1. The Composition and Structure of Enceladus’ Plume from the Solar Occultation
C. J. Hansen, L. Esposito, D. Shemansky, B. Lewis, A. I. F. Stewart, J. Colwell, A. Hendrix, R. West
January 2011

2. Outline
Using UVIS to observe occultations gives us data on the composition and structure of the gas flowing from Enceladus’ tiger stripe fissures
Plume Results
Composition
Mass flux
Temporal variability
Gas Jets
Structure
Mach number
Plume
Jets
Emphasize gas – UVIS doesn’t see the particles (although this of course is an image of the particles)
Say that “plume” refers to broad gas structure, “jets” are the collimated streams of gas coming from small source regions (<1 km^2)

3. UVIS Observations of Enceladus’ Plume
UVIS observes occultations of stars and the sun to probe Enceladus’ plume
Three stellar and one solar occultation observed to-date
Zeta Orionis
Feb lambda Sco
No detection (equatorial)
July gamma Orionis
Composition, mass flux
Oct zeta Orionis
Gas jets
May Sun
Composition, jets
The graphic is for the zeta Orionis occ, but it’s a nice illustration anyway
Bullet under date gives primary result from that occultation

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

5. 18 May 2010 - Solar Occultation
Two science objectives enabled by solar (rather than stellar) occultation:
1. Composition of the plume
New wavelength range: EUV
H2O and N2 have diagnostic absorption features at EUV wavelengths
The primary goal was to look for N2, on basis of INMS detecting a species with amu=28
2. Structure of the jets and plume
Higher time resolution, better snr
Note: projected size of sun = 1.2 km 5

6. Solar Occ results – Composition
H20 fit to absorption spectrum
Column density of H2O = 0.9 x 1016 cm-2
No N2 absorption feature -> N2 upper limit of 5 x 1013 cm-2
The (3 sigma) error bar is at the wavelength where we would expect to see the N2 absorption.
“IP” is the first Ionization Potential

7. Nitrogen feature at 97.2 nm not detected
Actual
No dip is seen at all at 97.2 nm
Upper limit < 0.5%
Consequences of no N2 for models of the interior
High temperature liquid not required for dissociation of NH3 (if there is NH3 in the plume)
Percolation of H2O and NH3 through hot rock is not required
Clathrate decomposition is not substantiated for N2 as the plume propellant
Importance of no N2
Could still have clathrate decomposition with CH4, but then lots of CH4 is needed
Predict
N2 feature at 97.2 nm fortuitously coincides with strong lyman gamma emission so lots of signal available
Very sensitive test!

8. Water Vapor Abundance
To calculate water vapor abundance in the plume the spectra are summed during the center 60 sec of the occultation, then divided by a 650 sec average unocculted sum to compute I / I0
I0 computed at two different times, results were the same
The extinction spectrum is well-matched by a water vapor spectrum with column density = 0.9 +/ x 1016 cm-2
Overall amount of water vapor is comparable to previous two (stellar) occultations
2005: 1.6 x 1016 cm-2
2007: 1.5 x 1016 cm-2 (maximum value of 3.0 x 1016 cm-2 at center)
Lower value in 2010 is at least partially attributable to the viewing geometry – the flux is in family with the previous results
The calculation for the last point on this page is shown on the next chart

9. ~Orthogonal Ground Tracks
Blue ground track is from zeta Ori occ on Rev 51
Orange is solar occ track, ~orthogonal
Plume is elongated.
If total flux is same then column density will be less by ~ 2/3
Solar occ result:
0.9 x1016 cm-2, ~2/3 of value in 2005
 Ingress
The purple ellipse encloses the jets
 Egress
Basemap from Spitale & Porco, 2007

10. Estimate of Water Flux 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 => FWHM
vth = thermal velocity = 45,000 cm/sec
for T = 170K
n = column density measured by UVIS
note that escape velocity = 23,000 cm/sec x v
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
Year n
(cm-2) y
(x 105 cm)
vth
(cm / sec)
Flux: Molecules / sec
Flux:
Kg/sec
2005
1.6 x 1016
80 (est.)
45000
5.8 x 1027
170
2007
1.5 x 1016
110
7.4 x 1027
220
2010
0.9 x 1016
150
6 x 1027
180 10

11. Solar Occ Jet Identificationsb c d e f
Definition of window back on slide 5
Minimum altitude
Window 0 and 1 matching features => jets
Repetition of features in window 0 and window 1 shows they are not due to shot noise, therefore likely to be real

12. Jets vs. Tiger Stripes
As before, gas jets appear to correlate to dust jets
Spacecraft viewed sun from this side
Ingress
Egress
Minimum Altitude 
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

13. Jet Structure Optical Depth
Higher snr enables better measurements of jets’ dimensions – more clearly distinguished from background plume
Density of gas in jets > 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 to the jets’ total equivalent width
The 3.4% is for all 6 of the jets
Density of gas in jets: 10 km / 150 km = 1/15; 20% more absorption over 1/15 scale -> 1/5 / 1/15 = 15/5 = 3
At 10%, 1/10 / 1/15 = 15/10 = 1.5
At 17%, 1/6 / 1/15 = 15/6 = 2.5

