1. Enceladus Dual Star Occultation
C. J. Hansen
6 January 2012

2. The Structure of Enceladus’ Plume from Cassini Occultation Observations
Gave AGU Talk, Dec. 2011
C. J. Hansen, L. W. Esposito, B. Buffington, J. Colwell, A. Hendrix, B. Meinke, D. Shemansky, I. A. F. Stewart, R. A. West

3. UVIS Observations of Enceladus’ Plume
Cassini’s Ultraviolet Imaging Spectrograph (UVIS) observes occultations of stars and the sun to probe Enceladus’ plume
Composition, mass flux, and plume and jet structure
Four stellar and one solar occultation observed to-date
Feb lambda Sco
No detection (equatorial)
July gamma Orionis
Composition, mass flux
Oct zeta Orionis
Gas jets
May Sun
Composition, jets
Oct – epsilon and zeta Orionis dual occultation
What do we observe

4. The Occultation Collection
gamma Orionis Occultation
The Occultation Collection
zeta Orionis Occultation
2011 occ was a horizontal cut through the plume also
Solar Occultation
Point is that we now have two horizontal cuts through the plume, as it turns out almost orthogonal to each other

5. Orion’s Belt Dual Occultation Geometry Rev 155
Dual stellar occultation by Enceladus’ plume, E15, 19 October 2011, of epsilon Orionis (blue) and zeta Orionis (white)
Horizontal cut through plume

6. Dual Occultation Eps Ori (Alnilam, B star) Zeta Ori (Alnitak, O star)
16.5 km at closest point
HSP centered on eps Ori
Dimmer star in uv by ~2x
Zeta Ori (Alnitak, O star)
37.9 km at closest point

7. The Plume: Water Vapor Column Density zeta Orionis
Ratio of occulted signal to unocculted signal: I/I0
From average of data records above FWHM
Compare to water vapor
Cross-sections from Mota, 2005
Same as we used for 2007 zeta Orionis occ
Don did f(time)
Best fit at H2O column density of 1.35 x 1016 cm-2
Larry got 1.3 x 1016 cm-2
Don’s plot of column depth as a function of time: Deepest absorption just after closest approach Similar situation in 2007, 2x difference

8. The Plume: Water Vapor Column Density eps Orionis
Same technique by CJH gave column density of 1.6 x 1016 cm-2
But Larry calculated column density of 1.4 x 1016 cm-2
I think my value is too high, re-doing this is on my to-do list…
Don did f(time)
As a function of time:
Deepest absorption just after closest approach

9. All Groundtracks In all occultations we look through the plume
Basemap from Spitale & Porco, 2007
Zeta Ori
2011
Solar occ
In all occultations we look through the plume
The groundtrack is the perpendicular dropped to the surface from the ray to the star
Zeta Ori
2007
Blue => zeta Orionis 2007
Red => Solar occ 2010
Green => zeta Orionis 2011

10. Estimate of Water Source Rate from Enceladus = 200 kg/sec
S = flux
= N * x * y * vth
= (n/x) * x * y * vth
= n * y * vth
Where
N = number density / cm3
x * y = area
y = vlos * t at FWHM
vth = thermal velocity = 45,000 cm/sec
for T = 170K
n = column density measured by UVIS
2011:
vlos = 7.48 km/sec y x v
Year n
(cm-2)
Uncert-ainty
+/- y
(x 105 cm)
vth
(cm / sec)
Flux: Molecules / sec
Flux:
Kg/sec
Fraction of orbit from periapsis
2005
1.6 x 1016
0.15 x 1016
80 (est.)
45000
5.8 x 1027
170
0.27
2007
1.5 x 1016
0.14 x 1016
110
7.4 x 1027
220
0.70
2010
0.9 x 1016
0.23 x 1016
150
6 x 1027
180
0.19
2011
1.35 x 1016
(prelim)
135
8.2 x 1027
240
Important message – flux has not changed much in 5 years. (Deviation is only 15%, not factors of 2)
Width (y) is at FWHM
Vlos is the line-of-sight velocity of the star across the plane of the sky
Given uncertainties our best estimate of water flux from Enceladus is 200 kg/sec 10

