67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 27

THE SIGNATURES OF HEAD LOBE STRETCH BEFORE SHEARING. 

  
Key:

Mauve, yellow, orange and dark green- the head rim match points of the now well-documented head-to-body matches in the sub series. 

Red- the head lobe strata lines, stretched into V-shapes.

Dark blue- rotation plane (the xy plane of 67P).

Brown- suggested paleo rotation plane.

White- head tip hinge gouges on head and body.

Fuchsia- Philae’s position (tiny dot next to the bottom-left red dots).

N and S- north and south. A line between these two points defines the z axis of rotation. 

Header unannotated:  

BACKGROUND

This is the second of two posts dedicated to the signatures of the head lobe stretching before it sheared from the body lobe of 67P. The first part listed the so-called vertical wall signatures. This part lists the actual head lobe stretch-before-shearing signatures. Stretch before shearing is distinct from stretch after shearing where the stretching process was immediately transferred to the weakest point- the growing neck. 

Parts 26 and 27 constitute the fifth and sixth sections of the sub series dedicated to proving that the head lobe stretched before shearing. If you haven’t read Part 26, it would be as well to read it first because it contains introductory information which is built upon in the descriptions of the signatures below. It’s not reproduced here because this is, for all intents and purposes, a continuation of that part. Just one point from that post’s introduction will be reiterated in this background introduction: all references to up, down, right and left are with respect to the ‘upright duck’ configuration with the head lobe on top, unless they are specifically stated as being with respect to the sides of an actual photo frame being viewed. 

The signatures of head lobe stretch before shearing are as follows. 

Signature 1- the mauve and yellow head matches are stretched into triangles.   
Photo 3- the mauve and yellow stretched triangles.

The mauve and yellow head lobe matches are stretched into points with striations on their upper faces due to that stretching. They are roughly the same length, showing that they stretched by about the same amount but the mauve triangle includes a big tear, probably because it had to stretch a little further. The mauve head ‘triangle’ isn’t so much a pointed triangle shape, more of a flat-tipped protrusion but it broadens at its base so it’s notionally a triangle or a truncated triangle. This localised stretching means that both these head matches gave up their grip on the body before the orange and dark green anchor points did so, allowing the entire vertical wall above them to start swinging clockwise (looking down on the head). That is to say, although the very tips of the triangles remained attached, the rest of the rim behind them including much of their anchor material, did stretch in sympathy with the wall swing. 

Although the mauve head match only vaguely resembles a triangle, it will be referred to as a triangle or stretched triangle along with the more obvious yellow triangle in this post. This is partly to save on yet more laborious differentiations but also because it reminds us that its very existence is due to the same movement and stretching process that shaped the more obvious yellow triangle. There will be a dedicated mauve match post around Part 30 because its detachment from the mauve anchor was quite complex. However, much or most of the mauve match detachment process creeps into this post because it’s relevant to these signatures. 

Understanding the tendency towards taking on a triangular or quasi triangular form is useful for understanding the mechanism that led to the mauve and yellow head matches getting stretched. The yellow triangle displays it quite beautifully, looking from above. It betrays the mechanism via the surface features that were imprinted on its top surface. These features are described in signature 5, below. As with so many other features on the comet, it’s puzzling that this mechanism signature imprint hasn’t been picked up on before now. 

The mechanism is the fact that the vertical wall swung in a clockwise direction in the final stages before head lobe shear and the mauve and yellow anchors moved with the wall at their bases while their tips remained joined to their body matches. Since the anchor bases remained attached to the base of the vertical wall, they stayed attached across the same width along the wall as they always had but the areas near the tips attached to the mauve and yellow body anchors were thinned both in plan view and cross sectional view. This would be similar to pulling chewing gum from a solid surface or perhaps more like sticking a block of wood to a door with epoxy glue and then pulling the block (i.e. the vertical wall) away again when the glue was only half dry. You would see ribbons of glue stretching between the two surfaces. If you pulled the block from just one end while keeping the other pressed against the door, you’d see ribbons of increasing length across the widening gap. In the case of the comet, the mauve and yellow triangles are the ribbons of glue and they are triangular because they were ‘glued’ all the way along their bases at the bottom of the vertical wall but only ‘spot-glued’ at their respective anchors. Pulling them back produced stretched triangles with wide bases and narrowed tips, rather than stretched ribbons. 

Signature 2- the dark green anchor was the last anchor to shear away. Full zoom needed.    

Photos 4 and 5- both showing how the head component of the green anchor was yanked down vertically. This is evidenced by the fact that it is more face-on to us than its body anchor match, which sits more in profile in both photos. Both photos need full zoom as small dots are used so as not to obscure detail.

The dark green anchor and matches are often referred to as just “green” but shouldn’t be confused with the bright green ridge at the opposite end of the vertical wall. That ridge always has the adjective ‘bright’ before the ‘green’. 

The dark green head match didn’t stretch laterally in the same manner that the mauve and yellow matches did but it did stretch downwards. The evidence for this is the fact that in the header photo to Part 24, the 3D anchor of the green head rim match presents itself to us almost square-on whereas its body match presents itself to us at about 45° profile. This isn’t a simple perspective issue due to the tipping of the head rim. In fact, this angle configuration of the two green match points runs counter to the prevailing perspective of the other three matches which present themselves in profile on the head and more square-on on the body. 

This shows that the green head rim match was yanked downwards markedly as it sheared away and that means that it must have been the last anchor to shear. Indeed it must have been the very last place on the head rim to detach, allowing the head to tip up. That’s because, if the head strata were pinned back by the vertical wall at their centres and therefore folding into V shapes prior to the head lobe detachment, they must have already sheared all the way down the soon-to-be head rim (the shear line) on both sides of the comet, either side of the vertical wall. They would have to shear in order to perform this sliding forwards motion into V shapes. That means the vertical wall was the last structure to shear away and the dark green anchor was therefore the very last place on the shear line to detach. This is notwithstanding the hinge at Bastet/Aker on which the head then tipped but the actual shearing process was complete by that point.

