Enceladus’ Plume and Jets:
UVIS Occultation Observations
June 2011
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
Coming up Next
• Opportunity for dual stellar occ by Enceladus’ plume, E15,
19 October 2011, epsilon Orionis (blue) and zeta Orionis (white)
~Orthogonal Ground Tracks
Blue ground track is from
zeta Ori occ on Rev 51
 Ingress
Orange is solar occ track,
~orthogonal
Gas jets appear to correlate
to dust jets in zeta Ori occ
Basemap from Spitale & Porco, 2007
 Egress
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 and Tiger Stripes
As before, gas jets appear to
correlate to dust jets
2010
Spacecraft viewed
sun from this side
Feature “a” would line up
better with Alexandria dust
jet if dust jet were actually
on the Alexandria tiger
stripe
Ingress

Minimum
Altitude
Feature “d” may correspond
to a new (fissure branching
off tiger stripe) hot feature
detected in CIRS thermal
data
2011 stellar occ will be similar
groundtrack, viewed from
opposite side
Egress
Basemap from Spitale & Porco, 2007
Jet Structure
• Higher snr enables better measurements of jets’ dimensions – more
clearly distinguished from background plume
• Density of gas in jets ~3 x the density of the background plume
(20% more absorption over 1/15 scale)
• 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
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 or 32 10
7 or 6
Baghdad VII
or new feature
12
e
39
40
10
8
Damascus III
13
f
47.5
49.7
14
7
Damascus II
14
*Average attenuation =17%
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
• Consistent with CDA model that gas is accelerated in nozzles to
surface to supersonic speeds
CDA – Composition partitioning in E
ring and plume (1)
• CDA detects small salt-poor
grains in the E ring, salt-rich
particles in Enceladus’ diffuse
plume
• Condensation of water molecules in
the high velocity gas jets produces
small salt-poor grains
Particle size 0.2 to 0.6 microns
•
Hypothesis is that high velocity jets
propel salt-poor grains to the E ring
•
Higher mass - fall back to surface
CDA – Composition partitioning in E
ring and plume (2)
Salt-rich
particles come
from water in
contact with
rocky core
• CDA model
• Hypothesize salt-rich particles with r > 0.6 micron are
aerosols sprayed from a subsurface ocean, ejected
along length of tiger stripe
•
Matson et al 2011 model puts
this all together, loosely
•
Subsurface ocean is charged
with dissolved gases
•
Bubbles come out of solution
as liquid rises, when they pop
(in the water/brine reservoir)
the salt-rich aerosols detected
by CDA are formed
•
Gas and salt-rich particles
escape along length of tiger
stripe
•
Gas also accelerated in
nozzles to surface, smallest
grains condense, CDA sees
salt-poor particles, we see
supersonic jets
•
Tiger stripe / nozzle physical
structure yet to be explained
The “Perrier”
Ocean
Comparison to INMS results from E7
• Highly collimated jets are consistent with INMS detection of enhanced
gas streams at higher altitude
• E7 INMS groundtrack at altitude of ~91 km (c/a) compared to UVIS profile
at altitude ~20 km (c/a)
• INMS and UVIS both detect Alexandria and Baghdad jets
Solar Occ results – Plume 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
(no need to explain N2 in
presence of NH3)
• 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
• The solar 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
• All flux values within 15% deviation suggests that Enceladus has
been erupting steadily for the last 6 years
• Water vapor flux in the jets = 30-50 kg/sec
–
(5 – 8) * 3.4% = 15-25% of 200 kg/sec
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
y
note that escape velocity = 23,000 cm/sec
x
v
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
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
Vertical cut through plume
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
Horizontal density profile
18 May 2010 - Solar Occultation
Key results:
• No N2 detected in plume; upper limit < 0.5%
• Water flux approximately constant over 6 year Cassini mission, 200 kg/sec
• Jets more collimated than previously reported, mach numbers = 5 to 8
• Suggests speed of 1 – 2 km/sec
• Alexandria jet correlates to enhanced INMS gas detection at high altitude
• Results documented in Hansen et al., GRL 38:L11202, 9 June 2011
Horizontal density profile
We conclude…
Supersonic gas jets are consistent with Schmidt et al.
model of nozzle-accelerated gas coming from liquid
water reservoir
High velocity jets are also consistent with CDA data
reported by Postberg et al. showing compositional
partitioning: small salt-poor particles reaching the E ring
and salt-rich particles in the diffuse component of the
plume close to Enceladus
Lack of N2 in presence of NH3 means that a relatively
cool liquid reservoir such as “Perrier Ocean” proposed
by Matson et al. is viable
Clathrate propellant highly unlikely
Backup Info
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
Solar Occultation Characteristics
Total duration of Solar Occ:
1min 35sec
Duration for full-width half max:
53 sec
FWHM
Line of sight velocity: 2.85 km/sec
Width of plume at FWHM:
56 sec * 2.85 = 150 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
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
2007 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
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
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