UVIS Satellite surfaces update

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

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 2.235 m • Verbiscer et al. 2006: possible detection of NH3 hydrate feature at 2.21 m

7. Enceladus: disk-integrated UVIS reflectance spectrum

8. Enceladus is much darker than pure H2O model

9. Enceladus is much darker in UV than in visible UVIS disk-integrated observation at ~2° compared with ground-based observation at ~2°

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: 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

13. mixture models: H2O + 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 0.2-0.3 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 0.2-0.5 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.