1. Enceladus Report
C. J. Hansen
January 2013

2. Deriving the Structure and Composition of Enceladus’ Plume from Cassini Occultation Observations
C. J. Hansen, L. W. Esposito, J. Colwell, A. Hendrix, D. Shemansky, I. A. F. Stewart, R. A. West
AGU
December 2012
Gave talk at AGU 2

3. 2011 Dual Occultation Eps Orionis (Alnilam, B star)
16.5 km at closest point
HSP centered on eps Ori
Dimmer star in uv by ~2x
Zeta Orionis (Alnitak, O star)
37.9 km at closest point

4. Updated Water Vapor Column Density - Dual Occ
Eps Orionis 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
Best fit based on minimizing the sum of the squares of the differences between the actual occ and water for a given column density, per discussion at our last team meeting
Inot_cal_vs_water_2.ps
Zeta Orionis I/I0
Best fit is 1.2 x 1016 cm-2 4

5. 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
(note that escape velocity = 24,000 cm/sec)
n = column density measured by UVIS
2011:
vlos = 7.48 km/sec y
The source rate has not changed much in >6 years
(deviation is <15%, not factors of 2) 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
1.2 x 1016
0.2 x 1016
135
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 5 5

6. However
There are lots of high phase angle observations designed for ISS dust jet observations
VIMS has been riding along – get integrated brightness of the plume particles
Matt Hedman has been analyzing this VIMS data
First, derived a phase function to normalize lots observations at different phase angles
Then plotted brightness as a function of orbital phase
There is an orbital dependence in the VIMS results
Brightness peaks at ~180 true anomaly (a rather narrow peak, not a sine wave)

7. Comparison to tidal energy model
VIMS now reporting that they see substantial brightening at high phase when Enceladus is at a true anomaly of 180 deg
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
Maybe we haven’t observed an occultation at the right place???
Position of Enceladus in its orbit at times of stellar occultations, and solar occultation
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
116
0.5
Apoapsis
-0.4
220
254.13
0.7
2007 and 2011
0.4
220, 220
270
0.75
Three quarter
0.6

8. All Horizontal Cuts
Basemap from Spitale & Porco, 2007
Zeta Ori
2011
Solar occ
In all occultations we look through the plume and jets
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

9. Eps – Zeta Comparison Two second integration time
Summed FUV over wavelength
Difference in altitude ~20.5 km B a c d
Zoom in on occultation time
Zeta Orionis shifted 4 sec
Should only be shifted 3.8 sec

10. Eps – Zeta Comparison to Dust JetsB a c d
Eps – Zeta Comparison to Dust Jets
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”
DIII differentiable from BI jet at 18 km, not at 40 km
Weak feature at “a” is not located at a published dust jet, but ISS and CIRS have reported enhanced activity here?

11. h1 h2 B h4 h5 h6 h7
No ramp
HSP - FUV
Geometry of groundtrack so closely paralleling the Baghdad tiger stripe means that jets are not well separated from gas release all along the fissure
Why is there more absorption in the HSP?
Level background for HSP replaced by a ramp, but still see more absorption in the HSP than in the FUV
Puzzled by depth of HSP absorption…
Keep using this - ramp calculation made no difference 11

12. HSP Did not make much difference
Computed ramp to use for background
Did not make much difference
Still see much more absorption in HSP than in FUV

13. HSP with Ramp
Ramp background
Straight-line background
Computed background for HSP with ramp instead of straight line
Somewhat better match (blue compared to red) but didn’t make much difference
Why is HSP absorption more than FUV?

14. Future Tasks
Need better water cross-sections to make progress on composition
Need ISS jet locations to make progress on jets
Need occultation at true anomaly = 1800 to look for orbit-related changes in source rate
Write CDAP for jet structure model
Write Icarus paper summarizing results from all occs

15. Comparison to INMS results from E7
Highly collimated jets are consistent with INMS detection of enhanced gas streams at higher altitude
E7 INMS groundtrack at altitude of ~91 km (c/a) compared to UVIS solar occultation profile at altitude ~20 km (c/a)
INMS and UVIS both detect Alexandria and Baghdad gas jets
Thanks to Ben Teolis, Brian Magee, and Hunter Waite for providing these plots, in our GRL 2011 paper

16. Supersonic Gas Jets and the E Ring
The UVIS detection of supersonic jets compares very favorably with CDA results
The full width half max (FWHM) of jet c (Baghdad I) is ~10 km at a jet intercept altitude of 29 km (z0)
Estimating the mach number as ~2 z0/FWHM the gas in jet c is moving at a Mach number of 6; estimates for the other jets range from 5 to 8
Supersonic gas jets are consistent with Schmidt et al. (2008)model of nozzle-accelerated gas coming from liquid water reservoir
Condensation of water molecules in the high velocity gas jets could produce small salt-poor grains (Postberg et al., 2011) detected by CDA
Particle size 0.2 to 0.6 microns
Compositional partitioning
Jet intercept altitude is where the ray to the sun intercepts the jet, accounting for its rotation in front of or behind the limb
High velocity gas streams propel smallest particles out to become Saturn’s E ring
Cassini ISS image PIA08321 16

17. What have we learned from occultations?
Composition
The plume is primarily composed of water vapor
Upper limits have been set for CO, N2, C2H4
Source rate
Flux of water is ~200 kg/sec
Range is from 170 kg/sec to 220 kg/sec
Suggests that Enceladus has been steadily erupting for past 7 years
Enough to explain all the water products observed in the Saturn system (H, O, OH)
Plume / supersonic jet structure
Collimated gas jets are detected with estimated mach number of > 5
Propel small ice particles out to become Saturn’s E ring 17

18. The Occultation Collection
gamma Orionis Occultation
The Occultation Collection
zeta Orionis Occultation
Solar Occultation 18

19. 2010 Solar Occultation
Still best data set for studying individual jets
High SNR
Geometry gave us well-separated jets
Clear separation allowed us to calculate spreading, and derive a mach number >5
Spacecraft viewed sun from this side
Ingress
Egress
Minimum Altitude 
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 19

20. Introduction Plume Jets
In 2005 we detected a plume of water vapor coming from Enceladus with a UVIS observation of a stellar occultation
UVIS has also detected collimated gas jets within the broad plume
Over the ensuing 6 years we’ve observed other occultations – but no more are planned
I’ll summarize the latest (last) data in the context of the entire occultation dataset
Plume
Jets
Shown in this Cassini ISS image are small particles – UVIS detects gas 20