Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0/A.COOPER
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0/A.COOPER
ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/A.COOPER
The gif is an update. It shows nice matching but the matches are fewer and more nuanced than in the descriptions below. Using the unannotated version is far more useful for seeing even more matches. The single red dot and green pair are starter dots for their respective features.
This part is primarily concerned with the morphological evolution of the two quasi-circular pancakes that are side by side at Bastet. In the process of describing the mechanisms that led to their present-day shape and position, other mechanisms in the neighbouring areas of Babi, Anhur, Ma’at and Khepry are invoked. This is to show that the pancake formation and slide mechanism wasn’t isolated but in keeping with and complementary to what was occurring next door on either side and below.
The global mechanisms of spin-up and the induced tensile stress and Coriolis forces are mentioned too. This is because the evolution of both the pancakes and their neighbouring regions were dictated by those forces. We’ll conclude with the observation that the symmetrical shape of Aker/Khepry and the slid crust material either side of it is effectively the signature of the tensile forces of stretch stamped on today’s comet. The same applies to the two pancakes at Bastet that straddled the rotation plane and the two head lobe crust slides either side of them.
Photo 1- original for photos 9-21.
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
(this credit applies to all unannotated versions of the original, cropped or otherwise until photo 21).
Photo 2- slightly cropped version used for photos 7-21.
Photos 3 to 6- Bastet close-up taken from above photo and 3 annotations with their joint key below.
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0/ A.COOPER
(This credit applies to all annotated versions of the original, cropped as here, or otherwise until photo 21).
Photo 3- original, cropped from photo 1.
Photo 4- the pancakes in red.
It’s worth noting for later that the top perimeters of the pancakes are zig zagged. In this photo, the red dots along the top are kissing the apex of each zig zag point insofar as they can be discerned with the resolution. There are other zig zag lines too because of extra delaminations of the pancakes so they too belong to them in notional terms. But we’re trying here to discern the most obvious zig zag perimeter. The two top perimeter zig zag lines here correspond to the zig zags that are dotted in a similar manner in the much higher resolution photo 24. The intervening photos don’t attempt to pinpoint the zig zags.
The two pancakes were described in Davidsson et al. 2016 as possibly being cometesimals that had splatted onto Bastet. They cite three such features, the third one being below the left hand one shown here. This shows some signs of matching the one above it but hasn’t been fully investigated yet. Since the objective of this part is to match this annotated pair, the third one will be ignored for now, investigated later and given its own part. Suffice to say, the upper-left pancake shows signs of sliding ‘upwards’ from the supposed pancake below it whether they match or not.
This matching and sliding behaviour will show that they’re not cometesimals but slid crust. They behaved in the expected manner during spin up: one piece of crust delaminating into two, the two pieces sliding neatly apart either side of the paleo equator or rotation plane.
Photo 5- the perimeters of the pancakes are dispensed with here and various features are matched between the two using various colours. These are bled matches which means one sat on top of the other before sliding away and the features, having some depth, were duplicated on both delaminations. Bled matches are less obvious than mirrored matches (as in head rim underside to body- Parts 1, 2, 5, 21 to name a few) or translational matches (as in the ‘red’ slide at Imhotep- Part 43). Bled matches have been found in many places especially at Serqet and Maftet (Part 29) and despite being potentially less obvious, they do remain very faithful.
Photo 6- these are additional matches overlaid on the photo 5 matches. Please ignore the terracotta match (see below). The light blue and two righthandmost yellow features appear to have delaminated along the line of the long, straight dark green line (it’s straight but for its small-scale zig zags). However, the bright green hole, left hand red line and left hand yellow feature seem to have slid with more of an upward component. This is in keeping with the left hand pancake moving up the head lobe more than the right hand one. The matches below the dark green line slid notionally south-north as expected for Coriolis forces during stretch. The left hand pancake then underwent a mini delamination from that neat, expected position and rode up the head a little further. This would probably be due to the vagaries of the Babi and Anhur slide head components tugging slightly differently on their respective pancake (see full explanation of this below).
Staying with photo 6, the right hand bright green feature is only a vague match placed here for future reference in case it gets forgotten and not subjected to further scrutiny. I don’t think it is a match but it was added in the early days of working out these matches. The main thing to remember is that, although it’s strictly speaking outside the right hand pancake, it’s a feature that matches so strongly to the body, one kilometre below, that it’s continually depicted in bright green in later photos as if it’s a protrusion to the pancake.
