The state of the plasma sheet and atmosphere at Europa D. E. Shemansky 1, Y. L. Yung 2, X. Liu 1, J. Yoshii 1, C. J. Hansen 3, A. Hendrix 4, L. W. Esposito 5 1 SET/PSSD, Pasadena CA 2 CIT, Pasadena CA 3 PSI, Ivins UT 4 PSI Los Angeles CA 5 LASP, UC Boulder CO Based on Astrophys. J., 797:84, 2014, Dec 20

Summary Analysis of a deep exposure of the plasma sheet at Europa on 2001 DOY 012 using a recalibration of the Cassini UVIS EUV spectrograph has allowed determination of the state of the plasma. The plasma is composed primarily of long-lived ions and electrons originating at the Io plasma torus. The interpretation of emission from Europa, first observed with HST in 1994, as originating from an O 2 atmosphere is disputed after reanalysis of the Cassini FUV emission spectrum. The present work concludes that O I is the primary species at a density two orders of magnitude below the previous O 2 model. Review of evidence from the time of Voyager through the present indicates that the stochastic train of measurements is consistent with the state of the plasma sheet obtained from the Cassini flyby. No evidence has been found for the presence of surface vents of H 2 O into the Europa atmosphere in the Cassini UVIS data. Io is the primary driver for variability in the system.

Cassini UVIS spectrograph exposure Jan Exposure position of UVIS slit on the Jupiter system with spacecraft pointing locked on Europa. Exposure provides image cube vector of 64 X 1024 elements. S/C – planet range: 16 Mkm.

Properties of the plasma environment

Fig 1: Exposure across latitude width of Europa plasma sheet Plasma sheet spectrum 148 hour exposure (red trace) compared to background spectrum (black trace) 398 hour exposure at latitudes above and below the plasma sheet. Identified features in the plasma are sulfur and oxygen ion emission.

Fig 2: Comparison of spectra of the Europa plasma sheet and the Io plasma torus. The Europa spectrum (brown trace) is compared to the Io torus spectrum (red trace) normalized at the 833 A (OII/OIII) feature. The emission features are identified as sulfur and oxygen ions excited by electrons at different temperatures.

Fig 3: The modeled Europa plasma sheet spectrum The observed spectrum (red trace) is compared to a model fit (light green trace) containing oxygen and sulfur ions in charge states n=1 to 4, at an electron temperature T e = 250,000 K. The O IV component is the light red trace, and the modeled LISM/IPM component is in light blue.

Fig 4: The modeled Io plasma torus spectrum The blue trace is the observed spectrum of the eastern ansa of the Io orbit. The red trace is the complete model, composed of sulfur and oxygen ions at charge level n = 1 to 4. The S IV model component is shown in light green. The electron temperature T e = 75,000 K. There are no neutrals (n = 0) in the model.

Io torusEuropa plasma sheet Enceladus plasma sheet T e (1000K) [e] (cm -3 )  [N i ] (cm -3 )  [N] (cm -3 ) [O II] (cm -3 ) [O III] (cm -3 ) [O IV] (cm -3 )< [S II] (cm -3 ) [S III] (cm -3 ) [S IV] (cm -3 ) [S V] (cm -3 ) E r a (yoctoW cm -3 ) ErT b (teraW) X  (yoctoW part -1 ) X S (10 25 atoms s -1 )700< Plasma properties | a) volumetric radiative loss. b) Total radiative loss

Conclusions: plasma properties The Cassini UVIS observation shows a low density high temperature plasma environment at Europa that originates mainly at the Io torus at 5.9 R J. The electron density at 9.4 RJ derived from the UVIS 2001 data is consistent within a factor of two of measurements from Voyager in 1979 and Galileo in 1996 – The electron temperature from the three encounters is the same within error. The results are consistent with the general conclusion drawn by Russell(2005) that Io is the primary source inserting ion mass for the accumulation of energy into the magnetosphere.

Properties of the Europa atmosphere

Fig 5: Cassini UVIS FUV spectrum of Europa OI 1304 A and 1356 A emission multiplets from the 3 S and 5 S states compared to optically thick and thin model calculations.

Fig 6: Observed Europa atmosphere emission brightness of the OI 3 S and 5 S states Disk averaged Rayleigh brightness of the 1304 A and 1356 A multiplets obtained from HST and Cassini UVIS from 1994 to The HST measurements require correction for the reflection of the underlying solar line at 1304 A.

Fig 7:Ratio of disc averaged OI 5 S and 3 S emission from Europa The emission ratio ( 5 S/ 3 S) measured in HST and Cassini UVIS observations compared to the ratio calculated from dissociative excitation of O 2 and directly excited OI. Optically thick gas increases the modeled ratio.

Fig 8: Latitudinal profiles of atomic hydrogen and oxygen emission Photometric profiles of H Ly  and OI 5 S and 3 S emission on a Europa radius scale. The upper limit for atomic hydrogen abundance is two orders of magnitude below the Smyth & Marconi(2006) H 2 O model.

Fig 9: Upper limit model spectra of H 2 emission from Europa Differential brightness spectra modeled for the plasma sheet and Europa atmosphere. The limits to H 2 abundance are two orders of magnitude below the Smyth & Marconi (2006) H 2 O model.

Conclusions: Europa atmosphere The Cassini UVIS observations indicate the atmosphere is an ionosphere extending to the surface, composed of 1 to 2 X 10 5 OI cm -3 and ~10 4 OII cm -3, constituting a hard vacuum. There is no evidence for products of atmospheric H 2 O physical chemistry on the scale of the Smyth & Marconi (2006) model of the atmosphere or the plasma sheet. Measurements of the magnetosphere electrons at 9.4 R J at Voyager encounter in 1979 and Galileo in 1996 to 1983 are consistent with the level of mass loading in the estimates from the Cassini UVIS observations.

Conclusions: Europa geophysical activity A single report of water product emission has been obtained in the published measurements from 1994 to 2014 (Roth et al 2014). Polar plumes in 2013 averaged over a 7 hour period contained an inferred 1.3 X H 2 O molecules. The dissociation rate into H + OH corresponding to HLy  emission is ~10 28 s -1, comparable to the continuous rate produced by Io. The H product would form a torus at the Europa orbit. The OH would be converted of O I in the atmosphere. The disk averaged atomic oxygen emissions in the 2013 plume observation are close in magnitude to the mean of all measurements since 1994, indicating that the global atmosphere was unaffected. The present model has ~10 30 atoms in in the atmosphere (100 sec of plume emission). This is a possible conflict. OH and O I physical chemistry impacting the surface is unknown. Atmospheric recycle time is unknown. This can be addressed only with atmospheric observation. Further modeling is needed. The Roth et al observation is critically important because it is the only direct indication of an H 2 O vent into the vacuum.. Magnetosphere energetic particle precipitation through surface sputtering is a source of atmospheric gas. Ip et al (1998) using Galileo EPD data concluded that sulfur and oxygen ions were the main sputtering species. We suggest that given the Io torus source these ions will build charge levels greater than 2 in their lifetime in the magnetosphere system. Sputtering through multiple fast charge exchange reactions near the upper surface molecular layer will have high efficiency and tend to send atomic species into the vacuum. Absolute efficiencies are uncertain.