67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 26

 
THE EVIDENCE FOR HEAD LOBE STRETCH BEFORE SHEARING. 
 Header, unannotated. 
 
Key: 

Mauve, yellow, orange and green dots- these are the head match components of the four matches in Part 24.

Red- these dotted lines follow the lines of the head lobe strata. They exhibit a distinct V-shape that’s centred on the dark green head match point. This is probably easier to see in the unannotated version. The lowermost red V is the conjectured Hatmehit slab hinge which is mentioned below and is where the lander, Philae, is located. 

Dark blue- rotation plane of 67P along its xy axis. The direction of rotation is towards the bottom of the frame. 

Brown- suggested paleo rotation plane that runs symmetrically through all major features relating to head lobe stretch that were presented in Parts 22 to 25 (the sub series) and Part 20 (the head lobe hinge). They total eleven stretch signatures aligned along one line (and plane). They run around the longest possible axis of the comet, which you’d fully expect if the signatures indicate it stretched along that line. 

White- the head lobe hinge. The rhombus shape is the body lobe hinge gouge; the line is the head lobe hinge gouge which is out of sight below the head rim. 

Fuchsia- Philae’s location. This is a very small dot just above one of the red dots in the lowermost V. It’s set against the black background i.e. in shadow. 

‘N’ and ‘S’- north and south pole directions. A line drawn between these two points would define the ‘z’ rotation axis which is, by definition, at 90° to the ‘xy’ rotation plane (dark blue).  

BACKGROUND

Part 26 is the fifth section in a sub series that builds proof for 67P’s head lobe stretching before shearing. The shearing away of the head resulted in the two lobes with the neck in between. The stretching and shearing process was due to spin-up of the comet at some time in its past due to asymmetrical outgassing or otherwise a Roche pass at Jupiter (see About page). This is the section which, as promised, collates all the evidence from the previous four sections and presents the case for head lobe stretch. However, there’s a small catch. There’s now so much evidence that it runs to around 15,000 words, so it’s going to be split into two posts, this post and Part 27. It can be split quite neatly. That’s because the various signatures of head lobe stretch before shearing will be laid out and they are divided into two parts. Firstly, those that are related to the ‘vertical wall’ which stayed resolutely anchored to the body and caused all the other strata to stretch around it. The vertical wall was introduced at the end of Part 25 and is elaborated on below. So those signatures are called ‘Vertical Wall Anchor Signatures’. Secondly, those signatures that are related to the actual head stretching process. They will be entitled, ‘Head Lobe Stretching Signatures’. The vertical wall anchor signatures will be laid out in this part. The head lobe stretching signatures will be laid out in Part 27. Both posts will include a lot of extra facts and concepts that sew all the previous evidence together. If you’ve read Parts 22-25 these will be easy to follow. If not, you will probably get the gist of it. 

Most aspects of the actual head lobe stretching process around the vertical wall are outlined in this post anyway but it’s their signatures, outlined in the next post along with their annotated photos, that prove it happened.

These won’t be the last sections in the sub series. In fact, they’ll be the springboard for all manner of ramifications when we eventually progress up and over the head lobe and all the way down to the head lobe’s ‘hinge’ at Bastet/Aker (Part 20). There are implications for the shape and stratification of the Hatmehit missing slab (slab C, Part 12) prior to its lift-off, the way it hinged on its own separate hinge point and the strata lines it revealed across the surface of Hatmehit after its departure. This in turn holds clues as to the nature of the morphology at Abydos, Philae’s final landing site after bouncing at Agilkia, the original target. That’s because Philae is sitting right inside the Hatmehit slab hinge point, a site where two strata layers were ripped out from between their neighbours. These strata left on the underside of the departing Hatmehit slab. This hinging scenario clearly has profound implications for what Philae will find. The process of grinding and ripping out a 170-metre-wide, 70-metre-deep chunk of strata is going to leave its own unique stamp on the terrain to say the least. This will inform virtually every data reading the lander takes. 

Abydos, Egypt is home to some notoriously deceptive hieroglyphics, one set overwriting the other, on stone, at different times, leading to some wildly speculative misdeciphering. The analogy could hardly be more apt. 

Head lobe stretch before shearing also answers questions over how the slab A extension came to have the appearance of being flayed (Part 22) and provides more insight into the appearance of the Hathor cliff. Doubtless, the arguments of several more of the previous 25 parts will be further informed too, in due course.

All references to ‘up’, ‘down’, ‘left’ and ‘right’ in this post are related to the ‘upright duck’ position with the head lobe sitting directly above the body lobe as we visualise the comet.

Some of the photo keys are long and narrative, since the key is sometimes the best place to point out a subtle feature, i.e. when scrutinising the photo. This means some keys are apt to appear to merge with the general prose. For this reason, the longer keys have a line of slashes after them, ‘/////’, to show the key has finished.

RECAPITULATION OF ‘TAKE HOME MESSAGES’

Photos 3/4- recap of the main features from the sub series. 

   

Mauve, yellow, orange and dark green (matching pairs)- the four matches from Part 24 shown here on both head and body.

Yellow arc- crater left by missing slab A.

Bright green- slab A extension.

Red- triangle of undisturbed craters (marked ‘T’).

Dark blue- rotation plane which is the xy plane of 67P. Direction of rotation is with the head lobe at the top moving into the frame, away from us, and the body lobe at the bottom moving out of the frame, towards us.

‘W’- the ‘vertical wall’.

Light blue- fracture plane on the body beneath the vertical wall. This is not annotated along its right hand end because it merges with the green anchor structure, which is already matched to the head lobe (Part 24). 

