1. UVIS Observations of Enceladus’ Plume
C. J. Hansen, I. Stewart, L. Esposito, A. Hendrix
June 2009

2. Zeta Orionis Occultation 2007 (Alnitak)
FUV and HSP data collected
FUV: 5 sec integration
HSP: 2 msec sampling
Horizontal density profile
True anomaly = 254
Results documented in Nature, 2008
gamma Orionis Occultation
(Bellatrix)
Vertical cut through plume
True anomaly = 98
Key results:
Dominant composition = water vapor
Plume column density = 1.6 x 1016 /cm2
Water vapor flux ~ 180 kg/sec
Documented in Science, Hansen et al, 2006 2

3. Cassini UVIS Characteristics
UVIS has 4 separate channels;
For star occultations we use the FUV and HSP:
Far UltraViolet (FUV) Spectrograph
1115 to 1915 Å
2D detector: spectral x 64 one-mrad spatial pixels
For occultations we use 512 spectral channels
5 sec integration time
High Speed Photometer (HSP)
2 msec time resolution
For the solar occultation we will use the EUV spectrograph
Extreme UltraViolet (EUV)
550 to 1115 Å
1 sec integration
[Not used for stellar occs:
Hydrogen – Deuterium Absorption Cell (HDAC)]

4. 1. Plume Composition and Column Density
Terminology:
Plume - large body of gas and particles
Jets - individual collimated streams of gas and particles
Plume
Jets

5. Rev 11 Gamma Orionis Occultation, FUV data
UVIS spectra, occulted and unocculted
Plot I/I0 to see absorption features
Compare I/I0 to water absorption spectrum
Water vapor: uses Mota cross-sections
Best fit column density = 1.6 x 1016 cm-2

6. Rev 51 zeta Orionis Occultation: FUV
Absorption is best fit by water vapor
Best fit average column density = 1.5 x 1016 cm-2
Error bar: +/- 1.4 x 1015 cm-2
Comparison to 2005 at 15 km altitude
2007 peak column density
= 3.0 x 1016 cm-2
2005 = 1.6 x 1016 cm-2
No detection of CO
formal 2-σ upper limit is 3.6 x 1014 cm-2
corresponds to mixing ratio with H2O of 3.0
Our nondetection of CO excludes 3% CO in the plume at the 2 sigma level
Used Mota (2005) cross-sections
I/I0

7. What else can UVIS detect?
Valuable to help resolve ambiguities in INMS detections
INMS detects a species with atomic mass = 28
Previously thought that this must be CO or N2
UVIS non-detection rules out CO at the 3% level
New idea (consistent with other INMS data) is that it could be ethylene
C2H4 = 2 * = 28

8. Ethylene at 3% H2O Column Density compared to H2O only
Rev 11 gamma Orionis occultation
Ethylene plus water compared to water only
C2H4 column density = 4.8 x 1014 cm-2
H2O column density = 1.6 x 1016 cm-2
Water only is still best fit to occulted spectrum although there are some interesting matches to small dips with ethylene added in

9. Methanol
Not likely to be detectable, doesn’t look like a good fit anyway…

10. Looking for Nitrogen: 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 10

11. 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 11

12. Water Column Density: FUV comparison to HSP 2007
FUV integrations are 5 sec duration
FUV spectrum shows gas absorption in Rev 51 time records 89 and 90
Higher time resolution of HSP data shows that the peak column density is about 2x higher than the 5 sec average calculated from the FUV data
FUV time record 89
FUV time record 90

13. 2. Looking for jets: 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 13

14. 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 14

15. 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

16. Gas 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 16

17. Gas Jet Model Key Result: Vvert / Vthermal = 1.5 Flow is supersonic
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.
The two deepest absorptions indicate jets are collimated, 10 km wide at 15 km altitude
In the plot the gas streams are shown as dashed vertical lines.
Key Result:
Vvert / Vthermal = 1.5
Flow is supersonic

