UVIS Icy Satellites Surface Studies Amanda Hendrix UVIS Team Meeting, St. George June 2013

Since the last team meeting (Jan 2013)… Flyby (R4)

Topics Io work Seasonal northern hemisphere brightening of Mimas? Tethys? Tethys LH-TH asymmetry Enceladus work H2O spectral characteristics

Io

Io observations Serendipitous We focus on the two best observations Io passed through slit while UVIS stared at ra/dec We focus on the two best observations Dec 30 2000 Jan 2 2001 There are a few other observations (including in eclipse) not considered here These are the first-ever EUV spectra of Io itself, along with the highest resolution full-FUV spectra of Io The community is excited for us to get them published We had been held up by a lack of appropriate rate coefficients to interpret/model the data Now Don has them!

Observation #1: Dec 30 2000 Io is bordering rows 32-33; Cassini-side of Jupiter (dusk side); anti-Jovian hemisphere (lIII~91°W), so Io is roughly in the middle of the torus

Observation #2: Jan 6 2001 Io is in row 30, on the far side of Jupiter (dawn); sub-jovian hemisphere; (lIII~115°W), so Io is roughly in the middle of the torus

Observation #1 Io Data plots (spectra)

Observation #2

Rate coefficients (a sampling)

MAIN RESULTS (from old version of paper – to be updated!) The 1666 Å feature is difficult to model. – to be fixed(?) with new 1999 FF/cal Put limit in e+SO2; most features are assumed to be due to e+S (faster rates). Derive S, O column densities Model our 2 spectra using electron densities and temperatures from our torus observations. Model Feaga et al. STIS spectrum with our model; their1479 interpretation is re-examined. We are puzzled about their 1485.6 Å feature – does not appear in model. Spatial distribution of species Chlorine? Wake/upstream differences? Neutral clouds? Other curiosities: row 31 rec 3 1250/1388 features; row 30/rec 2 1479 ratio.

Investigating seasonal changes on Mimas & Tethys

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

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

H2O2 formation modeling 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.

Tethys … seasonal changes (?) 180°W 048TE_ICYMAP002 (July 2007) The southern hemisphere is brightening up faster than I’d expected, though. So H2O2 is produced quickly, and destroyed slowly So soon-ish after the sun moves into the northern hemisphere (after Aug 2009 equinox), we should start seeing a darkening of the north The sun was primarily in the south roughly 2002-2009 So for Tethys obs in 2007, the sun had been producing H2O2 for ~5 years. For Tethys obs in 2012, the sun had been producing H2O2 for ~3 years. The dark southern hemisphere may take a while to brighten up since the H2O2 is slow to get destroyed. The bright northern hemisphere may get dark more quickly – so could end up with dark-ish north and south hemispheres. 210°W 180°W 210°W 164TE ICYLON002d (April 2012)

Tethys HST/STIS (Noll) Consider HST (Verbiscer – at least for Encel) in scaling IUE ISS (e.g. Greg, Paul S.)

T M E D R For both Dione and (especially) Tethys, the trailing hemispheres are much darker relative to the leading in the UV compared to the visible. (or, their leading hemispheres are relatively bright)

STIS UVIS

Enceladus Photometric characteristics Compared with other moons Spectral variations across the surface Signature of plume fallout

Verbiscer et al., 2005

Enceladus plume fallout

Schenk et al. 2011 Kempf et al. 2010

non-fallout region OLD 60°W fallout region Rev 121

OLD We have not done anything about solar variations (diurnal or cycle) OR photometry – so disregard y-scale. But spectral SHAPE should be ok.

H2O ice reflectance overplotted

non-fallout region NEW 60°W fallout region Rev 121

So this needs a lot of work yet to understand.

crystallinity, structure of H2O ice

Map the depth of this feature across different bodies Is the peak that appears in the Enceladus spectra from the south pole (esp. tiger stripes) related to the crystallinity of the ice? Map the depth of this feature across different bodies Water ice in reflectance (Pipes et al., 1974)

The upturn in H2O lab reflectance spectra toward short waves may be related to surface scattering – or to scattering from facets on the grains.

The fact that we don’t see the upturn in the Enceladus reflectance spectrum may be because we are sensing the fine E ring grains (& plume fallout grains) coating the surface – which are smaller than the facets on the grains in the lab

Upcoming activities & focus EPSC PSG/ISWG presentation? DPS? AGU? Enceladus – disk resolved spectral variations & photometric characteristics Io – wrap up paper Tethys – seasonal variations & LH/TH asymmetry