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 H 2 O 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 Range= 84,000-67,769 km Lon= 218°W Phase= 42.7° 012MI_ICYLON072_VIMS 18.7° between MI & SA Larger pixels Night/LH Day/TH

6. Mimas Day/TH 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 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 Night/LH

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/LHDay/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-SatDay/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/LHDay/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 H 2 O ice by ions –H 2 O + H + (O + ) -> H 2 O vapor, H, H 2 –Fraction of sputtered particles that are H atoms? Photodesorption of H 2 O ice –H 2 O + h -> H 2 O vapor, H, H 2 –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 H 2 O vapor –From sputtering, sublimation or photodesorption –H 2 O + h -> H + OH –Too cold for sublimation (T<110K) E-ring particle bombardment Micrometeoroid bombardment

12. Measured vs. Predicted a Fluxes a Jurac et al. (2001) H 2 O mol/cm 2 -sec for O + ions Predicted UVIS-Measured atoms/s Mimas 1.59e9 2+.5 e99.6e24 Enceladus 7.5e8 1+.5e95.9e24 Tethys 6.6-7.6e8 3.5+.5e8 1.2e25 Dione 3.9-6.5e8 <1e83.9e24 Rhea----- 4+.5e82.9e25 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