1. The Composition and Structure of Enceladus’ Plume from the Cassini UVIS Solar Occultation C. J. Hansen, L. Esposito, D. Shemansky, A. I. F. Stewart, A. Hendrix 23 May 2011

2. Outline UVIS occultations gives indicate 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 Characteristics UVIS has 4 separate channels For stellar occultations: 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: 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

4. UVIS Observations of Enceladus’ Plume UVIS occultations of stars and the sun 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

5. 2010 - Solar Occultation 2005 - gamma Orionis Occultation 2007 - zeta Orionis Occultation The Occultation Collection

6. 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

7. 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:

8. 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

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

10. Solar Occ results – Composition H 2 0 fit to absorption spectrum Column density = 0.9 +/- 0.23 x 10 16 cm -2 No N 2 absorption feature: N 2 upper limit of 5 x 10 13 cm -2

11. 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 (if there is NH 3 in the plume) 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!

12. Water Vapor Abundance Spectra are summed during the center 60 sec of the occultation, then divided by a 650 sec average unocculted sum to compute I / I 0 The 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 due to the viewing geometry – the H 2 O flux is in family with the previous results

13. ~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 x10 16 cm -2, ~2/3 of value in 2005  Ingress  Egress Basemap from Spitale & Porco, 2007

14. 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

16. Solar Occ Jet Identifications a b c d e f Window 0 and 1 matching features => jets are real Minimum altitude

17. Jets vs. Tiger Stripes As before, gas jets appear to correlate to dust jets Spacecraft viewed sun from this side Ingress Egress Minimum Altitude  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 Basemap from Spitale & Porco, 2007 * Altitude of ray to sun from limb

18. 2007 - Plume Structure and Jets Supersonic gas jets are consistent with Schmidt et al. model of nozzle-accelerated gas coming from liquid water reservoir 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

19. Jet Structure Higher SNR enables better measurements of jets’ dimensions – more clearly distinguished from background plume Density of gas in jets is 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 compared to the jets’ total equivalent width Optical Depth

20. Solar Occultation Jets

21. Comparison to INMS results from E7

22. 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

23. Summary Composition –Upper limit on N 2 of 5 x 10 13 cm -2 –H 2 0 column density = 0.9 x 10 16 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

24. Summary 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 differences: 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