1. Enceladus water jet models from UVIS star occultations
LW Esposito, AIF Stewart, CJ Hansen
19 June 2009

2. UVIS UVIS has 4 separate channels: Far UltraViolet (FUV)
110 to 190 nm
3 slit widths => 2.8, 4.8, 24.9 nm spectral resolution
2D detector: spectral x 64 one-mrad spatial pixels
Extreme UltraViolet (EUV)
55 to 110 nm
3 slit widths => 2.8, 4.8, 19.4 nm spectral resolution
Solar occultation port
High Speed Photometer (HSP)
2 - 8 msec time resolution
Hydrogen – Deuterium Absorption Cell (HDAC)
For the occultations we used the
HSP with 2 msec time resolution
FUV with 512 spectral channels (1.56 nm resolution), 5 sec integration time

3. Plume Composition is Water Vapor
I=I0 exp (-n*)
I0 computed from 25 unocculted samples
n = column density
 = absorption cross-section, function of wavelength
Water spectrum from Chan et al, Data taken at 298 K.
The absorption spectrum of water is shown compared to Enceladus’ plume spectrum (I/I0) for a water column density of n = 1.5 x 1016 cm-2

4. Estimation of Enceladus Water Flux
S = flux
= N * h2 * v
= n/h * h2 * v
= n * h * v
Where
N = number density / cm3
h2 = area
v = velocity
n = column density measured by UVIS
Estimate h from plume dimension, = 80 km
Estimate v from thermal velocity of water molecules in vapor pressure equilibrium with warm ice (600 m/sec for surface temperature ~ 180K – note that escape velocity = 230 m/sec) h v
S = 1.5 x 1016 * 80 x 105 * 60 x 103 = 0.7 x H2O molecules / sec
= 200 kg / sec

5. Can we detect Ethylene in Enceladus’ Plume?
INMS detects a species with atomic mass = 28
Previously thought that this must be CO or N2
New idea (consistent with other INMS data) is that it could be ethylene
C2H4 = 2 * = 28

6. Ethylene at 3% H2O Column Density is not detectable by UVIS
Rev 11 gamma Orionis occultation
Ethylene plus water
C2H4 column density = 4.8 x 1014 cm-2
H2O column density = 1.6 x 1016 cm-2

7. Plume Structure (2005)
Water vapor abundance calculated from each 5 sec spectrum.
The 2005 water profile is best fit by an exponential curve.
The best fit scale length is 80 km

8. Optical Depth vs. Rayheight (2007)
Minimum distance of rayheight above limb = 15.6 km
S/C velocity = km/sec
Best fit is tau = 64.4 x z
Density at jets is ~2x higher than “background” plume

9. 2007 High Speed Photometer (HSP) Data
HSP is sensitive to 1140 to 1900Å
Statistical analysis finds features that are statistically unlikely
Assumes signal is Poisson distribution
Compares to running mean
Six different bin sizes employed, absorptions compared, persistence of feature is part of test
m is the number of such events one would expect to occur by chance in the data set
m<<1 are likely to be real events
Possible real features:
1 (a) m = 0.032
2 (b) m =
3 (c) m =
6 (d) m = 0.026
Statistical analysis is applied to determine which features are likely to be real. The analysis assumes the signal is a Poisson distribution. A running mean is calculated over 1001 integration periods or binned equivalent time [6]. The mean is the baseline for point i. Six different bin sizes were employed. C is the binned stellar signal at a particular bin. The probability that the signal would be ≤C at that bin is given by the sum of the distribution over values ≤C. This step is performed for each bin, i, in the data set to find Pi = P(mi, ≤Ci). Pi is multiplied by the number of bins in the data set, N. This product gives us a value m, where m=NPi [7], or the number of such events one would expect to occur by chance in the data set.

10. Groundtrack of Ray
2005
2007

11. Enhanced HSP absorption features a, b, c, and d can be mapped to dust jets located by Spitale and Porco (2007) along the tiger stripes
Take time to explain this!

12. Absorption Features, Compared to Dust Jet Locations
Plotted here are:
Altitude above Enceladus' limb of the line-of-sight from Cassini to the star
Attenuation of the HSP signal, scaled by a factor of 300
Projections of the 8 jets seen by the ISS into the plane of the figure
Jets assigned a length of 50 km (for purposes of illustration)
C/A marks the closest approach of the line-of-sight to Enceladus.
The times and positions at which the line-of-sight intersected the centerlines of the jets are marked by squares.
The slant of the jets at Baghdad (VII) and Damascus (III) contribute to the overall width of the plume

13. Plume or jets?
The plume of gas and dust from Enceladus includes a number of individual jets seen by Cassini camera and by UVIS

14. Gas Jet Model
Gas Jets are idealized as sources along the line of sight with thermal and vertical velocity components
Source strength is varied to match the absorption profile.
The ratio of thermal velocity (vt) to vertical velocity (vb) is optimal at vt / vb = 0.65.
Higher thermal velocities would cause the streams to smear together and the HSP would not distinguish the two deepest absorptions as separate events.
At least 8 evenly-spaced gas streams are required to reproduce the overall width of the absorption feature (there may be more).
In the plot the gas streams are shown as dashed vertical lines.
Key Result:
Vthermal / Vbulk = 0.65
Flow is supersonic

