Enceladus’ Water Vapour Plumes
UVIS Observations in the Extended
Mission
C. J. Hansen, A. Hendrix, D. Shemansky, L. Esposito
June 2007
UVIS Science Objectives - Overview
Most Important
•
Observe occultation of gamma Orionis to
– Observe temporal variability
– Reconstruct vertical structure -> important constraint on what is happening in the
interior because we derive vertical velocity
– Calculate water flux
• Capacity to re-supply the E ring is still not completely certain
• (no question that the atomic oxygen in the system can be maintained by Enceladus)
•
Observe occultation of the sun to measure nitrogen
– Previously, presence of nitrogen was inferred from INMS measurements combined
with UVIS upper limit on CO
– Can definitively detect nitrogen in a solar occultation and get column density as a
function of altitude
Also
• Observe distant stellar occultations of the plume with the HSP to look for
density variations (like Rev 51)
• Observe system oxygen in Enceladus’ vicinity as a proxy for activity
Gamma Orionis Occultation - Rev 88
Gamma Orionis goes right behind the plume
Faster occ than in the primary mission
-> choice between snr and vertical resolution
Previous Stellar Occ Results
(brief reminder)
Composition of Plume is Water Vapour
I=I0 exp (-n*)
I0 computed from
25 unocculted
samples
n = column density
 = absorption
cross-section,
function of
wavelength
The absorption spectrum of water (pink line) is shown compared to Enceladus’
plume spectrum (I/I0) for a column density of n = 1.5 x 1016 cm-2
More Recent Fit
• Water absorption modelled
by Don Shemansky
• New water absorption
cross-sections used
• Better fit to data
• New column density =
1.6 x 1016 cm-2
• But no significant change
in results
• Old column density =
1.5 x 1016 cm-2
Structure of the Plume
The increase in water
abundance is best fit
by an exponential
curve – a comet-like
evaporating
atmosphere (1/R2)
does not fit the data
well, nor do global
hydrostatic cases
The best fit scale
length is 80 km
Monte Carlo model results - Predicted Plume Shape
The 0.1 contour => column density = 1016
Monte Carlo Model - Fit to Data
Best fit to UVIS column density as a function of altitude requires a vertical velocity of
300 to 500 m/sec
New calculation of water flux is 4 - 6 x 1027 molecules/sec, consistent with lower rate
from previous estimate, = 120 - 180 kg/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
Solar Occultation
• Reduced amount of water vapor
simulation
• (Further from source so need to
take this into account)
Enceladus Plume Occultation
In Rev 51 there is an opportunity to get a stellar occultation of Enceladus’
plume
This is the perfect geometry to detect density variations in the gas jets
We’d like more of these in the XM (greater distance is ok)
Changes in System Oxygen Content - Proxy for Activity
Changes at Enceladus
UVIS System Scans in the XM
•
MAG TWT included system scans in their request for apoapsis segments
•
We requested
– On all orbits with period > 15 days
• 8 hr/day
• Apoapsis +/- 4 or 5 days
•
Segmentation status is that there won’t be a cross-discipline TWT, but instead
large orbits will fall into either the Saturn or the MAG TWT
– They are all in the second year of the XM, beginning with Rev 109
– After Cassini is back in the equatorial plane
– This is assuming that we don’t get time in the 7 Rings equinox orbits
• Leaves ~20 orbits as opportunities
•
Likely competitors for apoapsis time will be ISS (but we can avoid them by
picking orbits with high phase angles) and CIRS
XM Orbit Petal Orientation
The sun is to the right
Backup Slides
Solar Occultation
Enceladus Stellar Occultation Geometries
February Lambda Scorpii
Occultation
July Gamma Orionis
Occultation
Egress
Ingress
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 (from scale
length) or 175 km (from horizontal distance
traversed)
h
v
Estimate v from thermal velocity of water
molecules in vapor pressure equilibrium with
warm ice (41,200 at 145 K or 46,000 cm/sec at
180 K – note that escape velocity = 23,000
cm/sec)
S = 1.5 x 1016 * (80 or 175) x 105 * (41 or 46) x 103 = 0.5 to 1.2 x 1028 H2O molecules / sec
= 150 to 360 kg / sec
Detection of Plume: High Speed Photometer
(HSP) vs. Time
• Clear indication of attenuation of signal during occultation ingress; egress is signature of HSP warmup
• Start to sense atmosphere ~24 sec prior to hard limb occultation, maybe as much as 30 sec (FUV)
• Ray height at –24 sec is ~ 155 km
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
“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
New Plume Model
• A new model has been
developed for Enceladus’
plumes by Tian, Toon,
Larsen, Stewart and
Esposito, paper submitted
to Icarus
• 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, separated 20 km
UVIS ray path across tiger stripes
Dust in the Plume - Is Enceladus the source of
Saturn’s E Ring?