‘Z’ and red line of dots- the rotation axis of 67P, which is at right angles to the xy rotation plane. It pierces the comet at its north pole in this photo and exits at its south pole. 

Fuchsia- crater or tray left by the missing Anubis slab (slab E, Part 23).

Single fuchsia dot at top-right of frame- Philae is just out of sight, behind a small ridge that’s about one dot width to the right of this dot.

/////

Although the first four parts of the sub series were worthy subjects for posts in their own right, the evidence they contained for head lobe stretch was included in a take home message at the end of each post. These pieces of evidence are listed below as a recap:

1) From Part 22, ‘The Slab A Extension’, which was the first section of the sub series: the slab A extension exhibits a very straight perimeter line (bright green) on the side adjoining the undisturbed triangle of craters (red). This perimeter appears to be a resilient ridge and a very sudden dividing line between the ‘flayed’ slab A extension area and the craters. This straight ridge runs up to join the mauve body match. It forms one side of the triangle of undisturbed craters. Being contiguous with the red triangle, it’s occasionally coloured red instead of bright green when annotating the triangle itself. 

2) from Part 23, ‘Missing Slab E- Anubis’: the crater or tray left by the missing Anubis slab exhibits a similarly straight line along its top edge (fuchsia). This line runs up to join the dark green body match in a mirror image to the slab A extension straight line running up to the mauve match. Together, the two straight lines enclose most of the two long sides of the red triangle of undisturbed craters and do so in perfect symmetry (length, angle and distance from the long axis of the triangle). It is in fact a long, thin isosceles triangle, hence the symmetry. And the mauve and dark green body matches are therefore found to be at either end of the triangle’s narrow base. As for the Anubis slab ridge, the straight, fuchsia perimeter line is contiguous with the red triangle so, like the slab A extension perimeter, it too is sometimes dotted red when annotating the triangle. 

3) From Part 24, ‘The Serquet-to-Seth Matches’: the four matches in this post included the already-mentioned mauve and dark green matches at either end of the red triangle’s base plus the orange and yellow matches spaced equally between them. Together, the four matches defined the base of the red triangle. It was also noted that all four matches were “defined by apparently resilient, ‘rocky’ outcrops on the body and a head lobe match that isn’t just confined to the head rim but penetrates deep into the head lobe structure.” It was suggested that the red triangle of undisturbed craters was somehow protected by this line of four unusually solid, rocky matches along its base while chaos ensued outside its well-defined perimeter lines.

4) From Part 25, ‘Tell-Tale Lines Across Anuket’: The long, rectangular fracture plane that runs below and in front of all four matches is the same length as the large, vertical section of head lobe directly above it. This vertical section, which will be dubbed the ‘vertical wall’, is two strata layers in height and set one stratum level above the head rim. There’s evidence of the other strata layers bending around it while it remained upright, unstretched and firmly attached to the fracture plane below. The fracture plane feature is also the same length as the base of the red triangle of undisturbed craters. By extension, the vertical wall is therefore also the same length as the base of the red triangle. This means that a resilient main anchor (the vertical wall) sprouting four deep-rooted sub-anchors (the four matches) protected the triangle of undisturbed craters behind it while everything else was stretched or ejected around it.

THE VERTICAL WALL

Photo 5- the vertical wall. 

 

The vertical wall is the obvious, white structure seen in profile at centre-left of the frame. It’s directly above the orange head rim match dot. Other colours are as for the photos above. Direction of rotation is towards the top blue dot i.e. largely into the frame. Although the wall looks uncannily close to vertical in the frame itself, it hasn’t been specially rotated to this point. It’s the way the NAVCAM camera took it but it has been rotated 90° for clarity, that is, so that it’s closer to the ‘upright duck’ orientation.  

So, the vertical wall was touched on at the end of Part 25, in preparation for this part. It has the adjective, ‘vertical’, partly to distinguish it from the ‘hollow-block’ wall in Part 18. It isn’t exactly vertical, with respect to the usual upright duck configuration, but is much closer to vertical than the surrounding, sloping strata. The name is applied to remind us of its more vertical character coupled with its double-strata height (compared with most strata thicknesses on the head lobe) and its wide, flat top that looks a bit like a horizontal ledge half way up the slope of the head lobe. These features give the game away that it never stretched, when anchored on the body, while all around it did so. It’s sometimes referred to below as ‘the wall’ for brevity. 

The vertical wall constitutes about one half of the region of Serqet, as named by ESA (see the ESA regions map below). The other half is the sloping rim stratum that incorporates the four head rim matches of Part 24, including the outline of the dark green and orange anchor structures emerging on its top surface. As with the Seth region, the name ‘Serqet’ won’t be used much here as there are these two distinct morphologies to deal with- the wall and the rim stratum. The rather less exotic naming of the ‘vertical wall’ was chosen here because the shape and structure of discrete comet features often betray their morphological evolution. The vertical wall’s key feature is that it’s much more vertical than the surrounding, sloping strata. It’s this trait that could allow one to argue that it’s actually the lack of any evolution in the vertical wall that speaks volumes- the fact that it remained vertical shows that it didn’t succumb to the head lobe stretching force vector. It held firm till the end whereupon it broke free and rose with the head. But it never stretched. 

Photo 6- ESA regions. 

 

ALIGNMENT OF THE SUB SERIES FEATURES ALONG ONE LINE: A PALEO ROTATION PLANE? 

 

Photo 7- The paleo rotation plane. 

