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Enceladus’ Plume and Jets: UVIS Occultation Observations

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

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Enceladus’ Plume and Jets: UVIS Occultation Observations

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

  2. Plume Jets 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

  3. UVIS Observations of Enceladus’ Plume Zeta Orionis • 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 UVIS observes occultations of stars and the sun to probe Enceladus’ plume Three stellar and one solar occultation observed to-date

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

  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 • Gas jets appear to correlate to dust jets in zeta Ori occ  Egress Basemap from Spitale & Porco, 2007

  7. f e a b d c Solar Occ Jet Identifications • 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 2010 Spacecraft viewed sun from this side Ingress 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  Minimum Altitude Egress Basemap from Spitale & Porco, 2007

  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 *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 (z0) • Estimating the mach number as ~2 z0/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 vsound = 320 m/sec (for ~170 K) then vvert = 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) Salt-rich particles come from water in contact with rocky core • CDA model • Hypothesize salt-rich particles with r > 0.6 micron are aerosols sprayed from a subsurface ocean, ejected along length of tiger stripe

  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 • H20 fit to absorption spectrum • Column density of H2O = 0.9 x 1016 cm-2 • No N2 absorption feature -> N2 upper limit of 5 x 1013 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 N2 for models of the interior • High temperature liquid not required for dissociation of NH3 (no need to explain N2 in presence of NH3) • Percolation of H2O and NH3 through hot rock is not required • Clathrate decomposition is not substantiated for N2 as the plume propellant • Predict • N2 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 1016 cm-2 • Overall amount of water vapor is comparable to previous two (stellar) occultations • 2005: 1.6 x 1016 cm-2 • 2007: 1.5 x 1016 cm-2 (maximum value of 3.0 x 1016 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 * vth = (n/x) * x * y * vth = n * y * vth Where N = number density / cm3 x * y = area y = vlos * t => FWHM vth = thermal velocity = 45,000 cm/sec for T = 170K n = column density measured by UVIS note that escape velocity = 23,000 cm/sec y x v

  20. 2007 - Zeta Orionis Occultation (Alnitak) 2005 - gamma Orionis Occultation (Bellatrix) • Key results: • Dominant composition = water vapor • Plume column density = 1.6 x 1016 /cm2 • Water vapor flux ~ 180 kg/sec • Results documented in Science, 2006 • Vertical cut through plume • Key results: • Average column density = 1.5 x 1016 cm-2 • Max column density = 3.0 x 1016 cm-2 • Gas jets detected, correspond to dust jets • Results documented in Nature, 2008 • Horizontal density profile • 18 May 2010 - Solar Occultation • Key results: • No N2 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 N2 in presence of NH3 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. 18 May 2010 - Solar Occultation Two science objectives enabled by solar (rather than stellar) occultation: 1. Composition of the plume New wavelength range: EUV H2O and N2 have diagnostic absorption features at EUV wavelengths The primary goal was to look for N2, on basis of INMS detecting a species with amu=28 2. Structure of the jets and plume Higher time resolution, better snr

  24. FWHM 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 • 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 Zeta Orionis occultation

  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 * Altitude is relative to limb of Enceladus

  27. Groundtrack of Occultation Enhanced HSP absorption featuresa, 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

  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 Tsurface = 180 K (from CIRS) V thermal = 406 m/sec V vertical = 624 m/sec

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

  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. FWHM 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 • 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 Zeta Orionis occultation

  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

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