Enceladus Report
C. J. Hansen
January 2013
Deriving the Structure and
Composition of Enceladus’ Plume
from Cassini Occultation
Observations
C. J. Hansen, L. W. Esposito, J.
Colwell, A. Hendrix, D. Shemansky,
I. A. F. Stewart, R. A. West
AGU
December 2012
Gave talk at AGU
2011 Dual Occultation
• Eps Orionis (Alnilam, B star)
– 16.5 km at closest point
– HSP centered on eps Ori
– Dimmer star in uv by ~2x
• Zeta Orionis (Alnitak, O star)
– 37.9 km at closest point
Updated Water Vapor Column Density Dual Occ
Eps Orionis I/I0
Best fit is 1.35 x 1016 cm-2
• Ratio of occulted signal to
unocculted signal: I/I0
• From average of data records above
FWHM
• Compare to water vapor
– Cross-sections from Mota, 2005
– Same as we used for 2007 zeta
Orionis occ
– Best fit based on minimizing the sum of
the squares of the differences between
the actual occ and water for a given
column density, per discussion at our
last team meeting
Zeta Orionis I/I0
Best fit is 1.2 x 1016 cm-2
Estimate of Water Source Rate from Enceladus =
200 kg/sec
S = flux (source rate)
= N * x * y * vth
= (n/x) * x * y * vth
= n * y * vth
Where
N = number density / cm3
2011:
x * y = area
vlos =
y = vlos * t at FWHM
vth = thermal velocity = 45,000 cm/sec for T = 170K
(note that escape velocity = 24,000 cm/sec)
n = column density measured by UVIS
7.48 km/sec
y
x
The source rate has not changed much in >6 years
v
(deviation is <15%, not factors of 2)
Year
n
(cm-2)
Uncertainty
+/-
y
(x 105
cm)
vth
(cm /
sec)
Flux:
Flux:
Molecules Kg/sec
/ sec
Fraction
of orbit
from
periapsis
2005
1.6 x 1016
0.15 x 1016
80 (est.)
45000
5.8 x 1027
170
0.27
2007
1.5 x 1016
0.14 x 1016
110
45000
7.4 x 1027
220
0.70
2010
0.9 x 1016
0.23 x 1016
150
45000
6 x 1027
180
0.19
2011 - e
1.35 x 1016 0.15 x 1016
120
45000
7.3 x 1027
220
0.70
2011 - z
1.2 x 1016
135
45000
7.3 x 1027
220
0.2 x 1016
However
• There are lots of high phase angle observations
designed for ISS dust jet observations
• VIMS has been riding along – get integrated
brightness of the plume particles
• Matt Hedman has been analyzing this VIMS data
– First, derived a phase function to normalize lots
observations at different phase angles
– Then plotted brightness as a function of orbital phase
• There is an orbital dependence in the VIMS
results
• Brightness peaks at ~180 true anomaly (a rather
narrow peak, not a sine wave)
Comparison to tidal energy model
• VIMS now reporting
that they see substantial
brightening at high
phase when Enceladus
is at a true anomaly of
180 deg
• Hurford et al 2007
model predicts tidallycontrolled differences in
eruption activity as a
function of where
Enceladus is in its
eccentric orbit
• Expect fissures to
open and close
Position of Enceladus
in its orbit at times of
stellar occultations,
and solar occultation
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
0.186
May 18, 2010
90
0.25
One quarter
-0.8
97.76
0.27
July 14, 2005
-0.77
0.5
Apoapsis
-0.4
254.13
0.7
2007 and 2011
0.4
270
0.75
Three quarter
0.6
180
170
116
180
• Maybe we haven’t
observed an occultation
at the right place???
Source rate
Kg/sec
220
220, 220
All Horizontal Cuts
Basemap from Spitale & Porco, 2007
Zeta Ori
2011
Solar occ
In all occultations we
look through the plume
and jets
The groundtrack is the
perpendicular dropped
to the surface from the
ray to the star
• Blue => zeta Orionis 2007
• Red => Solar occ 2010
• Green => zeta Orionis 2011
Zeta Ori
2007
Altitude (km)
Altitude of Ray
70
60
50
40
30
20
10
0
eps Ori
Zeta Ori
1
3
5
7
9 11
Sec after 640
13
Eps – Zeta
Comparison
15
• Two second integration time
• Summed FUV over wavelength
• Difference in altitude ~20.5 km
a
• Zoom in on occultation time
• Zeta Orionis shifted 4 sec
• Should only be shifted 3.8 sec
B
c
d
Eps – Zeta
Comparison to
Dust Jets
a
S/C
B
c
d
• Clear signal of gas from Baghdad
fissure (B), though no dust jet nearby
• New Gas Jet
• Damascus jets (DII and DIII): “c”, and
BI detected: “d”
• DIII differentiable from BI jet at 18 km,
not at 40 km
• Weak feature at “a” is not located at a
published dust jet, but ISS and CIRS
have reported enhanced activity here?
