1. Possible plumes at Europa, Observed by Cassini?
C. J. Hansen (+ Don + Amanda + Anya)
8 January 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 H2O
Plus 1304 and 1356 from bound O2 atmosphere
Roth et al., ScienceExpress

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
Detection at true anomaly = , ~apojove
Non-detection at true anomaly = and , perijove

4. 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: (2005).

5. 6 January 2001 Start time: 2001 006T0730 Duration: 4.7 hr
Range: 1.2 x 106 km
Integration time: 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:
Europa subs/c lon: W
True anomaly =
Pixel 63
Plume at 180
This is 3x higher signal than Jan 12
Our true anomaly = deg at time of this observation; Roth has no data here
Pixel 0

6. 12 January 2001 Start time: 2001 012T0630 Duration: 11.4 hr
Range: x 106 km
Integration time: 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:
Europa subs/c lon: W
Plume at 180W
Mean anomaly at time of our observation = ; flux decreased monotonically from 301 to 349
Roth non-detection

7. 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

8. 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

9. 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.

10. 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

11. 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: x 1014
Jan. 12: 7.4 x 1014

12. 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

13. 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.”

14. 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

15. 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

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

17. 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

18. 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?

19. Backup

20. 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.

21. 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: 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)

22. 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 , , and Å
The 1356 feature is a doublet at and Å
The UVIS is resolving the individual emission lines of the multiplets, consistent with observation of a point source

23. 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

24. 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.

25. 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.

26. 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.