As mentioned above, there is almost no sign of the green head match being yanked laterally, lending weight to the evidence that the vertical wall swung clockwise after detaching from the mauve and yellow anchors and swung on the green anchor as its fulcrum. And it has to be remembered that although the vertical wall itself detached from the mauve and yellow anchors, the top part of the anchors in the form of the stretched triangles on the rim stratum did stay attached to the anchors at their tips.

Signature 3- the stretched, V-shaped strata formations on the head lobe are centred on the green anchor point at the bottom (near the head rim) and on the paleo rotation plane at the top (from the vertical wall top to the Hatmehit crater rim).   
Photo 6- header, reproduced.

This is the most obvious of all the head lobe stretch signatures. That’s why it’s depicted in the header photo. 

The first two V-shapes in from the head rim are centred on the green head lobe match. In fact, they’re centred on a point just to the right of the classic match point. This is where the green anchor terminates- the classic green head match is always depicted in the centre of the anchor. The very slightly lighter area just below the first V-apex red dot in the photo (topmost red dot) is in fact the top-left extremity of the green anchor as depicted with green dots in photos 4 and 5. In those two photos, you can see it arcing left and into the base of the vertical wall so, by definition, it impinges on the vertical wall. It therefore runs a little way beyond the first red line in the header i.e. beyond it towards the bottom of the photo. 

If the vertical wall swung on the green anchor, as we suspect, the exact fulcrum wouldn’t be the middle of the anchor because that would involve grinding the anchor itself to shreds. That simply wouldn’t do for a feature that’s been touted for months as being one of the most resilient chunks of rock on the comet. So the wall would be expected to swing on the northern edge of the anchor and that is what we see. The northern edge is the right hand edge, looking down on the head lobe as depicted in the header. The V apex is right on the point where the anchor ends, as described in the previous paragraph. So the wall did indeed swing on the northern edge of the anchor. 

In Part 24, close-up photos of the body portion of the green anchor show very little debris inside that squared-off U-shape. That corroborates the idea that the head anchor portion was pulled up out of the body anchor U-shape, almost vertically, while its tip held on. If that’s correct, it was then plucked away at its tip with no grinding and sliding, just a break leaving that residue of detritus nestled in the apex of the body anchor. 

The right hand, northern arms of the first two V-shapes, (the ones centred on the green match) actually define the vertical wall itself. So, rather than talking about the “the first two V-shapes” we are really talking about the top and bottom of the vertical wall. No other strata layers bent around the green anchor, it was just the vertical wall that swung on it, as has been stated all along in the sub series. 

As we progress up the head lobe towards Hatmehit, the V-shapes jump across from being centred on the green match to being centred on the brown line. This is the hypothesised rotation plane that’s based on the alignment of stretch signatures. It was first introduced in Part 26. By the time we cross into the Hatmehit crater, there are six V-shapes between Hatmehit and the vertical wall that are centred on the paleo line. This would have been the actual rotation plane at that time, before and during the episode of head lobe stretch before shearing. And this was, of course, prior to head lobe shear and full-blown neck stretch. After shearing, the rotation plane shifted about 20° clockwise to its present-day location. That’s the theory if the paleo line is correct.

A further, isolated V-shape is to be found on the opposite rim of Hatmehit. This V-shape has its apex centred on the paleo plane as well. It has already been suggested as being the hinge on which the missing Hatmehit slab hinged up and departed from 67P. Philae is therefore sitting deep in the ripped-up strata of the southern arm of that V. 

The V’s continue across the Hatmehit crater in a spidery form, not visible in the header. All their apexes align with the paleo plane and, by extension, nestle neatly in a stack between the two V’s either side of Hatmehit. This will be discussed in an upcoming post. 

So this signature can be divided into two components: two V’s aligned on the green match that define the vertical wall on their northern arm; and V’s aligned along the paleo rotation plane from the 3rd to the eighth stratum layer up to Hatmehit. 

Together, these two types of alignment point to the same phenomenon: a tensile stress vector in the direction of prograde rotation causing the head strata to bend round a (temporarily) immovable object. For the V’s up to Hatmehit and centred on the paleo plane it was due to the head lobe strata being pinned back by the vertical wall at their centres while their sheared sides experienced the full tensile force towards Bastet without anything pinning them back. This meant that the six strata above the vertical wall were behaving as if they were bending round the vertical wall even though they were above it. As long as the wall was refusing to swing or even slide, it was protecting the apexes of the V’s above it from sliding while their exposed ends had sheared and were prone to sliding forward, hence the tendency to form V’s. This protection from sliding forward had the same effect as pinning the apexes back to the wall. Most or all of this bending would have occurred when the wall was very firmly in place, held by all four anchors. 

This scenario as betrayed by the V-shapes that are stamped on the head lobe is very neat. It’s not perfect though because we know the vertical wall didn’t fully pin down the stratum directly above it. The stratum slid forward revealing the top of the vertical wall. However, that first V-shape above the wall has a rather curved apex as you’d expect if it was being only half-pinned and sliding forward. So maybe it remains a neat scenario when looked at with a deeper resolution. Also, with a hint of what will emerge in future posts, there’s a superficial sliding element to these V’s as well as a deeper, V-shaped folding element. It’s the deeper element that’s thought to run to the bottom of the vertical wall and across the entire head lobe at this depth, even emerging at Hathor. This deeper mechanism presumably dominated proceedings for the protection or pinning of the V’s. The sliding of the stratum to reveal the top of the vertical wall was more related to the secondary, superficial sliding than the V-shape folding that held sway deep below the surface. That sub-surface mechanism was more dominant and therefore still bent the sliding stratum into a V-shape despite its willingness to slide.

