THE SLAB A EXTENSION
(Section 1 of a sub series building evidence for the head lobe stretching prior to shearing).
Yellow- crater left by slab A.
Orange- crater left by slab B.
Green- slab A extension .
Red- triangle of relatively undisturbed craters.
Mauve- single mauve dot is an isolated match, first noted in Part 6. The head lobe match to it is just off-frame above it.
Faint terracotta- shear line up to the beginning of slab A.
At the end of Part 20 there was a mention about new evidence for the head lobe stretching even prior to shearing itself from the body. Apart from a tentative reference to this possibility in Part 8, there has been scant evidence for it so far. Therefore, the assumption has been that all the action regarding stretch happened after the ‘unzipping’ along the shear line and was concentrated in the growing neck, not in the head (or body) lobe. Now there seems to be evidence for the head stretching. This would mean head stretch happening before any shearing had occurred or alternatively, after some but not all shearing had occurred.
I’ve tried to write up this evidence in one post but there are half a dozen concepts introduced all at once. This makes it difficult to emphasise which elements are the most important. Since some of these elements deserve posts in their own right and were sitting in the queue, I’ve decided to introduce some of them individually over the next few posts. However, they should now be considered as linked into a sub-series leading up to the head lobe stretch evidence. Whereas before, these posts would have been interesting discoveries in themselves, each adding to the general evidence for stretching in general, they will now all be adduced as evidence when the final post in the sub series is posted. The sub series will total three or four posts at least and this post on the slab A extension is the first of them.
THE SLAB A EXTENSION
The header photo shows a part of the body lobe which exhibits a somewhat different morphology from the neighbouring areas. This is proposed to be the extension to missing slab A (see Part 9 for missing slabs A and B). It’s dubbed the ‘slab A extension’ rather than ‘missing slab A extension’ which is more correct but a little ponderous.
It’s characterised as an extension because the curved part of its rim is exactly in line with the back curve of slab A’s residual crater. If anything, the right hand side of the slab A crater (as viewed in the header) could be described as being the geometric element that departs suddenly from this majestic arc along the back. So the continuous arc from slab A to its extension suggests a common mechanism for their appearance. The arc itself maintains a steady radius all the way before doubling back suddenly at 90° on the right hand side of the extension. After this remarkably sudden turn, the perimeter line follows a remarkably straight line all the way to the isolated, match point that was cited in Part 6 (mauve dot).
It’s hypothesised here that this area lost surface material at the same time that slab A (and slab B) departed but did so in a slightly different manner. It appears that it wasn’t a clean break of a monolithic slab but a crumbling away of a thinner layer of surface material. While slabs A and B might be likened to thick slabs of plaster that have blown from an old wall, the slab A extension could be likened to a thick layer of paint crumbling off the same plaster surface in large flakes, perhaps taking small lumps of the plaster surface with it, but not dislodging the main layer from the wall like slab A. So the plaster remained in place but its surface looks very roughed-up, almost scoured, by the flaking process.
The reasoning behind this is that there are features such as craters and ridges that are still apparent on the surface of the proposed extension. It’s as if the surface has been flayed: the vestigial craters have had their rims smoothed over whereas, in the adjacent area the crater rims are very defined (these adjacent craters are to the right in the header photo). Whilst the craters appear smoothed over, the hard ridges have resisted being smoothed resulting in jagged, saw-tooth lines and isolated ‘pimples’. Here’s a close-up view.
Green- the slab A extension. This continues only a small distance to the right, off-frame, before meeting the yellow-dotted crater rim.
Mauve- the isolated match.
Blue- saw-tooth line of a ridge (dots are to the right of each tooth so as not to obscure them). Also, isolated pimples (centre of frame). The pimples are more noticeable and more numerous in other photos with more oblique lighting angles.
Orange- a section that has collapsed in as a result of being on the edge of the extension. At first glance, it would appear that the green dots should follow the small, vertical cliff at the back of this section but the surface of the slumped portion can be seen to be pristine, not scalped and it has matches to the small cliff. The green line in this photo follows the same smooth arc as in the header, whereas if we included the orange-dotted area, we’d introduce a noticeable bump in the curve. Moreover, all terrain to the viewers side of the green dots looks decidedly more scalped and scree-strewn. The slumped orange area is whited-out in the header photo. It’s below the green dotted line in that picture.
If a monolithic slab had departed from the extension in the same manner as slab A, then it would have been 50-70 metres thick and taken all these vestigial craters and ridges with it on its surface. In that case, we would see a fracture plane left behind, flat like the remaining slab A crater, or resembling riven rock like the less obvious slab B crater.
So the slab A extension is characterised by the appearance of having been flayed or scalped. That’s just an analogy- it’s not suggested that something was actually dragged over it. But somehow that thin surface layer flaked away resulting in some smoothed-over terrain, some jagged terrain and lots of scree.
To the left of the extension in the header photo, we see the crater that remained after slab A departed (yellow dots). To the right, we see the area of apparently undisturbed craters, mentioned above, which extend across to the sharp ‘drop’ away to the Anubis region (red dots). This cratered area forms a thin triangle with the apex pointing towards the bottom-right of the frame and the short base running from the isolated match point to the Anubis drop-away. The base is contiguous with the shear line.
Below the slab A extension we see the relatively smooth, dusty region of Ash. Ash seems to be quite undisturbed as well, at least when compared to all the upheaval of the shear line and missing slabs occurring above it.
You may have noticed how the regional name for the area comprising the features in this post has been avoided thus far. The name is Seth and it’s actually not very instructive to use it for describing anything to do with the extension, or slab A, or the triangle of undisturbed craters.
The naming of regions was supposed to designate names to morphologically similar regions. What constitutes ‘morphologically similar’ is highly subjective, especially when going by appearances alone and not invoking mechanisms for how they differ. Also, the ESA people who did the naming couldn’t keep sub-dividing areas if sections within them looked slightly different. All in all, they struck a good optimum but Seth perhaps suffered from this process.
Seth has these three regions of differing appearance which were most likely subjected to subtly different processes to arrive at their present-day form: slab A, the extension and the long triangle. These different processes or mechanisms will eventually be adduced as the smoking gun for head stretch before shearing.
So we should try to forget about relating these areas to Seth. The Seth borders tacitly imply one morphology with one unknown, causal mechanism. These three areas should be seen as separate mini regions for our purposes in future posts in this sub series. The annotated photos are the best guide for delineating these regions and their morphology as related to the causal mechanisms suggested in those future posts.
The slab A mechanism has already been described in Part 9 and elaborated on in Part 14. The mechanism for the red triangle staying intact will get an airing. As for the slab A extension mechanism, it’s a work in progress. Although it’s likely to have been subject to much the same process as slabs A and B, there has to be more to it. Future posts will shed some light on it but it may remain partially unanswered.
Although the slab A extension is an interesting feature in its own right and deserved its own post, the take-home message from this post as regards the sub series and the evidence for head lobe stretch before shearing, should be emphasised here: it’s that sudden 90° turn at the end of the extension’s arc and the very straight run up to the Part 6 match.
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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All dotted annotations by scute1133.