UVIS Observations of
Enceladus’ Plume
C. J. Hansen, I. Stewart, L.
Esposito, A. Hendrix
June 2009
Zeta Orionis Occultation 2007
(Alnitak)
FUV and HSP data collected
FUV: 5 sec integration
HSP: 2 msec sampling
Horizontal density profile
True anomaly = 254
Results documented in Nature,
2008
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
Cassini UVIS Characteristics
UVIS has 4 separate channels;
For star occultations we use the FUV
and HSP:
• Far UltraViolet (FUV)
Spectrograph
– 1115 to 1915 Å
– 2D detector: 1024 spectral x 64
one-mrad spatial pixels
– For occultations we use 512
spectral channels
– 5 sec integration time
• High Speed Photometer (HSP)
– 1115 to 1915 Å
– 2 msec time resolution
For the solar occultation we will use
the EUV spectrograph
• Extreme UltraViolet (EUV)
• 550 to 1115 Å
• 1 sec integration
[Not used for stellar occs:
• Hydrogen – Deuterium Absorption Cell
(HDAC)]
1. Plume Composition and Column Density
Terminology:
• Plume - large
body of gas
and particles
• Jets individual
collimated
streams of gas
and particles
Plume
Jets
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
•
Rev 51 zeta Orionis Occultation: FUV
• Absorption is best fit by water vapor
• 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
• 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
I/I0
What else can UVIS detect?
• Valuable to help resolve ambiguities in INMS
detections
• INMS detects a species with atomic mass =
28
• Previously thought that this must be CO or N2
– UVIS non-detection rules out CO at the 3% level
• New idea (consistent with other INMS data) is
that it could be ethylene
– C2H4 = 2 * 12 + 4 = 28
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
Methanol
Not likely to be detectable, doesn’t look like a
good fit anyway…
Looking for Nitrogen: 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
• 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
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
2. Looking for jets: 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
Gas Jets
Density in jets is twice the
background plume
Gas jet typical width = 10
km at 15 km altitude
Ingress
a. Cairo (V)
b. Alexandria
(IV)
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)
Closest point
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
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 Simulations
•
Ian Stewart is modeling plume water vapor 2007 shown
•
Model shows that gas needs to come from
along tiger stripe, not just jets (based on 2005
results, where we went down to the surface,
but saw gas absorption between jets
Plume Picture
• Opportunity for dual stellar occ by Enceladus’ plume tweaked in,
19 October 2011, epsilon Orionis (blue) and zeta Orionis (white)
QuickTime™ and a
H.264 decompressor
are needed to see this picture.
2011 Occultation Objectives
Single stellar occultation
– Comparative data 2005 vs. 2007 vs. 2011
• Temporal variability
• Activity as f(orbital position)
– Better characterization of jets (no compression of HSP data)
Dual stellar occultation
– Vertical profile of column density
• Removes spatial / temporal ambiguity because profiles are
acquired at the same time
– Overall plume shape
– Contribution of jets to plume vs. gas leaking from tiger
stripes
– Degree of collimation of jets
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
Estimation of Water Flux from Enceladus
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
The biggest uncertainty is what to use for h
Estimate h from plume dimension, = 80 km
Estimate v = 60,000 cm/sec for surface
temperature ~ 180K
note that escape velocity = 23,000 cm/sec
h
v
S = 1.5 x 1016 * 80 x 105 * 60 x 103 = 0.7 x 1028 H2O molecules / sec
= 200 kg / 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
(z is minimum rayheight)
• Water vapor flux ~200 kg/sec
• No detection of CO, ethylene not definitive
“Search for the Missing Water Source”1
Neutral Species
• Water and its products are lost from the system by collisions, photoand electron- dissociation and ionization
•
Estimates of required re-supply rates, water molecules/sec:
•
•
•
•
2.8 x 1027
3.75 x 1027
1028
2 x 1028
1993
2002
2005
2005
Shemansky, et al.
1Jurac, et al.
Jurac and Richardson
Shemansky, et al.
E Ring
• Saturn’s E ring, composed primarily of 1 micron particles, is also
subject to erosion and loss due to sputtering of water from the surface
of the E ring’s dust particles and collisions of particles with Saturn’s
moons
•
Estimate of required re-supply rate:
• 1 kg / sec
2002
Juhasz and Horanyi
System Oxygen Content - Enceladus’ Influence
Backup slides
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
FUV analysis
Occ is easy to detect
Star drifted from pixel 13 to pixel
12 over the course of the
observation
Summary
2007 Occultation of Zeta Orionis - new results
•
•
•
•
•
•
•
•
Overall plume shape and density
Significant events are likely gas jets
UVIS gas jets correlate with dust jets in images
Previous Monte Carlo model updated with new data
We characterize jet widths, opacity, density
Ratio of thermal velocity to vertical velocity = 0.65, supersonic
Water vapor abundance derived from new FUV spectra, no CO
Comparison of 2005 to 2007 occultations does not substantiate
tidally-controlled energy-source models
Paper published in Nature 11-27-08
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.