Icy Moon Occultations: the
Search for Volatiles
C. J. Hansen, A. Hendrix, B. Meinke
6 January 2010
Outline
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Overview
Tethys
Dione
Iapetus
Rhea
Upper limits
• Enceladus
Enceladus’ Plume
UVIS Observations of Stellar
Occultations
Why do we observe occultations of
bright stars by Saturn’s icy moons
with UVIS?
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Typically to look for volatiles
– Dione, Rhea, Iapetus
– Enceladus’ plume
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Sometimes for rings
– Rhea
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UVIS Configuration:
– FUV
• 5 sec integration
• 512 spectral channels
– HSP
• 2 msec integration
Sigma SGR over Iapetus
Methodology
Step 1: Plot HSP data. The ingress is clean - this is the first place to look for any
attenuation. The egress is plagued by the instrument signal coming up again, so apparent
dips on egress are artifacts. To look for attenuation on egress must sum FUV over
wavelength, but this means temporal resolution is 5 sec, not 2 msec.
Step 2: Determine which spatial rows of the FUV the star is in and sum over those rows.
Generate I0 spectrum, the unocculted signal, pre-ingress and post-egress. Plot potentially
occulted signal I, with I0, to compare.
Step 3: Plot I / I0 to see absorption features. In the case of Tethys, Rhea, Iapetus, and
Dione we have found none. In the case of Enceladus we clearly see absorption by water
vapor by the plume.
Step 4: If absorption features are detected model gas column density and compare gas
absorption features to I / I0
Step 5: If no absorption features, calculate upper limits
Tethys
Goal: Search for volatiles in Tethys’ vicinity. Use this airless body to
compare others with more potential for gases.
Result: No evidence for gas absorption (as expected)
Occultations Observed
Orbit 15
2005-267T02:36
Orbit 24
2006-143T04:32
Orbit 27
2006-235T23:21
Orbit 15
beta Tau
66 Oph
8 nu Cap
Dione
Goal: Search for volatiles - MAPS instruments detect massloading of magnetosphere near Dione, similar to Enceladus but
down more than an order of magnitude? Look for small plumes.
May be very localized.
Result: No evidence for gas absorption (yet)
Occultations Observed
Observation start
Orbit 19
2005-358T16:40
Orbit 45
2007-147T12:12
Orbit 118
2009-263T17:00
Orbit 19
Star
66 Oph
alpha Leo
epsilon Cma
Ingress lat/long
-53.8 / 148.1
48.5 / 94.9
11.5 / 2.5
Egress lat/long
-52.4 / 327.9
61.9 / 304.3
26.3 / 163.1
Dione
Counts / 100 msec
Orbit 45
Orbit 118
Iapetus
Goal: Search for volatiles possibly being transported from the dark
side to the bright side of Iapetus - MAPS instruments report that
Iapetus has interesting / unusual interaction with solar wind.
Result: No evidence for gas absorption
Occultation Observed
Orbit 49
2007-253T12:55
sigma SGR
Rhea
Goal: Search for oxygen atmosphere that could cause plasma
absorption detected by MIMI. Search for ring material that could
cause plasma absorption detected by MIMI.
