Red- ridden-up layers.
Blue- matching holes that were nested on the head rim.
In Part 28 the final pieces of evidence for the head lobe stretching before it sheared were presented. These were the Hatmehit V’s, the stepped feature along the sliced rim of Hatmehit and finally, the onion layers which were said to have ridden up over each other during head herniation and stretch. No annotated photos were presented to support the riding up of the onion layers. The first batch of those photos will be presented in this post. There are too many for one post. It will take several, posted sporadically between other items.
Prior to the head shear, the concentric onion layers of the head didn’t just fold forward into upright folios, as described in Part 27, they actually rode up over each other. This is because the tensile stress due to comet spin-up was pulling them eastwards towards Bastet while the two lowest layers at the head rim were still pinned to the body (the so-called vertical wall stratum and Serqet rim stratum, described in Part 26). The folding of the layers into V’s relieved some of that ‘centrifugal’ tensile stress but it wasn’t enough, hence the next option, which was for them to ride up over each other.
The riding up of the onion layers would, in notional terms, be similar to a Venetian blind, except that it would be curved around the curving head lobe. That’s not a bad mechanistic analogy to bear in mind but perhaps it’s a broken Venetian blind with some layers angling up a bit more than others, some fused and a few torn in half, riding upwards but sideways a bit as well. However, when you see it in the round, the general idea of layers riding up over each other like a blind is unmistakable.
Photo 2- regional map of the comet for orientation.
FOUR RIDDEN-UP HOLES AT MA’AT/ MAFTET
The riding up of head onion layers is most evident in the form of four holes near the Ma’at/ Maftet border. The holes repeat on four successive layers.
Photo 3 shows the head lobe, including ridden-up layers with matching holes. Photo pairs with unannotated versions are numbered as one photo in this post. The NAVCAM took this picture at 90° to our usual ‘upright duck’ orientation, hence the vertical wall is facing upwards instead of to the left. It’s the obvious white area at top-centre and the head rim stratum flares out a little higher. Rotation direction is right side downwards.
Photo 3- four holes.
Light blue dots- holes that repeat on four successive ridden-up onion layers.
Two red dots- location of ridden-up features in the next photo.
Green, orange, yellow- these are the usual coloured head rim matches (Part 24) that match to similarly shaped areas below on the body. The dots are in fact just the most obvious sections of each matching area. They all join into a 1-kilometre-long matching line. The mauve match is almost completely obscured by the yellow one so it’s not shown here.
Terracotta- the sliced facet (Part 20) which defines the southern top edge of the head lobe.
Light orange- this broken line shows the exposed ends of onion layers that were broken off as collateral damage in the main head lobe slice (Part 20). They are hypothesised to have been pushed upwards, being on the top of the head but too near the edge of the slice to escape some damage. Although we’re calling them onion layers, they flaked away like puff pastry layers. They have left lower onion layers exposed, which are below the broken, light orange line. These exposed lower layers show up as a slightly rougher, more crumbled region than the non-flaked (but ridden-up) layers above the light orange line. That’s why this small area has been noticed as being different and therefore been designated as a separate region called Maftet. However, the reason for its different appearance (the probable head lobe slice) has not been identified by the Rosetta mission. Maftet may extend as far as the four blue dots because there’s some roughness up to that point too but Maftet is hard to see on the ESA regional map.
RIDDEN-UP LAYERS AT MA’AT
Photo 4- ridden-up layers at Ma’at. This is a close-up from the above photo but it focuses on a section of Ma’at on the border with Serqet. This photo is centred on the two features dotted red in that photo and has been swivelled 90° to ‘upright duck’ configuration.
Mauve- this is the very tip of the mauve head rim match peeping out from beyond the yellow match. Together, they form the tips of the pillars of the recently named Alexander Gate.
Dark green- perimeter of two matching features belonging to adjacent onion layers that have ridden up; and dark green is also used for the outline of the green head rim match itself, at the bottom on the rim line. The first and second ridden-up layers were nested over the head rim match before the stretch event.
Red- part of a curved, finger-like feature, which is an extension at the bottom of the first ridden-up onion layer. It’s picked out because it has a well-defined dark perimeter, which will be seen to be significant for the body match described further down.
Yellow- this is a quasi-rectangular area. It has the same width as the finger-like protrusion on the layer below it. It looks as if the two yellow dots at the bottom of this rectangle would have fitted somewhere close to the top of the red protrusion below, most likely at the green dot on the sharp turn into the finger. Indeed, in photo 11, further down we’ll be able to see the curved end of the matching finger-like protrusion below the yellow rectangle. This is all but whited out in this view but this photo was better for showing the general outline of the nesting layers.
