The Composition and Structure
of Enceladus’ Plume from the
Solar Occultation
C. J. Hansen, L.
Esposito, D. Shemansky,
B. Lewis, A. I. F. Stewart,
J. Colwell, A. Hendrix, R.
West
January 2011
Outline
Using UVIS to observe occultations gives us
data on the composition and structure of the
gas flowing from Enceladus’ tiger stripe
fissures
• Plume Results
– Composition
– Mass flux
– Temporal variability
• Gas Jets
– Structure
– Mach number
Plume
Jets
UVIS Observations of Enceladus’ Plume
• UVIS observes occultations of stars and the sun to probe
Enceladus’ plume
• Three stellar and one solar occultation observed to-date
Zeta Orionis
• 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
2005 - gamma Orionis Occultation
The Occultation Collection
2007 - zeta Orionis Occultation
2010 - Solar Occultation
18 May 2010 - Solar Occultation
Two science objectives enabled by solar (rather than
stellar) occultation:
1. Composition of the plume
New wavelength range: EUV
H2O and N2 have diagnostic absorption
features at EUV wavelengths
The primary goal was to look for N2, on basis of
INMS detecting a species with amu=28
2. Structure of the jets and plume
Higher time resolution, better snr
Note: projected size of sun = 1.2 km
Solar Occ results – Composition
• H20 fit to
absorption
spectrum
• Column density
of H2O = 0.9 x
1016 cm-2
• No N2
absorption
feature -> N2
upper limit of 5
x 1013 cm-2
Nitrogen feature at 97.2 nm not detected
Actual
• No dip is seen at all at 97.2 nm
• Upper limit < 0.5%
Consequences of no N2 for
models of the interior
• High temperature liquid not
required for dissociation of NH3
(if there is NH3 in the plume)
• Percolation of H2O and NH3
through hot rock is not required
Predict
• N2 feature at 97.2 nm fortuitously
coincides with strong lyman gamma
emission so lots of signal available
• Very sensitive test!
• Clathrate decomposition is not
substantiated for N2 as the
plume propellant
Water Vapor Abundance
• To calculate water vapor abundance in the plume the spectra are
summed during the center 60 sec of the occultation, then divided
by a 650 sec average unocculted sum to compute I / I0
– I0 computed at two different times, results were the same
• The extinction spectrum is well-matched by a water vapor
spectrum with column density = 0.9 +/- 0.23 x 1016 cm-2
• Overall amount of water vapor is comparable to previous two
(stellar) occultations
– 2005: 1.6 x 1016 cm-2
– 2007: 1.5 x 1016 cm-2 (maximum value of 3.0 x 1016 cm-2 at center)
• Lower value in 2010 is at least partially attributable to the viewing
geometry – the flux is in family with the previous results
~Orthogonal Ground Tracks
Blue ground track is
from zeta Ori occ on
Rev 51
 Ingress
Orange is solar occ
track, ~orthogonal
Plume is elongated.
If total flux is same then
column density will be
less by ~ 2/3
Solar occ result:
0.9 x1016 cm-2, ~2/3 of
value in 2005
Basemap from Spitale & Porco, 2007
 Egress
Estimate of Water Flux from Enceladus =
200 kg/sec
S = flux
= N * x * y * vth
= (n/x) * x * y * vth
= n * y * vth
Where
N = number density / cm3
x * y = area
y = vlos * t => FWHM
vth = thermal velocity = 45,000 cm/sec
for T = 170K
n = column density measured by UVIS
note that escape velocity = 23,000 cm/sec
v
x
Year
n
(cm-2)
y
(x 105 cm)
vth
(cm /
sec)
Flux:
Molecules /
sec
Flux:
Kg/sec
2005
1.6 x 1016
80 (est.)
45000
5.8 x 1027
170
2007
1.5 x 1016
110
45000
7.4 x 1027
220
2010
0.9 x 1016
150
45000
6 x 1027
180
Solar Occ Jet Identifications
f
a
e
b
c
d
Minimum altitude
• Window 0 and 1 matching features => jets
• Repetition of features in window 0 and window 1 shows
they are not due to shot noise, therefore likely to be real
Jets vs. Tiger Stripes
• As before, gas jets appear
to correlate to dust jets
Feature Altitude*
(km)
Dust
Jet
a
20
Alexandria
IV
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
Spacecraft viewed
sun from this side
Ingress

Minimum
Altitude
Egress
Basemap from Spitale & Porco, 2007
Jet Structure
Optical Depth
• Higher snr enables better measurements of jets’ dimensions – more
clearly distinguished from background plume
• Density of gas in jets > 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 to the jets’ total equivalent width
Gas Velocity
• 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
• Previously estimated mach number (from 2007 occultation) was 1.5
• Jets more collimated than previously estimated
• New estimate for vertical velocity: if vsound = 320 m/sec (for ~170 K)
then vvert = 1920 m/sec
• This is an upper limit because the gas will be cooled in a nozzle
• Water vapor flux in the jets = 20-40 kg/sec
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%
Summary
• Composition
– Upper limit on N2 of 5 x 1013 cm-2
– H20 column density = 0.9 x 1016 cm-2
• In family with previous occultations
• Suggests that Enceladus has been steadily erupting for
past 5 years
• Plume / jet structure
– Flux of water from 3 occultations is ~200 kg/sec
– Jets are more collimated than estimated from
2007 occultation
• Mach numbers of 5 to 8
Supersonic gas jets are consistent with Schmidt et al.
