1. UVIS Satellite surfaces update
Amanda R. Hendrix
UVIS Team Meeting
Berlin
8-10 June 2009

2. H2O ice Flat in visible Well-known 1.5, 2.0, 3.0 m features
Strong absorption edge in far-UV, ~0.165 m
Start by observing the reflectance spectrum of H2O ice. (varies of course in some ways with grain size and temperature)

3. Enceladus : previous work - 1
Buratti 1984
Voyager data
“the spectrum of Enceladus is flat, consistent with an almost pure H2O ice surface”

4. Enceladus: previous work - 2
Verbiscer et al. 2005
HST data
Consistent with Buratti 1984 results
One additional data point at 275 nm, suggesting a dropoff in reflectance

5. Enceladus: previous work - 3
Cruikshank et al. 2005
Ground-based data
“Consistent with the work of other investigators, we have detected no spectral features other than those of H2O ice”

6. Enceladus: previous work - 4
Emery et al. 2005
Ground-based data
“The spectrum of Enceladus shows activity near 2.25 m that we interpret as a possible signature of NH3 ice.”
NH3 ice feature at m
Verbiscer et al. 2006: possible detection of NH3 hydrate feature at 2.21 m

7. Enceladus: disk-integrated UVIS reflectance spectrum
We simultaneously noted 2 characteristics: the UVIS spectrum is darker than 1) the visible and 2) pure H2O ice model

8. Enceladus is much darker than pure H2O model
We simultaneously noted 2 characteristics: the UVIS spectrum is darker than 1) pure H2O ice model and 2) visible wavelengths.
The H2O ice edge is measured at 165 nm -- but it’s still dark nm

9. Enceladus is much darker in UV than in visible
UVIS disk-integrated observation at ~2° compared with ground-based observation at ~2°
Composite spectrum with UVIS + Enceladus data from Verbiscer et al 2005, 2006

10. what darkens Enceladus’ spectrum in the UV?
Not H2O
Must be something that is
bright in the visible
featureless in the visible
featureless (nearly) in the NIR

11. candidate species CO2: tholins:
The CO2 absorption edge occurs (in models) at ~175 nm -- too short for what we need
tholins:
Spectral mixing models with H2O ice & tholins did not suitably match UV data

12. NH3
NH3 is an attractive candidate because it has such a strong UV edge that you only need a little bit in your spectral model to be effective. And it’s bright and featureless throughout the visible.

13. mixture models: H2O + NH3
Blue line: intimate mixture of (95:5:1 1 m H2O: 12 m NH3:2 m tholin)
Light blue line: areal model of 20% 50 m H2O and 80% (86:1:13 1 m H2O: 16 m NH3:2 m tholin)
Red line: areal model of 23% 1 m H2O and 77% (99:1 1 m H2O: 100 m NH3)
Orange line: intimate mixture of (99:1 1 m H2O: 10 m NH3)
• So far, the models do not exactly replicate the Enceladus spectrum.
• They do mimic the spectral characteristics including the FUV “notch.”
• We do not have H2O ice or NH3 ice optical constants for appropriate temperatures.

14. more recent NH3 data
Dawes et al. 2007

15. one interpretation
There is a growing body of evidence that NH3 is present in small amounts (~1%) in the Enceladus plume, on the surface and in the environment
INMS (Waite et al.)
CDA (McBride et al. 2007)
CAPS (Smith et al. 2008)
NH3 is ejected through the plume into the system (E-ring)
Enceladus is bathed by E-ring grains, so the small amount of NH3 on the surface of Enceladus is continually replenished
also helps to continually keep the surface so visibly bright
Two options:
Redeposited globally through continual E-ring grain deposition/bombardment
We expect the other inner icy moons to exhibit similar spectral effects
Redeposited mainly in south pole region in plume fallout
Should see spectral variations with latitude on Enceladus

16. preliminary Mimas, Tethys composite spectra
* VIMS
* VIMS LH TH
Spectral modeling still to be done … but certainly something is absorbing strongly in the m region

17. preliminary Dione, Rhea composite spectra
* VIMS
* VIMS
Ground-based (Noll/Roush et al.)
Ground-based (Noll/Roush et al.)
HST (Noll et al.)
HST (Noll et al.)
Dione & Rhea might have less-steep slopes in the m region than Enceladus, Mimas, Tethys (more tholin, less NH3?)

18. Currently working up phase curves for the other moons …

19. So do we have evidence for NH3 on other moons?
Not sure yet.

20. So do we have evidence for more NH3 in Enceladus south polar region than elsewhere on surface?

21. Enceladus : disk-resolved example #1
004EN_ICYMAP010

22. Enceladus : disk-resolved example #2
011EN_ICYMAP015
Why do these spectra appear to have dip near 1830 Å while previous ones don’t?

23. disk-resolved: northern hem
Measured I/F spectrum is consistent with disk-integrated at ~115° (ratty though)

24. So far, we don’t seem to have any evidence for large long-wavelength brightness variations across the surface of Enceladus that could suggest variations in NH3 amounts.