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Cassini UVIS Measurements of Hydrogen Exospheres at the Icy Saturnian Satellites

This study presents the UVIS instrument measurements of hydrogen exospheres at the icy Saturnian satellites. The observations focus on Ly-α background and off-limb signals, as well as the potential sources of hydrogen. Preliminary results suggest that sputtering is the dominant source of hydrogen.

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Cassini UVIS Measurements of Hydrogen Exospheres at the Icy Saturnian Satellites

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  1. Cassini UVIS Measurements of Hydrogen Exospheres at the Icy Saturnian Satellites Amanda Hendrix Candy Hansen (with input from Charles Barth, Wayne Pryor, Don Shemansky) JPL/CalTech DPS 2005

  2. The UVIS Instrument • Long slit - 64 spatial pixels • Each spatial pixel is 1 mrad x (0.75, 1.5, 8) mrad • 1024 spectral pixels per spatial pixel • Images are made by scanning • Here we focus on observations when slit is held steady for ~10 min (increased SNR)

  3. Hydrogen measurements - 1 • Ly- (1216 Å) • Ly- (1025 Å) not discussed here • Disk itself is darker than background at 1216 Å • due to presence of H2O ice, which is very dark at short wavelengths • Signal off limb of icy satellite is higher than surrounding background Ly- • Background Ly- is combination of IPH + Saturn system H Icy satellites contribute in part to Saturn system H cloud

  4. Hydrogen measurements - 2 • The Ly- background does not vary significantly over the measured angular distance • Distribution is not due to sensitivity variations along slit • Off-axis scattering from body does not contribute to off-limb signal at Ly- • Results are preliminary • Still being worked • Background levels are still being studied

  5. Mimas Night/LH Day/TH Larger pixels Range= 84,000-67,769 km Lon= 218°W Phase= 42.7° 012MI_ICYLON072_VIMS 18.7° between MI & SA

  6. Mimas Night/LH Day/TH column emission rate = 0.051 kR slant column density= 2.52576e+12 atoms/cm2 volume density= 64432.7 atoms/cm3 escape flux = 2.2e+09 atoms/cm2-sec column emission rate = 0.057 kR slant column density= 2.80360e+12 atoms/cm2 volume density= 71520.3 atoms/cm3 escape flux = 2.4e+09 atoms/cm2-sec

  7. Enceladus Day/TH Night/LH column emission rate = 0.0328 kR slant column density= 1.62376e+12 atoms/cm2 volume density= 32475.2 atoms/cm3 escape flux = 1.06e+09 atoms/cm2-sec column emission rate = 0.0396 kR slant column density= 1.96040e+12 atoms/cm2 volume density= 39207.9 atoms/cm3 escape flux = 1.3e+09 atoms/cm2-sec

  8. Tethys Night/LH Day/TH column emission rate = 0.0232 kR slant column density= 1.14889e+12 atoms/sec volume density= 10838.6 atoms/cm3 escape flux = 3.6e+08 atoms/cm2-sec column emission rate = 0.0222 kR slant column density= 1.10219e+12 atoms/sec volume density= 10398.0 atoms/cm3 escape flux = 3.5e+08 atoms/cm2-sec

  9. Dione Night/Sub-Sat Day/Anti-Sat column emission rate = 0.0267 kR slant column density= 1.32603e+12 atoms/cm2 volume density= 11839.5 atoms/cm3 escape flux = 3.97e+08 atoms/cm2-sec column emission rate = 0.0179 kR slant column density= 8.84017e+11 atoms/cm2 volume density= 7893.01 atoms/cm3 escape flux = 2.6e+08 atoms/cm2-sec

  10. Rhea Night/LH Day/TH column emission rate = 0.0411111 kR slant column density= 2.03520e+12 atoms/cm2 volume density= 13302.0 atoms/cm3 escape flux = 4.5e+08 atoms/cm2-sec column emission rate = 0.034 kR slant column density= 1.68899e+12 atoms/cm2 volume density= 11039.2 atoms/cm3 escape flux = 3.7e+08 atoms/cm2-sec

  11. Possible Sources of H • Sputtering of H2O ice by ions • H2O + H+ (O+) -> H2O vapor, H, H2 • Fraction of sputtered particles that are H atoms? • Photodesorption of H2O ice • H2O + h -> H2O vapor, H, H2 • Westley et al. (1995) measure yields of ~0.5% (85K) - 0.7% (100K) • Photodesorption experiments studying H are lacking • Our measurements require yields of at least ~0.7% • This may be an unimportant source relative to sputtering • Photodissociation of H2O vapor • From sputtering, sublimation or photodesorption • H2O + h -> H + OH • Too cold for sublimation (T<110K) • E-ring particle bombardment • Micrometeoroid bombardment

  12. Measured vs. Predicteda FluxesaJurac et al. (2001) H2O mol/cm2-sec for O+ ions PredictedUVIS-Measuredatoms/s Mimas 1.59e9 2+.5 e9 9.6e24 Enceladus 7.5e8 1+.5e95.9e24 Tethys 6.6-7.6e8 3.5+.5e81.2e25 Dione 3.9-6.5e8 <1e83.9e24 Rhea ----- 4+.5e8 2.9e25 Measured fluxes are generally consistent with predicted -- good indicator that sputtering is dominant source -- may suggest additional source is secondarily important

  13. Conclusions • Clear Ly- signal above background • Escape fluxes are consistent with predicted sputtering rates • No significant leading-trailing variations so far • Suggests that gyroradii of scattering particles are as large as the body

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