1. UVIS Icy Satellites Surface Studies
Amanda Hendrix
Emilie Royer
Candy Hansen
Tim Cassidy
Also Keith Noll, John Spencer
UVIS Team Meeting, DLR
June 2014

2. topics Enceladus & Tethys Enceladus regional
Composite spectra: UVIS+ISS+VIMS+HST
Enceladus regional
Dark vs bright regions in south pole

3. Enceladus & Tethys
We know that UV is an absorbing region on satellites
What does this tell us about composition & processes?

4. Enceladus & Tethys Uppermost layers of regolith
Primarily E-ring grains coating surfaces
All over Enceladus
More on Tethys LH than TH
Other exogenic effects
UV processing
Radiolytic processing
Enceladus:
Plume fallout (especially in certain zones)
Tethys LH: should be mostly E-ring grains
Compare with Enceladus
Combine datasets from UVIS, HST/STIS, ISS, VIMS

5. Enceladus & Tethys STIS ISS, VIMS UVIS
Just focusing on UV dropoff region …
Note that Rhea LH NUV is similar to Dione LH NUV --- but that Rhea FUV is darker (but we need to confirm our Rhea phase curve with new cal)
UVIS

6. NH3
Enceladus models: H2O + 1% NH3 + 1% ice tholin
Abundances of NH3 may be small enough that they are not detectable by VIMS
Ammonia hydrate has been detected on Enceladus (Emery et al., 2005; Verbiscer et al., 2005) and Tethys TH (Verbiscer et al., 2008)
~1% NH3 in the Enceladus plume (Waite et al., 2009): a constant source
Hendrix et al. (2010)

7. adding in HST/FOS data (Noll et al., 1997)
Rhea & Dione:
• Leading & trailing hemispheres show O3-like absorption
• Roughly same absorption depth on both LH and TH
• Red slope similar to ice tholin

8. Jenniskens “organic refractory residue 1”

9. Jenniskens “organic refractory residue 1”
Multiplied by a red slope

11. Interpretation/conclusions
Enceladus disk-integrated:
E-ring grains + plume fall-out (constantly replenished, so not very processed)
Tethys LH:
E-ring grains (partly processed, but also constantly replenished)
Tethys TH:
E-ring grains, processed by cold plasma bombardment & UV; less replenished than LH
There are ~1% organics in the E-ring (Postberg et al., 2008, 2011)
Organics could be sourced directly from plume (Waite et al.) AND/OR could be processed hydrocarbons from plume
Upon further UV/radiolytic processing, organics become redder (yellow-red-brown-black) (e.g. Andronico et al., 1987; Jenniskens 1993)

12. Enceladus regional variations
Dark vs bright regions in south pole (Rev 88) – other observations too

13. ISS_088EN_ENCELCA001_PRIME 2008-283T19:21:40 EN+00:15
088EN_ICYMAP001_ISS

14. Bright regions
Dark regions
Water ice
Puzzling! Still in work.
Why don’t these show H2O ice dropoff (background subtraction)

15. Dark region in ISS – and UVIS- data
From P. Schenk: dark region are dark at phase angles smaller than ~40 °
Dark region in ISS – and UVIS- data