1. Possible plumes at Europa, Observed by Cassini? C. J. Hansen (+ Don + Amanda + Anya + Larry) June 2014

2. New HST Observations of Europa Roth et al. new result: surplus emission at Lyman alpha and 1304 Å near south pole interpreted as water vapor eruptions similar to Enceladus 1304 and 1216 emissions from e- on H 2 O Plus 1304 and 1356 from bound O 2 atmosphere Roth et al., ScienceExpress 1247051

3. New HST Observations of Europa 3 HST observations, only one showed enhanced emission Modeled as two 200 km high Enceladus-like plumes at 180W, 55S and 75S Variable emission attributed to tidal opening and closing of fissures in paper –Detection at true anomaly = 185-218, ~apojove –Non-detection at true anomaly = 343-13 and 289-318, perijove But no detection in subsequent HST observations Plumes are transient?

4. Did UVIS detect Europa’s Plumes when Cassini flew by? Can we provide a historic record? Hansen, C. J., D. Shemansky and A. Hendrix, “Cassini UVIS Observations of Europa’s Oxygen Atmosphere and Torus”, Icarus 176:305-315 (2005).

5. Cassini UVIS Observations Potential discovery of plumes at Europa motivated us to take another look at Cassini Europa data acquired as Cassini flew past Europa -> test idea that fissures open and close as a function of mean anomaly Three types of observations: 1.Targeted to Europa on approach ~Half were lost when s/c went into safe mode 2.System scans meant for Io torus but also including Europa’s orbit Provides more orbital longitude coverage 3.Two high spectral and full spatial observations were acquired Jan. 6 and Jan. 12 2001 to study Europa’s atmosphere and torus (FUV and EUV) Targeted to Europa to follow Europa around an arc of its orbit New unpublished EUV analysis carried out in the last few months by Don Shemansky

6. 1. Europa Targeted Observations TimeMean Anomaly 2000-348T0600112 2000-364T0754316.6 2000-366T0635151 2001-002T115420 2001-010T1026112 Five targeted observations spread out through orbit, however Known issue with interpretation of FUV data –Used occultation slit So CJH decided to look at EUV data instead (for enhanced 833 or 950 emission) –Also used the occ slit –Also has issues (row 5) –All 15 rows (except row 5) looked similar Tried just summing everything to use as a photometer to just look for differences but…

7. 1. Europa Targeted Observations Conclusion: EUV data not useful either Europa is in Io torus as seen from the s/c –That is why all rows looked the same (True of all of our observations – can’t separate changes in the torus from Europa)

8. 2. System Scan Observations System scans meant for Io torus but Europa orbit also captured in slit Inbound (Phase B) torus observation was centered on Jupiter with the slit perpendicular to Jupiter’s spin axis Selected 311T22:32 torus observation for analysis –1000 sec integration per footprint, 18 hr duration –32 spatial rows; 512 spectral elements –Ansae of Europa torus in pixels 3 and 29 –Ansae of Io torus in pixels 8 and 24 –Europa passed through mean anomaly at ~312T23:20 so this would be “normal” Europa To compare Europa at mean anomaly ~180 selected 312T16:19 torus observation –1000 sec integration per footprint –32 spatial rows; 512 spectral elements

9. 2. System Scan Observations Outcome: It is virtually impossible to detect Europa at 1356 or Europa’s torus at 1304 although the Io torus shows up nicely at 1304

10. 3. Using Europa to map out torus with IVP Pixel 63 Pixel 0 Start time: 2001 006T0730 Duration: 4.7 hr Range: 1.2 x 10 6 km Integration time: 1000 sec Number of records: 17 FUV 1.5 mR slit width 1024 spectral elements 64 spatial rows FUV range 1115 to 1914 Å Europa subtended 0.28 mR Europa phase angle: 94 0 Europa subs/c lat: -2.1 0 Europa subs/c lon: 201-220 W True anomaly = 50 – 70 6 January 2001

11. 12 January 2001 Start time: 2001 012T0630 Duration: 11.4 hr Range: 15.6 x 10 6 km Integration time: 1000 sec Number of records: 41 FUV 1.5 mR slit width 1024 spectral elements 64 spatial rows Europa subtended 0.20 mR Europa phase angle: 116 0 Europa subs/c lat: -2.8 0 Europa subs/c lon: 64 - 103 W Europa moved from the far side through the ansa to the near side of its orbit as seen from Cassini. True anomaly = 301 to 349

12. Europa Orbit Geometry - Both 0180 Roth detection 185 to 218 90 270 Roth non-detection 343 to 13 Roth non-detection 289 to 318 Cassini detection 50 to 70 Cassini non-detection (decreasing) 300 to 349 Ellipticity exaggerated

