Larry W. Esposito
3 May 2007
Enceladus SDT

• ~1 minute before closest approach the
Cosmic Dust Analyzer detected a peak in the number of small particles (blue diamonds), 460 km altitude
• 35 seconds before closest approach the Ion
Neutral Mass
Spectrometer measured a large peak in water vapor (yellow diamonds), 270 km altitude
• Gas and dust plumes are decoupled at these altitudes
Cassini clipped edge of plume:
INMS, CDA in situ Results

CDA Peak INMS Peak

Enceladus Stellar Occultation Geometries
February Lambda Scorpii
Occultation
July Gamma Orionis
Occultation
Egress Ingress

Localization of Enceladus’ Plume
(Not a global atmosphere)
• Ray intercepts were at latitude / west longitude:
15 / 300 Lambda Sco ingress (non-detection)
-31 / 141
-76 / 86
-0.2 / 28
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 Data: Comparison of Occulted to
Unocculted Spectra
FUV configuration:
• spectral channels binned by 2
(512 spectral channels from
113.5 nm to 191 nm)
• low resolution slit width
• 5 sec integration time
• full spatial resolution
Time record 33, the last full
5 sec integration prior to ingress, shows the deepest absorption. The ray altitude above Enceladus’ surface corresponding to time record 33 ranged from
30 to 7 km.
Clear signature of an atmosphere is present – both relatively narrow and broad absorption features

Composition of Plume is Water Vapour
I=I
0 exp (-n*

)
I
0 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/I
0
) for a column density of n = 1.5 x 10 16 cm -2

The increase in water abundance is best fit by an exponential curve – a comet-like evaporating atmosphere (1/R 2 ) does not fit the data well, nor do global hydrostatic cases
The best fit scale length is 80 km
Structure of the Plume

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

“Search for the Missing Water Source” 1
Neutral Species
• Water and its products are lost from the system by collisions, photo- and electron- dissociation and ionization
• Estimates of required re-supply rates, water molecules/sec:
• 2.8 x 10 27
• 3.75 x 10 27
• 10 28
• 2 x 10 28
1993
2002
2005
2005
Shemansky, et al.
1 Jurac, 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

Estimation of Water Flux from Enceladus
• S = flux
= N * h 2 * v
= n/h * h 2 * v
= n * h * v
Where
N = number density / cm 3 h 2 = 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)
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) h v
S = 1.5 x 10 16 * (80 or 175) x 10 5 * (41 or 46) x 10 3 = 0.5 to 1.2 x 10 28 H
2
O molecules / sec
= 150 to 360 kg / sec

• A new model has been developed for Enceladus’ plumes by Tian, Toon,
Larsen, Stewart and
Esposito, paper to appear in Icarus
• Monte Carlo simulation of test particles given vertical + thermal velocity, particle trajectories tracked under influence of gravity and collisions
• Assumes source of multiple plumes added together along each tiger stripe
New Plume Model
UVIS ray path across tiger stripes

Monte Carlo model results - Predicted Plume Shape

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
Water flux is 4 - 6 x 10 27 molecules/sec = 120 - 180 kg/sec
(consistent with initial estimate)

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 HST observed OH in 92
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

Changes in System Oxygen Content

0.8
0.7
0.6
0.5
3.0
2.8
2.6
2.4
2.2
1.2
1.0
0.8
1.1
2.0
1.8
1.6
1.4
1.0
0.9
0.4
0.3
0.2
0.1
0.0
0
(a)
(b)
100
200
Z: +5/-5 R s
, X: -6/-10 R
Z: +5/-5 R s
, X: 0/+6 R s
Z: +5/-5 R s
Z: +5/-5 R s s
, X: +6/+10 R
, X: 0/-6 R s s
500 300 400
Elapsed Time (days)
600 700

• UVIS measures water source large enough to create neutral oxygen cloud and to resupply E ring
• UVIS column density equal to about a single 1mm ice grain per square meter
• A uniform source could loft ice particles of radius about 1 micron; higher density jets could loft particles dangerous to Cassini

• Models
• Fumarole model. Misty vapor cools as it expands; ice particles condense. T ~ 170K.
• Geyser model. Local heating gives boiling water at depth, vent geometry gives vertical velocity, collimation; bubbles form and liquid freezes, effectively lofting larger particles to high speeds. T
~ 270K.
• Comet model. Sublimating vapor lifts ice grains from vent interface and carries them away. T ~
200K.

