Enceladus Plume Update
C. J. Hansen, I. Stewart, L.
Esposito, A. Hendrix
June 2009
Work-in-Progress
• Plume Composition
• Individual jets vs. gas release along
tiger stripe
Background
• INMS detects a species with atomic
mass = 28
• Previously thought that this must be CO
or N2
• New idea (consistent with other INMS
data) is that it could be ethylene
– C2H4 = 2 * 12 + 4 = 28
Ethylene Absorption Cross-sections
in FUV
• Don supplied absorption cross-sections for C2H4 from
Wu, 06/2004, data acquired at T = 150K
Water only, Rev 11 Best Fit
•
•
•
Rev 11 gamma Orionis occultation
Water only, uses Mota cross-sections
Column density = 1.6 x 1016 cm-2
Ethylene-only at 10% H2O Column Density
This is just for
illustration,
not really this
much
ethylene!
•
•
•
Rev 11 gamma Orionis occultation
Ethylene only
Column density = 1.6 x 1015 cm-2
Ethylene + Water at 3% H2O Column
Density
•
Rev 11 gamma
Orionis occultation
•
Ethylene plus water
•
C2H4 column density
= 4.8 x 1014 cm-2
•
H2O column density
= 1.6 x 1016 cm-2
Ethylene at 3% H2O Column Density
compared to H2O only
•
Rev 11 gamma Orionis
occultation
•
Ethylene plus water
compared to water only
•
C2H4 column density = 4.8
x 1014 cm-2
•
H2O column density = 1.6
x 1016 cm-2
•
Water only is still best fit to
occulted spectrum
although there are some
interesting matches to
small dips with ethylene
added in
Summary and Future Work
• Data has too much scatter to definitively say
yes or no to presence of ethylene
• Star does drift during the course of the
occultation - can we pull out a better I0 with
more careful selection of the records?
•
• Need to run Ian’s regression routines to
quantitatively compare the fit of water-only vs.
water + ethylene
FUV analysis
Occ is easy to detect
Star drifted from pixel 13 to pixel
12 over the course of the
observation
Methanol
• Not likely to be detectable, doesn’t look like a
good fit anyway…
Individual jets vs. broad
outgassing
• Debate about whether gas was coming
primarily from jets or all along tiger
stripes
• Implications for being able to compare
the 2005 occ to the 2007 occ
Zeta Orionis Occultation 2007
FUV and HSP data collected
FUV: 5 sec integration
HSP: 2 msec sampling
2007 - zeta Orionis
Horizontal density profile
True anomaly = 254
2005 - gamma Orionis
Vertical cut through plume
True anomaly = 98
Key results:
•
•
•
Dominant composition = water vapor
Plume column density = 1.6 x 1016 /cm2
Water vapor flux ~ 180 kg/sec
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
Groundtrack
of Occultation
• Blue line is
groundtrack
• Roman numerals
correspond to
ISS dust jet
sources
Gas Jets
Density in jets is twice the
background plume
Gas jet typical width = 10
km at 15 km altitude
Ingress
a. Cairo (V)
b. Alexandria
(IV)
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)
Closest point
a
b
c d
Absorption
Features,
Compared to
Dust Jet
Locations
Plotted here are:
•
Altitude above Enceladus' limb of the line-of-sight from Cassini to the star
•
Attenuation of the HSP signal, scaled by a factor of 300
•
Projections of the 8 jets seen by the ISS into the plane of the figure
•
Jets assigned a length of 50 km (for purposes of illustration)
•
C/A marks the closest approach of the line-of-sight to Enceladus.
•
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
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
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.
2007 Plume Simulation
• Ian has modeled plume
water vapor
• He now agrees that gas
needs to come from along
tiger stripe, not just jets
Backup slides
• 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
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
High Speed Photometer (HSP) Data
•
HSP is sensitive to 1140 to 1900Å
•
Statistical analysis applied to find
features that are probably real
– Assumes signal is Poisson
distribution
– Calculate running mean
•
Six different bin sizes employed,
absorptions compared,
persistence of feature is part of
test
•
m is the number of such events
one would expect to occur by
chance in the data set
•
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
Rev 3
•
CIRS_003EN_FP1FP3MAP
001 (CIRS is prime)
•
•
2005 048T00:15
UVIS observation:
UVIS_003EN_ICYLON004_
CIRS
•
•
•
Range: 79177 km
Subs/c lat: 0
Subs/c lon: 299
•
These values are for the
start times of the
observation
•
CIRS IFOV is shown
Rev 11
•
CIRS_011EN_FP3GLOBAL020
•
•
2005 195T13:54
UVIS observation:
UVIS_011EN_ICYLON003_CIR
S
•
•
•
Range: 198,220 km
Subs/c latitude: -37
Subs/c longitude: 144
•
CIRS IFOV shown
Rev 11
• CIRS_011EN_FP3REGION
021
• 2005 195T15:24
• UVIS observation:
UVIS_011EN_ICYLON006
• Range: 141,252 km
• Subs/c latitude: -41
• Subs/c longitude: 158
• CIRS IFOV
Rev 61
•
CIRS_061EN_FP34MAP001
•
2008 072T16:36
•
UVIS observation:
UVIS_061EN_ICYLON003_
CIRS
•
•
•
Range: 126,677 km
Subs/c latitude: 69
Subs/c longitude: 112
This is CIRS IFOV, so our slit will extend well off limb,
but probably too far north
Start
Rev 61
• UVIS
061EN_ICYMAP002_PR
IME (our observation)
• 2008 072T17:40
• Range: 73845 km
• Subs/c latitude: 69
• Subs/c longitude: 121
• Too far north
End
Rev 120 - November 2009
• CIRS is driver
• 2009 306T08:03
• UVIS observation:
UVIS_120EN_ICYMAP00
2
• Range: 9473 km
• Subs/c latitude: -1
• Subs/c longitude: 163
• Note that the observation
following this one is our
observation of the plume
with Saturn in the
background
Enigmatic Enceladus
High density dust jets
Are there corresponding high density gas
streams?
Future Plans
• Regression analysis
Ethylene at 10% H2O Column Density,
reduced by ~10%
•
Rev 11 gamma
Orionis occultation
•
Ethylene plus water
•
C2H4 column density
= 1.6 x 1015 cm-2
•
H2O column density
= 1.5 x 1016 cm-2
Ethylene at 10% H2O Column Density
•
Rev 11 gamma
Orionis occultation
•
Ethylene plus water
•
C2H4 column density
= 1.6 x 1015 cm-2
•
H2O column density
= 1.6 x 1016 cm-2