Red dots at top right- the Hatmehit hinge gouge. It doesn’t look V- shaped here because it’s being viewed side-on, not from above as it is in the header. Philae’s position is also annotated in Fuchsia, inside the hinge. It’s actually just a few metres over the horizon, probably a few tens of metres. 

Brown- paleo rotation plane. Direction of rotation is head into frame, away from us, body out of frame, towards us.

Light orange on head lobe- this is the perimeter of the vertical wall. 

Light orange line at bottom- the edge of Apis, just visible. The paleo rotation plane straddles it.  

Other colours are as for photos above.

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In total, eight stretch signatures symmetrically straddle the brown line in the photo. To be quite clear, for this post we are mostly concerned with signatures of stretching before the head lobe sheared from the body, not signatures of the head moving away from the body above a stretching neck after shearing. However, both types are found along the paleo rotation line. In the list of the 8 signatures below, the four coloured matches relate to stretch both before and after shearing. The rest are pure pre-shear stretch signatures. In other words, they wouldn’t exist if the the comet had not stretched prior to shearing. After the list of eight, some post-shear signatures on the paleo line are noted, none of which are discernible or indeed visible on the photo above. The signatures are listed as follows, from top to bottom of the photo:

1) Hatmehit hinge ‘V’ apex.

2) Fine V’s across Hatmehit (not visible; see Part 27).

3) The larger V’s, as annotated in the header. 

4) Vertical wall.

5) The four coloured matches on both head and body. 

6) The blue fracture plane.

7) The red triangle.

8) Apis.

Also, 9) and 10), the head and body lobe hinge gouges. They’re not in view here but are annotated in the header. These are related to post-shear stretching but are mentioned because they too are on the suggested paleo rotation plane. 

That totals 10 stretch signatures along one line (and in one plane) around two-thirds of the comet. Furthermore, they’re aligned along the longest possible axis of the comet. This is to be expected if it was the axis along which the signatures are saying it stretched. By definition, stretching means lengthening so it would be puzzling, to say the least, if we found that after all this discussion over five sub series sections, we discovered that the ‘stretch’ signatures were draped around the shortest axis. 

There’s also an eleventh signature that’s right on the paleo line and hasn’t yet had an airing. It’s at the end of this post. And there’s a possible twelfth signature on Imhotep, which will be the subject of a future post. 

The paleo rotation plane is made very evident by the cluster of stretch signatures above and below the vertical wall. Indeed, the wall is crucial to the understanding of both the paleo line and head lobe stretch before shearing. 

Not only did the vertical wall hold firm, acting as an anchor while everything stretched around it and past it. It was the actual reason for everything stretching around it. This is because it was pinning down the back of the soon-to-be head lobe firmly onto the body lobe while the head lobe itself was experiencing a substantial forward-sliding force vector. This is how the head lobe found itself to be stretching into V-shapes around the wall. This is probably the most crucial concept in the whole sub series and one from which everything else flows.

The forward-sliding force was due to the head lobe’s tendency to slide towards the end of the xy long axis during the comet’s spin-up, before tipping then lifting off (Part 10). This means that the shear line unzipped all the way down the sides while the wall held firm at the back of the head, refusing to shear. This unzipping must have happened for the V’s to have slid forward and visible evidence for it is apparent. There are signs of progressively greater upheaval as we progress along the shear line from the vertical wall to the Babi region. Firstly, there’s the material that is suggested to have been scalped from the slab A extension but just about held on to the ‘outside’ edge of the head lobe mauve match, rather like an outriding passenger. In doing so, it got plastered against the side of the neck and became part of Anuket (Part 25). Secondly, the rest of the slab A extension got flayed and ejected from the comet but it wasn’t a true slab i.e. a 50-metre-thick chunk of crust. Thirdly, we have missing slab A (Part 9), a proper chunk of crust. Fourthly, just beyond slab A, there’s a signature of massive outgassing (Part 7). This is at the exact centre of the unzipped side, the north pole, at 90° to the paleo rotation plane. That’s the most likely location from which you’d expect heat-generated gases and slurry from the sliding V’s to emerge. Due to the sudden shear and slide, these gases would result from being simultaneously heated and exposed to the the vacuum in the very small crevice between the newly created lobes or at least much lower pressures therein. Commenter, Marco, pointed out on the Rosetta blog that no heat was needed, just exposure to the vacuum. The blog post was “Inside Imhotep” dated 20/07/2015 and the comment thread contains a long discussion on the feasibility or otherwise of outgassing and slurry:

http://blogs.esa.int/rosetta/2015/07/20/inside-imhotep-2/

Marco was the first person to suggest stretch via spin-up due to asymmetrical outgassing and did so long before I was suggesting stretch via a Roche pass at Jupiter. 

The sliding force for the head lobe was in the forward direction of rotation and, as a result of this, all the features and force vector mechanisms described in the sub series can be seen to align along one line on the comet, the paleo line. And it should be along the xy rotation plane because the tensile force vector would have been along the rotation plane. The actual line of signatures we see today is close to the present-day rotation plane and runs all the way from the tip of the red triangle at one end of the body, up the neck, over the head, down the neck and down to the head lobe hinge on the opposite extremity of the body. 

The vertical wall is right next to today’s rotation plane. But the fact it straddles the line running through all the other stretch signatures suggests that the paleo rotation plane held sway when the head was still attached to the body. It’s positioned around 20° clockwise from today’s plane (looking from above the head lobe, see header photo).

