Transcript Slide 1

Enceladus water jet models from UVIS
star occultations
2 April 2013
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
• 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 – eps and zeta
Orionis dual occultation
Plume Composition is Water Vapor
I=I0 exp (-n* )
I0 computed from
25 unocculted
samples
n = column
density
= absorption
cross-section,
function of
wavelength
The absorption spectrum of water is shown compared to Enceladus’ plume
spectrum (I/I0) for a water column density of n = 1.5 x 1016 cm-2
Estimation of Enceladus Water Flux
•
S = flux
= N * h2 * v
= n/h * h2 * v
= n*h*v
Where
N = number density / cm3
h2 = area
v = velocity
n = column density measured by UVIS
Estimate h from plume dimension, = 80 km
Estimate v from thermal velocity of water
molecules in vapor pressure equilibrium
with warm ice (600 m/sec for surface
temperature ~ 180K – note that escape
velocity = 230 m/sec)
h
v
S = 1.5 x 1016 * 80 x 105 * 60 x 103 = 0.7 x 1028 H2O molecules / sec
= 200 kg / sec
2005 - gamma Orionis Occultation
The Occultation Collection
2007 - zeta Orionis Occultation
2011 occ was a
horizontal cut
through the plume
also
2010 - Solar Occultation
Can we detect Ethylene in
Enceladus’ Plume?
•
•
•
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 at 3% H2O Column Density is not
detectable by UVIS
•
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
Plume Structure (2005)
Water vapor
abundance
calculated from
each 5 sec
spectrum.
The 2005 water profile
is best fit by an
exponential curve.
The best fit scale
length is 80 km
Optical Depth vs. Rayheight (2007)
Minimum distance of rayheight above limb = 15.6 km
S/C velocity = 22.57 km/sec
Best fit is tau = 64.4 x z-2.33 - 0.007
Density at jets is ~2x higher than “background” plume
2007 High Speed Photometer (HSP) Data
•
HSP is sensitive to 1140 to 1900Å
•
Statistical analysis finds features that
are statistically unlikely
– Assumes signal is Poisson distribution
– Compares to 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
Groundtrack of Ray
2005
2007
Enhanced HSP
absorption
features a, b, c,
and d can be
mapped to dust
jets located by
Spitale and Porco
(2007) along the
tiger stripes
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
Plume
or jets?
• The plume of
gas and dust
from Enceladus
includes a
number of
individual jets
seen by Cassini
camera and by
UVIS
• 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
Best fit of 8 sources from Spitale & Porco to match UVIS occultation profile
Brightness of water vapor over Enceladus South pole from UVIS 8-jet model
Orion’s Belt Dual Occ Geometry
• Dual stellar occ by Enceladus’ plume, E15,
19 October 2011, epsilon Orionis (blue) and zeta Orionis (white)
• Horizontal cut through plume
Rev 155 Enceladus Dual Occultation
• Eps Ori (Alnilam, B)
– 16.5 km at closest point
– HSP centered on eps Ori
– Dimmer star
• Zeta Ori (Alnitak, O)
– 37.9 km at closest point
• Observed with UVIS Far Ultraviolet
channel (FUV: 1115 to 1915 Å) and
High Speed Photometer (HSP)
• Water vapor absorbs star signal in
the FUV
Plume Model for E7
• We still do not have enough measurements to
determine all model parameters [only 2 occ’s]
• Spacecraft trajectory uncertainty of 1 sec or
less [= 20km] can make jet identification
uncertain
• Prediction for 100km altitude: 1.1x109 for
Mach3, uncertain due to unknown eruption
velocity
S = flux
Estimate of Water Flux from Enceladus = 200
kg/sec
= 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
Year
n
(cm-2)
Uncertainty
+/-
2011:
vlos = 7.48 km/sec
x
v
y
(x 105
cm)
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
2007
The Jets
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)
• This time the HSP data was lower snr
– no features passed the rigorous statistical tests applied
– Unlike 2007 zeta Orionis occ
• Rely on FUV data, cross-correlation of absorptions in same place /
shifted in time
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
Eps and Zeta Orionis Comparison
• Damascus jets (DII and
DIII) identified
• Distinct at eps Ori
altitude, merged
higher
• Clear signal of gas from
Baghdad fissure (B-f),
though no dust jet nearby
?
B-f
DIII DII
•
•
•
• Possible activity at “?”
without associated dust
jet
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
HSP
• HSP targeted to eps Ori
?
• 0.008 sec integration
summed to 1 sec
Baghdad
Damascus
• Although features did
not pass our statistical
tests we can compare
to the FUV data set
• Good agreement with
eps Ori
a
b
c
HSP Data
Summary of Results: PLUME
• Attenuation in HSP data ~10% in 2007, ~6% in 2005
– Opposite of Hurford et al model of fissures opening and closing
• Plume column density goes as ~ z-2 or as exp(-z/H) (z is minimum rayheight)
• Water vapor flux ~200 kg/sec
• No detection of CO, upper limit 3% (3 sigma)
Summary of Results: JETS
• 2007 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 thermal velocity to vertical velocity in jet = 0.65
– Gas is supersonic: Mach 1.5
• Eight or more jets required to reproduce width and shape of absorption,
some evidence for diffuse sources
• Jet source area is smaller than 300 m x 300 m
Summary
• Mass flux determined, compared to other occs
• Jets tougher to identify because low snr
– HSP data did not pass statistical tests
• 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!