions at both Dione and Rhea (as reported by Desai et al, 2018) is somewhat surprising, as cross sections for plausible negative ion formation mechanisms (e.g., dissociative electron attachment to molecular oxygen) are much smaller than the relevant ionization cross sections (e.g., photo and electron impact ionization and charge exchange) for formation of O 2 + (Itikawa, 2009;Teolis, 2012;Teolis & Waite, 2016). However, it should be noted that these cross sections are given for gas phase interactions.…”

“…Indeed, in addition to the neutral particle observations by INMS, CAPS observed O 2 + and CO 2 + pickup ions during a close flyby of Rhea (Teolis et al, 2010). During this flyby, CAPS‐ELS also observed negative pickup ions that have been suggested to consist of O − (Teolis et al, 2010) and heavier carbon‐bearing species (Desai et al, 2018). These pickup ions were of likely exospheric origin and were found in ring‐like distributions with pitch angles close to 90°.…”

“…Such negative pickup ions were also reported during the Rhea R1 flyby that occurred on 26 November 2005 (day of year 300) (Teolis et al, 2010), and these were originally identified as O − . However, recent work by Desai et al (2018) has indicated that the feature seen at Rhea may instead be due to a negative ion with a mass of 23–29 u, tentatively identified as CN − , C 2 − , or C 2 H − . These authors also discuss a possible detection of O − pickup ions at a slightly earlier time during this flyby but were unable to make a conclusive identification.…”