The rotation plane change could be explained by the fact that the head lobe had nearly a quarter of its volume sliced off one side, the south pole side, as it tipped and lifted off the body (Part 20, photo 11). There’s some evidence that the south pole side of the body lobe also lost a lot of material but it wasn’t photographed in detail by the Rosetta NAVCAM prior to perihelion. And five smaller slabs were shed, of course, (slabs A-E; various Parts, commencing at Part 9). These probably had an effect on rotation plane change too. 

THE HEAD LOBE STRETCH-BEFORE-SHEARING SIGNATURES IN DETAIL

The first four sections of the sub series were mostly concerned with observations of features that were highlighted in the ‘take home messages’ as being relevant to head lobe stretch. The mechanism which brought them about was barely hinted at and has only been outlined in general terms so far in this post. Now we need to get to the bottom of the detailed mechanism. This mechanism shows that all the stretch signatures along the conjectured paleo rotation plane are indeed signatures of stretch before shearing and not something else. 

When the vertical wall did finally give way, it left signatures as to how it detached. It would be expected that one or other of the mauve and dark green anchors would detach first, introducing a swinging motion to that end of the wall before the other three matches followed suit, detaching in quick succession. This is because the chance of all four anchors giving way at the same time is very small. 

There’s good evidence that it was the mauve anchor that gave first. It exhibits no fewer than four matching tears shared between head and body, betraying a triple shear. This means there’s a double shear line signature on the head between the mauve match point and the yellow triangle. This complex behaviour of the mauve anchor will get its own post eventually. It’s the reason the head rim match appeared to be fudged in Part 24 by riding up the head lobe- but that upper, fork of the line is the initial tear line. The lower one peeled away shortly afterwards. This shows that the mauve anchor was under immense tensile stress. Despite shearing once to release tension, it wasn’t enough. 

The detachment of the mauve anchor before the others means that movement of the vertical wall and consequent stretching of the single rim stratum below it was (and still is) more emphasised at this end of the four anchors. There are at least six signatures of this gradation of stretching below the vertical wall as it swung round to join one arm of the V-shaped strata that had already stretched around it. It then finally detached at the other end, from the green match/anchor, allowing the commencement of head-tip. That detachment left a signature too. These signatures will be presented in Part 27 because they are not signatures of the vertical wall holding firm but signatures of the very final stage of head lobe stretch before shearing. 

These signatures betray the nature of the swing and the force vector pulling the vertical wall round in a clockwise direction. When considered alongside the V-shaped bends that the head strata make around the green match point, as depicted in the header photo, it shows that those V-shapes were caused by the same force in the same direction while the head lobe was still pinned at its back end. Indeed, when the vertical wall was firmly attached, the V-shapes would have had a flat bottom, instead of being pointed. The flat bottoms would have been the length of the wall as the V-shapes stretched around either end of it. These stubby V-shapes were therefore centred between the yellow and orange anchors, which is the midpoint of the line of four anchors. This is why the paleo rotation plane is suggested to have been along the line that runs through the middle of all the sub series stretch signatures including the vertical wall and the four anchors. 

If you were to scrutinise the header photo carefully, the stacked V-shapes can be seen to curve round subtly from being centred on the green head rim match point to being centred on the brown line that runs through the midpoint between the yellow and orange matches. That line also runs directly through the head lobe hinge gouge at Bastet and along the edge of the body gouge at Aker, lending weight to the fact that this was probably the paleo rotation plane. 

The paleo plane also runs dead through the centre of very fine V-shapes in the Hatmehit crater (not visible in the header). These are definitely folded strata lines because they can be traced up and over the south rim of the crater where each one meets up with its ‘sliced facet’ counterpart fracture on the south pole side of the head lobe. 

The first few V-shapes being aligned as they are on the dark green head match aren’t a fly in the ointment. They simply betray the swing of the vertical wall on what happened to be the strongest anchor point. That skewed the first few V-shapes away from the neat paleo rotation plane line that they had been on when the wall was still attached. 

These signatures, it must be remembered, were all stamped on the head lobe morphology when the head was still on the body, even though it was shearing, stretching, grinding, trying to break free. So when, for example, the mauve and yellow head matches are said to ‘give’ and ‘stretch’, there’s movement, yes, but they were still effectively held down by the green match point or anchor (and probably by the orange anchor for a while too). The second the head lobe sheared for good and started rising, these signatures were set in the head lobe morphology without any more deforming forces acting on them. Those forces were immediately transferred to the weakest point- the incipient, stretching neck. 

The signatures will be listed in two sets, as mentioned at the beginning of this post. Although they were listed for the purpose of providing evidence for the paleo rotation plane, the two sets will go into much more detail. They are actually greater in number than the paleo plane list because like everywhere else in this blog they are pinned down and scrutinised for mini signatures within the main signatures, just like the mini matches. The narrative for each signature reinforces a recurrent theme. That theme is that all the signatures bear the hallmark of being related to one another in sequence and arising as the result of tensile forces running along the line of the paleo rotation plane. And tensile forces along the line of the paleo plane are very closely related to the ‘centrifugal’ force due to rotation, ergo the stretch signatures arose due to comet spin-up. 

So 67P was spun up, stretched before shearing, eventually sheared and then entered a new phase of stretch with the head lobe rising on the stretching neck. 