Light blue- a triad of three small, scalloped dips. Notice there is a third set of them far above the two matched layers, denoting what is a third but smudged, matching onion layer. We know it’s there because the four holes in the previous photo dictate that there are at least four related layers. So we’ve found three of them matched a little way over, two of them very well matched. The fourth is the green head rim match itself. All four would have been nested against the Anubis slab a thousand metres below on the body. They were sitting right inside the rocky tray of the green body anchor, nested with the green head rim match at the bottom. The tray is the same shape as all these nesting features in this photo including the green head rim match:
Photo 5- the green body match, also called the green anchor tray.
The green head rim match is slightly smaller than its two matching layers that rode up above it. That’s because it sat at the bottom, inside the tray and its two companions were nested on top. They sat over the sidewalls. That ‘red’ protrusion above on the head probably sat in the dark, rounded dip along the front of the tray. The dip is favourably positioned and shaped to accommodate it. You can also see it is edged with the same dark-edged, rounded perimeter as is exhibited on the red finger tip on the head. Photo 11, later on shows the upper protrusion of the two ridden-up layers to be scalloped as well as rounded, which lends support to this dip accommodating it. See also the ‘advanced’ annotation of the green anchor tray at the bottom of this post.
Photo 6- a view of the dark green head and body matches together in the same photo. It’s inserted here just as a check that the green head and body matches are indeed the same size and shape and one above the other.
Below is another photo of ridden-up layers showing different matches and extra annotations to the matches above. It’s one photo, presented with five different annotations so as not to clutter it up with dots. The first one is unannotated as a dot-free reference photo, making six altogether. The second one just shows the paleo rotation plane for orientation purposes. The other four show the ridden-up layers.
Dark blue- key parts of the current rotation plane. The rest is left out so as to be able to focus on the paleo plane, which will become more and more important in upcoming posts. That’s because the paleo plane held sway during the riding up of the head layers and the current rotation plane wasn’t in any way involved at that time. The current plane is taken from the shape model equator on the ESA Rosetta blog. The paleo plane is judged from the 10 symmetrical stretch signatures (Part 26) running from Apis to Bastet and on down to the head lobe hinge at Anhur. Seeing as the stretch signatures in the photos are so symmetrical and at higher resolution than the shape model, the paleo plane is easier to judge than the current plane. They’re both accurate to about 100-150 metres, which translates to about 2-3 dot widths for the paleo plane due to the viewing angle and about 3-4 dot widths for the current plane.
Light blue dots- four holes that have ridden up with their four layers and were once nested together. Note that these are not the same four holes as in photo 3. They are a second batch, sited further along towards Serqet and were as good as completely whited out in photo 3. Here, they are thrown into relief by different lighting. The other four holes are in shadow to the bottom-right. These holes correspond, for the most part, to the same layers as the other four. They are just further over and slightly more angled away from the paleo rotation plane due to being forced over. We’ll see why they were forced over further down.
Small light blue- these define lines of travel up the head lobe for the four holes. They are actual, visible lines perhaps more visible in the unannotated version. They link the outside edges of each hole. The reason for the extra line in the middle is that some of the holes seem to have a double-lobed shape, the apex of which is also linked by one long line. The lines look like scrape marks but could be anything. For the most part, the two outermost ones follow the perimeter of a long depression that appears to have extended down from the originally nested four holes. The depression is the same width as the holes. So if they rode up, the depression rode up with them too. It would have gone from being one depression to four concertinaed-out depressions, which together now form a long, shallow trench.
The double-lobed holes and the line down the middle seem to suggest a slight sideways component to the sliding of strata. That would be a vector in the direction of Ma’at (in shadow, bottom right) while the bulk of the sliding was up the head lobe. Combing the two vectors betrays the direction of slide on this side of the rotation plane which is essentially in line with the rotation plane but biased slightly to the right of it. We’ll see in a future post that the layers riding up to the left of the rotation plane were biased slightly to the left in mirror fashion. This mirrored behaviour is entirely consistent with the head lobe layers folding into V-shapes under the tensile force of centrifugal spin-up (Part 27). The V’s had their apexes centred on the paleo rotation plane (Parts 26 and 27). The riding-up layers would exhibit this slight sideways sliding tendency as they were pulled forward, up the head, but were forced out sideways slightly, along the arms of the V’s. We can see this behaviour very clearly in photo 9, above. The red-dotted onion layer rims have not only ridden up but also spread towards the right of the frame, away from the rotation plane.
