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
The UVIS Instrument
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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)
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
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
Mimas
Day/TH
Night/LH
Larger pixels
Range= 84,000-67,769 km
Lon= 218°W
Phase= 42.7°
012MI_ICYLON072_VIMS
18.7° between MI & SA
Mimas
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
Night/LH
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
Enceladus
Day/TH
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
Night/LH
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
Tethys
Night/LH
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
Day/TH
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
Dione
Night/Sub-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
Day/Anti-Sat
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
Rhea
Night/LH
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
Day/TH
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
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
Measured vs.
aJurac
a
Predicted
Fluxes
et al. (2001) H2O mol/cm2-sec for O+ ions
Predicted
Mimas
1.59e9
Enceladus 7.5e8
Tethys
6.6-7.6e8
Dione
3.9-6.5e8
Rhea
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UVIS-Measured atoms/s
2+.5 e9
9.6e24
1+.5e9
5.9e24
3.5+.5e8
1.2e25
<1e8
3.9e24
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
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