1. Icy Moon Occultations: the Search for Volatiles
C. J. Hansen, A. Hendrix, B. Meinke
6 January 2010

2. Outline Overview Tethys Dione Iapetus Rhea Upper limits Enceladus
Enceladus’ Plume

3. UVIS Observations of Stellar Occultations
Why do we observe occultations of bright stars by Saturn’s icy moons with UVIS?
Typically to look for volatiles
Dione, Rhea, Iapetus
Enceladus’ plume
Sometimes for rings
Rhea
UVIS Configuration:
FUV
5 sec integration
512 spectral channels
HSP
2 msec integration
Sigma SGR over Iapetus

4. Methodology
Step 1: Plot HSP data. The ingress is clean - this is the first place to look for any attenuation. The egress is plagued by the instrument signal coming up again, so apparent dips on egress are artifacts. To look for attenuation on egress must sum FUV over wavelength, but this means temporal resolution is 5 sec, not 2 msec.
Step 2: Determine which spatial rows of the FUV the star is in and sum over those rows. Generate I0 spectrum, the unocculted signal, pre-ingress and post-egress. Plot potentially occulted signal I, with I0, to compare.
Step 3: Plot I / I0 to see absorption features. In the case of Tethys, Rhea, Iapetus, and Dione we have found none. In the case of Enceladus we clearly see absorption by water vapor by the plume.
Step 4: If absorption features are detected model gas column density and compare gas absorption features to I / I0
Step 5: If no absorption features, calculate upper limits

5. Tethys
Goal: Search for volatiles in Tethys’ vicinity. Use this airless body to compare others with more potential for gases.
Result: No evidence for gas absorption (as expected)
Occultations Observed
Orbit T02:36 beta Tau
Orbit T04:32 66 Oph
Orbit T23:21 8 nu Cap
Orbit 15

6. Dione
Goal: Search for volatiles - MAPS instruments detect mass-loading of magnetosphere near Dione, similar to Enceladus but down more than an order of magnitude? Look for small plumes. May be very localized.
Result: No evidence for gas absorption (yet)
Occultations Observed
Observation start Star Ingress lat/long Egress lat/long
Orbit T16: Oph / / 327.9
Orbit T12:12 alpha Leo / / 304.3
Orbit T17:00 epsilon Cma / / 163.1
Orbit 19

7. Dione
Orbit 45
Counts / 100 msec
Orbit 118

8. Iapetus
Goal: Search for volatiles possibly being transported from the dark side to the bright side of Iapetus - MAPS instruments report that Iapetus has interesting / unusual interaction with solar wind.
Result: No evidence for gas absorption
Occultation Observed
Orbit T12:55 sigma SGR

9. Rhea
Goal: Search for oxygen atmosphere that could cause plasma absorption detected by MIMI. Search for ring material that could cause plasma absorption detected by MIMI.
Only moon massive enough to retain a sputtered atmosphere
Recall:
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
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
MIMI also saw narrow dips in electron flux, attributed possibly to rings
Potential ring radii:
1610 km
1800 km
2020 km

10. Rhea Occultations Observed Orbit 47 2007-179T13:30 eps Ori
Orbit T16:37 beta, kappa Ori
Also, non-occultation data was analyzed to look for oxygen emission
Orbit 47

11. Rhea Rev 121 Beta Ori Occultation
Rev 121 features a beta Ori occultation ingress and partial egress, and a kappa Ori occultation egress
Ingress was close to equatorial
HSP data doesn’t show any obvious dips

12. Rhea Rev 121 Beta Ori Egress
Egress was at +80 latitude
Only one of the possible ring locations was crossed by the star

13. Rhea Kappa Ori Occultation Egress
HSP data, not equatorial
FUV data not analyzed yet
Bonnie Meinke ran her software to check for statistically significant dips that could be attributed to rings - none found in this observation -> no confirmation of rings

14. Rhea Oxygen Emission Results
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
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
The component broadening Rhea’s wake is not a sputtered O or O2 atmosphere

15.  I0() d Summary  I0 () exp {-n()} d I/I0 =
Since no volatiles have been detected in occultations we can calculate upper limits
Calculation of column density upper limits:
 I0 () exp {-n()} d
I/I0 =
 I0() d
Use I/I0 = 0.95 (reasonable for broad absorptions), plug in absorption crossections as a function of wavelength, solve for n (done numerically, by testing various values of n)

16. Column Density Upper Limits
All stars we use have a signal high enough to detect a 5% attenuation. This is not adequate for a single absorption feature, however for gases with broad absorptions in the FUV wavelength range it is a realistic threshold.
Gases with absorption features in the FUV: H2O, O2, CO, CO2, NH3, C2H4, methanol
Upper limits
H2O: 2 x 1015 cm-2
O2: 1.3 x 1015 cm-2
CO: 3.6 x 1014 cm-2
CO2: 1.3 x 1017 cm-2
NH3: 9.2 x 1015 cm-2
C2H4: 2.8 x 1015 cm-2
methanol: 4 x 1015 cm-2

17. Enceladus

18. Rev 120
Enceladus’ Plume against Saturn

19. Enceladus Rev 120
Enceladus flyby on Rev 120 used Saturn as source, looked for Saturn-light absorbed by Enceladus’ plume
Advantage is that plume could potentially be observed more often and over longer time scales than single stellar occultations
“Parked” the slit on Saturn (to minimize changes due to Saturn itself) x 3
The plume should be resolved horizontally
Reflected uv is not a very strong source however, so it was necessary to sum the data to increase the signal
Focus on the data over wavelength range 1646 to 1750 ang, where we have adequate signal from Saturn and a broad absorption feature

20. Geometer Image: ICYMAP Part 1
Calibrated data, plot shows 1646 to 1750 ang range
Expect to see plume as darker than Saturn because it is absorbing some of the reflected light

21. UVIS image of Enceladus’ Plume
Range:
At start = 18,810 km
At end = 32,478 km
Resolution:
At start = 19 km
At end = 32 km
Same as previous plot but with different color stretch and without ra/dec grid
ICYMAP Part 1

22. ICYMAP Part 2 Range: At start = 32,478 km At end = 46,227 km
Resolution:
At start = 32 km
At end = 46 km

23. ICYMAP Part 3 Range: At start = 46,227 km At end = 57,937 km
Resolution:
At start = 46 km
At end = 57 km

24. Raw data summed over 1646 to 1750 Range
Time
Pixels
Map shows pixel value divided by column average
Orange => pixel % lower than average
Blue => pixel 20-30% lower than average
Lilac => lowest pixel in column
Stipples show where plume is
Would like to see correlation of low counts with plume location

25. Conclusion This was an experiment worth attempting
But, not clear we can pull out any quantitative results
Standard deviation is ~ equal to the signal we are looking for
Sum more?