No ramp
h
1
h
2
B
h
7
h h
4 5
h
6
HSP - FUV
• Geometry of groundtrack so closely
paralleling the Baghdad tiger stripe
means that jets are not well separated
from gas release all along the fissure
• Why is there more absorption in the
HSP?
• Level background for HSP replaced
by a ramp, but still see more absorption
in the HSP than in the FUV
HSP
Computed ramp to use for background
• Did not make much difference
• Still see much more absorption in
HSP than in FUV
HSP with Ramp
Ramp background
Straight-line background
•
•
•
Computed background for HSP with ramp instead of straight line
Somewhat better match (blue compared to red) but didn’t make much
difference
Why is HSP absorption more than FUV?
Future Tasks
• Need better water cross-sections to make
progress on composition
• Need ISS jet locations to make progress
on jets
• Need occultation at true anomaly = 1800 to
look for orbit-related changes in source
rate
• Write CDAP for jet structure model
• Write Icarus paper summarizing results
from all occs
Comparison to INMS results from E7
• Highly collimated jets are consistent with INMS detection of enhanced
gas streams at higher altitude
• E7 INMS groundtrack at altitude of ~91 km (c/a) compared to UVIS solar
occultation profile at altitude ~20 km (c/a)
• INMS and UVIS both detect Alexandria and Baghdad gas jets
Thanks to Ben Teolis, Brian Magee, and Hunter Waite for providing these plots, in our GRL 2011 paper
Supersonic Gas Jets and the E Ring
• The UVIS detection of supersonic jets compares very favorably
with CDA results
– The full width half max (FWHM) of jet c (Baghdad I) is ~10 km at a jet
intercept altitude of 29 km (z0)
– Estimating the mach number as ~2 z0/FWHM the gas in jet c is moving at a
Mach number of 6; estimates for the other jets range from 5 to 8
• Supersonic gas jets are consistent with Schmidt et al.
(2008)model of nozzle-accelerated gas coming from liquid water
reservoir
• Condensation of water molecules in the high velocity gas jets
could produce small salt-poor grains (Postberg et al., 2011)
detected by CDA
– Particle size 0.2 to 0.6 microns
– Compositional partitioning
High velocity gas streams propel
smallest particles out to become
Saturn’s E ring
Cassini ISS
image
PIA08321
What have we learned from
occultations?
• Composition
– The plume is primarily composed of water vapor
– Upper limits have been set for CO, N2, C2H4
• Source rate
– Flux of water is ~200 kg/sec
• Range is from 170 kg/sec to 220 kg/sec
• Suggests that Enceladus has been steadily erupting for past 7 years
• Enough to explain all the water products observed in the Saturn system
(H, O, OH)
• Plume / supersonic jet structure
– Collimated gas jets are detected with estimated mach number of > 5
– Propel small ice particles out to become Saturn’s E ring
2005 - gamma Orionis Occultation
The Occultation Collection
2007 - zeta Orionis Occultation
2010 - Solar Occultation
2010 Solar Occultation
• Still best data set for
studying individual jets
• High SNR
• Geometry gave us wellseparated jets
• Clear separation allowed us
to calculate spreading, and
derive a mach number >5
Spacecraft viewed
sun from this side
Ingress

Minimum
Altitude
Egress
Basemap from Spitale & Porco, 2007
Introduction
In 2005 we detected a
plume of water vapor
coming from Enceladus
with a UVIS observation
of a stellar occultation
UVIS has also detected
collimated gas jets within
the broad plume
Plume
Jets
Over the ensuing 6 years
we’ve observed other
occultations – but no
more are planned
I’ll summarize the latest
(last) data in the context
of the entire occultation
dataset
Shown in this Cassini ISS image are small
particles – UVIS detects gas