Cassini UVIS Results on the Enceladus Plume and Spacecraft Safety Larry W. Esposito 3 May 2007 Enceladus SDT

Cassini clipped edge of plume: INMS, CDA in situ Results ~1 minute before closest approach the Cosmic Dust Analyzer detected a peak in the number of small particles (blue diamonds), 460 km altitude 35 seconds before closest approach the Ion Neutral Mass Spectrometer measured a large peak in water vapor (yellow diamonds), 270 km altitude Gas and dust plumes are decoupled at these altitudes Cassini actually flew through the outer edge of Enceladus’ plume Red line is closest approach to hot spot Green line is c/a to Enceladus Coma observed by INMS out to 4000 km. Densities in this extended region may be indicative of flux that varies on time scales of < 1 hr Hill sphere radius = 948 km

CDA Peak INMS Peak Outline of hot spot

Enceladus Stellar Occultation Geometries February Lambda Scorpii Occultation July Gamma Orionis Occultation Positive detection of plume on ingress! Tick marks every 5 sec Egress Ingress

Localization of Enceladus’ Plume (Not a global atmosphere) Ray intercepts were at latitude / west longitude: 15 / 300 Lambda Sco ingress (non-detection) -31 / 141 Lambda Sco egress (non-detection) -76 / 86 Gamma Ori ingress -0.2 / 28 Gamma Ori egress (non-detection) Consistent with localized plume or jet: Enceladus’ gravity insufficient to hold gravitationally bound sputtered atmosphere Also, the combination of other Cassini data sets are consistent with a plume of water vapor coming from Enceladus’ “Tiger Stripes” driven by the hot spot at the south pole detected by CIRS

FUV Data: Comparison of Occulted to Unocculted Spectra FUV configuration: spectral channels binned by 2 (512 spectral channels from 113.5 nm to 191 nm) low resolution slit width 5 sec integration time full spatial resolution Time record 33, the last full 5 sec integration prior to ingress, shows the deepest absorption. The ray altitude above Enceladus’ surface corresponding to time record 33 ranged from 30 to 7 km. Clear signature of an atmosphere is present – both relatively narrow and broad absorption features

Composition of Plume is Water Vapour I=I0 exp (-n*) I0 computed from 25 unocculted samples n = column density  = absorption cross-section, function of wavelength Water spectrum from Chan et al, 1993. Data taken at 298 K. The absorption spectrum of water (pink line) is shown compared to Enceladus’ plume spectrum (I/I0) for a column density of n = 1.5 x 1016 cm-2

Structure of the Plume The increase in water abundance is best fit by an exponential curve – a comet-like evaporating atmosphere (1/R2) does not fit the data well, nor do global hydrostatic cases The best fit scale length is 80 km

Neutral Species in Saturn’s System The Saturnian system is filled with the products of water molecules: H detected by Voyager in 1980, 1981 OH detected by Hubble Space Telescope in 1992 Atomic Oxygen imaged by UVIS in 2004 The water budget

“Search for the Missing Water Source”1 Neutral Species Water and its products are lost from the system by collisions, photo- and electron- dissociation and ionization Estimates of required re-supply rates, water molecules/sec: 2.8 x 1027 1993 Shemansky, et al. 3.75 x 1027 2002 1Jurac, et al. 1028 2005 Jurac and Richardson 2 x 1028 2005 Shemansky, et al. E Ring Saturn’s E ring, composed primarily of 1 micron particles, is also subject to erosion and loss due to sputtering of water from the surface of the E ring’s dust particles and collisions of particles with Saturn’s moons Estimate of required re-supply rate: 1 kg / sec 2002 Juhasz and Horanyi E ring particles are 0.3 to 3 microns Lifetime of 1 micron grains is ~50 yrs

Estimation of Water Flux from Enceladus S = flux = N * h2 * v = n/h * h2 * v = n * h * v Where N = number density / cm3 h2 = area v = velocity n = column density measured by UVIS The biggest uncertainty is what to use for h Estimate h from plume dimension, = 80 (from scale length) or 175 km (from horizontal distance traversed) Estimate v from thermal velocity of water molecules in vapor pressure equilibrium with warm ice (41,200 at 145 K or 46,000 cm/sec at 180 K – note that escape velocity = 23,000 cm/sec) h v S = 1.5 x 1016 * (80 or 175) x 105 * (41 or 46) x 103 = 0.5 to 1.2 x 1028 H2O molecules / sec = 150 to 360 kg / sec

New Plume Model A new model has been developed for Enceladus’ plumes by Tian, Toon, Larsen, Stewart and Esposito, paper to appear in Icarus Monte Carlo simulation of test particles given vertical + thermal velocity, particle trajectories tracked under influence of gravity and collisions Assumes source of multiple plumes added together along each tiger stripe UVIS ray path across tiger stripes

Monte Carlo model results - Predicted Plume Shape Simulations by Teddy Tian. Paper submitted to Icarus 10,000 test particles given a vertical velocity and a thermal velocity. Thermal velocity direction is chosen randomly and speed satisfies a Boltzmann distribution for Tthermal. Tthermal of 140 to 180K not much affect on results (these temps from Spencer). If T=273 then crack must be very narrow. Inferred surface density is 1010 to 1011 cm-3 Trajectories of particles tracked under the influence of gravity and collisions. Mean free path is calculated, increases with altitude. Number density near surface is high enough for collisions. Model which takes into account viewing geometry of tiger stripes suggest 10 to 100 % of stripes are venting Level 1.0 => 1017 cm2; Level 0.1 => 1016 cm2