18. Groundtrack of Ray
2005
2007

19. 2005 Jets Jets mapped to increases in opacity
In this occ we do not see B7 (star is occulted by limb before crossing B7)
Is it OK to compare 2005 and 2007?
IF individual jets are only source of plume then no
If gas from entire tiger stripe probably ok 19

20. Compare 2007 to 2005 - HSP 2005 attenuation <6% at 15 km
2007 attenuation at same altitude ~10%
Overall attenuation clearly higher in 2007 compared to 2005
The ratio of the opacity from 16 to 22 km between 2007 and 2005 is 1.4 +/- 0.4

21. 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

22. New Plume Simulations
Ian Stewart is modeling plume water vapor shown
Model shows that gas needs to come from along tiger stripe, not just jets (based on 2005 results, where we went down to the surface, but saw gas absorption between jets

23. Plume Picture
Opportunity for dual stellar occ by Enceladus’ plume tweaked in,
19 October 2011, epsilon Orionis (blue) and zeta Orionis (white)

24. 2011 Occultation Objectives
Single stellar occultation
Comparative data 2005 vs vs. 2011
Temporal variability
Activity as f(orbital position)
Better characterization of jets (no compression of HSP data)
Dual stellar occultation
Vertical profile of column density
Removes spatial / temporal ambiguity because profiles are acquired at the same time
Overall plume shape
Contribution of jets to plume vs. gas leaking from tiger stripes
Degree of collimation of jets

25. 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

26. Estimation of Water Flux from Enceladus
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
The biggest uncertainty is what to use for h
Estimate h from plume dimension, = 80 km
Estimate v = 60,000 cm/sec for surface temperature ~ 180K
note that escape velocity = 23,000 cm/sec h v
S = 1.5 x 1016 * 80 x 105 * 60 x 103 = 0.7 x H2O molecules / sec
= 200 kg / sec 26

27. Summary of Results PLUME: H2O column density in 2007 = 1.5 x 1016 cm-2
Density at 15 km altitude 2x higher
H2O column density in 2007 ~ 3.0 x 1016 cm-2
Attenuation in HSP data ~10% in 2007, ~6% in 2005
Difference contradicts Hurford et al model of fissures opening and closing
Plume column density goes as ~ z (z is minimum rayheight)
Water vapor flux ~200 kg/sec
No detection of CO, ethylene not definitive

28. “Search for the Missing Water Source”1
Neutral Species
Water and its products are lost from the system by collisions, photo- and electron- dissociation and ionization
Estimates of required re-supply rates, water molecules/sec:
2.8 x Shemansky, et al.
3.75 x Jurac, et al.
Jurac and Richardson
2 x Shemansky, et al.
E Ring
Saturn’s E ring, composed primarily of 1 micron particles, is also subject to erosion and loss due to sputtering of water from the surface of the E ring’s dust particles and collisions of particles with Saturn’s moons
Estimate of required re-supply rate:
1 kg / sec Juhasz and Horanyi
E ring particles are 0.3 to 3 microns
Lifetime of 1 micron grains is ~50 yrs 28

29. System Oxygen Content - Enceladus’ Influence
Units are Rayleighs
Point out Enceladus orbit, E ring 29

30. Backup slides

31. High Speed Photometer (HSP) Data
HSP is sensitive to 1140 to 1900Å
Statistical analysis applied to find features that are probably real
Assumes signal is Poisson distribution
Calculate 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. 31

32. FUV analysis Occ is easy to detect
Star drifted from pixel 13 to pixel 12 over the course of the observation

33. Summary 2007 Occultation of Zeta Orionis - new results
Overall plume shape and density
Significant events are likely gas jets
UVIS gas jets correlate with dust jets in images
Previous Monte Carlo model updated with new data
We characterize jet widths, opacity, density
Ratio of thermal velocity to vertical velocity = 0.65, supersonic
Water vapor abundance derived from new FUV spectra, no CO
Comparison of 2005 to 2007 occultations does not substantiate tidally-controlled energy-source models
Paper published in Nature

34. 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. 34