Enceladus water jet models from UVIS star occultations 2 April 2013

UVIS Observations of Enceladus’ Plume UVIS observes occultations of stars and the sun to probe Enceladus’ plume – Composition, mass flux, and plume and jet structure Four stellar and one solar occultation observed to-date Feb lambda Sco No detection (equatorial) July gamma Orionis Composition, mass flux Oct zeta Orionis Gas jets May Sun Composition, jets Oct – eps and zeta Orionis dual occultation

Plume Composition is Water Vapor The absorption spectrum of water is shown compared to Enceladus’ plume spectrum (I/I 0 ) for a water column density of n = 1.5 x cm -2 I=I 0 exp (-n*  ) I 0 computed from 25 unocculted samples n = column density  = absorption cross-section, function of wavelength

Estimation of Enceladus Water Flux S = flux = N * h 2 * v = n/h * h 2 * v = n * h * v Where N = number density / cm 3 h 2 = area v = velocity n = column density measured by UVIS Estimate h from plume dimension, = 80 km Estimate v from thermal velocity of water molecules in vapor pressure equilibrium with warm ice (600 m/sec for surface temperature ~ 180K – note that escape velocity = 230 m/sec) S = 1.5 x * 80 x 10 5 * 60 x 10 3 = 0.7 x H 2 O molecules / sec = 200 kg / sec h v

Solar Occultation gamma Orionis Occultation zeta Orionis Occultation The Occultation Collection 2011 occ was a horizontal cut through the plume also

Can we detect Ethylene in Enceladus’ Plume? INMS detects a species with atomic mass = 28 Previously thought that this must be CO or N 2 New idea (consistent with other INMS data) is that it could be ethylene –C 2 H 4 = 2 * = 28

Ethylene at 3% H 2 O Column Density is not detectable by UVIS Rev 11 gamma Orionis occultation Ethylene plus water C 2 H 4 column density = 4.8 x cm -2 H 2 O column density = 1.6 x cm -2

Plume Structure (2005) Water vapor abundance calculated from each 5 sec spectrum. The 2005 water profile is best fit by an exponential curve. The best fit scale length is 80 km

Optical Depth vs. Rayheight (2007) Minimum distance of rayheight above limb = 15.6 km S/C velocity = km/sec Best fit is tau = 64.4 x z Density at jets is ~2x higher than “background” plume

2007 High Speed Photometer (HSP) Data HSP is sensitive to 1140 to 1900Å Statistical analysis finds features that are statistically unlikely – Assumes signal is Poisson distribution – Compares to 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 = (b) m = (c) m = (d) m = 0.026

Groundtrack of Ray

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

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

Plume or jets? The plume of gas and dust from Enceladus includes a number of individual jets seen by Cassini camera and by UVIS

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 = 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

Best fit of 8 sources from Spitale & Porco to match UVIS occultation profile

Brightness of water vapor over Enceladus South pole from UVIS 8-jet model

Orion’s Belt Dual Occ Geometry Dual stellar occ by Enceladus’ plume, E15, 19 October 2011, epsilon Orionis (blue) and zeta Orionis (white) Horizontal cut through plume

Rev 155 Enceladus Dual Occultation Eps Ori (Alnilam, B) – 16.5 km at closest point – HSP centered on eps Ori – Dimmer star Zeta Ori (Alnitak, O) – 37.9 km at closest point Observed with UVIS Far Ultraviolet channel (FUV: 1115 to 1915 Å) and High Speed Photometer (HSP) Water vapor absorbs star signal in the FUV

Plume Model for E7 We still do not have enough measurements to determine all model parameters [only 2 occ’s] Spacecraft trajectory uncertainty of 1 sec or less [= 20km] can make jet identification uncertain Prediction for 100km altitude: 1.1x10 9 for Mach3, uncertain due to unknown eruption velocity

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 x v Yearn (cm -2) Uncert- ainty +/- y (x 10 5 cm) v th (cm / sec) Flux: Molecule s / sec Flux: Kg/sec Fraction of orbit from periapsis x x (est.) x x x x x x x x (prelim) x : v los = 7.48 km/sec

The Jets a b c d e f In the past we have identified collimated jets of gas from enhanced absorption features in the HSP (2007 zeta Ori occ) and the EUV (2010 solar occ) This time the HSP data was lower snr – no features passed the rigorous statistical tests applied – Unlike 2007 zeta Orionis occ Rely on FUV data, cross-correlation of absorptions in same place / shifted in time

All Groundtracks Blue => zeta Orionis 2007 Red => Solar occ 2010 Green => zeta Orionis 2011 In all occultations we look through the plume The groundtrack is the perpendicular dropped to the surface from the ray to the star Solar occ Zeta Ori 2007 Zeta Ori 2011 Basemap from Spitale & Porco, 2007

Eps and Zeta Orionis Comparison Average computed for each star Then ratios computed for each Time shifted to align enhanced absorption feature at B-f because geometry clearly correlated with fissure-crossing DIII DII B-f Damascus jets (DII and DIII) identified Distinct at eps Ori altitude, merged higher Clear signal of gas from Baghdad fissure (B-f), though no dust jet nearby Possible activity at “?” without associated dust jet ?

a b c HSP HSP targeted to eps Ori sec integration summed to 1 sec Although features did not pass our statistical tests we can compare to the FUV data set Good agreement with eps Ori Baghdad Damascus ?

HSP Data

Compare to Previous Occ’n

Summary of Results: PLUME Attenuation in HSP data ~10% in 2007, ~6% in 2005 – Opposite of Hurford et al model of fissures opening and closing Plume column density goes as ~ z -2 or as exp(-z/H) (z is minimum rayheight) Water vapor flux ~200 kg/sec No detection of CO, upper limit 3% (3 sigma)

Summary of Results: JETS 2007 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: Mach 1.5 Eight or more jets required to reproduce width and shape of absorption, some evidence for diffuse sources Jet source area is smaller than 300 m x 300 m

Summary Mass flux determined, compared to other occs Jets tougher to identify because low snr – HSP data did not pass statistical tests Determination of spreading at the two altitudes also limited by temporal resolution of the FUV (2 sec integration time) – 2 sec x 7.48 km/sec line-of-sight velocity = 15 km – That is the approx. width of the jets derived in earlier occultations Work in progress!