For the two V’s that are centred on the green match, it was due to the vertical wall detaching at the mauve and yellow anchors and swinging round in the final stages before shearing away for good. This means that the vertical wall had also originally been centred on the paleo rotation plane, straddling it as the very blunt ‘point’ of a flat-bottomed V-shape. It would be a flat-bottomed V due to the wall being pinned at both ends with two short arms bending round each end. The wall would have remained in position during the main period of head lobe stretch before shearing. But the V ended up centred on the green anchor after the wall swung round clockwise in the closing stages of head lobe stretch. The V became aligned along a true, pointed V-shape because it was now pinned solely at the green anchor. And really it’s the first two V’s that behaved this way, not just one. Together they define the bottom and top of the wall. Once their apexes had aligned on the green anchor, they remained that way until the final shear of the green anchor soon afterwards. They also remained in that same configuration after the final shear. 

So the V-shapes running from above the vertical wall up to Hatmehit had formed in line with the paleo plane when the wall was still attached and they remained in that alignment even after the wall swung because they were sufficiently far away not to be influenced by the jostling of the vertical wall swing. However, they would presumably have all moved en masse and in sympathy with the swing while maintaining their overall configuration centred on the paleo line. In other words it would have been a simple, translational shunt of the entire stack in the direction of Bastet because the thing that was pinning them back suddenly swung in that direction. This would explain the fact that the head lobe at Bastet is near-vertical and directly above Aker on the body. This is in contrast to the gentle slope of V’s on the opposite side that we’ve been discussing. 

This signature corroborates signature 2 above, which already furnishes evidence that the green anchor point was the last to shear. If it was the last to shear and the strata in its vicinity were experiencing the stretching force vector towards Bastet, those strata were surely going to bend round the green anchor until it finally sheared. That configuration is what we see today, written into the head lobe strata around the green head match point. 

Therefore, the green anchor was indeed the last anchor to give way, as well as being the fulcrum for the wall swing and the last place to shear along the shear line. That would explain the fact that the green anchor is by far the largest and most solid-looking, rocky anchor structure of the four. It has the deepest head lobe ‘roots’ and the largest body outcrop.

The following is an excerpt from an email to an acquaintance which sums up the above description in a slightly different way, along with additional points:

“This also illustrates the related fact that when the vertical wall finally gave, there had to be one of the four anchors that gave up last. It wasn’t going to be the yellow or the orange anchor points because the vertical wall was so solid. It was never going to split in two, so it had to swing from one end or the other and it happened to be the green end. This meant that when the wall gave, it ended up on one particular arm of the stacked V-shapes, the northern arm. It fell into line as dictated by the stretching force vector and the last-minute pinning of its green anchor point that held firm, allowing the clockwise swing. The stretching force was towards the Bastet head-tip hinge. This is why the Bastet head-tip hinge and the vertical wall are diametrically opposite each other on the head lobe with the paleo rotation plane running right through the middle of both. When the green anchor sheared, all the stretching forces were transferred instantaneously to the stretching neck and this meant the V-shaped signature of ‘head lobe stretch before shearing’ was set in the head lobe to this day.”

Signature 4- the tips of the mauve and yellow stretched triangles kiss the circularised rim.   
Photo 7- triangles kissing the rim. 

Mauve, yellow, orange, green- the usual head rim matches (very small, full zoom required). 

White- the circularised rim.

Red- the top and bottom V-shapes. 

There’s a big discrepancy between the white rim at top right of the photo and the arm of the first V-shape. But that arm defines the base of the vertical wall, which swung. The material between the wall and the rim (i.e. between these two dotted lines) stretched due to the mauve and yellow triangles staying attached to their respective anchors. Those anchors remained where the rim used to be on the body and so this stretched rim stratum literally flares out to the same circular line that the rim used to follow on the body and still does follow around today’s head rim. Although it appears as an anomaly to all the neatly stacked V shapes, it could be considered that all the V-shapes are the anomalous feature since they represent completely deformed strata. The stretched region between the white and red lines at least faithfully follows the old rim perimeter line at the mauve and yellow triangle tips even if their bases moved in with the vertical wall base, causing the triangles to stretch. 

The current perimeter line of the head rim at Serqet and along to Maftet (the other side of the green head match) is quite circular despite all the strata above the rim being bent into V-shapes. The reason for this happening at Serqet was touched on in signature 1 and the key for this signature’s photo. It’s because the mauve and yellow triangles remained attached at their tips until very near the last moment. So that rim stratum in the bottom half of Serqet now faithfully follows the original circular line of the rim that existed when it was on the body. 

The reason the rim appears more circular on the other side of the green head match is harder to explain. However, it has to be remembered that this exact section, from the green match, southwards to the ‘sliced facet’ (Part 20) was attached to the Anubis slab before it departed (Part 17). If you recall, the green match was described as the three-way junction between the head, the Anubis slab and the body. That would be when the head was attached to the body, of course. It was the unique point where all three touched each other. A large portion of this section beyond the green body match, which corresponds to the head rim seating position is a little way down the body lobe. This may have resulted in the rim stratum being pressed against the body rather than sliding forwards like all the other strata above it. You can see this lower-down body seating line in the header photo for Part 17. So, if it was pressed against the body it couldn’t slide forward into a V-shape. Additional evidence for this pressing against the body is to be found in the head lobe above the stepped-down section. This was already matched in Part 17. On the head lobe, the rim steps down in lock-step with the body seating line directly below it. The tops of the strata layers along this part of the rim edge and some way above it have been disturbed and flaked off. This corresponds to a roughed-up triangle whose base is the rim section that’s stepped-down. This roughed-up area is consistent with the first few layers of strata on the head lobe being flexed by being pressed against the body lobe. They would have been pressed against it by the forward tensile stress vector. This triangular area of rougher surface has been named as a separate region, ‘Maftet’ specifically because it’s rougher. This roughed-up triangle might be considered as a head lobe stretch signature in itself but it’s rather more tentative than the others, so it will remain as an adjunct to this signature, number 4.

