Enceladus Dual Star
Occultation
C. J. Hansen
6 January 2012
The Structure of Enceladus’ Plume from
Cassini Occultation Observations
Gave AGU
Talk, Dec.
2011
C. J. Hansen, L. W. Esposito, B. Buffington, J. Colwell, A. Hendrix, B. Meinke,
D. Shemansky, I. A. F. Stewart, R. A. West
UVIS Observations of Enceladus’ Plume
• Cassini’s Ultraviolet Imaging Spectrograph (UVIS) observes
occultations of stars and the sun to probe Enceladus’ plume
– Composition, mass flux, and plume and jet structure
• Four stellar and one solar occultation observed to-date
• Feb. 2005 - lambda Sco
• No detection (equatorial)
• July 2005 - gamma Orionis
• Composition, mass flux
• Oct. 2007 - zeta Orionis
• Gas jets
• May 2010 - Sun
• Composition, jets
• Oct. 2011 – epsilon and zeta
Orionis dual occultation
2005 - gamma Orionis Occultation
The Occultation Collection
2007 - zeta Orionis Occultation
2011 occ was
a horizontal
cut through
the plume also
2010 - Solar Occultation
Orion’s Belt Dual Occultation Geometry
Rev 155
• Dual stellar occultation by Enceladus’ plume, E15, 19 October
2011, of epsilon Orionis (blue) and zeta Orionis (white)
• Horizontal cut through plume
Dual Occultation
• Eps Ori (Alnilam, B star)
– 16.5 km at closest point
– HSP centered on eps Ori
– Dimmer star in uv by ~2x
• Zeta Ori (Alnitak, O star)
– 37.9 km at closest point
The Plume: Water Vapor Column Density
zeta Orionis
• 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 at H2O column density of
1.35 x 1016 cm-2
Larry got 1.3 x 1016 cm-2
Don’s plot of column depth as a function of
time: Deepest absorption just after closest
approach Similar situation in 2007, 2x
difference
The Plume: Water Vapor Column Density
eps Orionis
• Same technique by CJH gave
column density of 1.6 x 1016 cm-2
• But Larry calculated column
density of 1.4 x 1016 cm-2
• I think my value is too high, redoing this is on my to-do list…
As a function of time:
Deepest absorption just after
closest approach
All Groundtracks
Basemap from Spitale & Porco, 2007
Zeta Ori
2011
Solar occ
In all occultations we
look through the plume
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
Estimate of Water Source Rate from Enceladus =
200 kg/sec
S = flux
= N * x * y * vth
= (n/x) * x * y * vth
= n * y * vth
Where
N = number density / cm3
2011:
x * y = area
vlos = 7.48 km/sec
y = vlos * t at FWHM
vth = thermal velocity = 45,000 cm/sec
for T = 170K
n = column density measured by UVIS
Year
n
(cm-2)
Uncertainty
+/-
y
(x 105
cm)
y
x
v
vth
(cm /
sec)
Flux:
Molecule
s / sec
Flux:
Kg/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 1.35 x 1016
(prelim)
45000
8.2 x 1027
240
0.70
135
One more 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
• Expect fissures
to open and close
• Substantial changes
are not seen in the
occultation data,
although they would
be predicted, based
on this model
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
180
0.5
Apoapsis
-0.4
254.13
0.7
2007 and 2011
0.4
270
0.75
Three quarter
0.6
Source rate
Kg/sec
180
170
220, 240
Caveats
• How constant is the source rate?
• Is the source rate modulated by the position of Enceladus in its orbit?
From Larry:
• The previous derivations use slightly different approaches to find the column
density N:
2005 gOri: Last FUV spectrum measured, closest to surface. Best fit for N.
Use scale height H to estimate L.
2007 zOri: Two 5-second FUV spectra that span the entire occultation. Best
fit for N.
2010 Solar: 42 seconds of summed EUV spectra, covering the FWHM of the
occultation. Best fit for N.
2011 eOri, zOri: Mean of photometric analysis (match total attenuation with
H2O alone) of spectra within FWHM. This resembles DES approach for
Solar occ. To get the mass flux, multiply by the mass of H2O molecule and
FWHM. This gives
zOri: 1.3 E16 * 134km (9 spectra) = 236kg/sec
eOri: 1.4 E16 * 120km (8 spectra) = 227kg/sec
• Given the variety of approaches, the different channels, IP and occultation
tracks, it may be fortuitous that all 5 UVIS results give
208 ± 28 kg/sec. That is, a constant flux with standard deviation of 15%.
“Plume” refers to
the broad cloud of
dust and gas
emanating from
the south pole of
Enceladus
Nomenclature
Plume
“Jets” are the
highly collimated
streams of ice
particles (detected
by ISS) and gas
Jets
UVIS observes
the gas
component
2007
The Jets – Past Occs
2010
a
b
c
e
d
f
• In the past we have identified collimated jets of gas from
enhanced absorption features in the HSP (2007 zeta Ori occ)
and the EUV (2010 solar occ)
• Features in the 2007 HSP data were validated by Bonnie
Meinke using her F ring statistical test techniques
• Features in the 2010 solar occ were identified by looking for
matching absorptions in the two windows, and making the
argument that it was unlikely to be shot noise if they matched
Solar Occ
• Geometry gave us wellseparated jets
Spacecraft viewed
sun from this side
Ingress

Minimum
Altitude
Egress
Basemap from Spitale & Porco, 2007
The Jets – 2011 HSP Data
• This time the HSP data was lower snr
Plot note: elapsed times are incorrect
– Eps Ori instead of zeta Ori
– no features passed the rigorous statistical tests applied
• Rely on FUV data, cross-correlation of absorptions in same
place / shifted in time
Data
Bonnie’s Analysis
Visual inspection
• Human eye is good at picking out
features
Data
Data
Only one
time
integration
Interesting wide
hole
Binned Data
N
Y
?
