Multi-Instrument View of the
Enceladus Torus
Following H2O from Enceladus:
We see it in the plumes
As a ‘torus’ at Enceladus orbit
As OH following dissociation
As O
As plasma (H2O+, O+, H3O+, OH+)
Cassidy et al.
UVIS occultations
Herschel HIFI
HST, 90s
UVIS, 2004, 2007
CAPS
Herschel Observations of the
Enceladus H2O cloud
Paul Hartogh, Emmanuel Lellouch and others observed the
Enceladus H2O cloud in the sub-mm by the Herschel s/c. This
nicely complements HST and Cassini data, particularly UVIS
and INMS
Hartogh et al., 2011, A&A
We have been able to independently estimate the Enceladus
H2O source rate. Like UVIS occs, they measure a column
density from which a source rate is derived.
Cassidy et al.
1999: H2O emission from
Saturn’s atmosphere
556.935 GHz
SWAS telescope
2009: new spacecraft sees big
absorption on top of the H2O
emission
Herschel telescope
HIFI instrument
SWAS telescope (also
sensitive to the H2O
line) didn’t see H2O
torus.
SWAS/Earth direction
Herschel/Earth direction
Cassidy et al.
Detailed Look
Emmanuel Lellouch compared the data with a simple model torus:
2.5 RS wide with adjustable location
Variable scale height, density ~ exp(-z/H)
Speed distribution for Doppler shift:
H2O molecules on circular orbits + speed dispersion
Cassidy et al.
Torus position:
Cassidy et al.
Torus scale height:
Torus ‘temperature’ (speed dispersion):
Cassidy et al.
I’ve been modeling the neutral cloud and
comparing the results with HST observations of
OH (90’s) and UVIS observations of O (pre-SOI)
Enceladus
We used this model to compare Herschel obs. and get the H2O source
rate. How active are the plumes, does their output vary?
Conclusion from that: need neutral/neutral
collisions to help spread cloud
Conclusion: our one free parameter is the H2O source rate. We found
that we need about 1x1028 H2O s-1.
10
To compare with Herschel obs., I
simulated the Herschel field of view:
Herschel/Earth
direction
Model output:
H2O column (cm-2) in front of
Saturn for Herschel
observation geometry
Cassidy et al.
Herschel data. I adjusted the
H2O source rate until it matched
he observation. I lowered it
rom 1.0x1028 to 0.85x1028
One of 3 observation sets
1670 GHz H2O emission and
absorption
PSF
Again, H2O/H2O collisions drive the
cloud shape (and temperature).
Without collisions (below), model
does not match observations.
Observation summary
1990s
(OH obs by HST)
2004, 2007
(UVIS obs of atomic O)
~1028 H2O/sec
Estimates by Shemansky/Melin, Cassidy/Johnson,
Richardon/Jurac/Johnson
_________________________________________________
Herschel observations
June, 2009
July, 2009
June, 2010
Dec, 2010 (tentative)
July, 2011
January, 2012
8.5E27 H2O/s = 255kg/s
Estimates by Cassidy/Johnson model
7E27 H2O/sec =210 kg/s
still need to compare with model
Those source rates inferred from large-scale cloud
observations. Up close, UVIS occs also show fairly
constant source rate. Inferences from INMS and MAG
flyby data and other instruments suggest much larger
variability.
Other results from Herschel data
•Herschel-inferred scale height ~0.4 RS matches model, though
it does not match INMS inferred scale height (Perry et al., 2009)
•Cloud is ‘hot’: has a velocity dispersion of ~2-2.3 km/s (~30004000 K)
•But cloud is also ‘cold’ in terms of sub-mm emission (~16 K)
Model with H2O/H2O collisions predicts the ‘hot’ part.
But collisions are so rare (~1/day) that the molecules
don’t emit blackbody radiation. Talk about non-LTE!
•Model predicts that H2O from Enceladus hits Saturn—enough
to account for upper-atmosphere H2O.
Hartogh et al. (2011):
Model predicted H2O/OH/O flux onto Saturn, which
matched the flux necessary to explain stratospheric H2O
(flux required by Moses et al., Ollivier et al., 2000)
“ESA’s Infrared Space Observatory found the
water vapour in Saturn’s atmosphere. Then
NASA/ESA’s Cassini/Huygens mission found the
jets of Enceladus. Now Herschel has shown
how to fit all these observations together”
-Göran Pilbratt, Herschel Project Scientist
Model predicts that H2O/OH/O hits Saturn’s
equator, is there more stratospheric H2O tat
the equator? Preliminary results say yes
(Thiebault Cavalie) using H2O emissions
unaffected by torus absorption
Upcoming
•Herschel has looked for NH3 but did not detect any. We are working to turn
this upper limit on line-of-sight column density into an upper limit on the
NH3 source rate to compare with INMS detection of ~1%.
•We were granted one more observation in late June. This time we will look
away from Saturn’s disk to look for emission from the H2O torus rather than
absorption at 1670 & 1113 GHz.