The terracotta match in photo 6 was noticed as being an outright mistake due to a rare error in cross referencing photos and thinking the right hand one was whited out yet visible in a later photo. This occurred just before publishing and so it should be ignored.
Photos 7-10 the head-to-body matches (keys below).
Photo 7- original.
Photo 8- the most obvious matches are of certain features from the right hand head pancake matching to the characteristic right hand ‘dip’ at Aker. The bright green crescent is one of the most obvious of the matches. There are smaller bright green dots on both head and body depicting the interior crescent perimeter that’s a very faithful match. The head lobe component has a northward (rightward) delamination that’s in keeping with the slide vector of the delaminating pancakes.
The right hand dip at Aker is to the right of the the central ‘prow’ and it has an identical twin dip on the left of the prow. The prow runs down the exact centre line of today’s body lobe long axis. When the head was seated on the body, this prow line was where the paleo equator ran, precisely because the fast-spinning comet was pushing out the end of the long axis into a ‘v’ or prow shape (see Paleo Rotation Plane Adjustment page, photo 8 and text).
The short length of fuchsia dots on both the pancake and its seating correspond to the short perimeter section along which the pancake was attached to the fourth Babi cuboid (Part 40). This attachment was when the pancake was still in its seating on the body and therefore before head shear. The fourth Babi cuboid was part of the Babi slide in part 40. Its current position is just off-frame from a point towards the bottom right of the frame. An update to part 40 shows a faithful, hi-res match of the cuboid to the seating with the fuchsia dots. In that photo, there’s a slide track that betrays the sliding of the fourth cuboid over the pancake seating, so we know the pancake slid up the soon-to-be head before the cuboid slid down the body.
Photo 9- some slightly more subtle matches overlaid on photo 8 in light blue and dark green.
Photo 10- this is the same as photo 8 but shows a new feature in mauve. This is the first sign that the features in the right hand dip at Aker match those in the left dip. And both match to the ones on the two head pancakes. They weren’t annotated on the pancakes in previous photos. This means that the two pancakes on the head once sat in the two dips at Aker.
It also means the two dips delaminated from a single feature. It didn’t necessarily have to be a single dip but it had to have roughly the same shape and exhibit that same mauve feature in its bottom-right corner. It also must have exhibited the other more subtle features annotated further below which are now shared between the dips.
It would appear that the original feature was roughly the size and circularity of the largest pancake and had enough depth to create all four delaminations, two on the head and two on the body.
This feature would presumably have been acting like a more bulbous prow, being old crust that wouldn’t be very ductile. So it formed into a spherical cap on the stretching long axis tip. It then delaminated into two at Aker. That delamination would have been the two dips combined with their future pancakes. After this, the top layer i.e. the pancake pair delaminated from the dips and slid up the soon-to-be head lobe. The slightly more rounded seatings of the pancakes are clearly visible inside the dips.
The central prow then must have formed, as opposed to another spherical cap. This might have been because the next layer down, today’s Aker surface, was previously buried, more ductile and so more responsive to being bent into a prow. The bending would be due to the tensile forces of stretch turning into flexion forces as they rounded the stretching tip, creating a moment on the crust either side of the centre line.
There’s much more on the flexion forces rounding the long axis tips in the Paleo Rotation Plane Adjustment page. It explains the uncanny similarity in width of the two flattened tips at opposite ends of the body lobe diamond shape. Aker was the vestige which resisted tearing due to the flexion force and yielded by bending into a prow instead. It was narrow enough to resist the moment exerted and anything beyond its present-day perimeter couldn’t therefore resist that moment, hence its two snapped-away parallel sides and central prow. The snapped-away sides became the Babi and Anhur slides. They’re perfectly visible, flanking Khepry in the expected symmetrical manner: the Anhur v shape on one side and the fourth Babi cuboid on the other. The Anhur v shape obligingly took its Aker tear delamination with it just to make our matching job easier (photo 27- four blue dots)
The mauve feature in the left hand Aker dip is more obvious in some OSIRIS pictures, proving the match. A relevant photo may be added as an update in due course but the point being made here is that it’s a very clear match. It’s fairly clear here anyway. There are two interesting additional factors regarding this mauve match in the left hand dip. Firstly, there are three large boulders that lie just below the apparently truncated thin end at the bottom, suggesting they were broken off as the Khepry onion layers delaminated and slid eastwards (downwards in this view- see photos 21 and 27). You’ll need to look at the unannotated version to see the truncated end properly.