?
?
Optical Depth
M Values
No Significant Features
From Bonnie:
• Smallest m values are on the order of a few
• Optical depths are below 0.4
• Nothing passes more stringent tests of repeated
significance
• NOTE:
– Attenuation in plume ~5%, was ~10% last time
– Perhaps this is more diffuse overall compared to 2008
zet Ori occ’n
• Bonnie’s conclusions were verified by Bob West
with a different technique
FUV Data
• Two sec integrations
– Data is summed over all wavelengths, all
spatial pixels
• Zeta Ori trails eps Ori by ~4 sec
Groundtracks 2011
• Eps Ori is blue, zeta
Ori is green
• Three matching
features:
– “Split end” on Baghdad
fissure
– Crossing Baghdad
fissure
– Damascus jets
• In addition, eps Ori
likely distinguishes
Baghdad I
S/C
Eps and Zeta Orionis Comparison
• Signal of gas from
Baghdad fissure (B-f),
though no dust jet nearby
• Damascus jets (DII and
DIII) and BI identified
BVII?
• Feature at BVII?
?
B-f
DII&III
BI
•
•
•
• Weak feature at “?” is
not located at a published
dust jet, but ISS and
CIRS have reported
enhanced activity here
Average computed for each star
Then ratios computed for each
Time shifted to align enhanced absorption feature at B-f because geometry clearly
correlated with fissure-crossing (4 sec)
Direct
Comparison
S/C
?
Bf
DII&III
BI
• Clear correlation at fissurecrossing
• Slow return to unocculted
signal may be activity
between BI and BVII
HSP
• Now compare HSP (targeted to
eps Ori) to FUV for eps Ori
• 0.008 sec integration summed
to 2 sec to match FUV (plot time
ok now)
• Although features did not pass
our statistical tests we can
compare to the FUV data set
• Pretty good agreement with eps
Ori – more work to do on this to
resolve timing issues
• Then, bootstrap back to lesssummed HSP data to get better
time resolution
Summary
• Mass flux determined, comparable to other occs
– Work to do to better quantify uncertainties
• Jets tougher to identify because of low snr
– Features in HSP data did not pass statistical tests
– Geometry of occ along rather than across fissures may also have an
effect?
• Determination of spreading at the two altitudes also limited by
temporal resolution of the FUV (2 sec integration time)
– 2 sec x 7.48 km/sec line-of-sight velocity = 15 km
– That is the approx. width of the jets derived in earlier occultations
• Work in progress to better constrain jet structure
– HSP bootstrap from FUV
Back-up
Jets vs. Tiger Stripes
• As before, gas jets appear
to correlate to dust jets
Spacecraft viewed
sun from this side
Feature Altitude Dust
* (km)
Jet

Alexandria
IV
a
20
Closest
approach
19.7
b
21
Cairo V
and/or VIII
c
27
Baghdad I
d
30
Baghdad VII
e
38
Damascus
III
f
46
Damascus II
* Altitude of ray to sun from limb
Ingress
Minimum
Altitude
Egress
Basemap from Spitale & Porco, 2007
2007 - Plume Structure and Jets
Summary of 2007 results
•
•
•
•
•
Significant events are likely gas jets
UVIS-observed gas jets correlate with dust jets in images
Characterize jet widths, opacity, density
Density in jets ~2x density in background plume
Ratio of vertical velocity to bulk velocity = 1.5, supersonic
Supersonic gas jets
are consistent with
Schmidt et al. model of
nozzle-accelerated
gas coming from liquid
water reservoir
Jet Structure
Optical Depth
• Higher SNR enables better measurements of jets’ dimensions –
more clearly distinguished from background plume
• Density of gas in jets is twice the density of the background plume
• The jets contribute 3.4% of the molecules escaping from
Enceladus, based on comparison of the equivalent width of the
broad plume compared to the jets’ total equivalent width
Solar Occultation Jets
Comparison to INMS results from E7
Jet Properties
Feature
Altitude
of ray
relative
to limb
Z0:
Altitude
of ray
relative
to jet
source
FWHM:
full
width
half
max(km
)
Mach
number
~ 2 * Z0 /
FWHM
Associat
ed Dust
Jet
Excess
attenuation at
the jet
(%) –
for
density
calc*
a
21.3
21.6
7
6
Alexandria IV
27
Closest
approach
20.7
b
22
24
9
5
Cairo V and/or
VIII
17
c
28.4
29
10
6
Baghdad I
19
d
31.2
36
10
7
Baghdad VII
12
e
39
40
10
8
Damascus III
13
f
47.5
49.7
14
7
Damascus II
14
*Average attenuation =17%
CJH To Do List
1. Re-do calculation of eps Ori column density
2. Really rigorous determination of error bars for every
occultation we have observed
3. Re-do summed FUV vs. time for just spatial pixels 24, calibrated properly
4. Plot HSP with eps Ori – now that timing issues are
resolved!
5. Then go back to higher time resolution HSP data