Observations of
Enceladus’ Plume from
Cassini’s UltraViolet
Imaging Spectrograph
(UVIS)
June 2008
C. Hansen, L. Esposito, J. Colwell, A.
Hendrix, B. Meinke, I. Stewart
Overview
2007 Occultation of Zeta Orionis - new results
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Overall plume shape and density
Significant events are likely gas jets
UVIS gas jets correlate with dust jets in images
Previous Monte Carlo model updated with new data
We characterize jet widths, opacity, density
Ratio of thermal velocity to vertical velocity = 0.65, supersonic
Water vapor abundance derived from new FUV spectra, no CO
Comparison of 2005 to 2007 occultations does not substantiate
tidally-controlled energy-source models
Paper submitted, in review by Nature
Enceladus Plume Occultation of zeta Orionis
October 2007
• In October 2007 zeta Orionis was
occulted by Enceladus’ plume
• Perfect geometry to get a horizontal cut
through the plume and detect density
variations indicative of gas jets
• Objective was to see if there are gas
jets corresponding to dust jets detected in
images
Enceladus Plume Occultations
FUV and HSP data collected
FUV: 5 sec integration
HSP: 2 msec sampling
2007 - zeta Orionis
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Horizontal density profile
True anomaly = 254
2005 - gamma Orionis
Vertical cut through plume
True anomaly = 98
Key results:
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Dominant composition = water vapor
Plume column density = 1.6 x 1016 /cm2
Water vapor flux ~ 150 kg/sec
High Speed Photometer (HSP) Data
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HSP is sensitive to 1140 to 1900Å
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Statistical analysis applied to find
features that are probably real
– Assumes signal is Poisson
distribution
– Calculate running mean
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Six different bin sizes employed,
absorptions compared,
persistence of feature is part of
test
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m is the number of such events
one would expect to occur by
chance in the data set
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m<<1 are likely to be real events
Possible real features:
1 (a)
m = 0.032
2 (b)
m = 0.000008
3 (c)
m = 0.00056
6 (d)
m = 0.026
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
a
b
c d
Absorption
Features,
Compared to
Dust Jet
Locations
Plotted here are:
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Altitude above Enceladus' limb of the line-of-sight from Cassini to the star
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Attenuation of the HSP signal, scaled by a factor of 300
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Projections of the 8 jets seen by the ISS into the plane of the figure
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Jets assigned a length of 50 km (for purposes of illustration)
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C/A marks the closest approach of the line-of-sight to Enceladus.
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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
Gas Jets
Density in jets is twice the
background plume
Gas jet typical width = 10
km at 15 km altitude
Ingress
a. Cairo (V)
Feature Feature
Number Letter
m
Occultation Occultation
latitude
west longitude
Likely associated
dust jet
1
a
0.032
-79
82
Cairo (V)
2
b
0.000008
-86
112
Alexandria (IV)
3
c
0.00056
-86
192
Baghdad (VI)
6
d
0.026
-84
217
Damascus (II)
Egress
d. Damascus (II)
c. Baghdad (VI)
b. Alexandria (IV)
Closest point
• 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.
