Enceladus water jet models from UVIS star occultations2 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. 2005 - lambda Sco No detection (equatorial)

July 2005 - gamma Orionis Composition, mass flux

Oct. 2007 - zeta Orionis Gas jets

May 2010 - Sun Composition, jets

Oct. 2011 eps and zeta Orionis dual occultation

Plume Composition is Water VaporThe absorption spectrum of water is shown compared to Enceladus plume spectrum (I/I0) for a water column density of n = 1.5 x 1016 cm-2

I=I0 exp (-n*)

I0 computed from 25 unocculted samples

n = column density

= absorption cross-section, function of wavelength

Estimation of Enceladus Water FluxS = flux = N * h2 * v = n/h * h2 * v = n * h * v

WhereN = number density / cm3h2 = areav = velocityn = 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 1016 * 80 x 105 * 60 x 103 = 0.7 x 1028 H2O molecules / sec= 200 kg / sec hv

2010 - Solar Occultation2005 - gamma Orionis Occultation2007 - zeta Orionis OccultationThe Occultation Collection2011 occ was a horizontal cut through the plume also

Can we detect Ethylene in Enceladus Plume?INMS detects a species with atomic mass = 28Previously thought that this must be CO or N2New idea (consistent with other INMS data) is that it could be ethyleneC2H4 = 2 * 12 + 4 = 28

Ethylene at 3% H2O Column Density is not detectable by UVISRev 11 gamma Orionis occultation

Ethylene plus water

C2H4 column density = 4.8 x 1014 cm-2

H2O column density = 1.6 x 1016 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 kmS/C velocity = 22.57 km/sec

Best fit is tau = 64.4 x z-2.33 - 0.007

Density at jets is ~2x higher than background plume

2007 High Speed Photometer (HSP) DataHSP is sensitive to 1140 to 1900

Statistical analysis finds features that are statistically unlikelyAssumes signal is Poisson distributionCompares 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

Groundtrack of Ray20052007

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 LocationsPlotted 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 = 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.

At least 8 evenly-spaced gas streams are required to reproduce the overall width of the absorption feature (there may be more). Key Result:

Vthermal / Vbulk = 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

Orions Belt Dual Occ GeometryDual 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 pointHSP centered on eps OriDimmer star

Zeta Ori (Alnitak, O)37.9 km at closest point

Observed with UVIS Far Ultraviolet channel (FUV: 1115 to 1915 ) and High Speed Photometer (HSP)

Water vapor absorbs star signal in the FUV

Plume Model for E7We still do not have enough measurements to determine all model parameters [only 2 occs]Spacecraft trajectory uncertainty of 1 sec or less [= 20km] can make jet identification uncertainPrediction for 100km altitude: 1.1x109 for Mach3, uncertain due to unknown eruption velocity

Estimate of Water Flux from Enceladus = 200 kg/sec S = flux = N * x * y * vth = (n/x) * x * y * vth = n * y * vth

WhereN = number density / cm3x * y = areay = vlos * t => FWHMvth = thermal velocity = 45,000 cm/sec for T = 170Kn = column density measured by UVISxv2011: vlos = 7.48 km/sec

Yearn(cm-2)Uncert-ainty+/-y(x 105 cm)vth(cm / sec)Flux: Molecules / secFlux: Kg/secFraction of orbit from periapsis20051.6 x 10160.15 x 101680 (est.)450005.8 x 10271700.2720071.5 x 10160.14 x 1016110450007.4 x 10272200.7020100.9 x 10160.23 x 1016150450006 x 10271800.1920111.35 x 1016(prelim)135450008.2 x 10272400.70

The Jets20072010In 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 snrno features passed the rigorous statistical tests appliedUnlike 2007 zeta Orionis occRely on FUV data, cross-correlation of absorptions in same place / shifted in time

All GroundtracksBlue => zeta Orionis 2007Red => Solar occ 2010Green => zeta Orionis 2011In all occultations we look through the plume

The groundtrack is the perpendicular dropped to the surface from the ray to the starSolar occZeta Ori2007Zeta Ori2011Basemap from Spitale & Porco, 2007

Eps and Zeta Orionis ComparisonAverage computed for each starThen ratios computed for eachTime shifted to align enhanced absorption feature at B-f because geometry clearly correlated with fissure-crossing 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

?

HSPHSP targeted to eps Ori

0.008 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 OriBaghdadDamascus?

HSP Data

Compare to Previous Occn20082011

Summary of Results: PLUME

Attenuation in HSP data ~10% in 2007, ~6% in 2005Opposite 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 stripesJet opacity corresponds to vapor density doubled within jetsAlternate 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 = 0.65Gas is supersonic: Mach 1.5Eight or more jets required to reproduce width and shape of absorption, some evidence for diffuse sourcesJet source area is smaller than 300 m x 300 m

SummaryMass 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 kmThat is the approx. width of the jets derived in earlier occultations

Work in progress!

*What do we observe*Water spectrum from Chan et al, 1993. Data taken at 298 K.**Point is that we now have two horizontal cuts through the plume, as it turns out almost orthogonal to each other***Statistical analysis is applied to determine which features are likely to be real. The analysis assumes the signal is a Poisson distribution. A running mean is calculated over 1001 integration periods or binned equivalent time [6]. The mean is the baseline for point i. Six different bin sizes were employed. C is the binned stellar signal at a particular bin. The probability that the signal would be C at that bin is given by the sum of the distribution over values C. This step is performed for each bin, i, in the data set to find Pi = P(mi, Ci). Pi is multiplied by the number of bins in the data set, N. This product gives us a value m, where m=NPi [7], or the number of such events one would expect to occur by chance in the data set.

**Take time to explain this!***In the plot the gas streams are shown as dashed vertical lines.***Important message flux has not changed much in 5 years. (Deviation is only 15%, not factors of 2)Width (y) is at FWHMVlos is the line-of-sight velocity of the star across the plane of the skyGiven uncertainties our best estimate of water flux from Enceladus is 200 kg/sec **