1. Icy Satellites – Status and Science
C. J. Hansen, A. Hendrix
6 January 2005

2. Icy Satellite Science Objectives
UVIS Icy Satellite Science Objectives are to Investigate:
Surface age and evolution
UV albedo maps
Surface composition and chemistry
Reflection spectra
Tenuous atmospheres / exospheres
Emission spectra, occultations

3. Since our last meeting…
Cassini has had moderate-distance flybys:
Iapetus Oct x 106 km
Tethys Oct ,000 km
Dione Dec ~75,000 km
Iapetus Dec ,000 km

4. Phoebe
The Phoebe data set is complete (Cassini won’t be close to Phoebe ever again)
All three science objectives can be addressed with our data set
Considerable progress has been made in all three areas
Results have been presented at COSPAR, DPS, and AGU
High level results documented in UVIS team Science paper
Next on our list is a dedicated Phoebe paper (to submit to Icarus, Amanda Hendrix will be first author)
Most important finding: Evidence of water ice on Phoebe’s surface makes the outer solar system the most likely place of Phoebe’s origin

5. Example: Phoebe UV Albedo Map
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)

6. Composition: Phoebe’s Disk-Integrated FUV reflectance spectrum
H2O, CO2 frosts contribute to Phoebe’s FUV spectrum; other (dark)
materials unknown at this time - similar to C ring non-ice material

7. ISS_000PH_HIRESC Note brightness difference between
high and low latitudes

8. Phoebe’s spectral variations with latitude

9. No Emissions Detected above Background
A gas (oxygen) detection threshold of <2 x 1013 cm-2 is based on a minimum 2-sigma detectable level of 10 counts above
background, instrument sensitivity at nm of 3.4 cts/kR-sec, integration time of ~ 10 hours, and solar wind values for
the electron density and temperature at Saturn.

10. Phoebe UVIS Results - so far
Average FUV spectrum
Consistent with presence of CO2, H2O frosts
Phoebe is VERY dark <1650 Å; blocks IPH
Lack of emission features
No Chiron-like activity
Dissociative excitation of sublimated / sputtered H2O by solar wind is not a significant process (in contrast to Europa)
Latitudinal variation in reflectance spectrum
More H2O frost in S. polar region than at mid-latitudes

11. Iapetus
Distant Iapetus observations collected for longitudinal, phase coverage
(Most of the mission is required to fill in complete coverage)
Some observations were lost due to s/c pointing errors
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): – 0.6
Competing hypotheses: endogenic vs. exogenic origin
We can weigh in on this by comparing our spectra of both sides of Iapetus to Phoebe and Hyperion

13. Main Models for Albedo Dichotomy
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
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)

14. Iapetus: October 15, 2004
Range=1,222,000 km
Radius=730 km

15. Need much less H2O, more dark material to fit this
Iapetus spectrum, compared to Phoebe
a=88°

16. Iapetus UVIS Results - so far (October)
FUV spectrum of southern hemisphere region
Distant, sub-pixel observation
Both dark, light terrains in pixel
Reflected solar features do not appear as in Phoebe spectrum
May be due to lower spatial resolution, small size of body
May have overall slightly bluer spectrum than Phoebe; less steep at longer wavelengths
Less H2O
More dark material
Must wait for closer Iapetus observations to make more definitive statements

18. Sub-S/C location map

19. CIRS_00BIA_FP3MAP001_PRIME
Duration=00:57
AD=8.5 mrad
Sub S/C=(29, 070),
LT=14.4
=51o
Disk map; Cassini
Regio (subsolar
point Ts≈125 K)
(1)

20. Iapetus Bright-Dark Comparison

21. ISS_00CIA_GLOBMAPC001_PRIME (2004-366T14:00 UTC; 00:30 hrs)
Trigger #
Telemetry Rate S&ER3 (24)
number footprints
number of images
total Data Volume (predict) Mb
2+3x3 NAC clr mosaic
km alt.; 740 m/pxl;
d_IA=1950 pxl; 78 deg phase;
sub-S/C lat/lon = +54/50;
sun illuminates surface from left
2 NAC shutters per footprint (except #1 and2):
220 ms (bright) and 1.5 sec (dark terrain)
WAC 4-color context (Vio, Grn, Ir1, Ir3)

