Possible plumes at Europa, Observed by Cassini? C. J. Hansen (+ Don + Amanda + Anya) 8 January 2014

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 H2O Plus 1304 and 1356 from bound O2 atmosphere Roth et al., ScienceExpress 1247051

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 Detection at true anomaly = 185-218, ~apojove Non-detection at true anomaly = 343-13 and 289-318, perijove

Did we detect Europa’s Plumes when Cassini flew by? 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).

6 January 2001 Start time: 2001 006T0730 Duration: 4.7 hr Range: 1.2 x 106 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: 940 Europa subs/c lat: -2.10 Europa subs/c lon: 201-220 W True anomaly = 50 - 70 Pixel 63 Plume at 180 This is 3x higher signal than Jan 12 Our true anomaly = 50 - 70 deg at time of this observation; Roth has no data here Pixel 0

12 January 2001 Start time: 2001 012T0630 Duration: 11.4 hr Range: 15.6 x 106 km Integration time: 1000 sec Number of records: 41 True anomaly = 301 to 349 Europa moved from the far side through the ansa to the near side of its orbit as seen from Cassini. FUV 1.5 mR slit width 1024 spectral elements 64 spatial rows Europa subtended 0.20 mR Europa phase angle: 1160 Europa subs/c lat: -2.80 Europa subs/c lon: 64 - 103 W Plume at 180W Mean anomaly at time of our observation = 301-349; flux decreased monotonically from 301 to 349 Roth non-detection

Sub-pixel Europa 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

Full FUV Spectrum Ratio of 1304/1356 is diagnostic of O2 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 O2

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.

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 940 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% O2, 2% O Reflected and Emitted Oxygen

January 6 / January 12 Comparison Comparison of January 6 and January 12, 2001 exposures Flux from the later exposure reduced by a factor of ~3. 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. O2 Column densities (cm-2) Jan. 6: 12.4 x 1014 Jan. 12: 7.4 x 1014

Short Timescale Changes 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. We concluded that changes in flux are due to the environment, not Europa

Lyman Alpha Don Shemansky analysis summary “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.”

Jan. 6 vs. 12 Comparison of Jan. 6 to Jan. 12 shows a 3x difference in flux In our paper we attributed this partially to geometry, but that could only explain 1.5x difference Did we actually detect the plume(s)? Jan 6 plume at 180W would be ~sub-s/c Jan 12 plume would have been on limb

Europa Orbit Geometry - HST Ellipticity exaggerated 90 180 Roth non-detection 343 to 13 Roth detection 185 to 218 Roth non-detection 289 to 318 270

Europa Orbit Geometry - Cassini Ellipticity exaggerated 90 Cassini detection 50 to 70 180 Cassini non-detection (decreasing) 300 to 349 270

Europa Orbit Geometry - Both Ellipticity exaggerated 90 Cassini detection 50 to 70 180 Roth non-detection 343 to 13 Roth detection 185 to 218 Cassini non-detection (decreasing) 300 to 349 Roth non-detection 289 to 318 270

Thoughts Maybe we saw plume, but timing not particularly supportive of tidal model Tidal model doesn’t fit Enceladus very precisely either We also have unpublished EUV spectra of Europa at this time, basically consistent with FUV Should we write a quick Icarus note?

Backup

No Oxygen Torus Detected at Europa’s Orbit Oxygen is not a contributor to the torus identified by Mauk et al. (2003) at 9.5 RJ 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. 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 RJ in diameter, in order to obtain a null detection by UVIS.

UVIS Characteristics UVIS has 4 separate channels: Far UltraViolet (FUV) 110 to 190 nm 3 slit widths => 2.8, 4.8, 24.9 nm spectral resolution 2D detector: 1024 spectral x 64 one-mrad spatial pixels Extreme UltraViolet (EUV) 55 to 110 nm 3 slit widths => 2.8, 4.8, 19.4 nm spectral resolution Solar occultation port High Speed Photometer (HSP) 2 or 8 msec time resolution Hydrogen – Deuterium Absorption Cell (HDAC)

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

1304 and 1356 Spatial Distribution 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 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.

Looking for Europa’s Torus Pixel 0 Europa orbit in pixel 15 Io orbit in pixel 22 Pixel 63 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.

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/cm3, a factor of 5 less than the value postulated by Mauk et al. for a torus radius of 2 RJ.