Only moon massive enough to retain a sputtered atmosphere
Recall:
• Cassini’s flyby of Rhea in November 2005 was through the wake - a region in
which a depletion in charged particles is expected
• The expected depletion was detected, but over a larger expanse than
predicted
• MIMI observed a depletion of energetic electrons - their interpretation is that it
is due to a dust cloud, a neutral atmosphere or a combination of both - estimate
of required column density is 3 x 1016 cm-2
• MIMI also saw narrow dips in electron flux, attributed possibly to rings
Potential ring radii:
1610 km
1800 km
2020 km
Rhea
Occultations Observed
Orbit 47
2007-179T13:30
Orbit 121
2009-325T16:37
eps Ori
beta, kappa Ori
Also, non-occultation data was analyzed to look for oxygen emission
Orbit 47
Rhea Rev 121 Beta Ori Occultation
Rev 121 features a beta Ori
occultation ingress and partial
egress, and a kappa Ori
occultation egress
• Ingress was close to
equatorial
• HSP data doesn’t show
any obvious dips
Rhea Rev 121 Beta Ori Egress
• Egress was
at +80
latitude
• Only one of
the possible
ring
locations
was crossed
by the star
Rhea Kappa Ori Occultation Egress
• HSP data, not
equatorial
• FUV data not
analyzed yet
Bonnie Meinke ran her software to check for statistically
significant dips that could be attributed to rings - none
found in this observation -> no confirmation of rings
Rhea Oxygen Emission Results
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Rhea does not appear to have a neutral oxygen atmosphere
– Although 1304 and 1356 can be well above the background this occurs at
geometries where Rhea is viewed close to Enceladus
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This allows us to calculate upper limits for O and O2 associated with
Rhea, based on not seeing emission features
– O: for a 90 min observation, calculated for solar scattering at Saturn, gives
an upper limit of 4 x 1013 cm-2
– O2: for a 90 min observation, calculated for electron-impact excitation,
gives an upper limit of 1.5 x 1014 cm-2
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The component broadening Rhea’s wake is not a sputtered O or O2
atmosphere
Summary
Since no volatiles have been detected in occultations we can
calculate upper limits
• Calculation of column density upper limits:

I0 () exp {-n()} d
I/I0 =
 I0() d
• Use I/I0 = 0.95 (reasonable for broad absorptions), plug in
absorption crossections as a function of wavelength, solve for n
(done numerically, by testing various values of n)
Column Density Upper Limits
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All stars we use have a signal high enough to detect a 5% attenuation.
This is not adequate for a single absorption feature, however for gases
with broad absorptions in the FUV wavelength range it is a realistic
threshold.
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Gases with absorption features in the FUV: H2O, O2, CO, CO2, NH3,
C2H4, methanol
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Upper limits
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H2O: 2 x 1015 cm-2
O2: 1.3 x 1015 cm-2
CO: 3.6 x 1014 cm-2
CO2: 1.3 x 1017 cm-2
NH3: 9.2 x 1015 cm-2
C2H4: 2.8 x 1015 cm-2
methanol: 4 x 1015 cm-2
Enceladus
Enceladus’ Plume against Saturn
Rev 120
Enceladus Rev 120
• Enceladus flyby on Rev 120 used Saturn as source, looked for
Saturn-light absorbed by Enceladus’ plume
• Advantage is that plume could potentially be observed more
often and over longer time scales than single stellar occultations
• “Parked” the slit on Saturn (to minimize changes due to Saturn
itself) x 3
• The plume should be resolved horizontally
• Reflected uv is not a very strong source however, so it was
necessary to sum the data to increase the signal
Focus on the data over wavelength
range 1646 to 1750 ang, where we
have adequate signal from Saturn
and a broad absorption feature
H2O cross-sections from Mota
2.5E-17
2E-17
1.5E-17
H2O cross-sections from Mota
1E-17
5E-18
0
1
28
55 82 109 136 163 190 217 244 271 298 325 352 379 406 433 460 487 514
1115 to 1914
Geometer Image: ICYMAP Part 1
• Calibrated data, plot shows 1646 to 1750 ang range
• Expect to see plume as darker than Saturn because it is
absorbing some of the reflected light
UVIS image of Enceladus’ Plume
Range:
At start = 18,810 km
At end = 32,478 km
Resolution:
At start = 19 km
At end = 32 km
ICYMAP Part 1
ICYMAP Part 2
Range:
At start = 32,478 km
At end = 46,227 km
Resolution:
At start = 32 km
At end = 46 km
ICYMAP Part 3
Range:
At start = 46,227 km
At end = 57,937 km
Resolution:
At start = 46 km
At end = 57 km
Raw data summed over 1646 to 1750 Range
Time
Pixels 0 - 61
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Map shows pixel value divided by column average
Orange => pixel 10-20 % lower than average
Blue => pixel 20-30% lower than average
Lilac => lowest pixel in column
• Stipples show where plume is
• Would like to see correlation of low
counts with plume location
Conclusion
• This was an experiment worth attempting
• But, not clear we can pull out any quantitative
results
– Standard deviation is ~ equal to the signal we are
looking for
– Sum more?