Gas Jet Model
• 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
Tilted Jets
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Opening angle of
plume derived from
0.65 ratio of thermal
to bulk velocity,
projected to altitude
of occ, includes offvertical tilt of B7 and
D3 jets
May not need
additional arbitrary
jets
Work in progress
because timing not
consistent with
previous plot
Water Column Density: FUV comparison to HSP
FUV integrations are
5 sec duration
FUV spectrum shows
gas absorption in time
records 89 and 90
Higher time
resolution of HSP
data shows that the
peak column density
is about 2x higher
FUV time record 89
FUV time record 90
Water column density: FUV
• Absorption is best fit by water vapor
• Best fit column density = 1.3 x 1016 cm-2
• Error bar: +/- 1.4 x 1015 cm-2
• Comparison to 2005 at 15 km
altitude
• 2007 peak column density
= 3.0 x 1016 cm-2
• 2005 = 1.6 x 1016 cm-2
• No detection of CO
• formal 2-σ upper limit is 3.6 x 1014 cm-2
• corresponds to mixing ratio with
H2O of 3.0
• Our nondetection of CO excludes 3% CO
in the plume at the 2 sigma level
I/I0
Comparison to tidal energy model
• Hurford et al 2007
model predicts tidallycontrolled differences in
eruption activity as a
function of where
Enceladus is in its
eccentric orbit
• Substantial changes
are not seen in the
occultation data,
although they would be
predicted, based on this
model
• Expect fissures to
open and close
Position of Enceladus
in its orbit at times of
stellar occultations
Taken from Hurford et al,
Nature 447:292 (2007)
True Anomaly
(deg)
Fraction of orbit
from Periapsis
Position in Orbit
Stress
105 Pa
0
0.0
Periapsis
0.3
90
0.25
One quarter
-0.8
97.76
0.27
July 14, 2005
-0.77
180
0.5
Apoapsis
-0.4
254.13
0.7
October 24, 2007
0.4
270
0.75
Three quarter
0.6
Compare 2007 to 2005 - HSP
2005 attenuation
<6% at 15 km
2007 attenuation at
same altitude ~10%
Overall attenuation
clearly higher in 2007
compared to 2005
The ratio of the
opacity from 16 to 22
km between 2007
and 2005 is 1.4 +/0.4.
Groundtrack of Ray
2005
2007
2005 HSP data
• HSP data can
be fit by an
exponential
• Look for
departures due
to jets
• Appear to see
real features
2005 Jets
• Jets mapped to
increases in opacity
• In this occ we do not
see B7 (star is occulted
by limb before crossing
B7)
• Is it OK to compare
2005 and 2007?
• IF individual jets are
only source of plume
then no
• If gas from entire tiger
stripe probably ok
Plume
or jets?
• Attempt to pick
comparable
“box” between
2005 and 2007
• But different
jets visible
means
comparing
apples and
oranges
Summary of Results
PLUME:
• H2O column density in 2007 = 1.3 x 1016 cm-2
• Density at 15 km altitude 2x higher
– H2O column density in 2007 ~ 3.0 x 1016 cm-2
• Attenuation in HSP data ~10% in 2007, ~6% in 2005
– Difference contradicts Hurford et al model of fissures opening and closing
• Plume column density goes as ~ z-2
• Water vapor flux ~200 kg/sec
• No detection of CO
(z is minimum rayheight)
Summary of Results
JETS:
• 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 stripes
• Jet opacity corresponds to vapor density doubled within jets
– Alternate 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 = 0.6 5
– Gas is supersonic
• Eight or more jets required to reproduce width and shape of absorption
• Jet source is approximately 300 m x 300 m
Example Calculations
T surface = 140 K
V thermal = 359 m/sec
V vertical = 552 m/sec
For Tsurface = 180 K (from CIRS)
V thermal = 406 m/sec
V vertical = 624 m/sec
Backup Slides
Gas vs. Ice
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For reasonable mass ratios of ice to gas in
the jets (fI = 1) ice has too little opacity (tau
= 0.001/grain radius) to be detected by
HSP. Radius in microns
If jets are unresolved by HSP or have
spread significantly in reaching altitude
z=15km, the surface pressure at the vent
could be correspondingly higher
Plume Model
• Monte Carlo simulation of test particles
given vertical + thermal velocity, particle
trajectories tracked under influence of
gravity and collisions (Tian et al, 2006)
• Original model had arbitrary source
spacing along the tiger stripes
• Model now adapted for specific locations
of the 8 dust jets identified by Spitale and
Porco, actual viewing geometry of these
sources as seen from the spacecraft
The results shown here have
T surface = 140 K
V thermal = 359 m/sec
V vertical = 552 m/sec
For
Tsurface = 180 K (from CIRS)
V thermal = 406 m/sec
V vertical = 624 m/sec
Optical Depth vs. Rayheight
Minimum distance of rayheight above limb = 15.6 km
S/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