1. Dione’s O2 Exosphere
C. J. Hansen
January 2013

2. 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

3. 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
For scale length of 100 km bulk density is 1.5 x 1013 / 105 = 1.5 x 108 m-3

4. 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

5. Dione O2 Exosphere Next 3 pages are follow-up from team meeting
22 January 2013

6. Spectra Calibrated with cube generator
Summed over time then calculated average

7. 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

8. 2005-358T14:25 Phase angle: 106 Range: 151,120 km Duration: 30 min
Not clear where Dione is relative to rings
Phase angle: 106
Range: 151,120 km
Duration: 30 min

9. T00:47
Phase angle: 134
Range: 97,050 km
Duration: 42 min

10. T16:22
Phase angle: 81
Range: 300,695 km
Duration: 44 min

11. T0356
Phase angle: 81
Range: 52,668 km
Duration: 50 min

12. T0706
Phase angle: 21
Range: 54,414 km
Duration: 30 min

13. T0538
Phase angle: 68
Range: 237,601 km
Duration: 24 min

14. T1338
Phase angle: 4
Range: 677,100 km
Duration: 108 min

15. T0839
Phase angle: 13
Range: 146,146 km
Duration: 68 min

16. Summary
Time
Range (km)
Phase angle
Duration (min)
Typical 1304 (R)
Typical 1356 (R)
Ratio 1356/1304
T1425
151,120
106 30
3.3
2.7
0.8
T0047
97,050
134 42
4.8
1.0
T1622
300,695 81 44
2.3
0.9
T0356
52,668 50
3.6
T0706
54,414 21
6.0
5.3
T0538
237,601 68 24
T1338
677,100 4
108
5.6
2.6
0.5
T0839
146,146 13
8.8
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

17. 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

18. Backup

19. Summary Time Range (km) Phase angle Duration (min) Typical 1304 (R)
151,120
106 30
50/15 = 3.3
T0047
97,050
134 42
100/21 = 4.8
T1622
300,695 81 44
60/22 = 2.7
T0356
52,668 50
90/25 = 3.6
T0706
54,414 21
90/15 = 6
T0538
237,601 68 24
40/12 = 3.3
T1338
677,100 4
108
150/27 = 5.6
T0839
146,146 13
150/17 = 8.8