Photo 11- vertical wall matches and more green anchor matches.
Light blue- matching lines at the top and bottom of the vertical wall. The curve at the bottom isn’t very obvious here but is more so in other, more top-down photos. These will be shown in a future post because this curve, plus its ridge extension curving down to the orange match fits to the back of the fracture plane (Part 26) in Hapi, 1000 metres below.
Dark green- everything in dark green relates to features that have ridden up from the green match on the head rim. As stated above, all these similarly shaped layers were nested in the rocky, green anchor tray on the body before the head sheared. The green head rim match is delineated in small green dots at the bottom and is in partial shadow.
Large dark green- this dot is the centre of the head rim section of the green match and is the classic position for all single green dots when they depict the green match from a distance in other posts.
Medium-size dark green- these three dots depict an important match. It used to be attached to the end of one arm of the green anchor tray on the body and is now visible in its delaminated form, a bit like an exploded diagram of the original lump. Each central green dot is surrounded by very small dots to depict the perimeter of each component. The lump broke off and rose with the head. Before the shear, when the layers were riding up, it rode up with them, leaving a small residual lump at the anchor. This is angled away from us slightly in this photo hence its narrow appearance. It appears that the main part of the lump was deposited at the base of the vertical wall because that’s where its ridden-up layer decided to stop. The layer at the top of the wall exhibits the hollow from which that lump broke out. If you replayed the movie backwards, the vertical wall would be seen to slide down and out of view behind the rim stratum that has the lower blue curve. The top curve and bottom curve would nest together just before it disappeared behind the rim stratum. The wall would not only slide downwards but sideways towards us while the rim, too, eased towards us in sympathy with it (because the rim stretched away from us with the wall base just before head lift-off).
In reversing down at this angle, the hollow at the top would scoop up the main lump (middle green dot) on its way down. Then the two together would slide down a further stratum level to nest over the green head rim match. They would come to rest, slotted over the small residual lump sitting where it always had been, at the anchor. However, both lumps and the hollow travelled up with the head after shear. That means there was nothing of this broken-off end of the anchor left on the body.
Note the scalloped nature of the red finger next to the main lump in the middle. This finger matches the one already mentioned above in photo 4 and that one is seen here too, just above the shadowed head rim match. The scallop for the upper finger was mentioned above as making it a strong candidate for sitting in the dip that is sited in front of the the green anchor tray rim (photo 5). That’s the only physical place it could fit when these layers were nested in the tray. So, with the scalloped upper finger nesting over the curved, black-edged bottom finger, they must have sat in the only scalloped, curved, black-edged location they possibly could have- the dip at the front of the tray.
Photo 12- the ‘bright green ridge’ extension matches.
Bright green- these three matches are annotated in bright green because they’re a continuation of the very straight ‘bright green’ ridge that extends along one perimeter of the slab A extension on the body (Part 22). That ridge was described as extending up the first rim stratum above the mauve head rim match in Part 26 and you can just about see it here, a white strip extending down from the lowest of these three matches. It’s the white strip on the horizon. Although it’s only just visible here, it has a square cross section just like its rectangular, recoiled counterpart on the body (red triangle recoil, signature 6, Part 26). Its top end is annotated bright green and appears torn.
The second match is at the top of the vertical wall. Its tear that matches the ridge isn’t visible here because it’s facing down at the ridge end it tore from. If you slid the vertical wall down behind the first rim stratum, as described above, the two would join. This second match does, however, present to us the tear that married to the third match above it. It’s a long tear, directly facing us.
The third match is at the other side of the Nut depression. We’ve already discussed how the Nut depression isn’t an eroded section of Serqet (as the OSIRIS regions paper suggests), but the revealed top of the wall due to the stratum above sliding up the head. So the third match belongs to the slid stratum above the wall. It was formerly joined to the top of the wall, just like the first match below it, but in this case it was joined to that long tear on the side we see. There’s a spidery ridge line along which it slid and, if you were to reverse the slide, you would see it slide along that middle, wall-top match and click into place. This third match is clearer in top-down photos and will be revisited in more detail in future. Its tear is of course facing away from us so it doesn’t look very interesting here.
THE VERTICAL WALL PUSHED THROUGH THE LAYER ABOVE.
Since the lower, rim stratum match in photo 12 joins to the top of the vertical wall and the upper, Nut match joins to the same lump on the top, we come to the inescapable conclusion that the head rim stratum and the layer above Nut were once joined together as one stratum with the wall out of sight. The wall layer was buried one layer down beneath this single upper layer. There is also evidence for this on the body, 1000 metres below. It means that the wall tore its way through between the two layers and tipped upwards under the tension of stretch so as to become more vertical when viewed in upright duck mode. It had a greater tendency to do this than any other ridden-up stratum because it had its two ends ripped away at Anubis and Ma’at (when the wall and Ma’at were on the body).
The mechanism for this surgical excision of the wall from its respective layer is quite simple and based on the clear stretch signatures presented in Parts 22-28. It will be explained in an upcoming post.
The tipping-up of the wall also explains its wide, smooth expanse. Possibly even riven-looking. It’s because it was formerly the upper facet of an onion layer stratum that was buried in what was formerly the single-body comet.
We can now go back and see how the vertical wall pushed right through the onion layer above it. Photo 13 is photo 3 reproduced with different annotations and shows the vertical wall side-on.
Photo 13- the vertical wall pushing through the onion layer above it. This happened when still attached to the body while the head lobe was herniating under the tensile stress of stretch. So we are seeing the result, set in the head lobe after shear and will now reverse the process in our minds’ eye.
Bright green- the same matches as those shown in photo 11. Notice how neatly symmetrical they are in their separation when seen here in profile on the horizon. They’re spread along a line almost parallel to the paleo rotation plane (photo 8), as one would expect if they separated from one central lump under the tensile force of spin-up.
Light green- one of these lines runs along the bottom of the vertical wall which is the same as saying it runs along the top of the rim stratum. The other one runs along the back of Nut, which is defined by the onion layer that moved aside to make way for the wall pushing through. When that layer meets the rim layer back at the top of the wall, those two layers become one single layer.
The two light green lines form a very symmetrical shape with the top rim of the wall running right down the centre. The top rim of the wall is of course the join line where the two sections of the single onion layer will zip back together. The three bright green matches are perfectly silhouetted here, which helps to visualise the process.
If you played the movie in reverse and pressed the vertical wall back to where it used to be, under the surface, you would see the two outer, bright green dots move towards each other and merge with the central dot. That means the Nut layer and the rim layer would close together over the wall, meeting at its top rim, with all three bright green matches nesting together in the middle.
The two outer, sliding layers would become the one onion layer they always were when on the body. That single layer used to sit above and parallel to the wall when the wall wasn’t tipped up ‘vertically’. “Sit above” in this case means in the conventional strata sense i.e. with the wall stratum merged back with its respective onion layer, sitting deeper into the comet and the zipped-up layer sitting flat on top of it. It doesn’t mean ‘above’ as in ‘higher up the head lobe’ as we’ve been using the terminology for the ridden-up onion layers. Both layers, the wall and the zipped layer above it, sat flat on the body before shear (although there may well have been a proto-lump).
Today’s impressive-looking wall was therefore once just a buried section of an ordinary onion layer.
During the ‘reverse replay’ the two outer layers would perform their meeting back up in a perfectly symmetrical, equidistant slide. The two outer, bright green matches close either side of the central match as we view it here, one clamping in front, towards us (the Nut component) and one behind, away from us (the rim/ridge component). This, along with the fact that the long Nut match is sitting wholly within its respective layer section means that the two sections of the layer zip very tightly back together in the middle, along the top rim of the wall.
Photo 14- the vertical wall pushing through the layer above it now explains the ridden-up layers in photo 4.
Red- these three layers are (a) the two ridden-up layers from photo 4 that were carefully outlined along with the red finger and three blue scallops and (b) the smudged third layer that wasn’t outlined in that photo but had its matching three scallops annotated. Now that we can step back and see the vertical wall pushing through, we can see that these three layers were delaminating around the edge of the split so as to accommodate the wall coming through.
Regarding the main section of wall pushing through, there are just two sections of one single layer which unzipped above it and slid back spectacularly to where we see them today. But these three red-dotted layers at the side can now be seen to be extra delaminations concertinaing out across the smaller gap at the end of the split: one is from above the rim stratum section, i.e. from above the green head rim match; one is from below the Nut section; and one (the middle one) was sandwiched between the two. All three were nested at the rim over the green rim match and the full nest of four layers originally sat in the anchor tray on the body, as described above.
ADVANCED ANNOTATION OF THE GREEN ANCHOR TRAY.
Photo 15, below, is another view of the green anchor tray to complement photo 5. I’ve realised that those readers like Marco Parigi and Ramcomet who are very familiar with the matches, shear line and stretch mechanisms are capable of taking on much more information. So we’re going to start seeing more advanced annotations that are set aside or at the end of posts. They won’t be necessary for those readers new to stretch. The other photos in the post will be sufficient but careful scrutiny of the advanced pictures, and their unannotated versions will probably accelerate one’s understanding of stretch theory.
Photo 15 has a lot more information than photo 5, including mini matches, along with narrative matches for the ones that would be obliterated by dotting. You’ll be referred to photo 4 a few times for comparison and to photo 5, both of which are reproduced here first:
I believe most of the annotations in the tray are very close to where they sat but they can’t possibly be exact to more than about 20 metres due to the matches themselves stretching. The tray is about 350 metres across.
Large dark green- the classic annotated position of the green body match/anchor tray. Notice how it’s set back from the outer green line which is now reckoned to be where one or both of the layers on the head sat.The large green dot in photo 4 would sit exactly on top of this large dot.
Very small dark green (full zoom needed)- this line runs through the large green dot because it depicts the perimeter of where the head rim green match used to sit. Not to be confused with the four lime green dots that run parallel to and inside the nearside (see below).
Medium dark green- this is the perimeter of the upper layer(s) that slid up on the head. It’s set to the outside perimeter of the tray. For much of its path it follows an actual line.
Red- this is most probably the seating of the red end of the finger protrusion in photo 4, as also shown in photo 5. (Red is also used for the continuation of the shear line towards us because it’s been the traditional colour used for this section since Part 17, March 2015).
Yellow- the classic head lobe shear line going off into the distance.
Orange- the orange body match.
Light blue- these three holes seem to correspond to the three scallops in the ridden-up layers in photo 4. But in retrospect, the upper one is probably too far over. They are also visible in photo 5 where they are out of the shadow and show the same elongation as those on the head. Those three in photo 5 are more bunched together. There is an extra, faint one on the head on one layer which may correspond to this ‘misplaced’ higher one here. However, we’re right at the limit of resolution, not due to pixel size but due to the fact that these matches aren’t mirrored matches like the classic head rim to shear line matches. They’re bleeding through layers, like others we’ve seen before and so they change shape as they do so.
Lime green- these four holes are more like bi-lobed grooves running out to the small-green dotted perimeter. These four grooves would form mirrored teeth and dips along the corresponding perimeter on the head rim match. You can see three of them in photo 5 (unannotated version is best). They’re not dotted due to risk of being obliterated because they’re small.
Red- the continuation of the shear line as depicted in red in Part 17 but getting firmed up here due to seeing more close-ups of the corresponding head rim shape (including photo 4). Remember that if you are comparing the exact head rim line to the classic shear line on the body you follow the very small dark green dots inside the tray round to join the red line. The outer green line shows the line of the ridden-up layers and in that respect it doesn’t show the true shear line that broke away last of all. Satisfyingly, both green lines merge at the start of the red line.
There’s a rope-like or root-like feature inside the tray. It’s very obvious in photo 5 but here it’s in shadow except for the very nearest tip peeping from the shadow (second and third lime green dots up). You can just about see it in the shadow here but if you go to photo 5, fix the shape in your mind and compare it to the head rim match, you can see it on the head rim, photo 4, slightly more squared than it is in the tray (another bleed through match). It involves careful scrutiny of how the end nearer the orange match bends round to follow the perimeter of the green match on that side. It does so both on the head and inside the tray on the body. It also does a dog-leg towards the other side of the head rim match just like it does in the tray.
The sub series on head lobe stretch before shearing ran from Parts 22 to 28 but this Part furnished the photos for the onion layers riding up. So, with this post, the series that was started last May with the express intention proving the existence of onion layers riding up over each other, has concluded. Except this is just the start. This fact of riding-up layers opens up an Aladdin’s cave of mechanisms leading to an explanation for the morphology of every region and many sub-regions on the comet. Or rather, since this blog has already explained the large-scale morphology for every region except Ash, we’ll be moving on to explaining the differing morphologies within each region- as well as explaining why Ash looks so different from the rest of the regions. Every explanation is related directly or indirectly to the stretching of 67P.
There are many more ridden-up head layers that match. These will be posted between other items. Just the layers described in this post throw up all sorts of implications for what was going on in the body lobe when the layers were still attached there. This will probably be addressed before more layer matches are posted.
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All dotted annotations by scute1133.