model of nozzle-accelerated gas coming from liquid
water reservoir
Backup Info
Occultation is clearly visible
•
Window 0 has higher counts, but overall shape is the same
– Position of sun was slightly offset from center, but not an issue
•
•
•
•
•
•
Observation start time: 2010-138T05:51:44.45
Observation end time: 2010-138T06:10:36.45
Ingress: 2010-138T06:00:40.45
Egress: 2010-138T06:02:59.45
Velocity of sun across plane of sky ~ 2.75 km/sec
Data shown is summed over wavelength
Jet Identities
Feature
Altitude*
(km)
Dust Jet
a
20
Alexandria IV
Closest
approach
19.7
b
21
Cairo V and/or
VIII
c
d
27
30
Baghdad I
e
f
38
46
Damascus III
Baghdad VII
Damascus II
* Altitude is relative to limb of Enceladus
Groundtrack
of Occultation
Enhanced HSP
absorption features a,
b, c, and d can be
mapped to dust jets
(roman numerals)
located by Spitale
and Porco (2007)
along the tiger stripes
• Blue line is groundtrack
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 vertical velocity to thermal velocity in jet = 1.5
– 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
Plume Composition and Column Density
UVIS Ultraviolet Spectra provide constraints on:
• Composition, from
absorption features
• Column density
– Mass Flux
• Plume and jet structure
Plume
Terminology:
Jets
• Plume - large body of gas
and particles
• Jets - individual
collimated streams of gas
and particles
2005 - gamma Orionis
Occultation
2007 - Zeta Orionis
Occultation
(Bellatrix)
(Alnitak)
Key results:
•
Dominant composition = water vapor
•
Plume column density = 1.6 x 1016 /cm2
•
Water vapor flux ~ 180 kg/sec
•
Results documented in Science, 2006
Key results:
• Average column density = 1.5 x 1016
cm-2
• Max column density = 3.0 x 1016 cm-2
• Gas jets detected correspond to dust
jets
• Results documented in Nature, 2008
Vertical cut through plume
Horizontal density profile
18 May 2010 - Solar Occultation
Two science objectives enabled by solar (rather than stellar)
occultation:
1. Composition of the plume
New wavelength range: EUV
2. Structure of the jets and plume
Higher time resolution
Outline
• Plume Results
– Composition
– Mass flux
– Temporal variability
• Gas Jets
– Structure
– Mach number
New EUV Spectrum from Solar
Occultation
Navy is unocculted solar spectrum, with typical solar emissions
Red is solar spectrum attenuated by Enceladus’ plume
Outline
• UVIS Observations
– Occultations
– Instrument
• Plume Results
– Composition
– Mass flux
– Temporal variability
• Gas Jets
– Structure
– Mach number
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
Solar Occultation Characteristics
Total duration of Solar Occ:
2min 19sec
Duration for full-width half max:
56 sec
FWHM
Line of sight velocity: 2.85 km/sec
Width of plume at FWHM:
56 sec * 2.85 = 160 km
Compare to zeta Orionis Occ
– Zeta Orionis occultation lasted just 10
sec
– Line of sight velocity = 22.5 km/sec
– Width of plume at FWHM = 110 km
– HSP data summed to 200 msec so 50
samples
Zeta Orionis occultation
UVIS Characteristics
UVIS has 4 separate channels
For stellar occultations we use:
•
Far UltraViolet (FUV)
– 1115 to 1915 Å
– 2D detector: 1024 spectral x 64 onemrad spatial pixels
• Binned to 512 spectral elements
– 5 sec integration time
•
High Speed Photometer (HSP)
– 2 or 8 msec time resolution
– Sensitive to 1140 to 1915 Å
•
For the solar occultation we used:
Hydrogen-Deuterium Absorption Cell
(HDAC) not used
• Extreme UltraViolet (EUV) solar port
• 550 to 1100 Å
• 2D detector: 1024 spectral x 64 one-mrad spatial pixels
• No spatial information because signal from sun is spread across the detector
(deliberately)
• Spatial rows 5 - 58 binned to two windows of 27 rows each
• 1 sec integration