11. One more comparison to tidal energy model
Position of Enceladus in its orbit at times of stellar occultations, and solar occultation
Hurford et al 2007 model predicts tidally-controlled differences in eruption activity as a function of where Enceladus is in its eccentric orbit
Expect fissures to open and close
Substantial changes are not seen in the occultation data, although they would be predicted, based on this model
Taken from Hurford et al,
Nature 447:292 (2007)
True Anomaly
(deg)
Fraction of orbit from Periapsis
Position in Orbit
Stress
105 Pa
Source rate
Kg/sec
0.0
Periapsis
0.3
0.186
May 18, 2010
180 90
0.25
One quarter
-0.8
97.76
0.27
July 14, 2005
-0.77
170
0.5
Apoapsis
-0.4
254.13
0.7
2007 and 2011
0.4
220, 240
270
0.75
Three quarter
0.6

12. Caveats How constant is the source rate?
Is the source rate modulated by the position of Enceladus in its orbit?
From Larry:
The previous derivations use slightly different approaches to find the column density N: gOri: Last FUV spectrum measured, closest to surface. Best fit for N. Use scale height H to estimate L zOri: Two 5-second FUV spectra that span the entire occultation. Best fit for N Solar: 42 seconds of summed EUV spectra, covering the FWHM of the occultation. Best fit for N eOri, zOri: Mean of photometric analysis (match total attenuation with H2O alone) of spectra within FWHM. This resembles DES approach for Solar occ. To get the mass flux, multiply by the mass of H2O molecule and FWHM. This gives zOri: 1.3 E16 * 134km (9 spectra) = 236kg/sec eOri: 1.4 E16 * 120km (8 spectra) = 227kg/sec
Given the variety of approaches, the different channels, IP and occultation tracks, it may be fortuitous that all 5 UVIS results give 208 ± 28 kg/sec. That is, a constant flux with standard deviation of 15%.

13. Nomenclature Plume Jets
“Plume” refers to the broad cloud of dust and gas emanating from the south pole of Enceladus
“Jets” are the highly collimated streams of ice particles (detected by ISS) and gas
UVIS observes the gas component

14. The Jets – Past Occs
2007
2010 a b c d e f
In the past we have identified collimated jets of gas from enhanced absorption features in the HSP (2007 zeta Ori occ) and the EUV (2010 solar occ)
Features in the 2007 HSP data were validated by Bonnie Meinke using her F ring statistical test techniques
Features in the 2010 solar occ were identified by looking for matching absorptions in the two windows, and making the argument that it was unlikely to be shot noise if they matched
Assume Poisson distribution
Calculate number of events one would expect to occur by chance in entire data set, for several bins

15. Solar Occ Geometry gave us well-separated jets
Spacecraft viewed sun from this side
Ingress
Egress
Minimum Altitude 
Geometry gave us well-separated jets
Don’t dwell too much on this – just point out ground track and location of enhanced absorptions (big blue dots)
Basemap from Spitale & Porco, 2007

16. The Jets – 2011 HSP Data This time the HSP data was lower snr
Assume Poisson distribution
Calculate number of events one would expect to occur by chance in entire data set, for several bins
This time the HSP data was lower snr
Eps Ori instead of zeta Ori
no features passed the rigorous statistical tests applied
Rely on FUV data, cross-correlation of absorptions in same place / shifted in time
Plot note: elapsed times are incorrect

17. Data
Bonnie’s Analysis

18. Visual inspection
Human eye is good at picking out features

19. Data

20. Data
Only one time integration wide
Interesting hole

21. Binned Data N Y ? ? ?

22. Optical Depth

23. M Values

24. No Significant Features
From Bonnie:
Smallest m values are on the order of a few
Optical depths are below 0.4
Nothing passes more stringent tests of repeated significance
NOTE:
Attenuation in plume ~5%, was ~10% last time
Perhaps this is more diffuse overall compared to 2008 zet Ori occ’n
Bonnie’s conclusions were verified by Bob West with a different technique

25. FUV Data Two sec integrations Zeta Ori trails eps Ori by ~4 sec
Data is summed over all wavelengths, all spatial pixels
Zeta Ori trails eps Ori by ~4 sec