15. Best fit of 8 sources from Spitale & Porco to match UVIS occultation profile

16. Brightness of water vapor over Enceladus South pole from UVIS 8-jet model

17. Summary of Results: PLUME
Attenuation in HSP data ~10% in 2007, ~6% in 2005
Opposite of Hurford et al model of fissures opening and closing
Plume column density goes as ~ z-2 or as exp(-z/H) (z is minimum rayheight)
Water vapor flux ~200 kg/sec
No detection of CO, upper limit 3% (3 sigma)

18. Summary of Results: JETS
2007 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 thermal velocity to vertical velocity in jet = 0.65
Gas is supersonic: Mach 1.5
Eight or more jets required to reproduce width and shape of absorption, some evidence for diffuse sources
Jet source area is smaller than 300 m x 300 m

19. Backup Slides

20. Plume Model The results shown here have T surface = 140 K
Monte Carlo simulation of test particles given vertical + thermal velocity, particle trajectories tracked under influence of gravity and collisions (Tian et al, 2006)
Original model had arbitrary source spacing along the tiger stripes
Model now adapted for specific locations of the 8 dust jets identified by Spitale and Porco, actual viewing geometry of these sources as seen from the spacecraft
The results shown here have
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
Data shown as optical depth (inverted)
Point out non-alignment of b

21. Solar Occultation Rev 131
We have the opportunity to observe an occultation of the sun on Rev 131
New results are in the EUV, which gives us access to a different wavelength range than the FUV
The big scientific payoff is the chance to definitively detect / measure nitrogen in the plume - important for models of chemistry-driven dynamics in the interior

22. Solar Occultation
How well can UVIS measure N2 with a solar occultation?
Abundance of H2O measured by UVIS = 1.5 x 1016 cm-2
Mixing ratio of mass 28 in the INMS experiment at Enceladus was [M28]/[H2O]=0.036
A solar occultation has been simulated for our H2O optical depth assuming a commingled mixture of H2O and N2 in the spectral region of the H Ly line
The ability to measure N2 in a mixing ratio of [N2]/[H2O] = is indicated, for an abundance of N2 = 1 x 1014 cm-2
How do we know?
N2 was measured above the exobase in the UVIS T10 solar occultation observation using the measured extinction of the sol H Ly line by the N2 b(3,0) band.

23. Solar Occultation
This is a simulation of the results we could get from a solar occultation by Enceladus’ plume
UVIS can detect N2 absorption near 972 Ang
Mixing ratio for blue curve, showing clear absorption, is 0.05, close to the INMS derived value of [M28/H2O] = 0.036
Green curve shows likely detection limit with an order of magnitude less nitrogen, or [M28/H2O] = 0.005

26. Enceladus Plume Occultation of zeta Orionis October 2007
In October 2007 zeta Orionis was occulted by Enceladus’ plume
Perfect geometry to get a horizontal cut through the plume and detect density variations indicative of gas jets
Objective was to see if there are gas jets corresponding to dust jets detected in images

27. Changes in System Oxygen Content
Units are Rayleighs
Point out Enceladus orbit, E ring

28. Plume or jets? Attempt to pick comparable “box” between 2005 and 2007
But different jets visible means comparing apples and oranges

29. Gas Jets Density in jets is twice the background plume
Gas jet typical width = 10 km at 15 km altitude
Egress
Ingress
Because the dimension of the two prominents jets is about 1/8 the width of the largest plume, their gas density needs to be twice that of the surrounding plume at 15 km to yield the 12% opacity increase seen in the jets.
a. Cairo (V)
d. Damascus (II)
c. Baghdad (VI)
b. Alexandria (IV)
Closest point

30. Expect fissures to open and close
Comparison to tidal energy model
Position of Enceladus in its orbit at times of stellar occultations
Hurford et al 2007 model predicts tidally-controlled differences in eruption activity as a function of where Enceladus is in its eccentric orbit
Substantial changes are not seen in the occultation data, although they would be predicted, based on this model
Expect fissures to open and close
Taken from Hurford et al,
Nature 447:292 (2007)
True Anomaly
(deg)
Fraction of orbit from Periapsis
Position in Orbit
Stress
105 Pa
0.0
Periapsis
0.3 90
0.25
One quarter
-0.8
97.76
0.27
July 14, 2005
-0.77
180
0.5
Apoapsis
-0.4
254.13
0.7
October 24, 2007
0.4
270
0.75
Three quarter
0.6

31. New Plume Model
A new model has been developed for Enceladus’ plumes by Tian, Toon, Larsen, Stewart and Esposito, paper published in Icarus (2006)
Monte Carlo simulation of test particles given vertical + thermal velocity, particle trajectories tracked under influence of gravity and collisions
Assumes source of one plume is 2 x 2 km2, then multiple plumes added together along a tiger stripe, w/ 20 km separation
UVIS ray path across tiger stripes

32. Localization of Enceladus’ Plume (Not a global atmosphere)
Ray intercepts were at latitude / west longitude:
15 / 300 Lambda Sco ingress (non-detection)
-31 / 141 Lambda Sco egress (non-detection)
-76 / 86 Gamma Ori ingress
-0.2 / 28 Gamma Ori egress (non-detection)
Consistent with localized plume or jet:
Enceladus’ gravity insufficient to hold gravitationally bound sputtered atmosphere
Also, the combination of other Cassini data sets are consistent with a plume of water vapor coming from Enceladus’ “Tiger Stripes” driven by the hot spot at the south pole detected by CIRS
Gamma Orionis is a B2III star with a visual magnitude of 1.63
FUV summed over wavelength