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The Potential Hazard to Cassini from Small Dust in Enceladus Plumes Larry W. Esposito 23 August 2007 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 CDA Peak INMS Peak 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 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 UVIS Plume Model (Tian 2007) • A new model has been developed for Enceladus’ plumes by Tian, Toon, Larsen, Stewart and Esposito, paper 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 Monte Carlo Model - Fit to Data 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 1992 (Shemansky et al) The water source has not changed by any large factor. • 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 Oxygen atoms (x10-34) O1304 trend shows factor of 2x changes on weekly, monthly, yearly scales 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 (a) (b) 0 Z: +5/-5 Rs, X: -6/-10 Rs Z: +5/-5 Rs, X: 0/+6 Rs Z: +5/-5 Rs, X: +6/+10 Rs Z: +5/-5 Rs, X: 0/-6 Rs 100 200 300 400 Elapsed Time (days) 500 600 700 Enceladus Summary • UVIS measures water source large enough to create neutral oxygen cloud and to resupply E ring • UVIS column density equal to about a single 1/2 mm ice grain per square meter 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 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: these particles are not dangerous, by orders of magnitude Hazard calculation: Approach and assumptions • Plume has cylindrical symmetry about pole • Plume density is estimated along Cassini path from water column measured by UVIS star occultation • See following figures (from Spencer and Hansen): UVIS had a measurement at predicted highest density location for rev 61 Rev 61 plume max-----> 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 = fI* (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 fI: ratio of solid ice mass to water vapor q: power law size index Results • ND = 3E-9 fI • ND = 2E-3 fI for q = 6.4 (RPWS) for q = 4 (CDA) Values for fI, mass ratio solid/gas • Simple physical arguments of mass balance, force balance, growth of solids from vapor give fI < 1 • Comparing mass loss of solids published by ISS, CDA to vapor by UVIS gives fI ~ 0.01 • Recent ISS analysis gives fI ~ 1 • Schmidt physical model gives fI ~0.6 • Comparing UVIS and VIMS: fI > 0.01 H2O absorption at 2.7 microns H2O absorption at VIMS resolution 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 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 Why this is conservative • Physical models show it is much harder to loft larger particles: power law extrapolation is conservative • No evidence of big temporal variations, or of high pressure vents • This idealized model makes no specific claims about the exact plume mechanism: these are all included in the factor fI 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 of 0.2fI% or less • A physical model by Schmidt gives 10-5 • Better measurements of the size distribution and its opacity would improve the model