Enceladus as source of E Ring was first proposed in 1981 by Terrile and
Cook, based on coincidence of peak density of E ring with Enceladus’
orbit
Lifetime of 1 micron grains is short (<50 years)
E ring re-supply required is 1 kg/sec
• Comet analogy, using UVIS water vapor mass
• If dust-to-gas ratio is comet-like then ratio ranges from 0.1 to 2
• 0.1 x 150 kg/sec > 1 kg/sec required
• ISS results on dust densities suggest that the ratio is much lower - just
0.04 kg/sec of 1 micron particles
• From the rate of impacts on the CDA High Rate detector the particle
escape rate is inferred to be 5 x 1012 particles/sec or 0.2 kg/sec
assuming all grains have a radius of 2 microns
Detecting Temporal Variability
The water budget derived from the water vapor abundance
shows Enceladus supplies most if not all of the OH
detected by HST, atomic oxygen in the Saturn system
detected by UVIS
Implies activity for > 15 years
Since the oxygen in the system comes from Enceladus UVIS
may be able to remotely monitor Enceladus’ activity levels
by monitoring the system oxygen level
Oxygen atoms (x10-34)
Weekly O1304 Trend
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(a)
(b)
0
Z: +5/-5 Rs, X: -6/-10 Rs
Z: +5/-5 Rs, X: 0/+6 Rs
Z: +5/-5 Rs, X: +6/+10 Rs
Z: +5/-5 Rs, X: 0/-6 Rs
100
200
300
400
Elapsed Time (days)
500
600
700
Detectable Effects on Magnetosphere
At Enceladus
• Some of the neutrals in the plume are ionized by charge exchange
• Fast-moving co-rotating plasma overtakes Enceladus, gets slowed down by
mass-loading of slow-moving Enceladus plasma
• Plasma flow (sensed by CAPS) is slowed and diverted at a distance of 27 RE
In Saturn’s magnetosphere, away from Enceladus’ immediate vicinity,
• The neutral density dominates the plasma density
– Large tori of H, OH, O
•
•
Charge exchange is the primary ionization process and proceeds slowly,
resulting in relatively long average neutral lifetimes (~70 days)
Heavy ions supplied to inner magnetosphere are gradually accelerated to
corotation velocity, transported outward
Enceladus L shell crossings show
• Changes in absorption signatures of energetic electrons suggest plume
variability on time scales of days to weeks
2008 Enceladus Flyby
Cassini will fly by Enceladus again on March
12, 2008
This is a polar pass, with the closest approach
near the equator
We approach from the unilluminated side
Just minutes after Cassini passes to the
illuminated side Enceladus goes into eclipse
The closest approach distance has not yet
been finalized but will be < 100 km
We expect to clip the plume again on the
southbound asymptote
Intrigued by Enceladus
Enceladus will be an important target for more
flybys in Cassini’s extended mission
Enceladus should be the destination for a future
mission!
Neutral Species in Saturn’s System
• The Saturnian system is filled with the products of water molecules:
– H detected by Voyager in 1980, 1981
– OH detected by Hubble Space Telescope in 1992
– Atomic Oxygen imaged by UVIS in 2004
CO Limit
•
The Ion Neutral Mass Spectrometer (INMS) detected a species with mass = 28
amu, consistent with either N2 or CO
•
UVIS should have detected CO:
– Used absorption cross-sections from Eidelsberg, 1992
– “Require” 10% dip in signal for positive detection
I/I0 = 0.9 = exp (-nα)
for α = 820 x 10-18 at 1477.6 Å
->
n = 1.3 x 1014 cm-2 upper limit, ~1%
•
VIMS also places an upper limit of ~ 1014 cm-2
•
Implies an INMS detection of N2 since UVIS should have seen 3% CO
Enceladus Flybys in 2005
Orbit 3
Orbit 4
Orbit 11
Range = 1259 km
Subs/c lat = 200
Subs/c lon= 3080
Range = 497 km
Subs/c lat = -170
Subs/c lon = 2540
Range = 168 km
Subs/c lat = -510
Subs/c lon = 2180