1. Observations of Enceladus ’ Plume from Cassini ’ s UltraViolet Imaging Spectrograph (UVIS) C. Hansen, L. Esposito, J. Colwell, A. Hendrix, B. Meinke, I. Stewart June 2008

2. Overview 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 submitted, in review by Nature

3. 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

4. Enceladus Plume Occultations FUV and HSP data collected FUV: 5 sec integration HSP: 2 msec sampling 2007 - zeta Orionis Horizontal density profile True anomaly = 254 2005 - gamma Orionis Vertical cut through plume True anomaly = 98 Key results: Dominant composition = water vapor Plume column density = 1.6 x 10 16 /cm 2 Water vapor flux ~ 150 kg/sec

5. 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 = 0.000008 3 (c) m = 0.00056 6 (d) m = 0.026

6. 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

7. 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 bacd

8. Gas Jets a. Cairo (V) d. Damascus (II) c. Baghdad (VI) Closest point b. Alexandria (IV) Ingress Egress Density in jets is twice the background plume Gas jet typical width = 10 km at 15 km altitude

9. 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). Key Result: V thermal / V bulk = 0.65 Flow is supersonic

10. Tilted Jets Opening angle of plume derived from 0.65 ratio of thermal to bulk velocity, projected to altitude of occ, includes off- vertical tilt of B7 and D3 jets May not need additional arbitrary jets Work in progress because timing not consistent with previous plot

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

12. Water column density: FUV Comparison to 2005 at 15 km altitude 2007 peak column density = 3.0 x 10 16 cm -2 2005 = 1.6 x 10 16 cm -2 No detection of CO formal 2-σ upper limit is 3.6 x 10 14 cm -2 corresponds to mixing ratio with H 2 O of 3.0 Our nondetection of CO excludes 3% CO in the plume at the 2 sigma level Absorption is best fit by water vapor Best fit column density = 1.3 x 10 16 cm -2 Error bar: +/- 1.4 x 10 15 cm -2 I/I 0

13. Taken from Hurford et al, Nature 447:292 (2007) True Anomaly (deg) Fraction of orbit from Periapsis Position in OrbitStress 10 5 Pa 00.0Periapsis0.3 900.25One quarter-0.8 97.760.27July 14, 2005-0.77 1800.5Apoapsis-0.4 254.130.7October 24, 20070.4 2700.75Three quarter0.6 Position of Enceladus in its orbit at times of stellar occultations Comparison to tidal energy model 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

14. 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.

15. Groundtrack of Ray 20052007

16. 2005 HSP data HSP data can be fit by an exponential Look for departures due to jets Appear to see real features

17. 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

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

19. Summary of Results PLUME: H 2 O column density in 2007 = 1.3 x 10 16 cm -2 Density at 15 km altitude 2x higher –H 2 O column density in 2007 ~ 3.0 x 10 16 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 -2 (z is minimum rayheight) Water vapor flux ~200 kg/sec No detection of CO

20. 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 thermal velocity to vertical velocity in jet = 0.65 –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 T surface = 180 K (from CIRS) V thermal = 406 m/sec V vertical = 624 m/sec

22. Backup Slides

23. Gas vs. Ice For reasonable mass ratios of ice to gas in the jets (f I = 1) ice has too little opacity (tau = 0.001/grain radius) to be detected by HSP. Radius in microns If jets are unresolved by HSP or have spread significantly in reaching altitude z=15km, the surface pressure at the vent could be correspondingly higher

24. Plume Model 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 T surface = 180 K (from CIRS) V thermal = 406 m/sec V vertical = 624 m/sec

25. Optical Depth vs. Rayheight Minimum distance of rayheight above limb = 15.6 km S/C velocity = 22.57 km/sec Best fit is tau = 64.4 x z -2.33 - 0.007 Density at jets is ~2x higher than “background” plume