The first set of signatures, the ‘Vertical Wall Anchor Signatures’, comprises those signatures that show that the vertical wall held firm as all around it and past it, strata were stretching, the Anubis slab was being wrenched away and the slab A extension was being flayed. These are laid out in the next section of this post. The second set, ‘Head Lobe Stretch Signatures’, comprises those signatures that show the head lobe stretched due to being pinned down by the vertical wall at its rear/centre while having its two sides shunted forward by the comet spin-up force vector, that is, the sliding-forward force vector. This head lobe stretch therefore happened before the vertical wall detached from the body. As mentioned above, the second set, ‘Head Lobe Stretching Signatures’ will now be laid out in Part 27. Here is the first set.

VERTICAL WALL ANCHOR SIGNATURES:

Signature 1- The vertical wall is much more vertical than the surrounding sloping strata.  

 

Photo 8- the vertical wall (photo 5 reproduced).

Everything about the morphology of the head lobe suggests a stretching force towards Bastet. This is because at Bastet itself, the head slopes down sharply towards Aker whereas the sloping of the strata layers on the opposite side of the head running up to Hatmehit from Serqet are much shallower. These strata are divided very evenly in terms of their thickness and the slope is smooth with the greatest degree of shallowness exhibited along the line running up from the top of the vertical wall. The vertical wall itself flouts this rule by its very nature of remaining much nearer to vertical and not being pulled forward towards Bastet by the stretch force vector. 

Signature 2- The isolated fracture plane beneath the vertical wall in Hapi is virtually identical to the vertical wall in terms of its shape, size and orientation. 

 

Photo 9- The fracture plane.

Light blue- the fracture plane. Notice how its back edge follows a straight line. The right hand end is dotted blue here, unlike photo 3 where it was said to merge with the green anchor. You can see here that, to be exact, this end is contiguous with the rocky structure that runs between the orange and green anchor matches. This structure was meticulously matched to the head lobe in Parts 24 and 25. As such, it could be considered as a slightly raised continuation of the fracture plane because it too sits below the vertical wall. The front edge of the fracture plane follows a straight line as well but the line isn’t continuous. If the straight portions are joined, it runs right over the middle of the ‘rocky’ yellow anchor point just behind the yellow dot. That dot always marks the matching point for the tip of the triangle on the head rim above, a highly-detailed match that was set out in Part 24. It’s interesting that although it has no straight lines directly either side of it, the produced line running across the anchor kisses a lip in the rock itself that is oriented across it at the correct lengthwise angle to be part of the line. If this front edge of the vertical wall seating is correct, it shows that the stratum below the vertical wall, which did the stretching, was not flared out as it is now. There simply isn’t the space between the coloured matches and the front face of the wall when seated along this line. In other words, the stratum below the wall was stretched massively in the forward direction of the rotation plane. This caused that flared-out look we see on the portion of head rim where the four coloured matches of Part 24 reside (see the vertical wall photo, photo 8 above, for a view of the flared-out rim).

[key to photo 9 continued] Mauve, yellow, orange and green dots- body seating positions of the four matches from Part 24. 

Bright green- the slab A extension perimeter.

Red- one side of the red triangle of undisturbed craters. This could just as well be coloured bright green as it’s contiguous with the slab A extension perimeter.

Yellow- the edge of the crater left by missing slab A.

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This fracture plane, which was mentioned in Parts 24 and 25, is the same length as the base of the vertical wall. It runs from the mauve body match to the green body match, just as the base of the vertical wall runs from the mauve head match to the green head match. So there is an end-to-end match. For the vertical wall to have remained firmly embedded on the body, it needed to be well rooted in a ‘rocky’ structure (‘rocky’ is used for convenience to mean solid material- we know it’s not rock). 

Just in the place where such a structure would need to sit, we find not simply some bits of rock or a vast fracture plane extending over Hapi but an exact fit to the vertical wall above and nothing much to speak of at either end (except for the already matched mauve and green anchors that flank both the fracture plane and the vertical wall directly above it). Moreover, the back edge of the fracture plane runs parallel to the shear line, forming a rectangle that’s roughly the same width as the wide top of the wall (see below), though possibly a little wider. 

Finally, this rectangle is oriented along Hapi so as to be aligned at the same angle as the vertical wall, one kilometre above it. 

Signature 3- the top of the vertical wall is much wider and flatter than the slope of the other strata running past it and on up to Hatmehit. 

 

Photo 10- top of the vertical wall. 

Light orange- the perimeter of the top surface of the vertical wall. Note that it has more debris on it than any of the surrounding areas. 

Beige- these two dots show the top and bottom of the front surface of the vertical wall, which is just a thin line of white due to the extreme profile angle. 

Yellow, orange and dark green- these are the head rim components of three of the four coloured matches. The mauve head match is behind the yellow match and just a fraction over the horizon. 

Dark blue- present-day rotation plane. Direction of rotation is to the right. 

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This is also a head lobe stretch signature but belongs here. It should also be noted that the top of the vertical wall happens to be a separate ESA region called Nut. That’s because it’s thought to be a depression that’s full of rubble due to the erosion of Serqet. Presumably, the Nut ‘depression’ is therefore thought to be an erstwhile extension of Serqet that’s now been eroded into a dip. As with Seth and Serqet, the name, ‘Nut’ will be barely mentioned because the reason for it resembling a depression is the very fact that it’s the top of the vertical wall, revealed by head lobe stretch. And the provenance of the extensive rubble field that seems to have such a distinct affinity with the ‘depression’ is wholly different too. The reasoning for these points is as follows. 