The section of head rim beyond the dark green match was also attached to the Anubis slab, which could have departed at any point in the stretching process prior to head lobe shear. This attachment would have attenuated its forward slide into a V-shape as well. The attachment configuration was with the head lobe rebate attached to edge of the Anubis slab (see other Part 17 photos). The Serqet side of the three-way junction (or green anchor) wasn’t attached to the Anubis slab or pressed against the body. It was sitting on Seth and predisposed to sliding forwards with the rotation plane tensile force vector. But instead of a straightforward slide, it both slid and stretched while swinging round clockwise with the vertical wall. Hence the stretched mauve and yellow triangles. This therefore explains the rather different behaviour of the head rim at Serqet from that of the head rim at Maftet on the other side of the green anchor.

Whatever the reason for the section of rim along the Maftet perimeter being circular and not pressed into the southern arm of a V at the rim, it’s evident that interplay between head rim and body was interfering with the neat sliding forward into a V-shape at the rim. In the case of Serqet it’s clear, as described above. In the case of the southern section that was attached to the body and Anubis slab, it’s less clear. However, since this section faithfully continues the head rim curve from the Serqet curve it can be hypothesised that this is the original head rim line as it was when it was attached to the body. 

This means that the first true V shape is the one that runs along the bottom of the vertical wall, turns sharply round the green match point and continues beyond it at one stratum level up the head lobe.

If the V-shaped strata on the head were somehow circularised i.e. pushed back to their assumed original position prior to stretching, this outermost V shape would be rounded and run concentrically to and just inside the existing head rim curve we see today that includes the tips of the stretched triangles. This makes sense because the triangle tips represent the former perimeter line from which the triangles stretched.

This fact has a bearing on the statement made in signature 3 that the outermost V that bends round the green head match is more marked than the others. The stretched nature of the mauve and yellow triangles draws the eye outwards towards where the more circular rim used to be (and still is except for some dips between the ‘spot-glued’ triangle tips). But these two triangles and the stratum around them is a moderately thin, highly stretched section of rim. As already mentioned, it’s been stretched like pulling a piece of chewing gum that’s stuck to a solid surface. As such, it constitutes the most stretched portion of the whole comet. It flares out to where the rim used to be because it stretched like chewing gum from the old rim point instead of sliding. Thus, if you sliced off the flared rim with the two stretched triangles, you would see more clearly the true line of the first proper V, bending round the green head match. That’s the V that includes the base of the vertical wall and it can be seen to be the most marked V with the sharpest turn at its apex on the green match. You would expect it to be so if it was indeed subject to the prograde tensile force as well as being the first stratum directly above the green head match/anchor. 

The flared-out section with the mauve and yellow stretched triangles therefore has to be disregarded when visualising the V-shapes. This is due to the fact that being pinned, stretched and flared out, it didn’t undergo the bending round the green match in the classic V-shaped stretch scenario. That’s why the base of the vertical wall is considered here to be the line of the first true V-shape. And the pinning and stretching till the end is why the rim of the flared-out section follows the curved line that the head rim used to exhibit. This isn’t speculation, the intricate body matches to the head rim that were so painstakingly traced in Part 24 are also arranged in a curve directly below it on the body. They are not in a V-shape. 

Signature 5- the stretched head rim stratum above the four head rim matches shows a demarcation line between plastic stretch and stretch-to-failure. This is the stratum that includes the mauve and yellow stretched triangles. This demarcation line betrays the clockwise swing of the wall.  
Photo 8- the stretch-to-failure demarcation line, shown in faint pink.

The demarcation line starts at the orange match, extends diagonally across the stretched stratum and ends at the bottom of the vertical wall about halfway along the back of the stretched yellow triangle. If the vertical wall swung clockwise from the green anchor as its fulcrum, it means that, by definition, the green anchor point stayed put and refused to stretch. This in turn means that the orange anchor would want to swing just a small way, being close to the fulcrum. The yellow anchor would want to swing that much more, being further from the fulcrum. And the mauve match would want to swing the furthest, being at the other end of the vertical wall and farthest from the fulcrum. This is why the mauve and yellow head rim matches show the most obvious degree of stretch (into their obvious head rim triangles). They stretched so much that they couldn’t simply stretch plastically but developed striations too. They stretched to failure because their matrix of cometary material wouldn’t accommodate that degree of plastic stretch. The only way for these two matches to continue stretching to accommodate the greater swing of the vertical wall at that end was to develop striations- a pattern of cracks that took on part of the stretch load. 

Some of the cracks on the top surface of the yellow triangle are V-shaped in cross section, an indication that they were taking up much of the stress load, opening outwards from the base of their V-shapes. (These cross sectional V shapes on the yellow triangle striations have nothing to do with the big V-shaped strata pattern on the head lobe). The striations notionally run across the lines of force. They are parallel to the long side of the yellow triangle and slightly curved. This side is longer and more at right angles to the prograde rotation tensile force than the other side so it was subjected to more stress. This means the striations were at right angles to the tensile force and their V-shaped cross section was disposed to open up along the force vector. Other striations run straight across the triangle tip, betraying a similar mechanism at play. Their V-shaped cross sections were free to open up into wider V’s along those lines of force. This shows that the yellow triangle was having real difficulty holding onto its body anchor while remaining attached to the base of the vertical wall at the same time. It couldn’t stretch plastically so it resorted to these striations that were V-shaped in cross section. 

Incidentally, the single, obvious crevice that bucks the trend and runs across the striations to the tip isn’t a striation. It matches the sharp, straight edge of the main rocky protrusion on the yellow body anchor, one kilometre below. And it matches the neck protrusion that sits just below it and which used to nestle against the sharp edge of the anchor. 

As for the mauve head match, which swung round the most, it simply couldn’t arc round that distance even by stretching and forming striations. It exhibits a massive tear on the ridge just above the actual mauve head rim match. That ridge is the oft-mentioned ridge that’s usually annotated bright green and matches the bright green ridge on the body. The reason it could tear so spectacularly is that it is raised and of a square cross section. And the reason for such a well-defined cross section is that strata rifted straight down either side of it- another stretch signature which is described in signature 9, below. This tear was caused by events unfolding further above it on the head lobe and described in signature 6, below, but the net result was that the ridge was pulling massively on the mauve match, stretching it, striating it and, eventually, tearing it.

