1. UVIS Icy Satellites Surface Studies
Amanda Hendrix
UVIS Team Meeting, St. George
June 2013

2. Since the last team meeting (Jan 2013)…
Flyby (R4)

3. Topics
Io work
Seasonal northern hemisphere brightening of Mimas? Tethys?
Tethys LH-TH asymmetry
Enceladus work
H2O spectral characteristics

4. Io

5. Io observations Serendipitous We focus on the two best observations
Io passed through slit while UVIS stared at ra/dec
We focus on the two best observations
Dec
Jan
There are a few other observations (including in eclipse) not considered here
These are the first-ever EUV spectra of Io itself, along with the highest resolution full-FUV spectra of Io
The community is excited for us to get them published
We had been held up by a lack of appropriate rate coefficients to interpret/model the data
Now Don has them!

6. Observation #1: Dec
Io is bordering rows 32-33; Cassini-side of Jupiter (dusk side); anti-Jovian hemisphere
(lIII~91°W), so Io is roughly in the middle of the torus

7. Observation #2: Jan
Io is in row 30, on the far side of Jupiter (dawn); sub-jovian hemisphere;
(lIII~115°W), so Io is roughly in the middle of the torus

8. Observation #1 Io
Data plots (spectra)

9. Observation #2

10. Rate coefficients (a sampling)

11. MAIN RESULTS (from old version of paper – to be updated!)
The 1666 Å feature is difficult to model. – to be fixed(?) with new 1999 FF/cal
Put limit in e+SO2; most features are assumed to be due to e+S (faster rates). Derive S, O column densities
Model our 2 spectra using electron densities and temperatures from our torus observations. Model Feaga et al. STIS spectrum with our model; their1479 interpretation is re-examined. We are puzzled about their Å feature – does not appear in model.
Spatial distribution of species
Chlorine?
Wake/upstream differences? Neutral clouds?
Other curiosities: row 31 rec /1388 features; row 30/rec ratio.

12. Investigating seasonal changes on Mimas & Tethys

13. 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

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. H2O2 formation modeling
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

16. 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.

17. Tethys … seasonal changes (?)
180°W
048TE_ICYMAP002 (July 2007)
The southern hemisphere is brightening up faster than I’d expected, though.
So H2O2 is produced quickly, and destroyed slowly
So soon-ish after the sun moves into the northern hemisphere (after Aug 2009 equinox), we should start seeing a darkening of the north
The sun was primarily in the south roughly
So for Tethys obs in 2007, the sun had been producing H2O2 for ~5 years.
For Tethys obs in 2012, the sun had been producing H2O2 for ~3 years.
The dark southern hemisphere may take a while to brighten up since the H2O2 is slow to get destroyed.
The bright northern hemisphere may get dark more quickly – so could end up with dark-ish north and south hemispheres.
210°W
180°W
210°W
164TE ICYLON002d (April 2012)

18. Tethys HST/STIS (Noll)
Consider HST (Verbiscer – at least for Encel) in scaling
IUE
ISS (e.g. Greg, Paul S.)

19. T M E D R
For both Dione and (especially) Tethys, the trailing hemispheres are much darker relative to the leading in the UV compared to the visible.
(or, their leading hemispheres are relatively bright)

21. STIS
UVIS

22. Enceladus Photometric characteristics
Compared with other moons
Spectral variations across the surface
Signature of plume fallout

25. Verbiscer et al., 2005

26. Enceladus plume fallout

27. Schenk et al. 2011
Kempf et al. 2010

28. non-fallout region
OLD
60°W
fallout region
Rev 121

29. OLD
We have not done anything about solar variations (diurnal or cycle) OR photometry – so disregard y-scale.
But spectral SHAPE should be ok.

30. H2O ice reflectance overplotted

31. non-fallout region
NEW
60°W
fallout region
Rev 121

32. So this needs a lot of work yet to understand.

33. crystallinity, structure of H2O ice

34. Map the depth of this feature across different bodies
Is the peak that appears in the Enceladus spectra from the south pole (esp. tiger stripes) related to the crystallinity of the ice?
Map the depth of this feature across different bodies
Water ice in reflectance (Pipes et al., 1974)

35. The upturn in H2O lab reflectance spectra toward short waves may be related to surface scattering – or to scattering from facets on the grains.

36. The fact that we don’t see the upturn in the Enceladus reflectance spectrum may be because we are sensing the fine E ring grains (& plume fallout grains) coating the surface – which are smaller than the facets on the grains in the lab

37. Upcoming activities & focus
EPSC
PSG/ISWG presentation?
DPS?
AGU?
Enceladus – disk resolved spectral variations & photometric characteristics
Io – wrap up paper
Tethys – seasonal variations & LH/TH asymmetry