Icy Satellite Science Update
part 1:
Phoebe, Iapetus, hydrogen
(solar phase curve analysis not discussed yet)
C. J. Hansen, A. Hendrix
26 July 2005
Iapetus
•
Close Iapetus flyby on December 31
– 124,000 km distance
•
Big science question – Why is one hemisphere of Iapetus so much
darker than the other?
– Geometric albedo range (visible): 0.02 – 0.6
– Competing hypotheses: endogenic vs. exogenic origin
•
One hypothesis was that the dark side of Iapetus was contaminated by
material spiraling in from Phoebe
•
We can weigh in on this by comparing our spectra of both sides of
Iapetus to Phoebe and Hyperion
Main Models for Albedo Dichotomy
Model
Pros
Cons
Refs
Geologic
resurfacing
Infill of dark
crater floors;
clean edge (?)
Hemispheric
pattern
Smith et al.
1982
Accretion from
Phoebe
Hemispheric
pattern
Color
Soter 1974
As above +
volatization
Hemispheric
pattern; color?
The math
Cruikshank et al. 1983;
Squyres et al., 1984; Bell
et al. 1985; Buratti and
Mosher, 1995
Accretion from
Titan or Hyperion
Pattern and
spectra
Low albedo
Matthews 1992; Vilas et
al. 1996; Jarvis et al.
2000; Owen et al. 2001
Meteoritic
erosion
Hemispheric
pattern
Low density
(1.2gm/cc)
Cook and Franklin 1970;
Wilson and Sagan, 1995,
1996
Accretion of D-type
material from outer
retrograde satellites
Hemispheric
pattern and
spectra
Sufficient
source; Phoebe
not red
Cruikshank et al. 1983;
Buratti et al. 2002, 2004;
Black et al., 2004 (general
exogenic)
Phoebe
• June 11, 2004
• Solar phase angles:
– Inbound ~90°
– C/A ~25°
– outbound ~90°
• C/A altitude=2068
km
• Radius=107 km
Iapetus
• December 31, 2004
• Solar phase angles:
– Low phase inbound
– C/A a=94°
– High phase outbound
• C/A altitude=123,402
km
• Radius=730 km
Reflectance Models
Model = X*H2Oa + Y*H2Ob + Z*dark
X+Y+Z=100%
H2Oa=grain size #1
H2Ob=grain size #2
Dark=carbonaceous chondrite (Orgueil)
CIRS_000PH_FP3EWMAP001
Similar geometry
Time: C/A-01:22
Range: 31,300 km
Phase angle: 83°
Lat/Long: 21°S, 349°W
Blue/green=reflected solar
Red=background Ly-a (IPH)
Phoebe
Blue/green=reflected solar
Red=background Ly-a (IPH)
Long-l FUV (reflected solar)
Large crater walls reflect solar light
Phoebe (high res)
Brightness variations across the surface
are apparent
Long-l FUV (reflected solar)
Blue/green=reflected solar
Red=background Ly-a (IPH)
Phoebe: latitudinal variations
High latitude region is more H2O-ice rich; low latitude region has more non-ice material
Iapetus
Iapetus reflectance: dark and light terrains
Compositional variations in UVIS spectra could be due to
thermal segregation on Iapetus -- models are being tested
Raw Iapetus spectra: dark and light terrains
Iapetus compared with Phoebe
Results:
Relative Amounts of Model Components
Iapetus and Phoebe
Phoebe avg
Phoebe high lat
Phoebe low lat
Iapetus (bright)
Iapetus (dark)
Orgueil
~85%
~70%
~90%
~60%
~95%
%1 mm H2O %2 um H2O
~10%
~5%
~20%
~10%
~8%
~2%
~22% (200 um)
~18%
~5%
Pretty good fits are obtained with this model.
Need to model Iapetus dark terrain at different latitudes to check
segregation models.
Summary:
Iapetus vs. Phoebe
• What can we say about dark material on Iapetus?