The tear behind the mauve match was within what is called the stretched mauve triangle but when we start looking at it close up as opposed to in the header photo it starts looking pretty rough and not all that triangular. But it is a protrusion like the yellow triangle and that protrusion has striations plus this tear that’s flared to the right as we look from above. And to the left, the mauve match curves from the protrusion, round a dip and then out to the yellow match tip. This flaring on both sides plus the protrusion is what gives it the same basic stretched triangle characteristics as the yellow match even if its tip isn’t at all pointed. 

The diagonal dividing line running from the orange match is the point beyond which plastic stretch without recourse to striations was viable. From this point onwards, towards the green match, the head rim is smooth with a few bumps, resembling trowelled concrete. No striations are visible, suggesting wholly plastic stretch was at play. This demarcation line was manifested because the orange match had to swing much less of a distance as the vertical wall swung round, in fact, about a third of the distance of greatest swing at the mauve match. And by the same token, everything between the orange match and the dark green fulcrum was subjected to even smaller amounts of plastic stretch which also didn’t induce striations. 

The demarcation line is angled across this flared-out part of the rim below the vertical wall instead of running at 90° between wall and rim. This is consistent with the need for incorporating striations into the stretch being a gradual process. It started from a point of fully plastic stretch, continued through a short stretch where plastic stretch was gradually superseded by striations and on to the area of the yellow triangle where striations fully dominate. 

This pattern of differing types of stretch, even including a threshold line for the development of striations and a catastrophic tear at the mauve end is wholly consistent with a gradation of stretching distances. And a gradation of distances is consistent with the swing of the vertical wall being from the dark green fulcrum and the mauve match swinging round the most at the opposite end. This in turn shows there must have been a tensile force in the prograde direction of rotation that was responsible for pulling the mauve end of the vertical wall round as it remained pinned to the green anchor at the other end. Ergo, there was an ongoing prograde force acting on the head lobe prior to the vertical wall swing that caused the stretched V-shapes to bend at their apexes where they were being pinned to the vertical wall. Therefore, signature 5, the demarcation line between plastic stretch and stretch-to-failure is a strong signature for head lobe stretch before shearing. 

Signature 6- the tear behind the mauve head match was caused by the ridge above it trying to bend round and past the base of the vertical wall.   
Photo 9- the mauve head match tear. This is looking down the slope towards the head rim. The brown dot on the ridge coming towards us depicts the torn-away section and that happens to be in shadow. This fits to the other brown dotted section on the actual head rim mauve match point. The seating position is therefore further into the distance and lower down the head lobe than the peeled-back tear. Being just behind the mauve head match, it’s practically on the rim. Its left edge is probably right on the rim. 

Photo 10- the same mauve head match tear with different lighting (zoom for the same two brown dots).  

 This shows the ridge in question bending round the end of the vertical wall. The wall is bending too, yielding to the tensile stretch force vector. That force was in the direction of the red arrow which is parallel to the paleo rotation plane (brown). This end of the ridge appears to be torn as well in many photos from different angles. It may have torn from recesses in the base of the wall (last two pink dots). This would explain why the wall started bending- it was no longer anchored to the mauve match via the ridge. Bright green depicts the ridge. Pink depicts the base of the vertical wall. Blue at left, a single dot, shows today’s rotation plane barely peeping into view. It runs fairly parallel to the paleo line for this short stretch (actually ~20°), down and left.

In signature 5, we saw how the mauve head rim triangle couldn’t stretch the full span of the vertical wall swing at that end without undergoing a massive tear. That tear was just behind and above the mauve head rim match point and went across the ‘bright green’ ridge that matches the bright green ridge on the body. The body component of the ridge defines the border between the red triangle and slab A extension. The torn part of this ridge behind the mauve, head rim match point can be seen to match the tear around the perimeter of the hole it tore from just behind (above) the actual rim match. This will be elaborated on in the dedicated mauve match post around Part 30. For now, to show how everything fits together in sequence in stretch theory we can observe the features and deduce the tensile force vector that gave rise to them. The torn-away part of the ‘bright green’ ridge can be seen to be angled away towards Ma’at like a giant sheet of metal that was torn and peeled back. On closer inspection, it can be seen that this is because the bright green ridge it had been part of is also flaring away in a subtle curve in the same direction. This shows that the bright green ridge was yanking on the mauve head rim point and in this direction before the tear happened. Why in this direction? Because the ridge runs up to the left side of the base of the vertical wall (as per ‘upright duck’ orientation but to the right in photos 9 and 10, above). As it approaches this point it curves outwards as if trying to curve round the base of the wall. It’s quite clear that it was being pulled round and past the vertical wall. And where it was trying to get past the bottom end of the wall, the wall itself is bent round subtly at that end because even it couldn’t resist the prograde tensile force at that end. This was very likely because the mauve anchor was no longer fixed (except loosely at its tip) and therefore not protecting that end of the wall from the prograde tensile force vector. That’s three consecutive structures that are showing obvious tearing, stretching and bending in the same smooth curve, trying to get round the side of the firmly attached wall: the tear; the bent ridge; the bent wall-end. There may be a fourth to add into the neat line astern sequence: it’s the other end of the ridge that seems to have torn from the base of the wall and which was mentioned above. It’s harder to tell from these photos though, so not quite as compelling.

These stretch signatures, certainly the main three, could hardly be more obvious. They just need to be observed in unison instead of in isolation. And the most telling aspect of all is that they are all aligned exactly along the paleo rotation plane, which is the line of the prograde rotation tensile force vector. That’s why the red arrow in photo 10 has been placed where it is. It’s at the end of the line of features and points along that line. It’s also parallel to the brown paleo rotation line. The tensile force vector acted in this direction across the whole head lobe but in this spot it aligns perfectly with the line of stretching features.