The vertical wall must have had some reasonable thickness so as to stay rooted to its fracture plane match and resist the forward-sliding force vector due to spin-up of the comet. It would be quite reasonable to start with the working hypothesis that the thickness is the same as its fracture plane width seeing as it has the same length and orientation as the fracture plane. The thickness would originally have been largely hidden in the interior of the head lobe before the other strata stretched around and past it. That’s because the slope we see today between Serqet and Hatmehit would have been much less marked and we would likely have seen just the face of the wall. 

So if stretch did indeed happen and all the strata deformed around the resilient, immobile wall, you would expect the stratum above the wall to slide forward, revealing the top of the wall. That’s what we see. 

The wall-top is roughly the same width as the fracture plane, below on the body in Hapi. It’s not quite the same length as the base of the vertical wall and fracture plane, though. That’s due to the wall having a slope running down one side from its top to its base at the green head match. However, the wall-top does have a narrowing tail, so to speak, that extends over this sloped slide. This section of the stratum above the wall slid forward in sympathy but to a lesser extent than the portion directly above the flat top. This is consistent with the fact that the sloped section was still above the anchored base of the wall but, by virtue of being sloped, had a small triangle of extra strata between it and the main sliding stratum above it. This triangular portion took up some of the stretch, allowing the main sliding stratum to slide forward a little less above this point. You can see the way in which this extra triangular piece of stratum has deformed in a majestic dip around the resilient sloping edge of the wall. The dip itself is a big clue, since deforming an erstwhile flat plane round a sloping corner causes a dip due to the non-Euclidean nature of the 3D space through which the stretch is performed. The triangle is bounded by the sloping side of the wall below it and the lip of the thin ledge formed by the sliding stratum above it. The narrowing section of ledge due to the diminished degree of the main stratum sliding corresponds to the thin tail of the Nut region in ESA’s regional map. The triangle mentioned above should not be confused with the very important red triangle on the body. It probably won’t be mentioned again as it’s just incidental to the sliding of the wall-top stratum.

Also of note is the fact that the revealed top surface of the vertical wall is the only place down the head lobe slope with such a ‘horizontal’ facet, with respect to the upright duck. With respect to the direction of rotation, it’s poking up and facing east. It was therefore the most likely place to act as a ‘butterfly net’ to capture suborbital debris floating back down the rotation plane after the Hatmehit slab departed from 67P. The top of the wall is indeed strewn with debris, far more so than the surrounding areas. See Part 14, ‘Rock A’, for this debris capture mechanism. 

It’s also quite possible that the stratum sliding over the wall-top produced rubble too. However, this wouldn’t be consistent with the fact that there are other areas on this part of the head that are not subject to major stratum-sliding and yet seem to attract detritus by virtue of poking up and facing east. The wall-top rubble is also sitting on top of a blanket of dust, which is inconsistent with it being scoured out, leaving scree. All the other suggested ‘eroded’ features look like scree slopes with little or no dust. In the final analysis, the ‘Nut depression’ rubble is probably the result of suborbital capture with some remaining detritus due to the slide. But it certainly isn’t due to erosion to the point of achieving a ‘depression’. The depression is simply an optical illusion due to the displacement of the two strata. At best, erosion can amount to a few rocks popping out from below the sliding stratum with no dip forming at all. See photo 8 to get an idea of how the curved ‘depression’ is actually one stratum that’s slid over another.

The strong affinity of the rubble with the perimeter line of the so-called depression is what led to the scientific papers attributing it to erosion within the ‘depression’. However, the two mechanisms above are the real reason for that uncanny affinity. 

Incidentally, the following photo proves it’s a slid stratum and not an eroded depression. The key element here is almost invisible. It’s the very fine pair of dotted red lines in the middle (not the obvious red line on the left). These follow spidery stratum lines that can be traced all the way to the shadow at the bottom where they meet with their counterpart stratum lines on the ‘sliced facet’ (Part 20). These two lines clearly show one stratum layer that’s slid forward over another. That’s because their twists and turns mirror each other faithfully all the way from one end to the other.

Photo 11- proof that Nut is a slid stratum and not an eroded depression.  

 

Large red dots- head lobe perimeter as depicted in red dots in Part 17. 

Fine red dots (full zoom required)- these are at the centre of the photo and delineate the two stratum lines in question. Nut stops before the sliced facet but the stratum lines carry on to meet the sliced facet. 

Light orange- other spidery lines that run parallel to the two red lines. These show similar strata lines that haven’t slid as much. Sliding is a head stretch signature but it’s included here because its most obvious exponent is the stratum that slid forwards off the top of the vertical wall. So it’s also a vertical wall signature. 

Blue- present-day rotation plane. Direction of rotation is towards bottom-right.

Brown- suggested paleo rotation plane. This is glimpsed on the body too, running past the yellow body match. It runs straight through the middle of the vertical wall which is the obvious white area. Notice how the two planes gently converge. They cross in the middle of Hatmehit, off-frame to bottom-right (and also in the middle of Imhotep). They are flared out into free space as if projected out horizontally from the head rim. That flaring is not necessarily very accurate, it’s just done by eye. 

Signature 4- The four anchors.  

 

Photo 12- the four anchors, shown with the red triangle behind them. That is, behind with respect to the direction of rotation. 

As stated in Part 24, the mauve, yellow, orange and green matches were three-dimensional, rocky features that ran through both lobes when the head was seated on the body. In this respect they were unlike other matches around the shear line and their structure suggests they were sturdy, resilient features. Furthermore, they appear to have acted like roots for the vertical wall. The green and orange head matches have ridges that reach the base of the wall on the head. The orange anchor is imprinted on the fracture plane, below on the body, proving that it extended from deep within the body lobe to deep within the head lobe, all the way to the vertical wall. The yellow head rim triangle remained resolutely attached to the base of the wall as well while clearly having a vast anchor structure just below it. The mauve match isn’t quite as intimately attached but it still sat right in front of the wall, helping to pin it in place. All four anchors, evenly distributed across the front of the wall, could only help to pin down this solid immovable structure at this end of the comet. This is why it was the last point on the shear line to give way, allowing the rest of the head lobe rim to unzip down either side of the comet and slide forwards, folding itself around the wall in the process. 

