Enceladus Plume Simulations Feng Tian Ian Stewart Brian Toon

Enceladus Plume Simulations
Feng Tian
Ian Stewart
Brian Toon
Kris Larsen
Larry Esposito
July 11, 2006
ISS Observation
at 14-Jul-05 16:14
UT, range 112,340
All images are taken from http://www.planetary.org/saturn/enceladus_11.html
Observations Related to the Plume
• Magnetometer: B field lines of Saturn bending
around an extended atmosphere at Enceladus + a
"comet-like jet“
• CIRS: hot spot at temperatures >90K -- even more
than 140 K
• INMS: water maximum at ~37 s before its closest
approach (CA) -- nearly coincident with CA to the
CIRS hot spot.
• CDA: dust maximum at ~70 s before CA
• ISS: direct observation of icy particles in plumes
• UVIS Stellar Occultation Observations
Importance of the Plume on
• - source of icy particles in Saturn's Ering
• - source for atomic oxygen around
• - source for observed OH cloud around
• - THE missing source of water at the
orbit of Enceladus?
Questions to address :
• Constraints on water escape rate through
more detailed modeling
• Estimate deposition rate
• Physical characteristics of the plume
– is the UVIS data consistent with evaporation from
a large icy area near the south pole?
– if not, it it consistent with water escaping at some
bulk velocity through a vent or system of vents in
the surface?
Constant-number Time-driven
Monte Carlo Simulation
-- Marching through time in steps chosen so
that no more than one collision in dt
-- Assumption on density distribution to
compute collision frequency
-- Checks status of activities in each time step:
1. compute free path and the destination
2. If collision occurs, reset velocity at the collision site and
recalculate the destination
3. Check for multi-collision event
4. Return particles reaching 4 Re to origin
-- More efficient than Event-Driven algorithm in
the case of Enceladus
-- Can be visualized as process in time 
Geometry of the South Polar Region on Enceladus
Tian et al. Submitted to Icarus
Column Density Distribution for a Single Source
Average distance between adjacent sources along the Tiger Stripes is ~20 km.
Plumes merge together at high altitudes  exact locations for the point sources
not important.
Both vertical (altitude) and horizontal
displacements need to be considered
Occultation Geometry by Ian Stewart
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  vent instead of sublimation
Water escape rate is 4~6 x 1027 molecules/sec, = 120~180 kg/sec, consistent
with lower rate from previous estimate (Hansen et al. 2006). 1000 times
greater than the particles escape rate inferred from the ISS observations
(Porco et al. 2006)
Other Water Source Rates
• Amount required to supply OH:
– ~4x1027 s-1 = 120 kg/s (Jurac et al. 2002)
• Cassini Plasma Spectrometer (CPS): >=3x1027
s-1 = 100 kg/s mass loading rate (Tokar et al.
• INMS: 1.5~4.5x1026 s-1 <15 kg/s (Waite et al.
– highly variable source rate (1x1026 s-1 to 3x1027 s-1) in
time scales of less than 1 hour
• ISS: ~0.04 kg/s (Porco et al. 2006)
– Sensitive to large particles >0.1 m
– 1% ejected particles escaping
• Cosmic Dust Analyzer: ~0.02 kg/s (Porco et al.
Particles > 2 m
Water Deposition Rate
• Globally averaged: (2-7)x109 cm-2s-1
– Comparable to sputtering rate estimated (Shi et al.
1995, Jurac et al. 2001)
• Deposition rate in the South Polar Region
(latitude > 75o): (0.5-2)x1011 cm-2s-1
– Net growth than net erosion?
• High resurfacing rate in the south polar region:
(0.5-2)x10-4 cm/yr
– High albedo
• 1. Water source rate high enough to
supply water in the Saturn system.
• 2. Deposition rate higher than sputtering
rate in the polar region.
• 3. High surface velocity suggests vent
rather than sublimation.