22. ISS_00CIA_COMPD001_VIMS (2004-366T19:00 UTC; 02:00 hrs)
Trigger #
Telemetry Rate S&ER3 (24)
number footprints
number of images
total Data Volume (predict) Mb
1x1 NAC color mosaic
km alt.; 705 m/pxl;
d_IA=2050 pxl; 94 deg phase;
sub-S/C lat/lon = +64/24;
sun illuminates surface from left
10 NAC color frames + 3 polarizers
Center of ftprt over dark terrain
==> long exp times selected
Highest resolution color of BC flyby

23. More Iapetus in Nominal Tour
Planned Cassini flybys at Iapetus (selection)*
Date min. altitude best ISS pixel scale visible hemisphere**
Jul ,500,000 km km/pxl CR, southern bright terrain, SP
Oct ,110,000 km km/pxl anti-Saturn side (western CR), SP
31Dec04/ 01Jan ,500 km m/pxl northern CR, high northern latitudes
Nov ,000 km km/pxl eastern and central CR, north. bright terrain
Jan ,000 km km/pxl eastern CR and trans. zone, "moat" partially
Apr ,000 km km/pxl leading side, SP
Sep ~1,270 km# ~12 m/pxl CR (high phase), trailing side (low phase)
*...Included are all flybys in 2004 and all flybys below 106 km altitude until June 2008.
**...CR = Cassini Regio (dark material hemisphere); SP = south pole.
#...10 Sep 2007: targeted flyby altitude not fixed yet; might be between ~1000 and ~1400 km.

24. Dione Close flyby on 15 December 2004
Range: 72,912 km
Phase Angle: ~800
UVIS was riding along on three observations
Collected first observation (GLOCOL)
Got FUV data but not EUV
Range: 159,395 km (7 pixels)
Phase: 340
Partial collection of second observation (REGMAP), remainder lost due to error in how the SSR was configured
Next close Dione flyby is in Rev 16 (Oct. 2005)
Big science question for Dione post-Voyager was “What is the nature of the bright streaks?”

25. The nature of Dione’s streaks
Voyager
Cassini

26. UVIS Dione UV Albedo (GLOCOLOR)

27. Dione REGMAP

28. Tethys
Only fairly distant spectra so far, but significantly different than Phoebe
Upcoming, relatively close flybys:
Rev 4; ,975 km
Rev 7; ,990 km
Rev 15; 33,295 km
Rev 45; 97,131 km
Rev 47; 16,166 km
Rev 49; 48,324 km

29. Tethys: October 28, 2004 Range=255,000 km Radius=530 km

30. Tethys is ~10x brighter than Phoebe at similar phase angle
Need much more H2O, less dark material to fit this
Tethys spectrum, compared to Phoebe
a=50°
Model uses smaller H2O grain size

31. Tethys UVIS Results - so far
4 pixels on body
Reflected solar features do not appear as in Phoebe spectrum
May be due to larger amount of H2O ice - very dark
Tethys is ~10x brighter than Phoebe at similar phase angles
H2O is more dominant in Tethys spectrum than in Phoebe spectra
H2O grains may be smaller than on Phoebe - related to E-ring??

32. In the next 6 months…
First data from Mimas (range = 215,291 km) and Rhea (range = 144,171 km) in Rev 00C, right after completion of the Huygens Probe mission and playback (January 15)
We’ll get our first look at Enceladus in Rev 00C, at a range of 247,000 km (January 15)
Major upcoming close Enceladus flybys:
Rev 3; closest approach = 1179 km; February 17
But this is right after T3 so major pointing uncertainties
We will get stellar occultation of lambda Sco
Rev 4; targeted flyby, closest approach = 500 km; March 9