Dione’s O2 Exosphere
C. J. Hansen
January 2013
Background
• CAPS has detected O2+ in the vicinity of Dione
– Tokar, et al. (2012) reports CAPS results
• Detected O2+ with density of 0.01 to 0.09 cm-3
• From models derived column density of 0.9 to 7 x 1011 cm-2
• Likely due to sputtering and radiolysis
• INMS has also detected O2 and CO2
– Some evidence of CO2 seasonality
– Much higher density (in my notes)
• Can we detect O2 with UVIS?
– Nothing shows up in the occultation data (nor would that be
expected for the derived column density)
– Look for emissions at 1304 and 1356 Å
Detection limits for Atmosphere
• From stellar occs
– O2: 1.3 x 1015 cm-2
• From emission features
– Atomic oxygen solar scattering: 1.5 x 1013 cm-2
– O2 electron impact dissociation: 1.6 x 1014 cm-2
– Higher than CAPS number however INMS density
(in my notes) is higher
Approach
• Use ICYLON data sets with long duration, mostly
stationary
• Run cube generator for calibration
– Sum over time
– Sum over spectral channels for 1304 and 1356, plot vs
spatial rows
– Plot signal at long wavelength for first and last time
record to see where limb is
• Look for enhancement near limb
• Look at 1356 / 1304 ratio
– The 1356 / 1304 ratio for O2 is 1.8, for O it is 0.1
Dione O2 Exosphere
Next 3 pages are follow-up from team meeting
22 January 2013
Spectra
• Calibrated with cube generator
• Summed over time then calculated average
Conclusions
• We do not detect Dione’s O2 exosphere
reported by CAPS
• If 1304 is ~ same signal level as 1356
then all we are seeing is noise
• If 1304 is > 1356 then what we are
seeing is O in the system
2005-358T14:25
Phase angle: 106
Range: 151,120 km
Duration: 30 min
2006-325T00:47
Phase angle: 134
Range: 97,050 km
Duration: 42 min
2006-349T16:22
Phase angle: 81
Range: 300,695 km
Duration: 44 min
2007-273T0356
Phase angle: 81
Range: 52,668 km
Duration: 50 min
2007-273T0706
Phase angle: 21
Range: 54,414 km
Duration: 30 min
2008-083T0538
Phase angle: 68
Range: 237,601 km
Duration: 24 min
2009-360T1338
Phase angle: 4
Range: 677,100 km
Duration: 108 min
2010-097T0839
Phase angle: 13
Range: 146,146 km
Duration: 68 min
Summary
Time
Range
(km)
Phase
angle
Duration
(min)
Typical
1304 (R)
Typical
1356 (R)
Ratio
1356/1304
2005-358T1425
151,120
106
30
3.3
2.7
0.8
2006-325T0047
97,050
134
42
4.8
4.8
1.0
2006-349T1622
300,695
81
44
2.7
2.3
0.9
2007-273T0356
52,668
81
50
3.6
3.6
1.0
2007-273T0706
54,414
21
30
6.0
5.3
0.9
2008-083T0538
237,601
68
24
3.3
3.3
1.0
2009-360T1338
677,100
4
108
5.6
2.6
0.5
2010-097T0839
146,146
13
68
8.8
8.8
1.0
This is just a first cut – more work to do to understand the
implications of the geometry; background subtraction; why
we see roughly equal 1304 and 1356 emissions
Conclusions
• Some observations show elevated
1304, 1356 signal over what might be
generally expected for oxygen in system
• None show elevated signal near limb
• None show 1356 / 1304 = 1.8
• Conclusion: Dione’s bound O2
atmosphere has a column density below
our detection threshold
Backup
Summary
Time
Range
(km)
Phase
angle
Duration Typical 1304
(min)
(R)
Typical
1356 (R)
2005-358T1425
151,120
106
30
50/15 = 3.3
40 2.7
2006-325T0047
97,050
134
42
100/21 = 4.8
100 4.8
2006-349T1622
300,695
81
44
60/22 = 2.7
50 2.3
2007-273T0356
52,668
81
50
90/25 = 3.6
90 3.6
2007-273T0706
54,414
21
30
90/15 = 6
80 5.3
2008-083T0538
237,601
68
24
40/12 = 3.3
40 3.3
2009-360T1338
677,100
4
108
150/27 = 5.6
70 2.6
2010-097T0839
146,146
13
68
150/17 = 8.8
150 8.8