Cassini UVIS Results on the
Enceladus Plume and
Spacecraft Safety
Larry W. Esposito
4 June 2007
EPWG
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 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
• ND = 2E-3 f1
for q = 6.4 (RPWS)
for q = 4 (CDA)
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? 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
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