13. January 6 / January 12 FUV Comparison Comparison of January 6 and January 12, 2001 exposures With Europa at the ansa of its orbit as seen from Cassini, Io’s torus was not within the UVIS field of view on January 12. O 2 Column densities (cm -2 ) Jan. 6: 12.4 x 10 14 Jan. 12: 7.4 x 10 14 In family with original Hall (1994) detection

14. Jan 6 and Jan 12 FUV re-analyzed by Don Originally reported 3x difference in 1304 flux Jan 6 vs Jan 12, however better flat fields, new simulation and corrected computation of size difference led to no significant difference at all Same conclusion wrt lyman alpha So, we were not able to say we had seen evidence of a plume; No confirmation or rebuttal to the mean anomaly dependence from this data

15. New Result for Europa’s Atmosphere Re-analysis of the FUV data did pay off in a new result Europa was sub- pixel in both observations (this view was on Jan. 6) Centered in the ISS NAC field of view thus offset in UVIS slit

16. Oxygen Emission Features This raw segment of the Europa spectrum shows the oxygen emission multiplets at 1304 Å and 1356 Å The 1304 Å emission feature is a triplet of emissions at 1302.2, 1304.9, and 1306.0 Å The 1356 feature is a doublet at 1355.6 and 1358.5 Å The UVIS is resolving the individual emission lines of the multiplets, consistent with observation of a point source Atmosphere is predominantly O not O 2 with a column density of 4.7 x 10 12 cm -2 Based on relative strengths of 1302, 1304, and 1306 emission Consistent with opacity due to self-absorption and scattering New result from Don:

17. Io Torus EUV Work by Amanda and Don has established the Io torus features in the EUV

18. New EUV analysis for Europa Io’s torus species diffuse out Composition well-known from work on the torus spectra Can identify what is from Io at the distance of Europa By integrating time and spatial pixels across the Europa torus in the Jan 12 dataset a much longer equivalent integration time is achieved Io is the dominant source of ions in Europa’s torus

19. New Jan 12 EUV Results* Plasma species from the Io torus (5.9 R J ) slowly diffuse out to the Europa orbit (9.4 R J ), converting to higher order ions more rapidly than other loss processes Electron impacts increase the ionization levels, as shown in the table, which shows the ratio of the species at 5.9 R J to 9.4 R J ; at 5.9 R J lower order ions are dominant compared to the population at 9.4 R J, while at 9.4 R J the numbers are close to equivalent for the higher order ions Sharply rising temperature of outward diffusing plasma from Io with increasing charge state of existing ion particles indicates mass loading is drastically reduced from the level at 5.9R J. SpeciesO II O IVS IIS IIIS IVS V Ratio, 5.9 /9.4 14.411.0<0.758339.33.6 * Paper in prep, Shemansky et al, 2014

20. Conclusion given at Europa Plumes Workshop Cassini UVIS detection of higher order ions at Europa’s orbit likely precludes an eruption within the year preceding the Cassini flyby We conclude the Roth et al detection was a transient, not cyclic, event There is other evidence for transient events (Saur et al., 2011; Russell et al., 2005; Helfenstein and Cook, 1984) We recommend that the Europa Clipper project consider an observational strategy that looks for evidence of transient events rather than ongoing cyclical eruptions –Look for plumes in many different places, not just those implicated by the HST data

21. Backup

22. 6 January 2001 Pixel 63 Pixel 0 Start time: 2001 006T0730 Duration: 4.7 hr Range: 1.2 x 10 6 km Integration time: 1000 sec Number of records: 17 FUV 1.5 mR slit width 1024 spectral elements 64 spatial rows FUV range 1115 to 1914 Å Europa subtended 0.28 mR Europa phase angle: 94 0 Europa subs/c lat: -2.1 0 Europa subs/c lon: 201-220 W True anomaly = 50 - 70

23. 12 January 2001 Start time: 2001 012T0630 Duration: 11.4 hr Range: 15.6 x 10 6 km Integration time: 1000 sec Number of records: 41 FUV 1.5 mR slit width 1024 spectral elements 64 spatial rows Europa subtended 0.20 mR Europa phase angle: 116 0 Europa subs/c lat: -2.8 0 Europa subs/c lon: 64 - 103 W Europa moved from the far side through the ansa to the near side of its orbit as seen from Cassini. True anomaly = 301 to 349

24. Full FUV Spectrum Full UVIS FUV spectrum, filtered and flat- fielded, acquired on January 6, summed over 4.7 hrs The intensity scale is logarithmic to portray the full dynamic range, including Lyman alpha Atomic oxygen emission at 1304 Å and 1356 Å Other features in the spectrum are dominated by Io torus emissions in sulfur and LISM emission in the H Lya line Ratio of 1304/1356 is diagnostic of O 2

25. Components of the Spectrum The original spectrum and derived components are shown for the flat-fielded exposure for FUV row pixel 32. The flat field was generated using LISM data collected during cruise. The LISM background signal at the time of the observation was modeled using an average of pixels 52 to 56 (heavy blue). Along the bottom of the plot the best-fit model predicts for Europa oxygen emission features are shown.