Incidentally, the eastward delaminations at Khepry, along the paleo equator were core-directed. Core directed delaminations involved the core stealing matrix material from the short axis and donating it to the stretching long axis. This meant the crust had to yield by delaminating along the line of greatest stretch- the equator. So just like at Imhotep (Part 42), the red triangle (Part 26) and the head lobe layers (Part 29), any core-directed crust stretch was always east-west along the equator. Rather than actually stretching, it took the form of delaminations (Parts 29 and 38 among many) and sometimes rifting. Serqet/Nut is a lower onion layer poking through one such rift (bottom of part 29). As soon as any crust was completely loose or nearly so, Coriolis forces took over (see the Imhotep ‘blue’ slide in Part 42).
So right/left or north/south delaminations in photo 10 and subsequent photos are loosened crust and Coriolis directed. That’s what happened to the two pancakes when they were on the body. One slid and came to rest north of the paleo equator, in this case the prow, and still kissing it. The other one slid and came to rest south of the paleo equator and still kissing it too. Thus the two pancakes kissed each other with their kissing point right on the paleo equator at the prow. Then, just before the head lobe sheared for good, they recoiled up the soon-to-be head just like (and directly attached to) the head lobe components of the Babi and Anhur crust slides. Part 41 for Babi and the Paleo Rotation Plane Adjustment page for Anhur. The Babi slide’s corresponding head lobe components are the two delaminated layers curving beautifully all the way round from just above the right hand pancake to the so-called cove, one kilometre above the north pole. Between them they host (unsurprising for stretch theory) the much talked about ‘sunset jets’. The head lobe components of the Anhur slide (see below) were attached to the left hand pancake halfway up the south side but they don’t now extend to the corresponding place on the south pole due to material now lost (see photo 27 and Part 20). So it was one head lobe crust slide for each pancake, attached either side, north and south.
For a more graphic description, it was like a pair of goggles (the pancakes) being slid up onto the forehead of the head at Bastet with thick straps (the head lobe crust slide components) stretching round to the ears which would be the cove for the north side and material now lost on the south side. All completely symmetrical but for the left hand goggle glass rising a tad further up the forehead. Or perhaps one could say that seeing as the goggles were originally below the neck and the right hand goggle glass (from our view) is still near today’s head rim, perhaps the goggles were hanging round the neck and slipped up over the eyes but were skewed up at the left. Returning to photo 27 might now might elicit a smile.
A second observation regarding the mauve feature in the left hand dip is that it has four further delaminations. They’re spread leftwards (south) across the dip and are not annotated. The wide tops are the most obvious parts. They slid slightly downwards as well as left. If one produces the end of a line joining the delamination tops, that line crosses the Aker perimeter at the corner of the dip and into Anhur. It immediately joins the main Anhur V. That V appears to have pulled and bunched up the bottom of the dip down towards Khepry causing the higher bottom scarp of the dip. This is of course in keeping with what the Babi slide (Part 40) was doing in mirror fashion on its side of Aker. This caused the same effect in the right hand dip and a thicker scarp all the way across the bottom of both dips.
The two dips, the bottom scarp and the two flanking slides are completely symmetrical because the tensile forces of stretch were symmetrical. So none of this sliding is random. It’s entirely in keeping with the slide vectors that have been described here in many parts over many months. As for Anhur, it deserves its own post but it’s nevertheless described in the Paleo Rotation Plane Adjustment page, along with a short discussion on its mirror relationship to the Babi slide.
PHOTO UPDATE- the OSIRIS close up of the mauve match on the two dips at Aker (viewed from above the head lobe).
ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/A.COOPER
Photos 11 to 14- superimposing the paleo equator and the current equator.
Photo 11- the original reproduced for toggling comparisons.
Photo 12- the paleo equator is added here in brown. The paleo equator is more often called the paleo rotation plane on this blog because the paleo equator is where the paleo rotation plane emerges at the surface and is therefore the plane in which 67P stretched during spin-up. Calling it the paleo rotation plane constantly reminds us of its stretching influence. It’s being called the paleo equator more in this part because of the Coriolis sliding of the pancakes towards either side (north/south) of the actual equator line.