Monte Carlo Model - Fit to Data Zero Vz => sublimation -> column density declines more rapidly than observed Mass deposition is 2 orders of magnitude < escape rate, still adequate to resurface and account for high albedo Density distribution in plume as f(z): n(z) = nsurf * (1 + (z/(1+Vz/Vth))-2 Number density of plume consistent w/ column density = 1.5 x 1016 cm-2 is 108 - 109 cm-3, depending on assumed line of sight altitude range. If vertical velocity is same order of magnitude as thermal velocity the surface density is 1010 - 1011 cm-3 Best fit to UVIS column density as a function of altitude requires a vertical velocity of 300 to 500 m/sec Water flux is 4 - 6 x 1027 molecules/sec = 120 - 180 kg/sec (consistent with initial estimate)

Detecting Temporal Variability The water budget derived from the water vapor abundance shows Enceladus supplies most if not all of the OH detected by HST, atomic oxygen in the Saturn system detected by UVIS Implies activity for > 15 years, since HST observed OH in 92 Since the oxygen in the system comes from Enceladus UVIS may be able to remotely monitor Enceladus’ activity levels by monitoring the system oxygen level

Changes in System Oxygen Content Units are Rayleighs

Weekly O1304 Trend

Enceladus Summary UVIS measures water source large enough to create neutral oxygen cloud and to re-supply E ring UVIS column density equal to about a single 1mm ice grain per square meter A uniform source could loft ice particles of radius about 1 micron; higher density jets could loft particles dangerous to Cassini

Plume physical explanations Models Fumarole model. Misty vapor cools as it expands; ice particles condense. T ~ 170K. Geyser model. Local heating gives boiling water at depth, vent geometry gives vertical velocity, collimation; bubbles form and liquid freezes, effectively lofting larger particles to high speeds. T ~ 270K. Comet model. Sublimating vapor lifts ice grains from vent interface and carries them away. T ~ 200K.

Comparable mass In all these models, there is a close coupling between the ice and vapor Growth, lofting and/or evaporation involve an interchange between water molecules and solid ice particles For any significant interchange of mass or momentum, the column of water vapor incident on an ice grain’s surface area must have a comparable mass to the grain mass

Mass Balance N0 *  * a2 * H * mH20 =  * 4/3 *  * a3 For H ~ 40km,  ~ 1, we solve for a (in microns) a ~ N0/ (1012 cm-3) Thus, high pressure vents could loft or grow big particles, potentially dangerous to Cassini

Number of expected collisions Ncolumn/1.5 x 1016 f1 a0-3 A where a0, the particle radius, is measured in mm; and A, the Cassini sensitive area, is measured in m2. Ncolumn is the column density of gas along Cassini’s path through the plume. For E3, this would give about 10-3 f 1 hits (from INMS results). consistent with our safe passage.

Observational constraints The shape of the observed plumes shows V0 > Vth Tian etal can match the UVIS results with V0 ~ 400 m/s and N0 ~ 1010 – 1012 cm-3 This gives typical grain sizes a ~ 0.01–1, roughly consistent with photometry and CDA measurements Present analysis of Cassini data would give no indication of dangerous particles, by orders of magnitude

Hazard calculation: Approach and assumptions Plume has cylindrical symmetry about pole Estimate plume density integrated along Cassini path from water column measured by UVIS star occultation See Spencer and Hansen figures: UVIS had a measurement at location of highest density for rev 61

Rev 61 c/a ------->

Calculation If all water vapor along this line of sight to star (Ncol = 2E15/cm2) were swept up by Cassini’s sensitive area (0.8 m2), this would form a solid ice sphere of radius 500 microns Assume measured solid particle size distribution can be extended as a power law in radius to sizes dangerous to Cassini CDA: q = 4 RPWS: q = 6.4 (radius power law)

Number of dangerous particles Calculate the predicted number of hits by dangerous particles (r > 900 microns, Dave Seal) if Cassini flew a path with same minimum altitude: ND = f1* (4-q)/(q-1) * a03/(amax4-q - amin4-q) * (a*1-q - amax1-q)

Key parameters a*: dangerous particle radius, 900 microns a0: equivalent ice radius, 500 microns amin, amax: size range, radius 1-1000 microns f1: ratio of solid ice mass to water vapor q: power law size index

Results ND = 3E-9 f1 for q = 6.4 ND = 2E-3 f1 for q = 4

Values for f1, mass ratio solid/gas Simple physical arguments of mass balance, force balance, growth of solids from vapor give f1 < 1 Comparing mass loss of solids estimated by ISS, CDA to vapor by UVIS gives f1 ~ 0.01

But, what about small, high pressure vents But, what about small, high pressure vents? They could loft dangerous particles. Signal more variable within plume … Outside Within plume

Same number of high and low outliers

Entire Enceladus ingress cut (binned data) 3 features near limb found by ring feature search algorithm, but below significance threshold

Conclusions from 2 independent searches Sensitive to events as small as 50m; opacity as small as 10% We see no significant deviations from smooth variation Outlier events have width less than 1km and opacity less than twice mean

Conclusions Extrapolating Cassini plume measurements to rev 61 and to radius dangerous to Cassini, using the most optimistic size range, provides a conservative estimate of the number of hits expected The value is 0.2% or less No evidence for high pressure vents Better measurements of the size distribution and its opacity would improve the model