Enceladus: UVIS Constraints
and Modeling
C. J. Hansen, L. Esposito, D.
Shemansky, I. Stewart, A. Hendrix
19 July 2010
Outline
All plume models must ultimately be consistent
with data
• UVIS Observations
– What we observe: Occultations
– How we observe: Instrument
• 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
– Composition, mass flux, and plume and jet structure
• Three 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
2005 - gamma Orionis Occultation
The Occultation Collection
2007 - zeta Orionis Occultation
2010 - Solar 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
Outline
• UVIS Observations
– Occultations
– Instrument
• Plume Results
– Composition
– Mass flux
– Temporal variability
• Gas Jets
– Structure
– Mach number
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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
Plume: FUV Results from Stellar Occ’s
• Absorption is best fit by water vapor
• 2007 best fit average column density =
1.5 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
I/I0
• 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
New EUV Spectrum from Solar
Occultation
Navy is unocculted solar spectrum, with typical solar emissions
Red is solar spectrum attenuated by Enceladus’ plume
Composition
• 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
• No N2 (upper limit = 3 x 1013, so < 0.3%)
– AMU = 28 detected by INMS is not N2
– It is not CO in the plume (or UVIS would have seen it in our
stellar occs) or it is < 3%
– Maybe very little signal at amu=28 to worry about after all
per latest INMS results (personal communication, Waite
2010)
• N2
< 0.9%
• CO
< 0.9%
• C2H4 < 0.16%
Nitrogen feature at 97.2 nm not detected
Actual
• No dip is seen at all at 97.2 nm
• Upper limit < 0.3%
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
• A catalyst for decomposition is not
required at lower temperatures
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
Solar Occ
results: Water in
the Plume
• H20 fit to
absorption
spectrum
• Column density
of H2O = 0.9 x
1016 cm-2
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 +/- 0.1 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 may be at least partially attributable to the
viewing geometry
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
Preliminary result:
0.9 x1016 cm-2, ~2/3
 Egress
Water Vapor and other Gases
• Some evidence
that the mixing
ratio of water may
change
depending on
where you are in
the plume
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
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
160
45000
6.5 x 1027
170
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
• 2010 column density
very similar to 2005
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
October 24, 2007
0.4
270
0.75
Three quarter
0.6
Outline
• UVIS Observations
– Occultations
– Instrument
• Plume Results
– Composition
– Mass flux
– Temporal variability
• Gas Jets
– Structure
– Mach number
Detection of Gas Jets
• Perfect geometry to get a horizontal cut through the plume and detect density
variations indicative of gas jets
Zeta Orionis
Occultation
Density in jets is twice the
background plume
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)
Gas jet typical width = 10
km at 15 km altitude
Ingress
a. Cairo (V)
b. Alexandria
(IV)
Egress
d. Damascus (II)
c. Baghdad
(VI)
Closest point
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 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
correlate to dust jets
Spacecraft viewed
sun from this side
• Higher time resolution
because sun’s passage
behind the plume was
slower
Ingress

• We see gas between jets,
although no enhancement
as a function of depth in
the plume (time at which
sun was closest to limb is
not a deep attenuation)
Egress
Minimum
Altitude
Jet Identities
Feature
Altitude
(km)
Dust Jet
a
20
19.7
Alexandria IV
b
21
Cairo V and/or
VIII
c
d
e
f
27
30
38
46
Baghdad I
Closest
approach
Baghdad VII
Damascus III
Damascus II
Gas Velocity
• In the case of jet c (Baghdad I) the source of the jet is close
enough to the occultation ground track to calculate the height
and width
• Half-width of jet c = 2.5 sec * 2.85 km/sec = ~7 km (w) at ~27
km (h) altitude
w
w/h = tan ß = vthermal / vvert = 7 / 27
h
Mach number = vvert / vthermal = 3.9
• Previously estimated mach number (from 2007 occultation) was 1.5
• Jets more collimated than previously estimated
• New estimate for vertical velocity: if vthermal = 450 m/sec (for ~170 K)
then vvert = 1755 m/sec
Our maturing understanding…
• Composition
– Upper limit on N2 of 3 x 1013 cm-2
– H20 column density = 0.9 x 1016 cm-2
• In family with previous occultations
• Precisely corresponds to aspect ratio applied to 2005,
which was at a similar true anomaly
• 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 -> higher mach number of ~3.9
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
Altitude of Sun above Limb
f
e
a
c d
b
• Perpendicular distance of ray connecting UVIS and the sun from
the limb of Enceladus
• Attenuation at a and b bracket closest approach
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
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
a
b
c d
Gas 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
• 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.
• The two deepest absorptions
indicate jets are collimated, 10
km wide at 15 km altitude
Key Result:
Vvert / Vthermal = 1.5
Flow is supersonic
Groundtrack of Ray
2005
2007
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
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
Summary of Results
PLUME:
•
H2O column density in 2007 = 1.5 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, ethylene not definitive
(z is minimum rayheight)
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
Water Column Density: FUV comparison to HSP
2007
FUV integrations are 5
sec duration
FUV spectrum shows
gas absorption in Rev
51 time records 89 and
90
Higher time resolution of
HSP data shows that the
peak column density is
about 2x higher than the
5 sec average
calculated from the FUV
data
FUV time record 89
FUV time record 90
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
(Bellatrix)
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
Documented in Science, Hansen et al, 2006
Zeta Orionis Occultation 2007
(Alnitak)
Key results:
• Average plume column density = 1.5 x 1016 cm-2
• Maximum plume column density = 3.0 x 1016 cm-2
• Gas jets detected correspond to dust jets
Horizontal density profile
True anomaly = 254
Results documented in Nature, 2008
Rev 11 Gamma Orionis Occultation, FUV data
UVIS spectra, occulted and unocculted
Plot I/I0 to see absorption features
•
Compare I/I0 to water
absorption spectrum
•
Water vapor: uses Mota
cross-sections
Best fit column density = 1.6 x
1016 cm-2
•