None of this stretching and tearing behaviour behind the mauve head match (or mauve body match- signature 6, Part 26), could have happened after the head lobe sheared from the body because all the tensile forces were transferred to the incipient neck at the very moment of final shearing. That’s why it’s compelling evidence for the head lobe stretching before it sheared from the body. 

It should be noted that the bending/ stretching signatures at the other, sloping end of the vertical wall were dealt with in Part 26 because they were tacked onto a vertical wall signature. It was in signature 3, “The top of the vertical wall…” and it mentioned the concave structure trying to bend round the sloping side of the wall at that end. This is revisited briefly in Signature 7 below. 

Signature 7- the head lobe strata stretched past the top and sides of the vertical wall.  
Photo 11- the southern end of the vertical wall, which is at the opposite end to the mauve match and bright green ridge. The sloping perimeter at this end is defined by the beige line. The white curve is showing the concave deformation in the stratum that was trying to bend round at this end. The top of the wall, which was exposed by the forward slide of the stratum above, is outlined in pink. Other colours are as described in other photos. 

This is a duplicate signature because it’s a reminder of signature 3 in Part 26 in which the stratum above the vertical wall slid forward, exposing the wall-top and also the smooth, curved dip in the stratum on its right hand, sloped side. Moreover, the fracture planes curved upwards when the strata tried to get past the sides of the wall. The reason it’s a duplicate signature is that it’s both a vertical wall signature and a head lobe stretch signature so it appears in both Parts 26 and 27. However, signature 6, above in this post, is the signature of the stretching round the other side of the vertical wall, the left side. It wasn’t included in Part 26 as it wasn’t fully understood at the time. It too is a dual signature and it’s a more spectacular example than the gentle, concave fold around the sloped, right hand side. 

Signature 8- Ma’at rifted around 100 metres at the Ma’at-Serqet border.  
Photo 12- the rift at Ma’at-Serqet. Bright green depicts the top and bottom of the rift. It’s in bright green because the cliff formed by the rift is in fact the straight side of the classic bright green ridge. In other words, the raison d’être for the bright green ridge is that Ma’at rifted down leaving Serqet proud of it. The red arrow is the direction of the prograde tensile force vector. Mauve is the mauve head match. The single brown dot is the peeled-back, torn section from signature 6. Its seating is just out of sight behind the top mauve dot. Black dots sloping at the bottom show the head lobe rim. 

The cliff that runs along the Ma’at-Serquet border was formed as a result of the head lobe strata trying to stretch past the vertical wall when the wall was still resolutely sticking to its base and not swinging round on the green anchor. This is why this end of Ma’at looks like a sort of half-hexagon. It’s the only place on this stretched, sloping side of the head lobe, apart from the vertical wall itself, that doesn’t exhibit the very obvious, equal-thickness strata layers. This is because the stress was relieved by the rifting instead of stretching. In truth, the half-hexagon does actually have two ‘equal thickness’ strata layers like the rest of the head lobe on this side. The fracture plane is just about visible in a few photos. However, the upper stratum didn’t shear and slide forward over the lower stratum like everywhere else did with the exception of the vertical wall. It didn’t need to due to the rift which relieved all of the stress in this area. 

The rift is deeper towards the head rim where the mauve match is located and shallower up near the base of the vertical wall. It even continues along the top of the hexagon at a shallower depth. The deeper rifting at the bottom near the rim is about double the depth of the rifting at the top. This would be in keeping with it relieving two strata’s worth of stress. Since stress is a proxy for distance of induced stretch, all other things being equal, two layers would build up twice the stress and that therefore explains the double depth of the rift at the head rim. 

Notice how the cliff forming the rift is in line with the slab A extension perimeter line, below on the body. When the cliff formed by the rift is seen as part of the bright green ridge described in signature 6, it also matches the quasi rectangular block 1 kilometre below, behind the body mauve match point. That rectangular block is part of the bright green ridge on the body that runs along the slab A extension perimeter line. The two displaced sections match in terms of width, height, cross section, orientation and dusty, ‘white’ top. As stated, the bright green ridge on the head is the same one as in signature 6 in this post, the one that curved round the base of the wall. And that quasi rectangular block on the body is the one that slid back from the mauve body match during the red triangle recoil (signature 6, Part 26). It slid back along the perimeter of the slab A extension, which is usually dotted bright green like the head lobe ridge it matches.

Therefore, the cliff that is the rift along the Ma’at/Serqet border was once in line with the slab A extension perimeter when the head was attached to the body. This further reinforces the concept that this was a single, long, straight ridge spanning today’s head and body lobes- but in the past when the comet was a single body. Although it was said that the raison d’être for the bright green ridge at Ma’at-Serqet was the rift, the ridge itself must have existed in some form prior to the rift or it wouldn’t have rifted along that line. The same goes for the rectangular block. They were both part of the same straight, resilient feature even when it was disguised by material sitting against it. In the case of the head lobe, that material (Ma’at) rifted downwards. In the case of the body lobe, the slab A extension material was yanked away from the side of the rectangular block. That is why there is detritus along one side of the block. That detritus all but peters out halfway along the block, which may seem rather strange until we recall that it slid backwards in the red triangle recoil. We can now slide it back in our mind’s eye to its pre-recoil match at the mauve anchor. When in that position, the debris line stops at the end of the rectangular block.

Signature 9- The head rim between the mauve and yellow matches underwent a double-shear due to the premature delamination of the mauve anchor when still attached to the body (per signature 6 of this post).   
Photo 13- the double-shear at the mauve match. The lower mauve line is the main head lobe rim. The upper mauve line is the premature, supplementary shear line. This upper line is attached to, and indeed due to, the tear behind the mauve head match described in signature 6. The tear is dotted with a single, brown dot and this time it’s seen from the other side, from below. Its seating is just out of sight behind the mauve head match. The two shorter mauve lines are suggested mini matches. The yellow line is the continuation of the main head lobe rim around the yellow triangle base. Pink is the vertical wall base. Bright green is the ridge that rifted in Signature 8, rifting on its left side (out of view) from this viewpoint. 