– It does not look like Phoebe
• Phoebe material is too H2O-rich to match Iapetus dark spectrum
– We need very little H2O to model Iapetus dark material
• If Phoebe material (or any other material) coats Iapetus, then H2O
must be lost in the transport or impact process
• Our results do not discriminate between endogenic and exogenic
models for Iapetus dark material
– Loss of volatiles might be associated with either process
• Our results may be able to say something about thermal
segregation of species, however
– Formation of polar caps on Iapetus
• Papers in progress…
– Ultraviolet Observations of Phoebe from Cassini UVIS (Hendrix & Hansen)
– Iapetus’s Bright and Dark Terrains at FUV wavelengths (Hendrix & Hansen)
Hydrogen Exospheres
• Excess 1216Å signal is seen off the limb in nearly all
observations of icy satellites.
• Observations when slit is held steady across disk
• Ly-a distribution varies from observation to observation
• Does not appear to be due to row-to-row variations along slit
• Here we present results from Phoebe, Tethys,
Enceladus
• Similar distributions are also seen at Dione, Rhea
• Work in progress
• DPS presentation
• Wayne is providing background levels
• Charles Barth is providing advice
Phoebe: 1216 Å
Phoebe: 1216 Å
USC FF used for FUV data
Night side of disk
Illuminated
disk (spectra
on next page
Enhanced H off
limb (spectra on
next page
Data taken during ~15 min sit&stare observation after closest approach.
Slit was oriented across the disk, parallel to the equator.
Sum over spectrum, multiply by cal (.296367) to get kR, per Wayne’s suggestion.
Thus we have reflected solar on disk, but off disk signal is primarily Ly-a.
Red wiggly line is same data, but taken when not pointed at Phoebe (same period of
time).
Red straight line is average of red wiggly spectrum.
Low points (below bkgd) correspond to night side. High points correspond to day
side (long-wave signal).
Black line off dayside limb is higher than background, likely due to enhanced
hydrogen.
1/r fit
(need to figure out error bars)
Slant column emission rate at 107 km=0.178 kR
Column emission rate=0.034 kR
Slant column density=1.88 atoms/sec
Working on volume density…
Working on escape flux…
Phoebe: 1216 Å
USC FF used for FUV data
Phoebe’s disk is darker than background at Ly-a
Phoebe’s off-limb signal is higher than background at Ly-a
Phoebe: 1025 Å
LASP FF used for EUV data
See spectra on next page
Signal (cts/sec) at 1025Å during ~15 min stare at Phoebe with slit oriented
across Phoebe disk, parallel to equator.
Sun is to right.
Red dashed line is average signal at 1025Å when not pointing at Phoebe.
Data for this plot are from after closest approach.
Data are noisier than Ly-a but appear to show sunside limb enhancement of H
1/r fit vs pixels
Tethys
004TE_TETHYS004_VIMS
R= 83,234 km
Longitude= 357°W
Phase= 60.4°
icylon007
Tethys
004TE_TETHYS004_VIMS
icylon007
Wayne will provide background level
Enceladus
VIMS_003EN_enceladus006
Range= 41,345 km
Lon= 311°W
Phase= 25.4°
icylon005
Enceladus
(north)
(LH)
(south)
(TH)
Wayne will provide background level
What is process?
• It’s too cold for sublimation of H2O
– So escaping H likely comes directly from surface ice
• Photo-desorption
– For Phoebe + inner satellites
• Desorption by ions, electrons?
– For inner satellites
– Do we see a correlation with longitude?
What is result?
• Leftover oxygen in surface ice matrix
– O3 on Dione, Rhea (HST)
• Or does oxygen escape as well?
– No sign so far of 1304 Å escape
– Though oxygen is present in Saturn system
• Hydrogen injected into Saturn system
– Ionized - seen by CAPS?
– Stays neutral - gets mixed up with Wayne’s Saturn H?
Also in progress…
•
•
•
•
Io (with Don)
Io eclipses
Moon
Hope to get these submitted soon!!!