UVIS Icy Satellites Surface Studies
Amanda Hendrix
Emilie Royer
Don Shemansky
Candy Hansen
Tim Cassidy
UVIS Team Meeting, Pasadena
January 2014
topics
• Io
• Nugget ideas
• Composite spectra:
UVIS+ISS+VIMS+HST
• Enceladus regional
• Outer sats
Io
Io observations
• Serendipitous
– Io passed through slit while UVIS stared at ra/dec
• We focus on the two best observations
– Dec 30 2000
– Jan 2 2001
• There are a few other observations (including in eclipse)
not considered here
• Some of these slides were presented at Io workshop in
Boulder in October
• We are hoping to finish up modeling and get paper done
in next ~ month (!?)
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MAIN RESULTS (from old
version of paper – to be
updated!)
The 1666 Å feature is difficult to model. – to be fixed(?) with new 1999
FF/cal
Put limit in e+SO2; most features are assumed to be due to e+S (faster
rates). Derive S, O column densities
Model our 2 spectra using electron densities and temperatures from our
torus observations. Model Feaga et al. STIS spectrum with our model;
their1479 interpretation is re-examined. We are puzzled about their
1485.6 Å feature – does not appear in model.
Spatial distribution of species
Chlorine?
Wake/upstream differences? Neutral clouds?
Other curiosities: row 31 rec 3 1250/1388 features; row 30/rec 2 1479
ratio.
EUV, FUV dat
Dec 30,
2000
antiJovian
hemisph
ere
Jan 6,
2001
Jovian
hemisph
Observation 1, Row 32
1356Å
1304Å
1479Å
1720Å
1820Å
1388Å/1405Å
128 nm
191 nm
y-scale is time (11 1000-sec records); Io passed through FUV sli
Observation 2, Row 30
1356Å
1479Å
1304Å
128 nm
1388Å/1405Å
1720Å
1820Å
191 nm
y-scale is time (13 1000-sec records); Io passed through FUV sl
Observation 1 spectra: row 30
S++
S+
S+, S+++
S++
Observation 1 spectra: row 31
S++
S+
S+, S+++
S++
Observation 1 spectra: row 32
When Io is in the slit, the
neutral lines dominate
over the torus lines
Observation 1 spectra: row 33
Observation 1 spectra: row 34
Observation 1
1256 Å (mainly S+)
(1253Å + 1260Å) row 45
1720 Å (S++)
(1713Å + 1729Å) row 45
row 21
rec 1
row 21
rec 1
rec 11
time
rec 11
time
The ansa/ribbon of the torus was observed at the end of
Observation 1
1304 Å
rec 11
time
1356 Å
row 45
row 45
row 21
rec 1
row 21
rec 1
rec 11
time
Observation 1
1388 Å
rec 11
time
1477 Å
row 45
row 45
row 21
rec 1
row 21
rec 1
rec 11
time
Observation 1
1666 Å
1820 Å
row 45 (1807Å + 1820Å + 1826Å)
row 45
row 21
rec 1
rec 11
time
row 21
rec 1
rec 11
time
Observation 1
1430 Å
405Å + 1430Å + 1448Å)
row 45
row 21
rec 1
rec 11
time
This feature is a blend of torus + neutral lines.
Dec 30,
2000
antiJovian
hemisph
ere
Jan 6,
2001
Jovian
hemisph
Observation 2, Row 30
1356Å
1479Å
1304Å
128 nm
1388Å/1405Å
1720Å
1820Å
191 nm
y-scale is time (13 1000-sec records); Io passed through FUV sl
Observation 2 spectra: row 28
S++
S+
S+, S+++
S++
Observation 2 spectra: row 29
S++
S+
S+, S+++
S++
Observation 2 spectra: row 30
When Io is in the slit, the
neutral lines dominate
over the torus lines
Observation 2 spectra: row 31
Observation 2 spectra: row 32
Observation 2
1256 Å (S+)
(1253Å + 1260Å) row 45
1720 Å (S++)
(1713Å + 1729Å) row 45
row 21
rec 0
row 21
rec 0
rec 12
time
rec 12
time
Observation 2
1304 Å
rec 12
time
1356 Å
row 45
row 45
row 21
rec 0
row 21
rec 0
rec 12
time
Observation 2
1388 Å
rec 12
time
1477 Å
row 45
row 45
row 21
rec 0
row 21
rec 0
rec 12
time
Observation 2
1666 Å
1820 Å
row 45 (1807Å + 1820Å + 1826Å)
row 45
row 21
rec 0
rec 12
time
row 21
rec 0
rec 12
time
Observation 2
1430 Å
405Å + 1430Å + 1448Å)
row 45
row 21
rec 0
rec 12
time
This feature is a blend of torus + neutral lines.
Fig 1: Io Torus in vicinity of Io. Obs 2 IP= 40 RIo
Io torus fuv emission compared to model. No measureable S I or O I. Electron
impact model at Te = 75000 K is the red trace. The observed spectra shown
here are filtered and flatfielded signal counts.
Fig 2: Spectrum of Io; Obs 2: IP= 0 RIo.
Sub Jupiter Io atmosphere fuv emission, blue trace, compared to model total cyan trace.
Electron impact on ion population calculated at Te = 40000 K. Neutral emission model
calculated at Te = 30000 K. Calculated e + SO2 component plotted in light red. Neutrals and
ions are not necessarily co-located.
Fig 3: Comparison of Io Torus spectra East and West ansae
The fuv blue trace is the East exposure, the red trace is West. The volume
on the East is slightly cooler in electron temperature than the West,
accounting for the difference in emission brightness.
