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Enceladus: UVIS Constraints and Modeling

This article discusses the UVIS observations of Enceladus' plume, including composition, mass flux, temporal variability, gas jets, and structure. The article also explores the constraints and modeling of the plume based on the data collected.

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Enceladus: UVIS Constraints and Modeling

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  1. Enceladus: UVIS Constraints and Modeling C. J. Hansen, L. Esposito, D. Shemansky, I. Stewart, A. Hendrix 19 July 2010

  2. Plume Jets Outline All plume models must ultimately be consistent with data • UVIS Observations • What we observe: Occultations • How we observe: Instrument • Plume Results • Composition • Mass flux • Temporal variability • Gas Jets • Structure • Mach number

  3. UVIS Observations of Enceladus’ Plume • 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 Composition, mass flux, and plume and jet structure 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. 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

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

  7. 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 Two science objectives enabled by solar (rather than stellar) occultation: 1. Composition of the plume New wavelength range: EUV 2. Structure of the jets and plume Higher time resolution

  8. Plume: FUV Results from Stellar Occ’s • Absorption is best fit by water vapor • 2007best 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 I/I0

  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. Composition • 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 • No N2 (upper limit = 3 x 1013, so < 0.3%) • AMU = 28 detected by INMS is not N2 • It is not CO in the plume (or UVIS would have seen it in our stellar occs) or it is < 3% • Maybe very little signal at amu=28 to worry about after all per latest INMS results (personal communication, Waite 2010) • N2 < 0.9% • CO < 0.9% • C2H4 < 0.16%

  11. Nitrogen feature at 97.2 nm not detected Actual • No dip is seen at all at 97.2 nm • Upper limit < 0.3% Consequences of no N2 for models of the interior • High temperature liquid not required for dissociation of NH3 (if there is NH3 in the plume) • Percolation of H2O and NH3 through hot rock is not required • A catalyst for decomposition is not required at lower temperatures • 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!

  12. Solar Occ results: Water in the Plume • H20 fit to absorption spectrum • Column density of H2O = 0.9 x 1016 cm-2

  13. Water Vapor Abundance • To calculate water vapor abundance in the plume the spectra are summed during the center 60 sec of the occultation, then divided by a 650 sec average unocculted sum to compute I / I0 • I0 computed at two different times, results were the same • The extinction spectrum is well-matched by a water vapor spectrum with column density = 0.9 +/- 0.1 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 may be at least partially attributable to the viewing geometry

  14. 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 Preliminary result: 0.9 x1016 cm-2, ~2/3 of value in 2005  Ingress  Egress

  15. Water Vapor and other Gases • Some evidence that the mixing ratio of water may change depending on where you are in the plume

  16. 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

  17. 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 x v

  18. One more comparison to tidal energy model Position of Enceladus in its orbit at times of stellar occultations, and solar occultation • Hurford et al 2007 model predicts tidally-controlled differences in eruption activity as a function of where Enceladus is in its eccentric orbit • Expect fissures to open and close • Substantial changes are not seen in the occultation data, although they would be predicted, based on this model • 2010 column density very similar to 2005 Taken from Hurford et al, Nature 447:292 (2007)

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

  20. Detection of Gas Jets • Perfect geometry to get a horizontal cut through the plume and detect density variations indicative of gas jets

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

  22. 2007 - Plume Structure and Jets 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 Supersonic gas jets are consistent with Schmidt et al. model of nozzle-accelerated gas coming from liquid water reservoir

  23. 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

  24. Jets vs. Tiger Stripes Spacecraft viewed sun from this side Ingress • As before, gas jets correlate to dust jets • Higher time resolution because sun’s passage behind the plume was slower • We see gas between jets, although no enhancement as a function of depth in the plume (time at which sun was closest to limb is not a deep attenuation)  Minimum Altitude Egress

  25. Jet Identities

  26. w h Gas Velocity • In the case of jet c (Baghdad I) the source of the jet is close enough to the occultation ground track to calculate the height and width • Half-width of jet c = 2.5 sec * 2.85 km/sec = ~7 km (w) at ~27 km (h) altitude w/h = tan ß = vthermal / vvert = 7 / 27 Mach number = vvert / vthermal = 3.9 • Previously estimated mach number (from 2007 occultation) was 1.5 • Jets more collimated than previously estimated • New estimate for vertical velocity: if vthermal = 450 m/sec (for ~170 K) then vvert = 1755 m/sec

  27. Our maturing understanding… • Composition • Upper limit on N2 of 3 x 1013 cm-2 • H20 column density = 0.9 x 1016 cm-2 • In family with previous occultations • Precisely corresponds to aspect ratio applied to 2005, which was at a similar true anomaly • 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 -> higher mach number of ~3.9

  28. Backup Info

  29. 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

  30. Altitude of Sun above Limb • Perpendicular distance of ray connecting UVIS and the sun from the limb of Enceladus • Attenuation at a and b bracket closest approach f e c d a b

  31. 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

  32. 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

  33. a b c d 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

  34. 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 Gas Jet Model Key Result: Vvert / Vthermal = 1.5 Flow is supersonic

  35. Groundtrack of Ray 2005 2007

  36. 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

  37. 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

  38. 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

  39. 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 modelof 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, ethylene not definitive

  40. 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

  41. Water Column Density: FUV comparison to HSP2007 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

  42. 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

  43. Zeta Orionis Occultation 2007(Alnitak) 2005 - 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 • Key results: • Average plume column density = 1.5 x 1016 cm-2 • Maximum plume column density = 3.0 x 1016 cm-2 • Gas jets detected correspond to dust jets • Horizontal density profile • True anomaly = 254 • Results documented in Nature, 2008

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

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