1. Craters on Comets
J.-B. Vincent, N. Oklay, S. Marchi, S. Höfner, H. Sierks
33 (6 outlines) -> 27

2. Craters on comets Do they matter ? What do they look like ?
Do we understand them ?
Link to activity ?
What does it mean for Rosetta ?
Answers in PSS paper "Craters on Comets", Vincent et al 2014, in Press
ask me for a pre-print pdf

3. 1. Do they matter ? model by S. Marchi
67P spends 17% of its orbital time in the Main Belt
Intrinsic collision probability (ICP):
MBA/comet: km-2 yr-1
MBA/MBA: km-2 yr-1
Many "pits" are observed
on cometary nuclei
~ 5% could be caused
by impacts

4. 2. What do they look like ? Limited number of observations:
2 main morphologies:
- pit-halo
- sharp pit
Thomas 2007
Brownlee 2004

5. 2. What do they look like ? Pit-Halo
Central pit surrounded by an irregular halo surface with a rougher texture, seemingly rounded by erosion processes
Compatible with lab experiments of hypervelocity impacts on porous material over harder substrate
Sharp Pit
Deep depression circled with steep cliffs, rim sometimes raised above the surface, possible presence of debris piles on the floor
Compatible with lab experiments of hypervelocity impacts on hard crust covering loose material
(see Brownlee et al 2004 for references to experiments)

6. 3. Do we understand them ? gravity strength
Scaling Laws:
Semi empirical relationships linking experimental datasets, real craters, and material physics together, and with observations of real craters.
(K. Holsapple, K. Housen, J. Melosh, …)
gravity strength
Depends on physical parameters
+ material dependent constants, K1, μ, ν

7. 3. Do we understand them ?
For a typical comet density (500 kg.m-3), impactor density (2600 kg.m-3), impact velocity (10.5 km.s-1)
In agreement with Deep Impact experiment.
… but other scaling laws are equally valid depending on the measured crater size.
Scaling laws give the right order of magnitude for the crater, but do not describe its final shape.
Porosity taken into account on a first order approximation, other processes like sublimation not considered.
Need for more detailed tools => numerical simulations
R = 200 ai Y-0.2 [m] Vincent et al 2014, Eq. 6

8. 3. Do we understand them ? Hydrocode model:
We used the iSALE-2D code (Wünnemann et al 2006), including the latest improvements on porosity and compaction (Collins et al 2011).
Model: N. Oklay
"Pit-Halo" "Sharp Pit"

9. 4. Link to activity ?
Impact => extreme heat => loss of volatiles
Surface compaction => increased tensile strength => higher pressure required to lift up material
=> Cratered regions will show less activity (Wild 2)
But…
Impact crater morphology affects penetration depth of heat wave
Craters can act as heat traps => Higher temperature at larger depth
=> Cratered regions will show more activity (Tempel 1)
See talk "Thermal wave and geomorphologic features on cometary nuclei", by Sebastian Höfner on Thursday

10. 5. What does it mean for Rosetta ?
Rosetta will monitor the cometary activity and study the link to cratered areas.
Our numerical simulations show that a significant fraction of the projectile can survive the shock and remain buried in the pit.
=> change in composition of the crater floor
=> a 20m impactor will increase local gravity by a few %
=> Escape velocity increased by 10 cm.s-1
OSIRIS, VIRTIS, CONSERT will be able to test these hypotheses

11. Craters on comets - Conclusion
Do they matter ?
What do they look like ?
Do we understand them ?
scaling laws
numerical simulation
Link to activity ?
What does it mean
for Rosetta ?
YES
"Pit-Halo", "Sharp-Pits", + ??
Scaling laws fine for first interpretation, detailed num. simulations required to fully reproduce the morphology. Need more information on surface physical properties
Prevent or increase the activity, depending on local conditions
Look for craters !