Icy Satellites Update Amanda Hendrix UVIS Team Meeting, Orlando With contributions from Emilie Royer Tim Cassidy Candy Hansen Bonnie Buratti Ben Teolis Chris Paranicas UVIS Team Meeting, Orlando 7-9 January 2013

Since the last team meeting (Jan 2012)… Mimas paper was accepted at Icarus & published Icarus 220: 922-931. Zastrow Enceladus paper accepted at Icarus Icarus 220: 29-35. UVIS data (Mimas, Tethys) presented: LPSC PSG workshop EPSC DPS (Emilie) AGU (Emilie) Flybys (E17 on 27 Mar 2012; E18 on 14 Apr 2012; E19 on 2 May 2012… )

Topics Carbon models Enceladus forward scattering Orbital longitude relationship? Seasonal northern hemisphere brightening of Mimas? Tethys?

D&L 1984

Plot credit: Jeff Cuzzi

D&L 1984

From 2008 Icarus paper

Need to work on these models more (e. g Need to work on these models more (e.g. try intramixture model; add in ISS data) to try to fit broad peak in reflectance (~135 nm) … but getting there (there may be more shape in the spectrum at l<160 nm than assumed in our Icarus paper)

Plot credit: Roger Clark

The ~220 nm dip is probably too hard to get Perhaps the way to resolve the carbon vs iron problem is to look at FUV color ratios (130nm/150 nm) Especially in the rings The ~220 nm dip is probably too hard to get Applications to low-albedo asteroids as well In work

Enceladus: forward scattering (from disk? plume? E-ring grains?)

• Using mean anomaly values from Dave Seal • Values are not strictly disk-integrated reflectance (sum of rows 30-32) • Corrected for Sun-Saturn distance • ICYPLU observations only (revs 85-159) so far • So far, I see no sign of a peak near 180 nm

Mimas: 170-190 nm photolysis. H2O2 Co-rotation E-ring grains & neutrals Co-rotation Hot electrons Typical “cold” ions Hot electrons do NOT produce a significant “lens” shape on the leading hemisphere Brightening by E-ring grains on trailing hemisphere photolysis. H2O2 Why is it so much darker in the south than in the north?

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

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

Tethys Brightening by E-ring grains on leading hemisphere E-ring grains & neutrals Typical “cold” ions Hot electrons Co-rotation Hot electrons do NOT produce a “lens” shape on the leading hemisphere photolysis. H2O2 Why is it so much darker in the south than in the north?

H2O2 formation modeling For simplicity we analyze the concentration and time constant on the ice surface to get a rough idea of the concentrations and time scales. 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

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.

Dione, Rhea: no latitudinal variations due to less pure H2O ice on the surface (?) 30°W 129DI_ICYLON001 Normal albedo 180°W 018RH_ICYMAP006 Normal albedo

Any H2O2-related darkening in the north so far?... hard to say for Mimas 012MI_ICYLON011_PRIME 2005-214T05:34 167MI_ICYMAP001_CIRS 2012-157T07:02 TH illuminated LH illuminated

•Max brightness is very close; • Not looking at the same hemispheres • Need to work with geometer to carefully assess 012MI_ICYLON011_PRIME 2005-214T05:34 167MI_ICYMAP001_CIRS 2012-157T07:02 TH illuminated LH illuminated

Any H2O2-related darkening in the north so far Any H2O2-related darkening in the north so far?... still in work for Tethys 164TE_ICYLON002_CIRS 2012-106T00:32 This observation is a good option for testing and comparing to earlier observation; Need to get geometer up & running

Ongoing/future work Io paper (with Don, Andrew Steffl) Tethys paper (with Tim) Enceladus plume fallout material – spectrum Incorporate more recent flyby data UV-vis spectra Using ISS data, HST do spectral models with small amts of NH3 (after Tim's model) try Hodyss H2O absorbance values in reflectance models