Icy Satellites Update
Amanda Hendrix
UVIS team meeting
January 2008
Topics
• Iapetus
• Io
• Spectral models (Enceladus)
Iapetus Flyby
• Flyby Sept 10 2007 (1644 km altitude)
• UVIS results
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Press conference at DPS
DPS presentation
Workshop at October PSG
AGU presentation
Paper … special issue?
Thermal segregation is emerging as a
very important process on Iapetus …
UVIS can contribute by mapping
volatile concentrations.
UV 3-color image
Red=Ly-
Blue-green=long FUV (reflected
solar, H2O ice)
ISS map for context with UVIS
swath regions outlined.
Look at
different
regions across
the surface to
map H2O
content, explore
color of dark
material.
Note: Region 10 corresponds to the large bright crater.
COMPARING DARK REGIONS.
Regions 3, 6, 8 and 9 have ~identical spectra and are all more water ice rich than region 2. Region 7 also has H2O ice mixed in - more than the
other regions.
COMPARING DARK REGIONS.
The low-latitude apex region of the leading
hemisphere may be richer in a carbonaceous
material than a low-latitude region farther from
the apex (near 162°W). This color variation
could be the result of compositional differences
in exogenic and endogenic non-ice species.
COMPARING BRIGHT REGIONS.
The terrain in Region 10 is richer in H2O ice than
the bright polar region of the leading hemisphere.
… so even though the polar region is colder, it
appears to be less rich in water ice. There isn’t as
much thermal segregation happening there, since
the dark material doesn’t get to warm enough.
Iapetus: potential sources of
UVIS-measured H2O band
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We detect a weak H2O absorption band in UVIS spectra in the dark terrain
(Hendrix & Hansen in press)
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(1) Small fresh bright craters not seen in ISS images? (sub-ISS resolution)
(2) Subsurface volatiles diffusing through?
(3) Bound H2O?
(4) Volatiles mixed in with dark material?
=> Band depth vs latitude & longitude can help constrain the situation
– (1) More small craters expected at apex of LH (but random) (If dark material is
coming in currently/recently, could be coating over these craters)
– (2) Would occur most in warmest dark layer … at low latitudes; (possibly more in
south - warmer longer)
– (3) Don’t expect any variation with latitude or longitude
– (4) latitude-dependent, as T decreases with latitude; (no seasonal north-south
variation)
Iapetus models
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MODEL #1 (pre-2007 flyby) = DUST COATING from exogenic source
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The albedo pattern is due only to exogenic dust coming in, then the dark material expected to
be thickest at LH apex - greatest flux of incoming exogenic stuff
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At low latitudes, temperatures are relatively high, allowing for max sublimation
so volatiles within/below dark terrain at apex low lats will sublime away
MODEL #2 (Spencer/Denk) = THERMAL SEGREGATION
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only a little bit of dark exogenic material was needed initially. The addition of that dark material
triggered the sublimation of volatiles (due to slow rotation, the surface can warm up). As
sublimation progresses, more dark material (endogenic) is exposed. So the dark material we
currently see at Iapetus is a mixture of exogenic and endogenic material.
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So the low latitudes at the LH apex do not necessarily have a thicker layer of dark material than at low
latitudes away from the apex.
Stellar Occultation. A stellar occultation of Sigma Sgr was performed to
determine the upper limits on gases in Iapetus’ vicinity. We will determine upper limits for the
following gases: O2, H2O, NH3, CO and CO2.
Warm-up Artifact
Star in UVIS slit at limb of Iapetus on
egress.
HSP (high-speed photometer) occultation data. No
signal attenuation is measured on ingress.
Io
• Recall that we have 2+ very nice Io
observations
• Working with Don to model the spectra
– e-+S
– e-+O
– e-+SO2
• Hope to finish up in next month or so.
Observation #2: January 6, 2001
The slit is held steady on a fixed ra/dec.
1000-sec integration periods
Io passes through row 30, during record 3.
Jan 6 2001
Record 3
Icy Satellite Spectral Models
Water Ice in the FUV
Models of H2O ice, using optical constants from Warren (1984).
OPTICAL CONSTANTS: Warren (1984)
Icy Satellites - Comparisons
(this is suspiciously low)
• Interesting spectral variations between satellites!
• Will be working on phase curves (CDAP)
• Still puzzling over long- shape … (not flat like H2O ice model)
Icy Satellites do not look just like
H2O Ice - long 
• Instrumental effects
– Cannot be ruled out
– (but the long- shape is not always the same)
• Optical constants
– Difficult to measure where ice is transparent
– Unreliable??
• Non-ice species are also present and are spectrally active in
this region?
– Expect that Enceladus is the most H2O-ice-pure, but:
• Darker than predicted
• Spectral shape
• (By the way, Enceladus is also brighter than H2O ice model
at short wavelengths … but not focusing on that here. May
be the result of non-H2O ice stuff mixed in.)
Enceladus compared with pure
H2O
Enceladus average TH
spectrum at 30° phase
(Higher albedo is measured
than predicted at <160 nm)
Notice that the albedo of
Enceladus is ~0.3 at 180
nm, compared with ~0.65
at 439 nm (Verbiscer et al.
2005)
So we need to add in some stuff that is spectrally active in the 180-439 nm region
Spectra of Candidate Species
NH3
Verbiscer et al. (2006)
modeled the NIR spectrum of
Enceladus using a mixture of
H2O and ammonia hydrate
(NH3-H2O).
We do not have FUV optical
constants for ammonia
hydrate so use pure NH3 to
exhibit its spectral
characteristics.
NH3 is spectrally bland
throughout the NUV-VIS.
It has a strong absorption
feature in the FUV.
Sample Enceladus model:
H2O and NH3
Could this upturn be due to NH3?
Summary of spectral models
• Still puzzling over how much to believe the long-
shape
– I need to put error bars on spectra (15% for cal) and see
how things look
• Regardless of long- shape, we still need to
account for large difference in Enceladus I/F in
visible and in FUV
– Need to add in to model something spectrally active in
180-440 nm region (that doesn’t have distinct bands
elsewhere)
– NH3?? Something else??