Possible plumes at Europa,
Observed by Cassini?
C. J. Hansen (+ Don + Amanda + Anya + Larry)
June 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 eon 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 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?
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).
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• New unpublished EUV analysis carried out in the last few months by Don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
1. Europa Targeted Observations
• 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
Time
Mean Anomaly
2000-348T0600
112
2000-364T0754
316.6
2000-366T0635
151
2001-002T1154
20
2001-010T1026
112
• Tried just summing
everything to use as a
photometer to just look for
differences but…
1. Europa Targeted Observations
• 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)
Conclusion: EUV data not useful either
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
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
3. Using Europa to map out torus with IVP
6 January 2001
Pixel 63
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 0
12 January 2001
True anomaly = 301
to 349
Start time: 2001 012T0630
Duration: 11.4 hr
Range: 15.6 x 106 km
Integration time: 1000 sec
Number of records: 41
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
Europa Orbit Geometry - Both
Ellipticity exaggerated
90
180
Cassini detection
50 to 70
0
Roth non-detection
343 to 13
Roth detection
185 to 218
Cassini non-detection
(decreasing) 300 to 349
270
Roth non-detection
289 to 318
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.
• O2 Column densities
(cm-2)
Jan. 6: 12.4 x 1014
Jan. 12: 7.4 x 1014
• In family with original
Hall (1994) detection
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
New Result for Europa’s Atmosphere
• Re-analysis of the
FUV data did pay off
in a new result
• Europa was subpixel in both
observations (this
view was on Jan. 6)
• Centered in the ISS
NAC field of view thus
offset in UVIS slit
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
New result from Don:
Atmosphere is predominantly O not O2 with a column density of 4.7 x 1012 cm-2
Based on relative strengths of 1302, 1304, and 1306 emission
Consistent with opacity due to self-absorption and scattering
Io Torus EUV
• Work by Amanda and Don has established the Io torus features in the EUV
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
New Jan 12 EUV
Results*
* Paper in prep,
Shemansky et al,
2014
• Plasma species from the Io torus (5.9 RJ) slowly diffuse out to the Europa
orbit (9.4 RJ), 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 RJ to 9.4 RJ; at 5.9 RJ lower order ions
are dominant compared to the population at 9.4 RJ, while at 9.4 RJ the
numbers are close to equivalent for the higher order ions
Species
O II
O II
O IV
S II
S III
S IV
SV
Ratio,
5.9 /9.4
14.4
11.0
<0.7
58
33
9.3
3.6
• 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 5.9RJ.
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
Backup
6 January 2001
Start time: 2001 006T0730
Duration: 4.7 hr
Range: 1.2 x 106 km
Pixel 63
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 0
12 January 2001
True anomaly = 301
to 349
Start time: 2001 012T0630
Duration: 11.4 hr
Range: 15.6 x 106 km
Integration time: 1000 sec
Number of records: 41
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
Full FUV Spectrum
• Full UVIS FUV
spectrum, filtered and flatfielded, 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
Ratio of 1304/1356 is diagnostic of O2
• Other features in the
spectrum are dominated
by Io torus emissions in
sulfur and LISM emission
in the H Lya line
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
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
Don Shemansky analysis summary
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.”
Europa Orbit Geometry - HST
Ellipticity exaggerated
90
180
0
Roth non-detection
343 to 13
Roth detection
185 to 218
270
Roth non-detection
289 to 318
Europa Orbit Geometry - Cassini
Ellipticity exaggerated
90
180
Cassini detection
50 to 70
0
Cassini non-detection
(decreasing) 300 to 349
270
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.
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.