Cassini UVIS Icy Satellites:
Update and Progress on Analysis
Amanda Hendrix & Candy Hansen
UVIS team meeting
January 2006
*** This version of the presentations was modified as noted
after the team meeting, based on discussion at meeting.***
Since last UVIS team meeting…
• 4 icy satellite flybys
–
–
–
–
Tethys (September 2005)
Hyperion (September 2005)
Dione (October 2005)
Rhea (November 2005)
TETHYS
015TE_ICYLON019_CIRS
015TE_EWSCAN001_CIRS
Ithaca Chasm
crater
long- image
(1710-1900 Å)
R= 49658 km
Longitude= 347°W
Phase= 21°
HYPERION
015HY_ICYMAP008_UVIS
Ly- image
(1710-1900 Å)
R= 55,679 km
Phase= 51.7°
long- image
(1710-1900 Å)
HYPERION
015HY_ICYMAP013_UVIS
long- image
(1710-1900 Å)
R= 17,883 km
Phase= 126.3°
DIONE
016DI_icymap003_prime
long- image
(1710-1900 Å)
(no ISS rider)
R= 103,024 km
Longitude= 160°W
Phase= 22.9°
016DI_icymap005_iss
(part 30; 17:16:21)
DIONE
016DI_targflyby001_iss - part 4
~250°W, 20°N
240°W, 7°S
start
R= 64,749 -> 3585 km
Longitude= 166°W
Phase= 22.7°
end
RHEA
018RH_FP3MAP002_CIRS
long- image
(1710-1900 Å)
1304Å image
R= 87,493 km
Longitude= 170°W
Phase= 18.8°
018RH_ICYMAP006_CIRS
330T19:03
spectral analysis -- history
•
•
•
•
•
We’ve known for a long time (since Phoebe) that the shape of the icy
satellite (and rings) spectra are consistent with H2O ice, due to upturn in
brightness for >~1650 Å.
In trying to get the Phoebe paper out, we’ve been struggling to interpret
the shape of the spectrum as well as the magnitude of the spectrum
We look at the other icy satellites to see if that helps (their spectra may be
more easily understood - more similar to pure H2O ice)
We have been carefully analyzing the data to make sure we’ve got
everything right - in terms of factors of , 4, 2 (low-res vs high-res slits) to make sure that the absolute reflectance (or albedo) is well-understood
We’ve been looking into different spectral modeling types (i.e.,
checkerboard, intimate) and using accurate representations of candidate
species (now using single-scatter parameters from Gary Hansen at many
grain sizes and wavelengths to get out the bidirectional reflectance of the
model parameters) for direct comparison with UVIS data
Raw spectrum
Flat-fielded
1 row average
cts/sec
Simulated solar spectrum; Flatfielded; 1 mrad filled
Output in cts/sec
(input was (F/r2)4e-6)
From Tomasko,
Imeas=p()()(F/2)4e-6
 Imeas (cts/sec) = p()()
solaruvis (cts/sec)
= geometric albedo at some phase angle
Note shape of reflectance
spectrum
Raw spectrum
Flat-fielded
1 row average
cts/sec
Simulated solar spectrum; Flatfielded; 1 mrad filled
Output in cts/sec
(input was (F/r2)4e-6)
Raw spectrum/simulator output=
I/F=reflectance
Note shape of reflectance
spectrum
More examples of reflectance spectra
Notice these
are >1 at
longest 
This is OK.
This is the geometric
albedo and therefore
this means that
the satellites do
not scatter like
Lambertians.
BUT: if this is right, then
geometric albedo at 1900 Å may
be larger than at longer 
Could be interpreted to mean that EN and DI have similar H2O grain sizes and Dione has more non-ice material (more scooped out)
More examples of reflectance spectra
Notice these
are >1 at
longest 
This is NOT OK.
This is I/F and NOT the
geometric albedo.
Could be interpreted to mean that EN and DI have similar H2O grain sizes and Dione has more non-ice material (more scooped out)
And more examples of reflectance spectra
Phoebe, Iapetus, Hyperion
H2O spectra flatten out
shortward of 1900 Å
H2O
H2O reflectance at different grain sizes for
a particular set of , i, e angles
Could the difference in shape between H2O and icy satellites
be due to instrument response to NUV solar and H2O (both bright)??
Evidently not.
Comparisons with Longer
Wavelengths
•
•
•
•
Compare with HST at NUV wavelengths to see if they agree
– Rhea, Dione, Iapetus (Noll et al.)
Use a sub-pixel, disk-integrated UVIS observation for comparison with
HST
When comparing UVIS and HST, must consider filling factor and phase
correction
This comparison should really help us understand whether we’ve got the
correct magnitude, and all factors of 2, , 4 or other accounted for.
Disk-Integrated Phase Curve: All longitudes at 1820 Å
Disk-Integrated Phase Curve: All longitudes at 1900 Å
Test Case 1: Rhea
004RH_icylon003
(targeted using
UVIS_FUV)
Test Case 2: Rhea
010RH_zerophase
Rhea at 0° phase (geometric albedo)
HST
UVIS
We are clearly not comparing apples and apples - so the UVIS spectrum is likely not
geometric albedo after all, and we need to scale UVIS data to match up with HST.
Rhea at 0° phase (geometric albedo)
The UVIS spectrum (sub-pixel) needs to be adjusted properly
for comparison with HST
HST
UVIS
We are clearly not comparing apples and apples - so the UVIS spectrum is likely not
geometric albedo after all, and we need to scale UVIS data to match up with HST.
Rhea: FUV - NIR (disk-integrated)
something other than H2O is present
1 um H2O, scaled
arbitrarily
UVIS + HST +
Clark groundbased
Rhea: FUV - NIR (disk-integrated)
1 um H2O +
tholin, scaled
arbitrarily
UVIS + HST +
Clark groundbased
Rhea: FUV - NIR (disk-integrated)
1 um H2O + tholin, scaled arbitrarily
UVIS (scaled) + HST
Spectral Models - Intimate
Mixtures
Spectra of Candidate Species - 1
CO2
H2O
NH3
Spectra of Candidate Species - 2