1. Enceladus’ Plume and Jets: UVIS Occultation Observations June 2011

2. 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

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 Feb. 2005 - lambda Sco No detection (equatorial) July 2005 - gamma Orionis Composition, mass flux Oct. 2007 - zeta Orionis Gas jets May 2010 - Sun Composition, jets Zeta Orionis

4. 2010 - Solar Occultation 2005 - gamma Orionis Occultation 2007 - zeta Orionis Occultation The Occultation Collection

5. Coming up Next Opportunity for dual stellar occ by Enceladus’ plume, E15, 19 October 2011, epsilon Orionis (blue) and zeta Orionis (white)

6. ~Orthogonal Ground Tracks Blue ground track is from zeta Ori occ on Rev 51 Orange is solar occ track, ~orthogonal  Ingress  Egress Basemap from Spitale & Porco, 2007 Gas jets appear to correlate to dust jets in zeta Ori occ

7. Solar Occ Jet Identifications a b c d e f 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 Minimum altitude

8. Jets and Tiger Stripes As before, gas jets appear to correlate to dust jets Feature “a” would line up better with Alexandria dust jet if dust jet were actually on the Alexandria tiger stripe Feature “d” may correspond to a new (fissure branching off tiger stripe) hot feature detected in CIRS thermal data 2011 stellar occ will be similar groundtrack, viewed from opposite side Spacecraft viewed sun from this side Ingress Egress Minimum Altitude  Basemap from Spitale & Porco, 2007 2010

9. Jet Structure Higher snr enables better measurements of jets’ dimensions – more clearly distinguished from background plume Density of gas in jets ~3 x the density of the background plume (20% more absorption over 1/15 scale) 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

10. Jet Properties FeatureAltitude of ray relative to limb Z 0 : Altitude of ray relative to jet source FWHM: full width half max(km ) Mach number ~ 2 * Z 0 / FWHM Associat ed Dust Jet Excess attenu- ation at the jet (%) – for density calc* a21.321.67 6 Alexandria IV 27 Closest approach 20.7 b222495 Cairo V and/or VIII 17 c28.429106 Baghdad I 19 d31.236 or 32107 or 6 Baghdad VII or new feature 12 e3940108 Damascus III 13 f47.549.7147 Damascus II 14 *Average attenuation =17%

11. Gas Velocity The full width half max (FWHM) of jet c (Baghdad I) is ~10 km at a jet intercept altitude of 29 km (z 0 ) Estimating the mach number as ~2 z 0 /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 v sound = 320 m/sec (for ~170 K) then v vert = 1920 m/sec This is an upper limit because the gas will be cooled in a nozzle Consistent with CDA model that gas is accelerated in nozzles to surface to supersonic speeds

12. CDA – Composition partitioning in E ring and plume (1) CDA detects small salt-poor grains in the E ring, salt-rich particles in Enceladus’ diffuse plume Condensation of water molecules in the high velocity gas jets produces small salt-poor grains Particle size 0.2 to 0.6 microns Hypothesis is that high velocity jets propel salt-poor grains to the E ring Higher mass - fall back to surface

13. CDA – Composition partitioning in E ring and plume (2) CDA model Hypothesize salt-rich particles with r > 0.6 micron are aerosols sprayed from a subsurface ocean, ejected along length of tiger stripe Salt-rich particles come from water in contact with rocky core

14. The “Perrier” Ocean Matson et al 2011 model puts this all together, loosely Subsurface ocean is charged with dissolved gases Bubbles come out of solution as liquid rises, when they pop (in the water/brine reservoir) the salt-rich aerosols detected by CDA are formed Gas and salt-rich particles escape along length of tiger stripe Gas also accelerated in nozzles to surface, smallest grains condense, CDA sees salt-poor particles, we see supersonic jets Tiger stripe / nozzle physical structure yet to be explained

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 profile at altitude ~20 km (c/a) INMS and UVIS both detect Alexandria and Baghdad jets

16. Solar Occ results – Plume Composition H 2 0 fit to absorption spectrum Column density of H 2 O = 0.9 x 10 16 cm -2 No N 2 absorption feature -> N 2 upper limit of 5 x 10 13 cm -2

17. 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 N 2 for models of the interior High temperature liquid not required for dissociation of NH 3 (no need to explain N 2 in presence of NH 3 ) Percolation of H 2 O and NH 3 through hot rock is not required Clathrate decomposition is not substantiated for N 2 as the plume propellant Predict N 2 feature at 97.2 nm fortuitously coincides with strong lyman gamma emission so lots of signal available Very sensitive test!

