Rhea Oxygen?
Units on plots need to be fixed
C. J. Hansen
9 January 2007
Background
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Rhea is the largest of Saturn’s icy moons, and the only one massive
enough to hold on to a sputtered atmosphere (analogous to Europa)
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Cassini’s flyby of Rhea in November 2005 was through the wake - a
region in which a depletion in charged particles is expected
The expected depletion was detected, but over a larger expanse than
predicted
In particular MIMI observed a depletion of energetic electrons - their
interpretation is that it is due to a dust cloud, a neutral atmosphere or a
combination of both - estimate of required column density is 3 x 1016
cm-2
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With UVIS we can look for the signature of a bound oxygen
atmosphere
– Look for emission at 1304 (O) and 1356 (O2)
– This is a progress report on that effort
Changes in System Oxygen Content
Data Processing
• Ten data sets were selected for analysis
– Field of view centered on Rhea, not slewing, long integration
• Records summed over time
• Calibrated, flat fielded (using AS with NaN’s), but no red patch
• Summed wavelength channels corresponding to 1304, 1356,
and background set of channels at intermediate wavelength
• Subtracted background, plotted signal vs. pixel number (fix
units: not kR/ang)
• Plotted one wavelength channel where reflected Rhea data will
show position of Rhea limb
• Divided by duration to get rate (fix units - s/b Rayleighs: R/angsec)
• Most results show no Rhea-associated presence of oxygen
• Some do, but it is a geometric effect
Little / No Oxygen
(There is oxygen throughout the system - but is it associated with
Rhea?)
Rev 5
Rhea Rev005
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Range
Phase
Direction to Saturn
Duration = 66 min
Rhea Rev005
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Range
Phase
Direction to Saturn
Duration = 66 min
Rev 5
Rhea Rev005
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Range
Phase
Direction to Saturn
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Duration = 58 min
# spatial pixels = 64
Rhea Rev005
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Range
Phase
Direction to Saturn
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Duration = 58 min
# spatial pixels = 64
Rev 12
Rhea Rev012
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Range
Phase
Direction to Saturn
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Duration = 38 records x 120 sec = 76 min
# spatial pixels = 57
Rhea Rev012
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Range
Phase
Direction to Saturn
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Duration = 38 records x 120 sec = 76 min
# spatial pixels = 57
Lots of Oxygen
Rhea Rev018
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Range
Phase
Direction to Saturn
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Duration = 178 records x 30 sec = 89 min
# spatial pixels = 64
Rhea Rev018
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Range
Phase
Direction to Saturn
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Duration = 178 records x 30 sec = 89 min
# spatial pixels = 64
Rev 18
Rhea Rev004
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Range
Phase
Direction to Saturn
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Duration = 20 records x 120 sec = 40 min
# spatial pixels = 64
Rhea Rev004
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Range
Phase
Direction to Saturn
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Duration = 20 records x 120 sec = 40 min
# spatial pixels = 64
Conclusions
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Rhea does not appear to have a neutral oxygen atmosphere
– Although 1304 and 1356 can be well above the background this occurs at
geometries where Rhea is viewed close to Enceladus
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This allows us to calculate upper limits for O and O2 associated with
Rhea, based on not seeing emission features
– O: for a 90 min observation, calculated for solar scattering at Saturn, gives
an upper limit of 4 x 1013 cm-2
– O2: for a 90 min observation, calculated for electron-impact excitation,
gives an upper limit of 1.5 x 1014 cm-2
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The component broadening Rhea’s wake is not a sputtered O or O2
atmosphere
Backup Slides
Rhea Rev012
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Range
Phase
Direction to Saturn
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Duration = 38 records x 120 sec = 76 min
# spatial pixels = 57
Rhea Rev005
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Range
Phase
Direction to Saturn
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Duration = 110 min
# spatial pixels = 64
Rhea Rev005
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Range
Phase
Direction to Saturn
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Duration = 110 min
# spatial pixels = 64
Rhea Rev006
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Range
Phase
Direction to Saturn
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Duration = 49 records x 120 sec = 98 min
# spatial pixels = 24
Rhea Rev006
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Range
Phase
Direction to Saturn
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Duration = 49 records x 120 sec = 98 min
# spatial pixels = 24
Rev 6
Rev 11
Rhea Rev012
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Range
Phase
Direction to Saturn
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Duration = 38 records x 120 sec = 76 min
# spatial pixels = 57
Rev 12
Rhea Rev011, DOY 195
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Range
Phase
Direction to Saturn
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Duration = 17 records x 120 sec = 34 min
# spatial pixels = 57
Rhea Rev011, DOY 195
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Range
Phase
Direction to Saturn
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Duration = 17 records x 120 sec = 34 min
# spatial pixels = 57
Rhea Rev011, DOY 195
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Range
Phase
Direction to Saturn
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Duration = 52 records x 120 sec = 104 min
# spatial pixels = 57
Rhea Rev011, DOY 195
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Range
Phase
Direction to Saturn
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Duration = 52 records x 120 sec = 104 min
# spatial pixels = 57
Rev 11
Rhea Rev018
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Range
Phase
Direction to Saturn
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Duration
# spatial pixels
Rhea Rev018
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Range
Phase
Direction to Saturn
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Duration
# spatial pixels
Rev 18
Rhea Rev012
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Range
Phase
Direction to Saturn
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Duration = 38 records x 120 sec = 76 min
# spatial pixels = 57
Rev 18
Rhea Rev026
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Range
Phase
Direction to Saturn
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Duration = 83 records x 120 sec = 166 min
# spatial pixels = 57
Rhea Rev026
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Range
Phase
Direction to Saturn
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Duration = 83 records x 120 sec = 166 min
# spatial pixels = 57
Rhea Rev026
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Range
Phase
Direction to Saturn
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Duration = 76 records x 120 sec = 152 min
# spatial pixels = 57
Rhea Rev026
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Range
Phase
Direction to Saturn
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Duration = 76 records x 120 sec = 152 min
# spatial pixels = 57