Photo 14- the same photo in close-up.   

Photo 15- the double-shear from above. Same key as above.  

The segment of the shear line between the mauve match and the yellow match underwent a double shear on the head lobe side. This was due to the double detachment at the mauve head match. That would be the tear behind the head rim match point, mentioned in signature 6 of this post. As such, this signature could be considered as an extension of signature 6 even though it contains a wholly new feature that corroborates signature 6, thereby providing more evidence for head lobe stretch before shearing.

The basic scenario is that the torn section, the ragged ‘sheet metal’ section that was peeled back with the head lobe ridge behind the mauve match, lofted a supplementary section of rim between it and the yellow triangle. This was before the main shear line detached and so it had no choice but to stretch a section of head lobe material between the supplementary rim and the still-attached main rim as the supplementary rim rose. Meanwhile, the yellow match held firm until it detached just the once without any similar premature shearing. 

The supplementary rim line initially runs along almost horizontally from the peeled-back tear. It runs towards the yellow stretched triangle and then gently curves down and out towards the tip of that triangle, out on the main rim. It melds with main rim at or just before the yellow tip and that means that it slopes down the short side of the triangle. The reason it can slope down and outwards is that the stretched yellow triangle is stretched upwards as well as outwards so it has the form of a pyramid or cone that’s flared at its base, but only in very approximate terms. This means the supplementary rim slips round and down in one smooth arc that takes in both the downward and outward components needed to reach the triangle tip. This is not really visible in the above photo but those photos that do show it don’t show the rest of the supplementary line so well. 

This vertical stretching between the supplementary section of rim and the true rim resulted in what appears to be half of a vertical crater between the mauve and yellow matches on the head. It’s the smooth, ‘white’ area (actually black to the human eye, ~0.04 albedo, but highly exposed and lighter than its surroundings). It looks like half a crater but it’s simply a stretched section of material between the main rim and the secondary rim above it. It matches a similar ‘semi-crater’ between the mauve and yellow matches on the body but only in the sense that it was joined to that area. Again, this body match looks like half a crater but isn’t. It was created by the red triangle recoil so it’s just a 200-metre gap that has opened up. The gap happens to resemble a crater because it has a curved back section. That back section sprung back violently in comparative terms, from a point right up front, next to the mauve match. The red triangle recoil is described in signature 6, Part 26.

So where is the actual match to this ‘snowy white’ vertical semi-crater on the head? True, it does match the pseudo semi-crater on the body but that area doesn’t have the same smooth, white finish. It’s riven at the front and scree-covered towards the back. 

The actual matching material that tore away from the semi-crater on the head is in fact the curved back section of the pseudo crater. It’s the slumped material that looks like the folds of a blanket:      
Photos 16,17,18- these are all the same photo with different annotations, key below.

The first of the three photos shows the match line along the back of the pseudo crater on the body (red) and the line it tore from (yellow). The second shows the same two lines along with the perimeter of the scree that appears to have been exposed from underneath the ‘blanket’ during the recoil event. The third is unannotated for clarity due to the dots obscuring detail. 

The match to the head’s semi-crater is the smooth, light coloured section sloping down from the quasi rectangular block that slid back along the bright green ridge. The block’s backward slide also happened during the red triangle recoil. The central ‘crater’ section next to it recoiled too but even more violently, which is why it sprung back into a curve and resembles a badly folded blanket. It’s flared-up along the front perimeter (the match line), it has a gentle, longitudinal fold along its middle and a deep, shadowed overhanging fold all along its lower, back perimeter. The extra-violent recoil of the ‘blanket’ is why the rectangular block’s matching tear (to the mauve anchor) is yanked even further back on that side. And that, in turn, is why it’s significantly angled with respect to its mauve anchor seating. Again, that’s described in signature 6, Part 26. The recoil was so violent that the blanket even tore itself from the front, inside corner of the block and remains in a peeled-back configuration some way further back down the long side of the block. It’s sticking up as a ragged, sloping edge of the blanket that matches the ragged, sloping edge of the mauve body match. This violent recoil also explains why the ‘blanket’ doesn’t have a front perimeter line that matches the bottom of the head rim semi-crater all that well. However, both the blanket and the head’s semi-crater, which is really the other half of the blanket, can be matched to a line on the body. The line is on the scalped pseudo crater, dotted yellow, and it’s the line along which the blanket tore in two. The two halves tore from each other and, prior to tearing, they were stretching apart as one single blanket. The tear must have happened just before or at the point of final shearing at the main shear line. This will be explained fully in the dedicated mauve match post. Here are some more photos from different angles:    
Photos 19/20- The blanket tear line. The view is from behind the mauve anchor, towards Hapi. 

Red- the recoiled blanket front perimeter line.

Yellow- the tear line of the blanket (the same yellow line as in photos 14 and 15).

Bright green- the matching tears for the end of the rectangular block and the back of the mauve anchor. The mauve anchor isn’t shown mauve because it would involve mixing mauve dots in with both green and yellow because the latter two are contiguous with large parts of the mauve anchor. But the anchor can be described as having its centre point at the join between the green and yellow lines and it’s just out of view, down the slope into Hapi. Its top and right hand edges are, for all intents and purposes, the shadowed frill.  

Photo 21- the ‘blanket’ matches shown together in one photo: 

 Notice, the lower mauve line on the head’s semi-crater has been extended by two dots up the side of the mauve head match. This is because the blanket material on the body extended up to this point on the counterpart side of the mauve body match counterpart. The true match to this body counterpart is actually the supplementary rim above the mauve head match, that is, the peeled-back tear up behind it on the ridge. However, the tear was originally seated on the mauve head anchor anyway. So it followed along this short line of two extended mauve dots, after all. The reason for this distinction is that the supplementary rim did the stretching of this half of the blanket, not the classic head rim match point so it might seem strange to see the body half of the blanket matched to something that appears not to have the capacity to rise and stretch the head half of the blanket. In a sense, the mauve match did have the capacity to do so- it simply delaminated giving rise to the peeled-back tear and supplementary rim. 