The paleo equator line in photo 12 is faithfully represented down the central prow of Aker and on further down Khepry where it bisects the next paleo equator/plane signature. That’s the Khepry flattened tip, one of the 17 paleo plane signatures mentioned in the Paleo Rotation Plane Adjustment page. The brown line isn’t traced up the neck between the lobes because the neck didn’t exist when the paleo plane held sway. It held sway only when the head was attached hence its being traced over the lobes only. After the head sheared, probably immediately after, the paleo plane started precessing. It precessed 12° to 15° anticlockwise to today’s plane (see next photo). Precession was about the long axis so the long axis tips at Hatmehit and Apis acted as gimbals.
So after head shear, the paleo plane, along with its currently visible 17 signatures, became a fossil plane and no longer held sway. However, the precessing rotation plane would have been close to (only a few degrees off) the paleo plane in the early stages of the head lobe rising on the growing neck. This would be the case even if precession was at play immediately on the commencement of shear and head lobe rise.
It has to be noted here that the the paleo plane line going up between the pancakes on the head lobe is not truly representative of the paleo plane. This point is rather nuanced. It does show the actual line that was produced by being draped over the prow so it used to be truly representative when it sat there. However, just before head shear, the two pancakes rose up the head and the left hand one rose higher in the process. This in effect means that the pancake pair rotated clockwise by perhaps 40°. This was during (and superimposed on) their translational slide from Aker and up the soon-to-be head lobe at Bastet. The result is that the paleo plane signature we still see today isn’t representative of its former position or orientation anymore. It’s now skewed by the same 40° clockwise because it’s attached to the middle of the two pancakes. It’s nevertheless useful as a guide because we can rotate the pancake pair back anticlockwise by 40° in our mind’s eye. This would be in order to seat the old signature over the paleo plane position. This still wouldn’t be the place the signature originally came from (which was the prow at Aker) but since the translational component of the pancake slide was westward, straight up the paleo plane, the signature line from the prow stayed on the paleo equator line during the slide- but for the rotational component which we’ve accounted for by rotating the pancake pair back again. The result of this operation is shown in photo 26.
Photo 13- the current equator is superimposed in dark blue. The current equator is also the current rotation plane. It’s at an angle of 12°-15° to the paleo plane. The blue line is always an informed guess based on the low resolution shape model. There aren’t many detailed features to relate from the model to the photos so it’s never quite as accurate as the paleo plane based on the 17 signatures of stretch. However, it’s fairly accurate, to within perhaps 50-100 metres, which is enough to show the degree of precession that occurred after head shear.
Photo 14- with the shear line on the body and the head rim shown in terracotta. This is to show how the left hand pancake is rather further up the head lobe from the rim than the right hand one. Also, notice how the top edge of the pancakes’ seating in the dips at Aker run along the line of circles that denote the shear line across the top of Aker. These circles were noted in Part 21 and match to the cylindrical shapes running along the head rim directly above. The top edges of the pancakes are zig zagged as mentioned for photo 4. There are also zig zags between the shear line circles by virtue of their arrangement in a line. The circle zig zags on the body haven’t been matched one-for-one to the pancake tops on the head in any certain manner yet. However, even in this fairly low resolution photo you can see that there appear to be three triangular shapes between the circles along the top of the right hand seating between the last fuchsia dot and the turn to descend the v-shape to the Aker prow. Similarly on the head version there are three or four similarly sized triangles in the same place depending on which delamination you choose.
It’s telling that we’re even able to discuss these similarly sized, roughly congruent triangles of the same number, arranged in a similarly straight line of the same length and for a tiny run along the perimeter (that’s constrained by larger scale matches either side). That’s as opposed to completely different-sized square shapes or oblongs at different angles or different perimeter lengths of any shape at all. Or an infinite permutational array of amorphous blobs matching nothing anywhere- as one would expect on two lobes that had formed billions of kilometres apart and slowly drifted together.
Photo 15- the original, reproduced for toggling comparison.
Photo 16- having matched the right hand pancake to its seating below on Aker and to its left hand partner, it remains to add in the left hand pancake and its seating.
The left hand seating was also matched to the right hand seating so it’s a physical impossibility for the left pancake not to have been seated in the left hand seating. All other colours are as for previous photos.
Photo 17- this photo shows the triangle at the top of the Aker prow.
It’s a flat triangle sitting in front of the line of circles that delineate the shear line along the top rim of Aker. This triangle is the same size as the triangle between the two pancakes along their top perimeter. It’s also exactly where you’d expect such a triangle to be if the two pancakes came from the two dips. It’s better to look at the original, unannotated version because the dots partially obscure the triangle.
The two pancake seatings shown here don’t quite reach the line of circles at the shear line. This was an experiment, a slight adjustment to the seating. This is because there are clear delaminations above the pancakes (shown in red). These are zig zagged too but arranged along straighter lines on a larger scale. This suggests it was they that sat along the line of circles at the shear line and the pancakes sat just behind them. However the delaminations are so thin, they probably constituted so little volume that they just merged under the pancakes that thus did indeed kiss the circles. This is almost certainly a dud experiment because the pancakes have zig zags anyway. This was dawning on me whilst thinking it through again but the annotation is done and serves to introduce the triangle match and upper delamination lines.
The experiment was also instigated to help explain the somewhat more circular pancakes fitting to the curved areas in the somewhat squared-off dips. However, the slight anomaly between squared-off and rounded is largely explained via the eye being drawn away from the obvious rounded seating by the prow and a few other straight lines.
Photo 18- this shows the two pancakes as in photo 16 but with the shear line and head rim added in terracotta.
The shear line is now kissing the pancake seatings as before and is thus not very obvious on the body. The head rim and shear line are shown here to give context regarding visualising the sliding and clockwise rotating of the pancakes. They also aid us in seeing the movement of the head after shear.
The shear line riding off wildly into the distance on the right isn’t a vague, optimistic guess. It’s been finely matched to the head rim 1 kilometre above it and the Babi cuboid slide 800 metres below it, all explained by and within the constraints of the tensile force vectors at play during stretch. This short section of shear line has many parts dedicated to it including Parts 38 to 41.
Photos 19-20 original reproduced for toggling and additional matches between the left and right pancake seatings (in orange, dark green and light blue).
It’s advised to fly solo by referring to the original as well rather than relying on the dots.
Photo 21- The slide delaminations down Khepry (medium green lines and mauve feature slide). Also light blue dots showing tear line matches where the Anhur crust tore away from Aker. The left hand pancake therefore suffered this tear along with its seating tear when seated in the left hand dip.
In photo 22, below, the blue-dotted matches are very well-defined triangular tear features. For the body, they’re more obvious in photo 27. That photo includes a further delamination down the body of the blue dots.
Photo 22- original for the Bastet pancake close ups. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DyASP/IDA/A.COOPER
The head lobe horizon (not the rim) is shown in small, yellow dots. This was the only relevant Bastet close up in the 2000 OSIRIS photos from MTP 07, taken from about 30km with the narrow angle camera (NAC). There were in fact five almost identical versions sitting together. For this reason the pancakes themselves are so foreshortened that they themselves bear little fruit in terms of showing the matches between them. However, in the foreground (south of the left hand pancake) there are some “off stage” matches which follow the same delamination vector. This is a mirror image (in terms of position and force vector, not shapes) to the bright green delamination that went in the opposite direction, north, from the right hand pancake. Both delaminations are away from the top, outside ‘corner’ of their respective pancake.
Photo 23- the zoomed-in version used for annotation.
Photo 24- the delamination from the top-left of the left hand pancake, going in a southwards direction (towards us). Key below.
Before we describe the delamination, the red perimeters are the actual pancakes as before and the two farthest bright green features are the two obvious holes as before. The red dots along the tops of the pancakes are each placed on a point of a zig zag as was attempted in photo 4. You can see they are very obvious for the right hand pancake (at the top of the picture). There are five points dotted here instead of the supposed three or four suspected earlier. However, if you check photo 22, it looks more as though there are three along the top with one rather stunted one (second one up in this view) and the top one being some way down what was the fuchsia line in other photos. More close-up work on the circles along the shear line is needed to firm up these mini-matches. The upper delaminated lines of zig zags are also visible here. They form lines that sit above their respective pancake tops, especially noticeable for the three (or four) zig zags above the right hand pancake. One faint, long line of zig zags runs across the top of the triangle that fits to the prow triangle. There’s another very vague line above that, which was annotated earlier in the lower res photos but it looks as if it’s not a line of zig zags.
The delamination, which is the main focus of photo 24, has its seating within the left hand pancake but slides to a point outside it. The features that slide are denoted by mauve, bright green, orange and yellow. Their visibility is in the same order as the colours listed, mauve first, yellow last. You’ll need to refer to the original, especially for the yellow match.
Photo 25- this shows an additional slid feature in red, which delaminated from the red perimeter.
The red v shape on the perimeter seating is one of the Anhur tear triangles, single-dotted in blue above and in photo 27. The long yellow line with a dog-leg is relevant to photo 27 and not to this delamination.
Photo 26- this shows the paleo equator or paleo rotation plane adjusted by the imaginary anticlockwise rotation adjustment of the pancake pair. This process was described above. Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
You can see the paleo plane crosses through the left hand pancake instead of trying to follow the skewed version between today’s pancakes. This adjustment sends the line straight at the centre of the Bastet dip that sits above the Hatmehit cliff. The v apex on the cliff (just over the horizon) is the next paleo plane signature after the prow. This is by far the largest expanse between two signatures on the whole comet. This is precisely because the pancakes slid up Bastet and swivelled in the process. They even carried on delaminating further up. This completely smudged the paleo plane signature.
The probable reason for the swivel of the pancake pair is the fact that the left hand pancake was and is still firmly attached to its Anhur slide head lobe component. Although the goggles analogy had the Babi slide head lobe component acting as a strap attached to the right hand pancake, it isn’t attached today so it might have pulled on that pancake less or not at all, leaving the sympathetic tensile forces alone to send them up together along with all the vagaries of varying friction. The Anhur head component is a very solid chunk and probably resisted friction somewhat more so it slid higher.
Photo 27- this shows the pancake pair and their seating from a south pole viewpoint. Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
You can now see the Anhur slide v shapes and their matching head lobe components. These are also v’s of course. The thick, chunky one is attached to the left hand pancake and the attachment point is denoted by the orange dot. This yellow v-shaped line attachment is shown as the yellow dog-leg line in photo 25. That line doesn’t have an orange dot at its end because it’s a close up. The seating point for the chunky head lobe v shape is denoted by matching its orange dot to the middle orange dot next to the pancake seating. This automatically seats it to the yellow v shape at the seating. However, the seating itself delaminated down the body to the point where the lower orange dot sits with its own delaminated v shape.
The faint blue dots (this is an old annotation, faded with additions) denote the ragged, torn edge of four points that was annotated in photo 21. This torn edge of the pancake seating also delaminated and slid with the yellow v shape and the orange dot that denotes the v shape’s attachment point to the ragged edge. The four seating dots match to the four dots down the left side of the left pancake in photo 21 (not shown here because they’re whited out). It should be stressed that the four pancake zig zags in photo 21 are not definitively matched to the four main zig zags at the seating but it’s a working hypothesis that they do so because it’s the only place they could have come from and both seating and pancake exhibit the same size, shape and apparent number of zig zags. This is the same argument as for the zig zags along the top of the pancakes matching to the circles along the shear line.
The Anhur slide is a mirror slide to the Babi slide in terms of its body slide vectors down Anhur towards the base of the comet and its head components sliding up Bastet with the pancakes. This is why the medium green delaminations down Khepry, also in photo 21, are perfectly believable. They were doing what everything on either side of them was doing and they followed suit.
Similarly, the two pancakes were caught in the middle between the Babi and Anhur head components and drawn up with them by the same slide vector. All this happened when the head was still attached to the body along the shear line as shown in previous photos. It probably happened only shortly before the head actually sheared. From that point onwards, the tensile forces of stretch were transferred to the growing neck and the slides stopped.
The symmetry of the tensile forces of stretch during this whole scenario is the reason we see Aker with two symmetrical dips either side of a prow and with torn sides of equal length equidistant from the prow. It also explains the pancake pair sitting directly above the symmetrical dips, the two symmetrical slides either side of Aker down the body and their corresponding symmetrical slides up the head. And finally, it explains the neat, flat, symmetrical shape of the Khepry surface below Aker and the fact that its about the same width as Aker too.
In the process of describing the morphological evolution of the two pancakes on Bastet we have managed to knit that evolutionary scenario seamlessly into the already known processes in neighbouring regions that were brought about by stretch. Therefore the evolution of the pancakes is complementary to that ongoing, internally consistent hypothesis that 67P stretched.
All the processes described for the pancakes and for the neighbouring regions are constrained by and consistent with the tensile force vectors of stretch as brought about by spin-up of the comet. This has resulted in the symmetrical shape we see at Aker/Khepry and its environs as well as on the head lobe at Bastet and its environs.
The signatures of the symmetrical tensile forces of stretch are literally stamped on the comet because those symmetrical forces pulled the comet into a symmetrical shape.
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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All dotted annotations by A.Cooper
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DyASP/IDA/A.COOPER