• In all these models, there is a close coupling between the ice and vapor
• Growth, lofting and/or evaporation involve an interchange between water molecules and solid ice particles
• For any significant interchange of mass or momentum, the column of water vapor incident on an ice grain’s surface area must have a comparable mass to the grain mass

N
0
*  * a 2 * H * m
H20
=  * 4/3 *  * a 3
• For H ~ 40km,  ~ 1, we solve for a (in microns) a ~ N
0
/ (10 12 cm -3 )
• Thus, high pressure vents could loft or grow big particles, potentially dangerous to
Cassini

N column
/1.5 x 10 16 f
1 a
0
-3 A where a
0
, the particle radius, is measured in mm; and A, the Cassini sensitive area, is measured in m 2 . N column is the column density of gas along Cassini’s path through the plume. For E3, this would give about
10 -3 f
1 hits (from INMS results). consistent with our safe passage.

• The shape of the observed plumes shows V
0
> V th
• Tian etal can match the UVIS results with V
400 m/s and N ~ 10 10 – 10 12 cm -3
0
~
0
• This gives typical grain sizes a ~ 0.01–1  , roughly consistent with photometry and CDA measurements
• Present analysis of Cassini data would give no indication of dangerous particles, by orders of magnitude

• Plume has cylindrical symmetry about pole
• Estimate plume density integrated along
Cassini path from water column measured by UVIS star occultation
• See Spencer and Hansen figures: UVIS had a measurement at location of highest density for rev 61

Rev 61 c/a ------->

• If all water vapor along this line of sight to star (N col
= 2E15/cm 2 ) were swept up by
Cassini’s sensitive area (0.8 m 2 ), this would form a solid ice sphere of radius 500 microns
• Assume measured solid particle size distribution can be extended as a power law in radius to sizes dangerous to Cassini
– CDA: q = 4
– RPWS: q = 6.4 (radius power law)

• Calculate the predicted number of hits by dangerous particles (r > 900 microns, Dave
Seal) if Cassini flew a path with same minimum altitude:
• N
D
(a
*
1-q
= f
1
* (4-q)/(q-1) * a
0
3 /(a max
4-q - a min
4-q ) *
- a max
1-q )

• a
*
: dangerous particle radius, 900 microns
• a
0
: equivalent ice radius, 500 microns
• a min
, a max
: size range, radius 1-1000 microns
• f
1
: ratio of solid ice mass to water vapor
• q: power law size index

• N
D
• N
D
= 3E-9 f
1
= 2E-3 f
1
for q = 6.4
for q = 4

• Simple physical arguments of mass balance, force balance, growth of solids from vapor give f
1
< 1
• Comparing mass loss of solids estimated by
ISS, CDA to vapor by UVIS gives f
1
~ 0.01

But, what about small, high pressure vents? They could loft dangerous particles. Signal more variable within plume …
Outside Within plume

Same number of high and low outliers

Entire Enceladus ingress cut (binned data)
3 features near limb found by ring feature search algorithm, but below significance threshold

• Sensitive to events as small as 50m; opacity as small as 10%
• We see no significant deviations from smooth variation
• Outlier events have width less than 1km and opacity less than twice mean

• Extrapolating Cassini plume measurements to rev 61 and to radius dangerous to Cassini, using the most optimistic size range, provides a conservative estimate of the number of hits expected
• The value is 0.2% or less
• No evidence for high pressure vents
• Better measurements of the size distribution and its opacity would improve the model