26. Reflected and Emitted Oxygen The average of rows 29, 30, 33, 34 superposed on the Europa row to show the contribution of Io plasma torus emission after flat-fielding and LISM removal, illustrated with the dark blue spectrum, the sum of pixel 31 and 32 The red spectrum shows the oxygen flux from Europa’s sunlit surface and atmosphere at 1304 and 1356 Å The weak solar reflection component is based on the reflected carbon feature at 1335 Å, which enabled determination of the albedo of Europa at 94 0 phase angle The turquoise line shows subsequent calculation of the contribution of reflected sunlight to the 1304 Å, 1356 Å and 1216 Å features Best fit is 98% O 2, 2% O

27. The 11.4 hr data set for January 12 divided into * far side (integrations 0 – 16), * ansa (integrations 17 – 33), and * near side (integrations 34 – 40) The spectra show the 1356 Å emission feature is constant over this interval (longer than one Jovian rotation), while the 1304 Å flux monotonically declines in intensity. Short Timescale Changes We concluded that changes in flux are due to the environment, not Europa

28. Lyman Alpha “The Europa 2001 JAN 06 data, obtained near 4 RJ impact parameter, shows more variation in the SW data than the JAN 12 data, but is very similar in shape to the torus stare results obtained just prior to the Europa observation. One significant deviation in the Europa JAN 06 SW data occurs at the location of Europa with an enhancement at rows 32 and 33, correlating with an enhancement in these rows in the Europa JAN 06 LW data. This result is consistent with H emission from Europa. No detectable corresponding enhancement occurs in the Europa JAN 12 data, where the SW data is very flat.” Don Shemansky analysis summary

29. Europa Orbit Geometry - HST 0180 Roth detection 185 to 218 90 270 Roth non-detection 343 to 13 Roth non-detection 289 to 318 Ellipticity exaggerated

30. Europa Orbit Geometry - Cassini 0180 90 270 Cassini detection 50 to 70 Cassini non-detection (decreasing) 300 to 349 Ellipticity exaggerated

31. Oxygen is not a contributor to the torus identified by Mauk et al. (2003) at 9.5 R J at levels detectable by UVIS. The atomic oxygen in the Europa’s extended atmosphere is subject to ionization and loss from photoionization, charge exchange, and electron dissociation. The rate of ionization from these three processes is ~1.6 x 10 -6 per sec, thus the lifetime for an oxygen atom in Europa’s exosphere is estimated to be 7.2 days. No Oxygen Torus Detected at Europa’s Orbit Enough oxygen atoms are lost from Europa’s atmosphere to account for the total number required by Mauk et al., but the distribution would have to be over a torus at least 5 R J in diameter, in order to obtain a null detection by UVIS.

32. Oxygen Emission Features This raw segment of the Europa spectrum shows the oxygen emission multiplets at 1304 Å and 1356 Å The 1304 Å emission feature is a triplet of emissions at 1302.2, 1304.9, and 1306.0 Å The 1356 feature is a doublet at 1355.6 and 1358.5 Å The UVIS is resolving the individual emission lines of the multiplets, consistent with observation of a point source

33. Distribution of the 1304 Å and 1356 Å oxygen emission in the January 6, 2001 data set along the slit spatial dimension. The 1356 Å feature is sharply peaked at the position of Europa. The diffuse 1304 Å feature source persists across all the illuminated spectral pixels (crossing the Io torus) and is detectable in row 28, which corresponds to the opposite side of Europa’s orbit 1304 and 1356 Spatial Distribution

34. Oxygen line emissions for the first 4.7 hrs of the January 12 data set Most of the January 12 data set was collected near the ansa of Europa’s orbit as seen from Cassini. Both 1304 Å and 1356 Å are sharply peaked at Europa’s position. 1304 and 1356 Spatial Distribution

35. Looking for Europa’s Torus Outbound Jupiter system data collected February 11 – 13, 2001, was analyzed to search for the presence of a Europa oxygen torus The slit was oriented perpendicular to Jupiter’s spin axis. Data was collected for 28 hours however the slit was slewed from north to south by 4 slit widths in a 30 min repeat cycle, thus actual integration time was ~ 7 hrs. Pixel 0 Pixel 63 Europa orbit in pixel 15 Io orbit in pixel 22

36. Oxygen in Io’s Torus, not Europa The Io torus ansa spectrum at row 22 is shown compared to the Europa orbit ansa at row 15. Oxygen lines are not detectable in the Europa row. The density that UVIS could detect in a 7 hr integration is ~8 atoms/cm 3, a factor of 5 less than the value postulated by Mauk et al. for a torus radius of 2 R J.