On the Possibility of a Plume Traversal During Galileo's E12 Encounter With Europa
Abstract
On 16 December 1997 the Galileo spacecraft flew by Europa's sunlit trailing hemisphere with altitude at closest approach about 200 km. It is conjectured that near closest approach the spacecraft flew through an ionized plume thus providing direct evidence of ejection of volatiles from Europa's surface [Jia et al., Nature Astronomy, 2018]. This reasoning is based on two lines of evidence. One is an interpretation of PWS plasma wave data exhibiting an emission near 400 kHz at 12:02 UT. If this is the upper hybrid resonance, then it represents a localized increase in plasma density by a factor of 3, or more. The second is an MHD model which better accounts for some elements of the observed magnetic perturbations if a plume with approximately that plasma density was present at that location. In this presentation we discuss additional context provided by plasma measurements from the Galileo plasma analyzer (PLS). Flow vectors derived from measurements of the ions show evidence of a significant deflection of plasma away from the moon starting at 12:01 UT, one minute prior to the reported time for the plume traversal, and persisting over a period of 8 to 10 minutes. Ion densities during the 44-minute duration of the encounter are in good agreement with electron densities inferred from the plasma waves, with the exception of the 12:02 observation. The PLS ion measurements do not substantiate elevated density at that time. In this presentation, we discuss the PLS observations. We investigate the possibility that PLS may have missed the plume, and we discuss constraints on the magnitude and the spatial extent of plume-related plasma phenomena for this encounter. Whether this plume existed, or not, these observations have important implications for future exploration of this water world and others where ionospheric currents affect the magnetic environment.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMSM44A..08P
- Keywords:
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- 2732 Magnetosphere interactions with satellites and rings;
- MAGNETOSPHERIC PHYSICS;
- 6218 Jovian satellites;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 6250 Moon;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 6280 Saturnian satellites;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS