1. Enceladus Dual Star Occultation Update
C. J. Hansen
19 June 2012

2. Plume
jets

3. 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

4. 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

5. 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

6. Eps Ori I/I0 Best fit is 1.35 x 1016 cm-2
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
Inot_cal_vs_water_2.ps
Calibrated data
Summed spatial rows 2-4
I0 calculated from 11 samples before and after the occultation
I calculated from records (records with absorption > FWHM)
Best fit for wavelength channels (broad absorptions) was 1.5 x 1016, but that resulted in too much absorption at deepest feature

7. Zeta Ori I/I0 Best fit is 1.25 x 1016 cm-2 Calibrated data
Zeta/cal spectra/Inot_cal_vs_water.ps
Calibrated data
Summed spatial rows 2-4
I0 calculated from 11 samples before and after the occultation
I calculated from records (absorption > FWHM)
Best fit for wavelength channels (broad absorptions) was 1.3 x 1016, but that resulted in too much absorption at deepest feature

8. Estimate of Water Source Rate from Enceladus = 200 kg/sec
S = flux (source rate)
= 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 e
1.35 x 1016
120
7.3 x 1027 z
1.25 x 1016
133
7.5 x 1027
224
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 8

9. All Horizontal Cuts 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. 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

11. 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
Are these features real?
This time the HSP data was lower snr
Eps Ori instead of zeta Ori
no features passed the rigorous statistical tests applied
Must rely on FUV data, cross-correlation of absorptions in same place / shifted in time

12. No Statistically Significant Features in HSP
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 zeta Ori occultation
Also, geometry made jets harder to distinguish from plume
Bonnie’s conclusions were verified by Bob West with a different technique

13. 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

14. FUV Data vs. Time Two sec integrations
Data is summed over all wavelengths, all spatial pixels
Zeta Ori trails eps Ori by ~4 sec
Enhanced absorption times input to groundtrack plot

15. Eps – Zeta Direct Comparison
Zeta Ori is green; eps Ori is blue
Enhanced absorption shown as dots
Clear correlation at Baghdad fissure-crossing
Damascus II and III (“c”), Baghdad I detected (“d”)
Slow return to unocculted signal may be activity between BI and BVII
S/C
c/a

16. Eps and Zeta Orionis ComparisonB a c d
c/a
Calibrated data summed over rows 2-4 and over wavelength
Average computed for each star, then ratios computed for each
Time shifted to align enhanced absorption feature at B because geometry clearly correlated with fissure-crossing (4 sec)
Also aligns egress

17. Eps – Zeta Direct ComparisonB a c d
Eps – Zeta Direct Comparison
S/C
Clear signal of gas from Baghdad fissure (B), though no dust jet nearby
New Gas Jet
Damascus jets (DII and DIII): “c”, and BI detected: “d”
Weak feature at “a” is not located at a published dust jet, but ISS and CIRS have reported enhanced activity here

18. Altitude of ray for eps and zeta Orionis
Ingress
Egress
Not shifted by 4 sec (yet)
Need to adjust timing for fact that zeta trailed eps Ori

19. Altitude of ray for eps and zeta Orionis
Not shifted by 4 sec (yet)
Altitude of zeta Ori shifted 4 sec
Offset of the two stars is ~ constant over the same territory
Delta ~ 20.5 km

20. HSP Comparison of HSP (targeted to eps Ori) to FUV for eps Ori
0.008 sec integration summed to 2 sec to match FUV
Although features did not pass our statistical tests we can compare to the FUV data set
Then, bootstrap back to less-summed HSP data to get better time resolution
Time correct on HSP

21. Now consider HSP Reconstructed trajectory FUV eps Ori is blue
HSP eps Ori is red
Zeta Ori is green
Dots show enhanced absorption in HSP data summed to 1 sec
HSP groundtrack = eps Ori
All channels show enhanced absorption at Baghdad fissure; Align data from each channel there
What do I have to do in geometer save file to get HSP instead of FUV?

22. HSP and FUV Why is the HSP signal more attenuated than the FUV?
(Probably due to I0 calculation – it really needs to be a ramp?) h8 h1 h2 B h7 h6
Puzzled by depth of HSP absorption… h4 h5
Eps Ori FUV and HSP collected at the same time
HSP summed to 1 sec could shift +/- 1 sec relative to FUV
Aligned at new Baghdad jet B

23. h1 h2 B h4 h5 h6 h7
HSP - FUV
HSP enhanced absorption features do not pass statistical tests, however some appear to be correlated to jets:
h2: feature “a” in FUV
h4: Damascus III
h5: Damascus II
h6-h7: Baghdad I
h8: Baghdad VII
Puzzled by depth of HSP absorption…

24. FUV Plume: Water Vapor Column Density eps OrionisB
Baghdad I
From Don, column density as a function of time:
New Baghdad jet (“B”) emitting gas with column density = 1.6 x cm-2
Deepest absorption (2.4 x 1016 vs 1.35 x 1016), due to Baghdad I
Don did f(time)

25. FUV Plume: Water Vapor Column Density zeta OrionisB
Baghdad I
Don did f(time)
Don’s plot of column depth as a function of time
Zeta Orionis signal attenuation ~ 20 km higher than eps Ori

26. Gas Dissipation Column Density: Be = 1.6 x 1016 Bz = 1.0 x 1016
eps BI B
zeta BI
Don did f(time)
Column Density:
Be = 1.6 x 1016
Bz = 1.0 x 1016
Bz/Be = 0.6
Range delta =
= 20.7 km
BIe = 2.4 x 1016
BIz = 1.65 x 1016
Biz/BIe = 0.7
Range delta = 37.8 – 18.9 = 18.9 km

27. 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
But overall width of plume does not expand much

28. Summary
What can we learn from two cuts through the plume at different altitudes?
Overall width of plume is not very different (120 vs. 133 km)
Consistent with gas leaving at escape velocity on ~linear trajectory
Close to surface see less gas between jets/fissures than at higher altitude (gas is collisionless, diffusion is from slightly different trajectories leaving fissure)
DIII differentiable from BI jet at 18 km, not at 40 km
Can compare column density at jets at two altitudes -> dissipation -> spreading

29. Back-up

30. 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

31. 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…

32. 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

33. Solar Occultation Jets

34. Comparison to INMS results from E7

35. 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%

36. 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

37. 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

38. 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

39. Data
Bonnie’s Analysis

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

41. Data

42. Data
Only one time integration wide
Interesting hole

43. Binned Data N Y ? ? ?

44. Optical Depth

45. M Values

46. 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%.

47. 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)

48. 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