THE MISSING SLABS
Throughout the first eight parts of this series I’ve made passing references to “missing slabs” that were broken from the flanks of the body lobe of the comet and lost to deep space when the head lifted away from the body. Just like the outgassing in part 7 and the Dykes in part 8, this is a working hypothesis which needs more proof. However, as is the case with those two other posts, the existing evidence for this is favourable.
In the header photo, there are two distinct arcs on the body which curve down and up along its flank and roughly parallel to the ‘shear line’ where the rim of the head broke free from the body. One arc comprises the whole of site A the very flat crater. This is denoted by yellow dots. Let’s call that slab site A, which is to say, this is the area where the lost slab A once sat. The other is less pronounced in this photo but more pronounced in the photo below. Let’s call that slab site B (orange dots). The terracotta dots run along the shear line for all photos in this post.
Notice how slab site B’s floor is quite flat, rather like slab site A’s but without the dust covering. It’s somewhat riven but essentially flat with an appearance reminiscent of cleaved rock as if there was once a vast slab, some tens of metres thick, sitting above this stratum layer. In stark contrast to this, the appearance of the curved perimeter couldn’t be more different. It has a crystalline quality reminiscent of fractured marble. That’s not so noticeable in the header photo but obvious in others, below. Taken together, the cleaved stratum running down the flank of the body from the shear line and the crystalline, fractured arc suggest that if there indeed was a slab there, it was levered up along the shear line and broke with a brittle fracture along the arc. Rather like the process of quarrying rock on the Earth, it appears there was little adhesive resistance between the two layers along the cleavage but the lifting or snapping forces along the arc were across the stratum layers of the slab, hence the brittle fracture.
The same principle applies for slab site A. Although you can’t see a cleaved surface due to the dust layer it is very flat so it’s in keeping with the same cleavage scenario as suggested for slab site B. It also exhibits the same crystalline fracture along the arc.
Here’s an additional photo showing slab sites A and B in profile:
And a photo showing the crystalline nature of the break around the arc of slab site B with its key below:
This photo is culled from Part 7 with added annotation. These are the annotations relevant to this post and are all down the right hand side of the photo:
Bright yellow: slab site A perimeter
Bright orange: slab site B perimeter
Light blue: the visible part of the crystalline appearance of the arc (but only about a third of the arc is visible here).
Notice the three orange dots in the bottom right corner showing the arc perimeter coming back into ‘view’- it’s actually a tiny bit further right but the three dots serve to orientate the viewer. I know the visualization is tricky here. Other annotations are with the photo in Part 7 but the terracotta dots showing the shear line are worthy of mention here as well, especially if you try to relate them to the terracotta dots in the header photo. The head lobe obscures the shear line in the upper part before it resumes at the flat-edged crater next to slab site A (that crater was viewed in detail from the opposite side in part 8)
Additional evidence for the brittle nature of the fracturing around the arcs is listed below:
1) The sloping nature of the fracture. This is common when levering cleaved slabs of rock on the earth or breaking highly stratified slabs of slate or sandstone.
2) That said, when the slab is almost free at the last moment, the resistance along the very top edge of the rim of the arc is near to zero and so the fracture can become vertical. We see this very low, vertical ridge around parts of the very top rim of slab site A. (Not very easy to discern in the photos for this post).
3) The very fact that the breaks are arced suggests the slabs were levered up from the opposite side, along the shear line. If levered from a point source, the leverage force is the same along concentric circles from the source. If levered up in sympathy with the rim of the uplifting head, as is proposed, the leverage points (along the lifting head rim) would be spread along various straightish lines as opposed to being point sources. The forces would still fan out in concentric circles but they would be slightly flatter, more oval-shaped. Many other factors to do with the comet structure will muddy this ideal shape (ridges, slab thickness etc.) but these flattened arcs are what we see and are consistent with the leverage force coming from the shear line.
4) Exposed subsurface showing pock marks of lighter material. This is apparent across one side of the slope of the brittle fracture in slab site A. It was remarked upon in one of the Rosetta blog posts before Christmas 2014, which suggested it was newly exposed material. This is of course evidence in its own right for a slab to have been overlying that area. But the pock-marked nature of the whiter material is also suggestive of a brittle break. Here is the photo from that Rosetta blog post:
5) Detritus on the slopes of both fractures. This would suggest they were brittle in nature. Whilst the slabs would have been lifted at one end by the uplifting head, much of the detritus from the brittle break wouldn’t have been disturbed in the sense of being lifted away from the centre of gravity with sufficient velocity to escape the gravity well. Some detritus would have been barely disturbed at all, perhaps snapping and grinding off the underside of the slab on the slope of the fracture and scraping along the surface a few metres, across the arc. That would be the debris we see today all around these two fracture arcs. Some debris however would have been lifted into suborbital trajectories and landed elsewhere on the comet. This could be the reason for the random distribution of so many boulders across all parts of the comet. Another group of rocks, however small it might have been, would have escaped the gravity well along with the slabs. A fourth group would have fallen between the suborbitals and the escapees, achieving orbital velocity but not escape velocity. There is circumstantial evidence for this. The American Astronomical Society 225th meeting in January 2015 hosted presentations on 67P. One mentioned that there were golf ball and baseball-sized rocks orbiting the comet. I would expect a few bigger ones too but perhaps they broke apart via spin-up or micrometeoroid stream encounters which are known to exist between the Earth and Jupiter.
As for evidence that the slabs were levered up from a fracture plane with little adhesive resistance, the following evidence is offered:
1) There is evidence along the shear line that sublimated gases preferentially followed the very obvious fracture planes of the comet (Part 7). Indeed the head broke away from these fracture planes (Part 6). Whatever the deep history of these planes and their strata is, it remains to be discovered but they are undeniably there in the photos and were also mentioned at AAS 225 in January 2015. If gases were finding their way through these fracture planes quite easily it suggests those planes were already weak due to whatever ancient process laid them down. It would follow that they were therefore very susceptible to any uplift.
2) In several of the previous parts I’ve referred to the 300-metre long slurry pile that pushed up the frilly rim of the cliff when it once sat along that line. I proposed that this slurry pile was deposited by gases exiting at what was once a long fissure across the back perimeter of the oft-mentioned rectangle and that this fissure constituted the shear line. This pile now sits marooned like a piece of rope laid across the fracture plane. It even curves round the back of the hillock mentioned in Part 2 (which sits at 2 o’clock from ‘B’ in this post’s header photo) and pretty well traces the shear line for another half a kilometre after that. There are other elongated ridges of a very similar appearance in the same area of slab site B. They also have a rope-like look to them, low and rounded (to the right of the ‘B’ in the header photo). If these were all slurry piles they are the fingerprints of what were once longitudinal fissures, just like the well-documented one in the rectangle. Today, they are sitting pretty atop the fracture plane but if they do represent ancient fissures then, by definition, a slab albeit cracked by these fissures, was sitting on slab site B and these ‘ropes’ were at the bottom of those fissures.
There will be more posts on missing slabs including one site that is appearing slowly from behind the terminator as the comet approaches the sun. As things stand at this date, it looks to have very similar characteristics to slab site B. Its presence is unsurprising, given that it is sited below the most tipped-up portion of the head lobe (so possibly experiencing the biggest uplift force) and is in the most favorable spot on the rotation plane to fling its slab free.
Apropos the above paragraph, I mentioned in a recent post that the cove was around the most tipped up part of the head because it was around the central portion of the cliff but I wasn’t taking into consideration the more dominant plane of tipping which means the cove is really around half way along the slope of the tip.
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO