Enceladus water jet models from UVIS
star occultations
LW Esposito, AIF Stewart, CJ Hansen
19 June 2009
UVIS
UVIS has 4 separate channels:
• Far UltraViolet (FUV)
• 110 to 190 nm
• 3 slit widths => 2.8, 4.8, 24.9 nm
spectral resolution
• 2D detector: 1024 spectral x 64 onemrad spatial pixels
•
Extreme UltraViolet (EUV)
• 55 to 110 nm
• 3 slit widths => 2.8, 4.8, 19.4 nm
spectral resolution
• 2D detector: 1024 spectral x 64 onemrad spatial pixels
• Solar occultation port
•
High Speed Photometer (HSP)
• 2 - 8 msec time resolution
•
Hydrogen – Deuterium Absorption
Cell (HDAC)
For the occultations we used the
• HSP with 2 msec time resolution
• FUV with 512 spectral channels
(1.56 nm resolution), 5 sec
integration time
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
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
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
Backup Slides
Plume Model
• Monte Carlo simulation of test particles given
vertical + thermal velocity, particle trajectories
tracked under influence of gravity and collisions
(Tian et al, 2006)
• Original model had arbitrary source spacing
along the tiger stripes
• Model now adapted for specific locations of
the 8 dust jets identified by Spitale and Porco,
actual viewing geometry of these sources as
seen from the spacecraft
The results shown here have
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
Solar Occultation
Rev 131
• We have the opportunity to observe an occultation of the sun on Rev 131
• New results are in the EUV, which gives us access to a different wavelength
range than the FUV
• The big scientific payoff is the chance to definitively detect / measure nitrogen
in the plume - important for models of chemistry-driven dynamics in the interior
Solar Occultation
How well can UVIS measure N2 with a solar occultation?
• Abundance of H2O measured by UVIS = 1.5 x 1016 cm-2
• Mixing ratio of mass 28 in the INMS experiment at Enceladus
was [M28]/[H2O]=0.036
• A solar occultation has been simulated for our H2O optical depth
assuming a commingled mixture of H2O and N2 in the spectral
region of the H Ly line
• The ability to measure N2 in a mixing ratio of [N2]/[H2O] = 0.005
is indicated, for an abundance of N2 = 1 x 1014 cm-2
How do we know?
• N2 was measured above the exobase in the UVIS T10 solar
occultation observation using the measured extinction of the sol
H Ly line by the N2 b(3,0) band.
Solar
Occultation
• This is a simulation of the
results we could get from a
solar occultation by Enceladus’
plume
• UVIS can detect N2
absorption near 972 Ang
• Mixing ratio for blue curve,
showing clear absorption, is
0.05, close to the INMS derived
value of [M28/H2O] = 0.036
• Green curve shows likely
detection limit with an order of
magnitude less nitrogen, or
[M28/H2O] = 0.005
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
Changes in System Oxygen Content
Plume
or jets?
• Attempt to pick
comparable
“box” between
2005 and 2007
• But different
jets visible
means
comparing
apples and
oranges
Gas Jets
Density in jets is twice the
background plume
Gas jet typical width = 10
km at 15 km altitude
Ingress
a. Cairo (V)
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)
b. Alexandria (IV)
Closest point
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
• Substantial changes
are not seen in the
occultation data,
although they would be
predicted, based on this
model
• Expect fissures to
open and close
Position of Enceladus
in its orbit at times of
stellar occultations
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
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
New Plume Model
• A new model has been
developed for Enceladus’
plumes by Tian, Toon,
Larsen, Stewart and
Esposito, paper published
in Icarus (2006)
• Monte Carlo simulation
of test particles given
vertical + thermal velocity,
particle trajectories tracked
under influence of gravity
and collisions
• Assumes source of one
plume is 2 x 2 km2, then
multiple plumes added
together along a tiger
stripe, w/ 20 km separation
UVIS ray path across tiger stripes
Localization of Enceladus’ Plume
(Not a global atmosphere)
•
Ray intercepts were at latitude / west longitude:
15 / 300
-31 / 141
-76 / 86
-0.2 / 28
Lambda Sco ingress (non-detection)
Lambda Sco egress (non-detection)
Gamma Ori ingress
Gamma Ori egress (non-detection)
Consistent with localized
plume or jet:
– Enceladus’ gravity
insufficient to hold
gravitationally bound
sputtered atmosphere
– Also, the combination of
other Cassini data sets are
consistent with a plume of
water vapor coming from
Enceladus’ “Tiger Stripes”
driven by the hot spot at
the south pole detected by
CIRS
FUV summed over wavelength