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

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

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

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

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

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)

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

Jenniskens “organic refractory residue 1”

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

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)

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

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

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

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