Fast and Slow Water Ion Populations in the Enceladus Plume

Abstract

Ion velocities have been measured during the Enceladus E3 and E5 flybys using the Cassini Plasma Spectrometer (CAPS) instrument on the Cassini spacecraft. Data from three sensors in the CAPS instrument have been examined from two flybys that occurred during 2008. Positive ion measurements from the CAPS Ion Beam Spectrometer and Ion Mass Spectrometer have been used to measure positive ion velocities. The CAPS Electron Spectrometer has been used to complement the positive ion findings with measurements of negative ion velocities. Two velocities for the positive ions are found, with the fast ions (2.3–5.8 km/s) originating from the high‐speed neutral gas emission and slow ions (0.2–2.2 km/s) associated with the low‐speed thermal gas emission from Enceladus. Negative ions were found to be near stationary or northerly traveling, implying a deceleration mechanism within the plume. A tentative detection of fast negative ions was also recorded for one of the flybys. These findings will aid in future modeling of plume dynamics.

Resolving Space Plasma Species With Electrostatic Analyzers

“…The instrument observed fluxes of negative ions in the Enceladus plumes during the Enceladus encounters in 2008 (Coates et al, 2010). The analysis of the observations revealed the abundance of water group negative ions (Haythornthwaite et al, 2020). In Figure 7, we show ELS observations during the Cassini's E3 flyby, in terms of logarithm of the count rates (C/Δτ), recorded in azimuth anode 5, as a function of energy and time.…”

“…In another example, observations by the electron spectrometer of Cassini Plasma Spectrometer (CAPS/ELS, Young et al, 2004) allow the detection of heavy negative ions in the vicinity of Titan (e.g., Coates et al, 2007;Desai et al, 2017;Wellbrock et al, 2013;Wellbrock et al, 2019) and Enceladus plume (e.g., Coates et al, 2010;Haythornthwaite et al, 2020). In these cases, the heavy ion plasma is quasistatic and the bulk velocity of the ions in the spacecraft frame is the spacecraft ram speed, therefore, it is the same for all plasma species.…”