1. Jean-Baptiste Vincent updated 08 Oct. 2014
Activity and pits on 67P
Jean-Baptiste Vincent
updated 08 Oct. 2014

2. Is there such a thing as an active area
Is there such a thing as an active area ? Are there features directly linked to activity, either as source or consequence ? The beginning of an answer to the activity question is the deep cylindrical hole seen close to the North pole of the comet.

3. Many jets can be linked to this hole, apparently starting from the bright walls on both sides. Note that the illuminated side of the hole in the image below is the opposite of the one seen in the image above and on previous slide. There is almost continuous activity during a comet day as each side gets illuminated.
The hole has a very peculiar morphology, with horizontal layers, but also vertical striations and maybe small spherical features yet to be explained.

4. Is this pit a typical "active area"
Is this pit a typical "active area" ? If yes, there should be others, with similar morphology and activity.
We have two more in the same image,
and they show some faint structures which are NOT compression artifacts (we have some in this image but nowhere near the jets).
Jets ?

5. SHAP5 images. Some shadows show signal (blue), some not (red), are we seeing dust from active spots or just faint signal from the surface itself ?

6. Looking at it from a different angle, it’s as if the 3 depressions are slowly evolving into one common pit. Their floors have different elevation, though, which could be a signature of their relative age: the deepest = the oldest, or an intrinsic feature predating the activity.

7. Pit forms and expands radially
Since all walls of the pits seem to be active, it's natural to think that they will expand radially. If a pit is initially formed on a sloped/terraced terrain, the circular shape will disappear after a while and only a fraction of the circle will remain. The downward expansion is more difficult to explain, as we need to be sure that any excavated material will escape the pit and not fall back on the floor. The depth of the pit could be indicative of the strength of the activity/gas pressure below the floor/volatile content.
Poor artistic skills !
Pit forms and expands radially
Pit edge cuts through existing cliff/slope,
circular shape is lost

8. Physics ! 200 m Can erosion alone creates such a pit ?
We take Seth_00 as an example.
V = π r2 h = 6.28e6 m3
ρ = 400 kg/m3 => M = 2.5e9 kg
200 m
Assuming Qdust = Qgas in mass, and Qgas from TN-5556
Qdust_perihelion = 255 kg/s => Terosion = 9.8e6 s = 3.7 months*
Qdust_3.5AU = 9.3 kg/s => Terosion = 269e6 s = 8.5 years*
(*) … of constant activity
These numbers are an optimistic lower limit. If we take into account the latitude, we see that a pit like Seth_00 can only be active for a few months per orbit.
Erosion is taking place but cannot be the only process forming the pit
Need for a short-time, catastrophic event.

9. Source of outburst ?
If erosion is not sufficient, we need to trigger an explosion somewhere below the surface. (I'm ruling out pits as primordial features)
which mechanism ?
how deep is the explosion taking place ?
how deep are we excavating ?
is d/D = 1 primordial ?
Source ?
super volatiles building up pressure below the surface ?
cannot be too far deep, thermal gradient very steep
exogenic phase transition ?
see Dennis' talk

10. Explosive events. Methane sink holes in Siberia
Explosive events ? Methane sink holes in Siberia. 30m diameter/ >70m depth
K. Moskvitch, Nature, 31st July 2014

11. What about other pits/holes ?
I started to prepare a catalog of all the pits seen in SHAP4S/SHAP5 images. I named the first one "Seth_00". I use it as a reference and every other depression is compared to this one.
Selection criteria:
circular or quasi-circular shape
layers and/or vertical striations
rough "cliffs", with possible debris field at their feet
smooth floor
(activity)

12. Seth Regio

13. Seth_00 to Seth_04 4 2 1 3 mean d/D = 0.93
Pit #
Diameter
Depth
Depth/Diameter
Seth_00
213
176
0.83
Seth_01
114 91
0.79
Seth_02
118 99
0.84
Seth_03
126
138
1.10
Seth_04
221
198
0.90
mean d/D = 0.9
Note: depth estimated from shadows and visible cliffs on flat images. Needs to be measured again on a proper DTM.

14. There are many "half-pit" structures in the same region.
I'd like to think they use to be similar to Seth_00 but they could be formed by a completely different process.
Former pits ?

15. Seth_05 Next to landing site C.
Circular hole, vertical striations along the cliffs, smooth floor with some debris, no activity visible in this image

16. Ash Regio

17. Ash_01 & Ash_02 2 1
Tail side. Pits which resemble the other ones, but are partly embedded in dust mantle. Not clear to me if they are active spots or not. The floor of pit Ash_01 is brighter than the surroundings, fresh material ?
Note that this is potentially the secondary source of activity predicted by Björn Davidsson and myself from inversion of jets and coma water density in the approach phase.

18. Ash_03-06 03 04 05 06
Right wing. Again many depressions and half-pits are seen there.
Ash_03 & Ash_04 are much shallower than the reference pit, with a bowl shape instead of a cylindrical morphology. Probably not active pits, maybe these are signatures of inner voids, as Simone suggested.
Ash_05 & Ash_06 are more like the reference, but the high curvature of the surface in this region complicates the interpretation.

19. Imhotep Regio

20. Imhotep regio
A few depressions which could be related to activity (blue lines)
Many circular features, described as consolidated terrain around impact craters by myself, or alternatively mud volcanoes by Anne-Thèrèse.
Probably not active.

21. Ma'at Regio

22. Ma'at_01 & Ma'at_02
Two pits on the left side of the head, same morphology, jets detected 2 1
Many small depressions and granular flows can be seen in between the two pits, possible activity ongoing !

23. Ma'at_01 & Ma'at_02 2 1 More flows visible in high resolution
(SHAP5 images)

24. Possible jets coming out of these pits.
Single image inversion gives sources in the vicinity of Ma'at_01 and Ma'at_02 but we are not yet able to link this unambiguously to these two pits. Jet-like features coming out of the neck could look similar due to projection effects.
Very close to the nominal landing site (J).

25. Ma'at_03 Several examples on the right side of the head.
Morphology somewhat different from the reference pit: less deep, and with a rougher floor. It seems to me that if these were active pits, they have not been releasing gas/dust for a long while.
They look older and the debris field is in my view more characteristic of rim failure and mass wasting than fresh dust.

26. Bastet Regio

27. Bastet_01
Right side of the head. Possibly a very old pit. Impressive set of striations on the surface, and significant mass wasting.

28. Map of pits
Pits positions close to the neck are not well projected but this view gives you an idea of where they are. Exact positions given in separated XLS file.

29. Statistics

30. Summary
Many pits on the comet, very specific morphology. Several of them show evidence for activity.
Active pits appear to have a d/D close to 1 (depth = diameter), while inactive ones are much shallower
Current activity/erosion alone is not sufficient to form these pits. They are more likely to start from an outburst/sink by surface collapse, and expand later with the sublimation of the inner walls.
Pits can continue to evolve even without volatiles, but on much longer time scale like traditional craters (mass wasting, rim failure, deposit of ejecta from nearby active sources, …).
Dust which cannot escape the pits accumulates at the bottom and forms a smooth layer, which can be moved or lifted later by complex gas flows.
The walls (or ice layers within them) seem to be the main source of activity.
Pit erosion by activity is an ongoing process. We are monitoring these areas since August, and might have already detected changes. Careful comparison between all images will tell us…