1. Icy Satellites Update Amanda Hendrix UVIS Team Meeting, Orlando
With contributions from
Emilie Royer
Tim Cassidy
Candy Hansen
Bonnie Buratti
Ben Teolis
Chris Paranicas
UVIS Team Meeting, Orlando
7-9 January 2013

2. Since the last team meeting (Jan 2012)…
Mimas paper was accepted at Icarus & published
Icarus 220:
Zastrow Enceladus paper accepted at Icarus
Icarus 220:
UVIS data (Mimas, Tethys) presented:
LPSC
PSG workshop
EPSC
DPS (Emilie)
AGU (Emilie)
Flybys (E17 on 27 Mar 2012; E18 on 14 Apr 2012; E19 on 2 May 2012… )

3. Topics Carbon models Enceladus forward scattering
Orbital longitude relationship?
Seasonal northern hemisphere brightening of Mimas? Tethys?

4. D&L 1984

5. Plot credit:
Jeff Cuzzi

6. D&L 1984

7. From 2008 Icarus paper

8. Need to work on these models more (e. g
Need to work on these models more (e.g. try intramixture model; add in ISS data) to try to fit broad peak in reflectance (~135 nm)
… but getting there (there may be more shape in
the spectrum at l<160 nm than assumed in our
Icarus paper)

9. Plot credit:
Roger Clark

10. The ~220 nm dip is probably too hard to get
Perhaps the way to resolve the carbon vs iron problem is to look at FUV color ratios (130nm/150 nm)
Especially in the rings
The ~220 nm dip is probably too hard to get
Applications to low-albedo asteroids as well
In work

11. Enceladus: forward scattering (from disk? plume? E-ring grains?)

12. • Using mean anomaly values from Dave Seal
• Values are not strictly disk-integrated reflectance (sum of rows 30-32)
• Corrected for Sun-Saturn distance
• ICYPLU observations only (revs ) so far
• So far, I see no sign of a peak near 180 nm

13. Mimas: 170-190 nm photolysis. H2O2 Co-rotation
E-ring grains & neutrals
Co-rotation
Hot electrons
Typical “cold” ions
Hot electrons do NOT produce a significant “lens” shape on the leading hemisphere
Brightening by E-ring grains on trailing hemisphere
photolysis.
H2O2
Why is it so much darker in the south than in the north?

14. H2O2: a UV darkening agent
Carlson et al. 1999
The Mimas observation was made in Feb 2010, not long after equinox; the southern hemisphere of Mimas had been experiencing summer

15. MIMAS
TETHYS
Tethys gets darker (toward TH) than Mimas
Tethys’ LH (part of it) is as bright as Mimas’s TH
Tethys’ bright LH seems to be offset toward north

16. Tethys
Brightening by E-ring grains on leading hemisphere
E-ring grains & neutrals
Typical “cold” ions
Hot electrons
Co-rotation
Hot electrons do NOT produce a “lens” shape on the leading hemisphere
photolysis.
H2O2
Why is it so much darker in the south than in the north?

17. H2O2 formation modeling
For simplicity we analyze the concentration and time constant on the ice surface to get a rough idea of the concentrations and time scales.
On the Mimas leading hemisphere, from electrons & ions
We estimate an average surface H2O2 concentration from electrons and ions of only ~0.008%
Considering UV photons: ~0.13 % at 45 deg latitude

18. H2O2 timescales The time constants are
~8 years for dark surfaces on the leading hemisphere, and
~65 days on illuminated surfaces at 45 deg latitude.
consistent with slow H2O2 destruction by electrons and ions in the shadowed northern latitudes during the ~7 year winter timeframe
followed by a several month recovery in peroxide during the transition to summer as surfaces are newly illuminated.
The months-long recovery time scales would imply a time lag in the northern latitude albedo, possibly accounting for the north-south asymmetry seen by UVIS ~6 months after equinox.

19. Dione, Rhea: no latitudinal variations due to less pure H2O ice on the surface (?)
30°W
129DI_ICYLON001
Normal albedo
180°W
018RH_ICYMAP006
Normal albedo

20. Any H2O2-related darkening in the north so far?... hard to say for Mimas
012MI_ICYLON011_PRIME
T05:34
167MI_ICYMAP001_CIRS
T07:02
TH illuminated
LH illuminated

21. •Max brightness is very close;
• Not looking at the same hemispheres
• Need to work with geometer to carefully assess
012MI_ICYLON011_PRIME
T05:34
167MI_ICYMAP001_CIRS
T07:02
TH illuminated
LH illuminated

22. Any H2O2-related darkening in the north so far
Any H2O2-related darkening in the north so far?... still in work for Tethys
164TE_ICYLON002_CIRS
T00:32
This observation is a good option for testing and comparing to earlier observation;
Need to get geometer up & running

23. Ongoing/future work Io paper (with Don, Andrew Steffl)
Tethys paper (with Tim)
Enceladus plume fallout material – spectrum
Incorporate more recent flyby data
UV-vis spectra
Using ISS data, HST
do spectral models with small amts of NH3 (after Tim's model)
try Hodyss H2O absorbance values in reflectance models