Signature 5- The red triangle of undisturbed craters. 

 

Photo 13- the red triangle. The four coloured matches form the base of the triangle. 

The triangle of undisturbed craters on the body, annotated with red dots in the sub series, was once sitting directly behind the vertical wall and its four anchors. Its short base is exactly the same length as the vertical wall as well as the fracture plane the wall sat on and the four anchors. This strongly suggests that it was largely protected from the stretching force vector. That’s because its craters are visibly less disturbed than its surroundings. Indeed, with the force vector being in line with the rotation plane, this triangle would need to be long and thin and have its long central axis in line with the rotation plane as well. It’s very nearly in line with today’s rotation plane. Since all the stretch signatures suggest that the paleo rotation plane was skewed about 20° clockwise (looking down on the head), this would place that plane right down the middle of the triangle. The evidence for rotation plane swing will be presented in future posts or in a dedicated post. 

The reason the triangle would be long and thin is because the tensile stress force vectors would need to be directed around the immobile vertical wall and join up somewhere further back down the body lobe. They wouldn’t join back up just behind the wall. This phenomenon would be akin to the long triangular shape of a sand dune that’s protected from the wind behind a rock and called a wind tail. 

And the red triangle certainly is long and thin. The two long sides are very straight. They are contiguous with the straight perimeter of the slab A extension and the top perimeter of the crater or tray left behind by the missing Anubis slab. These two straight, contiguous sides were flagged up as being important in the first two take home messages from Parts 22 and 23 and reproduced at the beginning of this part. That’s because the two perimeter lines are literally where the tensile stress vector passed along like the edge of the wind current past the rock (i.e. the vertical wall) and the sand dune (the red triangle). On one side of each line there was tensile stress and on the other there was little or no tensile stress. Hence the mayhem visited on the slab A extension on one side and Anubis on the other versus the comparative tranquillity inside the triangle. It’s because there was a dramatic shearing event along both lines. The shear came about due to the slide induced by having a longitudinal release of tensile stress forces along one side of the dividing line and practically no force released on the other side of it. There would be little or no force released on the inside of the line because of there being little or no tensile stress force inside the red triangle in the first place (actually there must have been some small residual force inside the triangle- see signature 6). This would apply in exactly the same way to both of the long sides of the red triangle. 

In summary, both the Anubis slab and the slab A extension slid sideways along the edge of the red triangle as they departed the comet. This was due to the shearing process that was in turn due to the difference in longitudinal tensile forces along the long sides of the triangle. The red triangle is the visible representation of those force lines, imprinted on the body lobe.

This shearing event along the sides of the triangle could only have happened if the vertical wall had kept itself anchored firmly to the body while the tensile forces worked their way around it in a long, triangular shape. This is why the red triangle of undisturbed craters is a strong signature for the vertical wall remaining anchored till only very shortly before head lobe lift-off.       

Signature 6- The red triangle recoil. 

   

Photos 14/15- red triangle recoil. These are body-to-body matches. The dots sometimes obscure the detail despite being small. It’s best to use the annotated version for familiarising yourself with the matches and then fly solo, as it were, using the unannotated version. 

Red arrows- direction of recoil, circa 200 metres. The red line used to be contiguous with the yellow line (which is predominantly the shear line), before springing back violently when the head lobe finally sheared for good. 

Large yellow dot- the classic yellow match point, which is always placed at the apex of the yellow triangle. The yellow triangle exhibits mini matches to the head rim triangle around its perimeter and also to the recoil line on the body (annotated in red). 

Medium yellow dots- this is predominantly the shear line. It runs from the base of the yellow triangle at top right, has a small gap (no recoil matches), and then extends all the way to the orange match point and beyond. However, since it is matching features to the red line (of recoil features) it’s doing so without any respect for the shear line per se. Therefore, there’s a gap at the top end where there is no discernible matching line to the recoil line. And there’s an additional spur running about 90° in from the shear line that does match a corresponding spur off the recoil line. That spur, which runs towards the obvious shadow, would have been sitting under the head triangle before it rose, rather than on the perimeter of the triangle. Indeed, this very line is matched to the underside of the head triangle in Part 24. It’s one of the ‘double lines’ in that part.

Mauve- the mauve body match that matches to the head. 

Orange- the orange body match that matches to the head. The dark green match is just off-frame to the right. 

Red line- the recoil line that faithfully shadows the shear line at a distance of some 200 metres behind it. The only reason the three obvious curves in the yellow line are less pronounced in the red line is that the red line is on a slope along this stretch. That foreshortens its respective curves from our viewpoint. From other angles they are more pronounced. 

Small red and yellow dots- these are additional mini matches that were either just inside or just outside the shear line.

Bright green- this is annotated in this colour because this resilient ridge along the slab A extension perimeter is always annotated bright green. The two sections at top left are two matching sections along the ridge that tore away from each other. The section at top right is the continuation of the ridge down into Hapi.