Determining the Rate that Mass Flows Away
from the Planet
• Assume source of plasma is located inside – say, in a ring
around Enceladus’ orbit and outwards – 4 to 7 Rs
• Total mass of plasma produced = dM/dt ~10s kg/s
Flux out of the cylinder = dM/dt=Vr 2p R p1/2 H No
H
R
No = N(R) x exp (z/H)2 where N(R) is measured density profile
H = measured scale height
dM/dt = rate of mass production in volume (units=kg/s)
Vr(R) = radial transport rate - derived by assuming dM/dt
Determining the Rate that Mass Flows Away
from the Planet
Flux out of the cylinder = dM/dt=Vr 2p R p1/2 H No
H
R
No = N(R) x exp (z/H)2 where N(R) is measured density profile
H = measured scale height
dM/dt = rate of mass production in volume (units=kg/s)
Vr(R) = radial transport rate - derived by assuming dM/dt
OR
2p3/2 R H N(R) exp (z/H)2 = dM/dt / Vr(R)
We plot this from CAPS data
• Next we look at some CAPS data
• These are fits to ion spectra done by Rob
Wilson
• Error bars are uncertainties in individual
fits.
• Yes, there are some bad fits – needs
further sorting
– NOT numerical moments from Michelle Thomsen
2005
2006
2007
2008
2009
3
OH+ R*H*n*exp((Z/H)2)
10
Black = fiducial lines – same for all plots
2
10
1
10
0
H+ R*H*n*exp((Z/H)2)
103
10
2
10
1
10
0
10
4
6
8
10
12
R (RS)
14
16
18
20
2005
2006
2007
2008
2009
3
OH+ R*H*n*exp((Z/H)2)
10
2004
2
10
1
10
0
H+ R*H*n*exp((Z/H)2)
103
10
2
10
1
10
0
10
4
6
8
10
12
R (R )
S
14
16
18
20
2005
2006
2007
2008
2009
3
OH+ R*H*n*exp((Z/H)2)
10
2005
2
10
1
10
0
H+ R*H*n*exp((Z/H)2)
103
10
2
10
1
10
0
10
4
6
8
10
12
R (R )
S
14
16
18
20
2005
2006
2007
2008
2009
3
OH+ R*H*n*exp((Z/H)2)
10
2006
2
10
1
10
0
H+ R*H*n*exp((Z/H)2)
103
10
2
10
1
10
0
10
4
6
8
10
12
R (R )
S
14
16
18
20
2005
2006
2007
2008
2009
3
OH+ R*H*n*exp((Z/H)2)
10
2007
2
10
1
10
0
H+ R*H*n*exp((Z/H)2)
103
10
2
10
1
10
0
10
4
6
8
10
12
R (R )
S
14
16
18
20
2005
2006
2007
2008
2009
3
OH+ R*H*n*exp((Z/H)2)
10
2008
2
10
1
10
0
H+ R*H*n*exp((Z/H)2)
103
10
2
10
1
10
0
10
4
6
8
10
12
R (R )
S
14
16
18
20
2005
2006
2007
2008
2009
3
OH+ R*H*n*exp((Z/H)2)
10
2009
2
10
1
10
Something wacky here….
0
H+ R*H*n*exp((Z/H)2)
103
10
2
10
1
10
0
10
4
6
8
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12
R (R )
S
14
16
18
20
2p3/2 R H N(R) exp (z/H)2 = dM/dt / Vr(R)
We plot this from CAPS data
So – if the plotted quantity does not very much with
time then
• dM/dt and Vr are pretty constant with time
• They both vary together with time
Variability of plasma from CAPS
Cassidy et al.
Extra
Cassidy et al.
Comparing model and data
Emmanuel developed his own relatively simple model of the torus before
comparing with our Monte Carlo cloud model. It is a torus of H2O, 2 RS
wide. He varied the diameter, density, scale height and velocity dispersion
to match the data (see plots on later slides).
Included absorption cross sections and Doppler shift to calculate
absorption line shapes, which required a separate code to calculate
molecular excitation levels under the influence of solar and Saturnian
blackbody radiation.
These were compared with my model output. His derived velocity
dispersion (2.0-2.3 km/s, 3000-4000K) agrees with my model. We went a
step further with H2O column density: I sent him line-of-sight densities,
which he convolved with the Herschel PSF to compare directly with data.
He still used the fitted velocity dispersion, though. It remains to calculate
the line profiles directly from my model output.
Cassidy et al.
Cassidy et al.
North
• SS
Center
South
Cassidy et al.
Extra: slides on cloud spreading
Cassidy et al.
Water vapor is ejected from Enceladus far too slowly to explain its breadth:
Molecules leave Enceladus on
nearly circular orbits:
we found that a combination of spreading processes could reproduce the
observations fairly well:
•Neutral/neutral collisions
•Ion/neutral collisions
•Dissociation (by electrons and UV photons)
(this produces OH and O from the H2O)
Each of these adds or subtracts speed to orbiting molecules:
Cassidy et al.
Cassidy et al.
Cassidy et al.
Line of sight
Column Density (cm-2)
Line of Sight O Column Density
(UVIS observations of fluorescent O)
Data
RS
Cassidy et al.
Model