The Potential Hazard to Cassini from Small Dust in Enceladus Plumes

The Potential Hazard to Cassini
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
• 35 seconds before
closest approach the Ion
Neutral Mass
Spectrometer measured
a large peak in water
vapor (yellow
diamonds), 270 km
• Gas and dust plumes
are decoupled at these
CDA Peak INMS Peak
Composition of Plume is Water Vapour
I=I0 exp (-n*)
I0 computed from
25 unocculted
n = column density
 = absorption
function of
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
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
• Assumes source of
multiple plumes added
together along each tiger
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
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
Elapsed Time (days)
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 ~
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
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----->
• 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
• 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
• ND = 3E-9 fI
• ND = 2E-3 fI
for q = 6.4 (RPWS)
for q = 4 (CDA)
Values for fI, mass ratio
• 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 …
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
• 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