14. Gas Velocity
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
Previously estimated mach number (from 2007 occultation) was 1.5
Jets more collimated than previously estimated
New estimate for vertical velocity: if vsound = 320 m/sec (for ~170 K) then vvert = 1920 m/sec
This is an upper limit because the gas will be cooled in a nozzle
Water vapor flux in the jets = kg/sec
Jet intercept altitude is where the ray to the sun intercepts the jet, accounting for its rotation in front of or behind the limb

15. 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%

16. Summary Composition Plume / jet structure
Upper limit on N2 of 5 x 1013 cm-2
H20 column density = 0.9 x 1016 cm-2
In family with previous occultations
Suggests that Enceladus has been steadily erupting for past 5 years
Plume / jet structure
Flux of water from 3 occultations is ~200 kg/sec
Jets are more collimated than estimated from 2007 occultation
Mach numbers of 5 to 8
Supersonic gas jets are consistent with Schmidt et al. model of nozzle-accelerated gas coming from liquid water reservoir

17. Backup Info

18. Occultation is clearly visible
Window 0 has higher counts, but overall shape is the same
Position of sun was slightly offset from center, but not an issue
Observation start time: T05:51:44.45
Observation end time: T06:10:36.45
Ingress: T06:00:40.45
Egress: T06:02:59.45
Velocity of sun across plane of sky ~ 2.75 km/sec
Data shown is summed over wavelength

19. Jet Identities Feature Altitude* (km) Dust Jet a 20 19.7 b 21 c 27 d
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
* Altitude is relative to limb of Enceladus

20. Groundtrack of Occultation
Enhanced HSP absorption features a, b, c, and d can be mapped to dust jets (roman numerals) located by Spitale and Porco (2007) along the tiger stripes
Take time to explain this!
Blue line is groundtrack

21. Summary of Results JETS:
HSP data shows 4 features with m < 0.1 (probability of chance occurrence). Typical half-width: 10 km at z = 15 km.
Gas jets can be correlated with dust jets mapped in images on Cairo, Alexandria, Damascus and Baghdad tiger stripes
Jet opacity corresponds to vapor density doubled within jets
Alternate explanation: no excess gas, with all increase due to dust. Then, dust opacity peaks at 0.05 in the jets. This would give 50x more mass in dust compared to vapor within the jet.
Ratio of vertical velocity to thermal velocity in jet = 1.5
Gas is supersonic
Eight or more jets required to reproduce width and shape of absorption
Jet source is approximately 300 m x 300 m
Example Calculations
T surface = 140 K
V thermal = 359 m/sec
V vertical = 552 m/sec
For Tsurface = 180 K (from CIRS)
V thermal = 406 m/sec
V vertical = 624 m/sec

22. Plume Composition and Column Density
UVIS Ultraviolet Spectra provide constraints on:
Composition, from absorption features
Column density
Mass Flux
Plume and jet structure
Terminology:
Plume - large body of gas and particles
Jets - individual collimated streams of gas and particles
Plume
Jets
What do we learn

23. 2007 - Zeta Orionis Occultation (Alnitak)
gamma Orionis Occultation
(Bellatrix)
Key results:
Dominant composition = water vapor
Plume column density = 1.6 x 1016 /cm2
Water vapor flux ~ 180 kg/sec
Results documented in Science, 2006
Vertical cut through plume
Zeta Orionis Occultation (Alnitak)
Key results:
Average column density = 1.5 x 1016 cm-2
Max column density = 3.0 x 1016 cm-2
Gas jets detected correspond to dust jets
Results documented in Nature, 2008
Horizontal density profile
18 May Solar Occultation
Two science objectives enabled by solar (rather than stellar) occultation:
1. Composition of the plume
New wavelength range: EUV
2. Structure of the jets and plume
Higher time resolution 23

24. Outline Plume Results Gas Jets Composition Mass flux
Temporal variability
Gas Jets
Structure
Mach number

25. New EUV Spectrum from Solar Occultation
The data
Navy is unocculted solar spectrum, with typical solar emissions
Red is solar spectrum attenuated by Enceladus’ plume

26. Outline UVIS Observations Plume Results Gas Jets Occultations
Instrument
Plume Results
Composition
Mass flux
Temporal variability
Gas Jets
Structure
Mach number

27. 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…

28. Solar Occultation Characteristics
FWHM
Total duration of Solar Occ:
2min 19sec
Duration for full-width half max:
56 sec
Line of sight velocity: km/sec
Width of plume at FWHM:
56 sec * 2.85 = 160 km
Compare to zeta Orionis Occ
Zeta Orionis occultation lasted just 10 sec
Line of sight velocity = 22.5 km/sec
Width of plume at FWHM = 110 km
HSP data summed to 200 msec so 50 samples
Zeta Orionis occultation

29. UVIS Characteristics UVIS has 4 separate channels
For stellar occultations we use:
Far UltraViolet (FUV)
1115 to 1915 Å
2D detector: spectral x 64 one-mrad spatial pixels
Binned to 512 spectral elements
5 sec integration time
High Speed Photometer (HSP)
2 or 8 msec time resolution
Sensitive to 1140 to 1915 Å
Hydrogen-Deuterium Absorption Cell (HDAC) not used
For the solar occultation we used:
Extreme UltraViolet (EUV) solar port
550 to 1100 Å
2D detector: spectral x 64 one-mrad spatial pixels
No spatial information because signal from sun is spread across the detector (deliberately)
Spatial rows binned to two windows of 27 rows each
1 sec integration