26. Groundtracks 2011 Eps Ori is blue, zeta Ori is green
Three matching features:
“Split end” on Baghdad fissure
Crossing Baghdad fissure
Damascus jets
In addition, eps Ori likely distinguishes Baghdad I
S/C

27. Eps and Zeta Orionis Comparison
Signal of gas from Baghdad fissure (B-f), though no dust jet nearby
Damascus jets (DII and DIII) and BI identified
Feature at BVII?
Weak feature at “?” is not located at a published dust jet, but ISS and CIRS have reported enhanced activity here
DII&III BI
B-f
BVII? ?
Average computed for each star
Then ratios computed for each
Time shifted to align enhanced absorption feature at B-f because geometry clearly correlated with fissure-crossing (4 sec)

28. Direct Comparison Clear correlation at fissure-crossing
DII&III BI
B-f ?
Direct Comparison
S/C
Clear correlation at fissure-crossing
Slow return to unocculted signal may be activity between BI and BVII

29. HSP Now compare HSP (targeted to eps Ori) to FUV for eps Ori
0.008 sec integration summed to 2 sec to match FUV (plot time ok now)
Although features did not pass our statistical tests we can compare to the FUV data set
Pretty good agreement with eps Ori – more work to do on this to resolve timing issues
Then, bootstrap back to less-summed HSP data to get better time resolution
Time correct on HSP

30. Summary Mass flux determined, comparable to other occs
Work to do to better quantify uncertainties
Jets tougher to identify because of low snr
Features in HSP data did not pass statistical tests
Geometry of occ along rather than across fissures may also have an effect?
Determination of spreading at the two altitudes also limited by temporal resolution of the FUV (2 sec integration time)
2 sec x 7.48 km/sec line-of-sight velocity = 15 km
That is the approx. width of the jets derived in earlier occultations
Work in progress to better constrain jet structure
HSP bootstrap from FUV

31. Back-up

32. Jets vs. Tiger Stripes
Spacecraft viewed sun from this side
Ingress
Egress
Minimum Altitude 
As before, gas jets appear to correlate to dust jets
Feature
Altitude* (km)
Dust Jet a 20
Alexandria IV
Closest approach
19.7 b 21
Cairo V and/or VIII c 27
Baghdad I d 30
Baghdad VII e 38
Damascus III f 46
Damascus II
Don’t dwell too much on this – just point out ground track and location of enhanced absorptions (big blue dots)
* Altitude of ray to sun from limb
Basemap from Spitale & Porco, 2007

33. 2007 - Plume Structure and Jets
Summary of 2007 results
Significant events are likely gas jets
UVIS-observed gas jets correlate with dust jets in images
Characterize jet widths, opacity, density
Density in jets ~2x density in background plume
Ratio of vertical velocity to bulk velocity = 1.5, supersonic
Supersonic gas jets are consistent with Schmidt et al. model of nozzle-accelerated gas coming from liquid water reservoir
New solar occ data is better resolution, gives us better numbers for all of these results…

34. Jet Structure Optical Depth
Higher SNR enables better measurements of jets’ dimensions – more clearly distinguished from background plume
Density of gas in jets is twice the density of the background plume
The jets contribute 3.4% of the molecules escaping from Enceladus, based on comparison of the equivalent width of the broad plume compared to the jets’ total equivalent width
The 3.4% is for all 6 of the jets

35. Solar Occultation Jets

36. Comparison to INMS results from E7

37. Jet Properties
Feature
Altitude of ray relative to limb
Z0: Altitude of ray relative to jet source
FWHM: full width half max(km)
Mach number ~ 2 * Z0 / FWHM
Associated Dust Jet
Excess attenu-ation at the jet (%) – for density calc* a
21.3
21.6 7 6
Alexandria IV 27
Closest approach
20.7 b 22 24 9 5
Cairo V and/or VIII 17 c
28.4 29 10
Baghdad I 19 d
31.2 36
Baghdad VII 12 e 39 40 8
Damascus III 13 f
47.5
49.7 14
Damascus II
*Average attenuation =17%

38. CJH To Do List Re-do calculation of eps Ori column density
Really rigorous determination of error bars for every occultation we have observed
Re-do summed FUV vs. time for just spatial pixels 2-4, calibrated properly
Plot HSP with eps Ori – now that timing issues are resolved!
Then go back to higher time resolution HSP data