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Icy Satellites Update: Mimas, Tethys, Enceladus, and more

Updates on recent research and observations of icy satellites Mimas, Tethys, and Enceladus, including new findings and ongoing/future work.

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Icy Satellites Update: Mimas, Tethys, Enceladus, and more

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

  2. 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… )

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

  4. D&L 1984

  5. Plot credit: Jeff Cuzzi

  6. D&L 1984

  7. From 2008 Icarus paper

  8. 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)

  9. Plot credit: Roger Clark

  10. 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

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

  12. • 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

  13. Mimas: 170-190 nm 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?

  14. 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

  15. 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

  16. 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?

  17. 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

  18. 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.

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

  20. 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

  21. •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

  22. 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

  23. 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

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