18. Water Vapor Abundance The solar extinction spectrum is well-matched by a water vapor spectrum with column density = 0.9 +/- 0.23 x 10 16 cm -2 Overall amount of water vapor is comparable to previous two (stellar) occultations –2005: 1.6 x 10 16 cm -2 –2007: 1.5 x 10 16 cm -2 (maximum value of 3.0 x 10 16 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 All flux values within 15% deviation suggests that Enceladus has been erupting steadily for the last 6 years Water vapor flux in the jets = 30-50 kg/sec – (5 – 8) * 3.4% = 15-25% of 200 kg/sec

19. Estimate of Water Flux from Enceladus = 200 kg/sec S = flux = N * x * y * v th = (n/x) * x * y * v th = n * y * v th Where N = number density / cm 3 x * y = area y = v los * t => FWHM v th = 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 Yearn (cm -2) y (x 10 5 cm) v th (cm / sec) Flux: Molecules / sec Flux: Kg/sec 20051.6 x 10 16 80 (est.)450005.8 x 10 27 170 20071.5 x 10 16 110450007.4 x 10 27 220 20100.9 x 10 16 150450006 x 10 27 180 y

20. 2007 - Zeta Orionis Occultation (Alnitak) Key results: Average column density = 1.5 x 10 16 cm -2 Max column density = 3.0 x 10 16 cm -2 Gas jets detected, correspond to dust jets Results documented in Nature, 2008 Horizontal density profile 2005 - gamma Orionis Occultation (Bellatrix) Key results: Dominant composition = water vapor Plume column density = 1.6 x 10 16 /cm 2 Water vapor flux ~ 180 kg/sec Results documented in Science, 2006 Vertical cut through plume 18 May 2010 - Solar Occultation Key results: No N 2 detected in plume; upper limit < 0.5% Water flux approximately constant over 6 year Cassini mission, 200 kg/sec Jets more collimated than previously reported, mach numbers = 5 to 8 Suggests speed of 1 – 2 km/sec Alexandria jet correlates to enhanced INMS gas detection at high altitude Results documented in Hansen et al., GRL 38:L11202, 9 June 2011 Horizontal density profile

21. We conclude… Supersonic gas jets are consistent with Schmidt et al. model of nozzle-accelerated gas coming from liquid water reservoir High velocity jets are also consistent with CDA data reported by Postberg et al. showing compositional partitioning: small salt-poor particles reaching the E ring and salt-rich particles in the diffuse component of the plume close to Enceladus Lack of N 2 in presence of NH 3 means that a relatively cool liquid reservoir such as “Perrier Ocean” proposed by Matson et al. is viable Clathrate propellant highly unlikely

22. Backup Info

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

24. Solar Occultation Characteristics Total duration of Solar Occ: 1min 35sec Duration for full-width half max: 53 sec Line of sight velocity: 2.85 km/sec Width of plume at FWHM: 56 sec * 2.85 = 150 km FWHM Zeta Orionis occultation 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

25. 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: 2010-138T05:51:44.45 Observation end time: 2010-138T06:10:36.45 Ingress: 2010-138T06:00:40.45 Egress: 2010-138T06:02:59.45 Velocity of sun across plane of sky ~ 2.75 km/sec Data shown is summed over wavelength

26. Jet Identities FeatureAltitude* (km) Dust Jet a20 Alexandria IV Closest approach 19.7 b21 Cairo V and/or VIII c27 Baghdad I d30 Baghdad VII e38 Damascus III f46 Damascus II * Altitude is relative to limb of Enceladus

27. 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 Blue line is groundtrack Groundtrack of Occultation

28. 2007 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 T surface = 180 K (from CIRS) V thermal = 406 m/sec V vertical = 624 m/sec

29. Plume Composition and Column Density 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 UVIS Ultraviolet Spectra provide constraints on:

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

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

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

33. Solar Occultation Characteristics Total duration of Solar Occ: 2min 19sec Duration for full-width half max: 56 sec Line of sight velocity: 2.85 km/sec Width of plume at FWHM: 56 sec * 2.85 = 160 km FWHM Zeta Orionis occultation 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

34. UVIS Characteristics UVIS has 4 separate channels For stellar occultations we use: Far UltraViolet (FUV) –1115 to 1915 Å –2D detector: 1024 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: 1024 spectral x 64 one-mrad spatial pixels No spatial information because signal from sun is spread across the detector (deliberately) Spatial rows 5 - 58 binned to two windows of 27 rows each 1 sec integration