Fig 4: Analysis of the Io Torus East ansa spectrum
The cyan fuv trace shows the model fit to the observed spectrum, at an
electron temperature Te = 75000. K.
Fig 5: Io Torus 12/05/00 E_W ansae
Corrected euv signal spectra of the East and West ansae of the Io Plasma
Torus. Brightness difference is attributed to electron temperature.
Fig 6: Analysis of the East ansa Io Torus spectrum 12/05/00
The red trace is the complete emission model. The light green trace is
the SIV component of the model. Electron temperature Te = 75000 K. [e]
= 1419 cm-3.
Fig. 7: Comparison of Io exposures
Fig. 8: Comparison of Io exposures
Fig. 9: Model fit in SI and SII to fuv2001 spectrum of Io
Electron excited SI and SII emission used to fit the FUV2001 exposure in
Fig. 2
•
fuv analysis with file e:\cassini\code\cal\fuv_14641_fpn_c1_pre.csv based on USC analysis of
Stewart reduced LISM data; requires 14641 pre smoothing.
•
The SII multiplet 1250 A shows an anomalous emission enhanced over the model (Fig 3) at the
western torus ansa, and in Fig 7 for the Io exposure. See fig 9. The phenomenon is physically real.
The ratio of SIII lines at 1713 and 1729 A for fuv2000 is anomalous, and does not fit the model.
The 1667 A feature is produced by e + SO2.
nugget ideas
The Full Mimas
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We know that Saturn’s moon Enceladus is the
brightest object in the solar system … at visible
wavelengths, that is
Studies using Cassini’s UVIS instrument find
that Mimas is actually brighter than Enceladus
at far-UV wavelengths (light not seen by the
human eye)
The is particularly obvious at small phase angle
… when Mimas becomes a “full Moon”
It is believed that this is due to the photometric
behavior (opposition surge) of Mimas vs
Enceladus… Enceladus’s surface may be
covered with such fine plume and E-ring grains
that there isn’t much of an opposition surge
Enceladus
toward full moon
Mimas
Seasons on Saturn’s moon Tethys
detected by Cassini’s UV spectrograph
A UV observation of
Tethys (July 2007) shows
the brightest region in the
northern hemisphere.
The sub-solar point was
south of the equator at this
time.
180°W
210°W
210°W
180°W
A later observation (April
2012), when the sub-solar
point had shifted
northward, indicates that
the brightest region seems
to have shifted southward.
UVIS+ISS+VIMS+HST
(AGU)
procedure
• Start with disk-integrated observations
– To get identical phase angle, longitude coverage
• Try for simultaneous observations (must be distant
for UVIS to be disk-integrated) – but highresolution observations are better for VIMS
– So if simultaneous doesn’t work, we try for similar
geometry
– If similar geometry doesn’t exist, we try to similar
longitude and do a phase correction
• For UVIS, we use phase curves of Royer & Hendrix (see next
talk)
• Study hemispheric albedo/spectral variations,
where possible
UVIS
diamonds - ISS
ISS error
bars are
approximate
VIMS
Tethys
trailing
338 nm
leading
Schenk et al,
2011
LH: simultaneous UVIS-ISS
obs
TH: not simultaneous, but
same geometry
Dione
trailing
leading
Schenk et al,
2011
LH: UVIS obs was at 34°phase, correct to 26°
TH: not simultaneous but same phase angle
Rhea
trailing
leading
Schenk et al,
2011
30 ° phase
LH: observed
simultaneously by ISS and
UVIS.
TH: UVIS observed at
13°phase, scaled down
30 ° phase
?
?
adding in HST/FOS data
(Noll et al., 1997)
Rhea & Dione:
• Leading & trailing
hemispheres show
O3-like absorption
• Roughly same
absorption depth on
both LH and TH
• Red slope similar to
ice tholin
adding in HST/STIS data
(Noll et al., 2008)
So in modeling these spectra, we need a species that does NOT
decrease ~linearly in reflectance (190-260 nm)… rather it has a
~bowl shape in the ~210-300 nm region and then drops off
NH3?
Enceladus models: H2O + 1% NH3 + 1% ice thol
Abundances of NH3 may be small enough that they are not detectable by VIMS
Ammonia hydrate has been detected on Enceladus (Emery et al., 2005; Verbiscer et al., 2005) and
Tethys TH (Verbiscer et al., 2008)
~1% NH3 in the Enceladus plume (Waite et al., 2009): a constant source
Hendrix et al. (2010)
Enceladus plume fallout regions
Plume Fallout
IR-green-UV
Schenk et al. 2011
Kempf et al. 2010
Rev 4 mosaics
Rev 4 brightness map
~175-190 nm
LommelSeeliger
photometric
correction
(m0/(m+m0)
Rev 11 mosaics
Rev 11 brightness map
~175-190 nm
LommelSeeliger
photometric
correction
(m0/(m+m0)
Rev 3 mosaic
Rev 3 brightness map
~175-190 nm
LommelSeeliger
photometric
correction
(m0/(m+m0)
Possible sources of UV
brightness variations
• Grain size
– Likely not
• scattering behavior
– Increased backscattering from fresh plume grains?
• Composition
– Salts?
– Epsomite is quite bright throughout UV-vis
• Why do boundaries not match CDA/vis?
– Competing effects of E-ring grains?
– At shallower plume fallout regions, UVIS is more
sensitive to E-ring grains?
Outer Satellites
UVIS data smoothed by 7
UVIS data smoothed by 11
• Plans for 2014 papers:
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Io
Enceladus photometry
Tethys: albedo dichotomy + seasonal variations
Composite spectra (UVIS+ISS+VIMS+HST + models)
Enceladus regional variations