1. 33 (6 outlines) -> 27

2. Objectives
Understanding the formation and evolution of dust jets and dust coma structures at all scales.
In the context of the Rosetta mission, we are developping a numerical model that can be used to :
invert physical parameters from observations of dust jets (mainly images acquired with the OSIRIS camera system)
simulate the evolution of dust coma structures and predict activity patterns during the mission.

3. spacecraft, short distance (OSIRIS will reach cm/px resolution)
9P/Tempel 1
spacecraft, long distance
2000 m
7500 km
Farnham et al 2007
ground based observation
(filtered image enhancing the jets)

4. 9P/Tempel 1
Vincent et al, A&A, 2010
(model description)
81P/Wild 2
Lin et al, A&A, 2011
67P/CG
Vincent et al, A&A, 2012
(accepted)

5. COSSIM: COma Structures SIMulator
3. Ballistic trajectory
2. Dust accelerated by gas drag above the surface or in the subsurface
v = v0 β1/6 (Fulle 87)
β = / grain size (Burns 79)
grain size = power law
emission direction = perpendicular to local surface
1. Description of 3D surface and local conditions: T°, illumination, dust size distribution, gas species
COSSIM: COma Structures SIMulator

6. direction of the jet
intersection “jet plane”/surface

7. Location of source region from jet stereoscopy

8. Going further
With OSIRIS we are ready to track the jets in 3D down to the surface and we want to understand the local physical processes.
=> COSSIM version 2
New parameters:
Gas flow ! Not only pure dust model.
Local topography is used to constrain the gas flow, not only illumination
Several gas emitting sources (can be different gas)
Surface temperature depending on presence of dust and ice, porosity, illumination conditions, ...
From a known surface (topography, material) and local conditions (illumination, temperature, pressure), we calculate gas flows with a DSMC code.
When the gas flow is simulated, we add dust particles and model their acceleration.

9. 1. An impact can excavate the dust layer and reveal ice underneath.
In order to test the new model, we focus on three “realistic” test cases:
1. An impact can excavate the dust layer and reveal ice underneath.
2. Farnham et al, and Vincent et al have linked some of the Tempel 1 jets to the edge of one of the smooth regions, where a cliff has been seen to recede from 2005 to 2011.
3. We discuss the possibility that sublimation takes place below the surface and gas escapes through cracks. Many small jets can be formed this way and later merge into a larger collimated structure.
Belton 2010

10. Simulation parameters:
Each simulation covers a domain of a few hundred meters diameter, 100m above nucleus surface
For 67P, typical outgassing for H2O at 2 AU is 1x1027 molecule.s-1
Considering emission only from the illuminated hemisphere, this translates into a surface outgassing of x 10-6 kg.m-2.s-1
The flux from the active area is defined as 10 times the flux in the surrounding region
We run the DSMC model until the empirical stability criterion
mcs/mfp < 0.3 is reached
the drag force is then calculated on dust particles at each step

11. 80m
200m

12. 80m
400m

13. 100m
100m

14. Conclusions
From our simulations we find that the terminal velocity of the gas varies between 500 m.s-1 and 1 km.s-1
The visualization of dust particles trajectories is not fully implemented yet but our simulations derive a terminal velocity of a few 10s of m.s-1 for 10 um particle, at ~2km from the nucleus. This is also similar to what was obtained from ground based studies (Tozzi et al 2010, Vincent et al 2012)
We are now able to simulate dust jets from the surface to infinity, taking into account the physics of the acceleration region.
From OSIRIS images we will invert the location of jets sources and use COSSIM to describe the acceleration and trajectories of dust particles.
To be added: photometric model of the dust, grain outgassing and fragmentation, simple coma model

15. spacecraft, short distance
200 m
spacecraft, short distance
9P/Tempel 1
2000 m
spacecraft, long distance
ground based observation
(filtered image enhancing the jets)