Photo 22- this shows the semi-crater on the head from a different angle. 

It’s the area between the two mauve lines. This is a good photo for showing the line along which the blanket tore, which is the bottom line. Notice how there is an arc of scree (perimeter dotted white) in front of this half of the blanket, just as there is in front of its body counterpart section of blanket. This is extra evidence that both sections are part of the same thin blanket-like material and that both sections underwent stretch and recoil in a similar manner, leaving scree from their undersides in their wakes. The head lobe semi-crater/blanket perimeter line can be seen to exhibit an upwards curve in the middle. This may represent a small amount of recoil in that portion while being pinned at either end by the mauve and yellow head anchors/match points.

It’s probably best to visualise the pseudo crater on the body as having had a blanket or skin of pliable material, perhaps 20 metres deep covering it. And at that time, the pseudo crater would have been a much smaller area behind the mauve match. It first of all allowed its blanket to stretch up with the secondary head rim. The blanket then tore in half on final shearing and the body half was dragged across the opening gap of the pseudo crater as it opened in the red triangle recoil event. Today, the pseudo crater appears to have been skinned because the blanket material recoiled to its back perimeter, leaving a stark, lightly riven rocky surface with the arc of scree towards the back.

This signature proves that there was also an upward component to the stretch at the mauve match as the vertical wall swung round clockwise. So whilst the entire head lobe was still notionally pinned to the body by the green match/anchor at this time, the head was nevertheless trying to lift off as the wall swung round. The term “lift-off” really refers to head tip, which involved this back end of the head rim tipping up, essentially vertically, while the front end hinged on the Bastet/Aker hinge diametrically opposite. This implies that the head lobe had slid forward towards the xy long axis extremity as far as it needed to- it had found somewhere to wedge against and hinge up (see Part 10 for a full explanation of this mechanism). This means that a strong head-tip tendency was at play by this stage. Head lobe slide had just finished or was about to finish with the last stages of swing by the vertical wall. The only thing now attenuating the runaway tip-up was the last-minute pinning down by the green anchor. And since the green anchor was slightly offset to south of the paleo rotation plane, there was an unbalanced component to the head-tip. It would have had a south-pole-directed component. So this is yet more evidence for the southward head-tip required for the head to grind on the body causing the head lobe slice. The detailed evidence at this end of the shear line supports all the evidence found at the other end and along the south pole.

Further evidence for the upward stretch of the head rim at this time is the fact that the yellow triangle is stretched somewhat upwards as well as laterally. However, due to its ability to accommodate that movement with stretching (along with striations described in signature 5), the yellow triangle didn’t need to perform a catastrophic tear or double-shear causing a supplementary rim. However, it did experience the effects of the double shear in a small way but incorporated it into its striations. It had to accommodate the last curving stretch of the supplementary rim. The supplementary rim appears to become one of the yellow triangle striations as it descends to the triangle tip. 

Summarising signature 9, the mauve head match had to stretch further when the wall swung round, which explains its inability to accommodate that distance of swing with stretch and striations only. The double-shear was the way in which that excess movement could be accommodated and it was achieved with what was effectively a delamination of the mauve anchor prior to the head lobe shearing from the body completely. The torn section that delaminated then hoisted the entire supplementary rim from one side only, allowing it to curve round and down to the yellow triangle tip. 

You may recall that Part 24 mentioned that the header photo appeared to fudge a head-body match by drifting inexplicably up the head lobe and that this apparent fudge would be explained in this Part. The above is that explanation. Although this upper line is the supplementary shear line, it is the one that matches to the body most clearly between the mauve and yellow body matches, very noticeably so in plan view. Despite all the explanation for signature 9, this matching body line hasn’t even been mentioned in this post and doesn’t need to be. Remember that the yellow line denoting the tear line for the blanket in all the photos above is for the final shear line, not the premature or supplementary shear line. The matching body line for the supplementary shear line has its own distinctive features and a very neat mini match. That will be discussed in the mauve match post along with a recap of the final shear line along which the blanket sheared.

Though the above explanation for signature 9 is laborious due to the complexity of the shearing in this area, it is far from exhaustive and not simply because of the omission of the supplementary shear line body match. The double shear will be fully explained with more photos in the mauve match post.

CONCLUSION

There are six vertical wall signatures and nine head lobe stretch-before-shearing signatures. If we remove the duplicate signature 8 in this post for the purposes of counting unique signatures, that’s 14 signatures that show the head lobe stretched before it sheared from the body. 

That concludes the sixth part of the sub series devoted to proving head lobe stretch before shearing. It is a pivotal point in the sense that all the evidence gathered during Parts 22-25 has now been adduced to prove that thesis in Parts 26 and 27. The next few posts will build on the fact that the head lobe stretched before shearing but won’t be trying to prove it did so. Doubtless, a few more gems of evidence will turn up but they will just be incidental extra proof. Those posts could be considered as sections of the sub series but this part rounds off the six parts that were dedicated to proving the thesis. 

FUTURE POSTS

We’ll be looking at how the V-shapes continue across Hatmehit and how they join up with the fracture planes between the strata on the sliced head lobe facet. That slice is on the south side of the head lobe. Once that relationship is established, we’ll be able to identify clearly the hinge on which the Hatmehit missing slab (Part 12) tipped up. From that, we’ll find that Philae is sitting in a ~70-metre-deep trough that is the ripped-out strata of the hinge.

Since other areas on 67P have been neglected while concentrating on the sub series, other news is pressing. For instance, there appears to have been a large slurry outflow on Imhotep. Also, the paleo rotation plane needs a post of its own, as does the mauve match. So we may break away from the head lobe for one post. In the meantime, please see the page (not a part yet) named, “Paleo Rotation Plane 360° Projection”. The link is on the bar at the top of the page. 

PHOTO CREDITS

Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

To view a copy of this licence please visit:

http://creativecommons.org/licenses/by-sa/3.0/igo/

All dotted annotations by scute1133.

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