Icy Satellites – Status and
Science
C. J. Hansen, A. Hendrix
6 January 2005
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
Since our last meeting…
Cassini has had moderate-distance flybys:
–
–
–
–
Iapetus
Tethys
Dione
Iapetus
Oct. 20
Oct. 28
Dec. 15
Dec. 31
1.2 x 106 km
255,000 km
~75,000 km
124,000 km
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
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)
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
ISS_000PH_HIRESC
Note brightness
difference between
high and low latitudes
Phoebe’s spectral variations with latitude
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 130.4 nm of 3.4 cts/kR-sec, integration time of ~ 10 hours, and solar wind values for
the electron density and temperature at Saturn.
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
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): 0.02 – 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
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
Sufficient source;
Phoebe not red
Cruikshank et al. 1983;
Buratti et al. 2002, 2004;
Black et al., 2004 (general
exogenic)
Hemispheric
pattern and
spectra
Iapetus: October 15, 2004
Range=1,222,000 km
Radius=730 km
Need much less H2O, more dark material to fit this
Iapetus spectrum, compared to Phoebe
a=88°
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
Sub-S/C location map
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)
Iapetus Bright-Dark Comparison
ISS_00CIA_GLOBMAPC001_PRIME
(2004-366T14:00 UTC; 00:30 hrs)
Trigger #
Telemetry Rate
number footprints
number of images
total Data Volume (predict)
2159
S&ER3 (24)
11
24
97 Mb
2+3x3 NAC clr mosaic
122500 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)
ISS_00CIA_COMPD001_VIMS
(2004-366T19:00 UTC; 02:00 hrs)
Trigger #
Telemetry Rate
number footprints
number of images
total Data Volume (predict)
1x1 NAC color mosaic
117500 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
2998
S&ER3 (24)
1
13
76 Mb
More Iapetus in Nominal Tour
Planned Cassini flybys at Iapetus (selection)*
Date
min. altitude best ISS pixel scale
visible hemisphere**
----------------------------------------------------------------------------------------------------------------------------Jul 2004
2,500,000 km 15 km/pxl
CR, southern bright terrain, SP
Oct 2004
1,110,000 km
6.7 km/pxl anti-Saturn side (western CR), SP
31Dec04/ 01Jan05
117,500 km
710 m/pxl
northern CR, high northern latitudes
----------------------------------------------------------------------------------------------------------------------------Nov 2005
416,000 km
2.5 km/pxl eastern and central CR, north. bright terrain
Jan 2006
879,000 km
5.3 km/pxl eastern CR and trans. zone, "moat" partially
Apr 2006
603,000 km
3.6 km/pxl leading side, SP
Sep 2007
~1,270 km#
~12 m/pxl CR (high phase), trailing side (low phase)
----------------------------------------------------------------------------------------------------------------------------*...Included are all flybys in 2004 and all flybys below 10 6 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.
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?”
The nature of Dione’s streaks
• Voyager
• Cassini
UVIS Dione UV Albedo (GLOCOLOR)
Dione REGMAP
Tethys
•
Only fairly distant spectra so far, but significantly different than Phoebe
•
Upcoming, relatively close flybys:
–
–
–
–
–
–
Rev 4;
Rev 7;
Rev 15;
Rev 45;
Rev 47;
Rev 49;
82,975 km
64,990 km
33,295 km
97,131 km
16,166 km
48,324 km
Tethys: October 28, 2004
Range=255,000 km
Radius=530 km
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
Model uses smaller H2O grain size
a=50°
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??
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