Direction of comet rotation is in the opposite direction to the recoil direction arrows, as you would expect, given the tensile force vector direction. This force vector was aligned along the rotation plane and it was probably along the paleo rotation plane at the time of shearing rather than today’s rotation plane, circa 20° offset from it. So the tensile or pulling force yanked the red triangle backwards with respect to the forward rotation direction. 

The region between the two bright green tears and the commencement of the red and yellow lines does in fact exhibit the same recoil signature, but it’s smudged due to the double-shear at the mauve match. It’s not worth annotating before the dedicated mauve match post comes out at a later date. That will make it clearer how and why it’s smudged. 

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The red triangle of undisturbed craters is sometimes referred to as being “relatively” undisturbed. This is because it didn’t quite escape unscathed. We wouldn’t expect the head lobe to stretch before shearing without the body exhibiting the same stretching behaviour. One would expect the body lobe to stretch out into something approaching a point towards Apis at the xy extremity of the rotation plane and that is what we see today in the shape of the body lobe. We even see in Apis a tiny portion of the original crust right on the apex. It had slabs ripped away from either side and so was spared. You wouldn’t expect a neat cone shape to slide off the back of the comet so slabs departing from two sides, possibly three sides, would be a good approximation to that perceived ideal. It’s understandable, though not entirely expected, that a small remnant might remain at the tip despite it being at the xy, long axis extremity. 

The present-day rotation plane runs just to one side of Apis, on the Anubis side. The suggested paleo rotation plane runs plumb through the middle of Apis, which is at the apex of the long axis of the body lobe. You would expect this to be the case if the comet’s body lobe was stretching in line with the spin-up force vector. Apis is at one end of that long paleo line of stretch signatures that all align along or symmetrically across it. The stretch signatures dictate that line and yet the line just happens to run exactly round the longest axis. This is why it’s suggested as the paleo rotation plane. The present-day rotation plane is just offset from it like a piece of string that was tied around the longest possible axis but slipped to one side at one end, Apis and to the opposite side at the other end, Khepry, so as to compensate. As mentioned above, it was probably the head lobe slice that kicked the rotation plane round clockwise.

The red triangle is long and thin. It extends most of the way to Apis and its shape roughly mirrors the overall stretched shape of the body lobe. It’s just offset from the current rotation plane and symmetrically straddles the suggested paleo rotation plane. When looked at together, all the above evidence suggests that the red triangle did succumb to some stretching but not the sort that would flay it like the slab A extension or eject it like the Anubis slab. This is because, though protected by the vertical wall, there would have been some creep of the longitudinal force vector lines beyond the perimeter and into the triangle. They would be diminished but still enough to stretch it gently. 

The vertical wall protection and subtle stretching scenario would also have applied to some of the core volume just below the cratered crust we see. However, when the vertical wall finally gave way, the surface crust of the red triangle recoiled. It sprung back by 200 metres or so while the core beneath it didn’t or did so to a lesser extent. This recoil is evident in the fact that three of the four body anchor points have matches on the red triangle that are situated some 200 metres behind them in the direction of Apis. 

This has nothing to do with the classic head-to-body matches. These are a new phenomenon: body-to-body matches due to crust recoil. The red triangle base tore away from the anchors and slid back. The body components of the anchors opted to stay rooted to the solid fracture plane mentioned above. The head lobe anchor components sheared and rose with the head rim. So the red triangle tore from the shear line at the same moment that the head lobe rim did. The head rose and the red triangle, delivered from the tensile stress, recoiled like a rocket-launch gantry. The four body anchor remnants were left, marooned between the two. 

You can probably toggle between photo 7 and the recoil photo and see that the direction of the recoil arrows is along the rotation plane but in a backwards direction. It looks to be slightly more in line with today’s plane than the paleo plane. However, one would expect it to be exactly in line with the paleo plane. But since the mauve match detached just before the yellow and orange matches, the recoil had a head start at that end. This could account for the slight swing of the recoil ‘front’ as depicted by the red line. That swing was perhaps 10-15 degrees and resulted in the red recoil line not quite straddling the rotation plane at 90° when it came to rest. You can see that the red recoil line is slightly closer to the orange match than it is to the yellow and mauve matches, which lends weight to this supposition. What we are seeing here is the mirror image of the vertical wall swing. Both the vertical wall and the red recoil line detached from the mauve anchor (and therefore from each other) first and that caused them to swing away from each other in opposite directions under the influence of the longitudinal tensile forces. 

The clearest of these recoil body matches in terms of the violent tear and recoil signature is the match behind the mauve anchor, annotated in bright green in the photo. You can even see the line along which it slid backwards and the debris it scoured out as it did so. Its top area has the same width and neat, ‘white’ surface as the mauve match. Even more tellingly, that width and white surface is the same as on the ridge on the head lobe that runs up the border of Ma’at and Serqet, directly behind the mauve head match. Both these head-body features have already been highlighted as being linked (Parts 22 and 24, annotated in bright green) because they lined up as one long line when the head was attached to the body. 

The same applies to the yellow body anchor. Its actual shear line imprint, as described in great detail in Part 24, is triangular. That triangle matches a similarly shaped triangle the same 200-metre distance back along the body as that of the recoil match to the mauve anchor. Both triangles exhibit the characteristic yellow ‘mini matches’ which are the three distinct arcs on the side of the triangle that runs towards the orange match. The orange recoil match behaves similarly, and includes its own mini matches. 

This recoiling of the red triangle would account for the fact that the craters are still intact but a bit roughed up as if a cometquake had hit them. They are strewn with rubble yet they weren’t flayed like the slab A extension and evidently weren’t flung from the comet like the Anubis slab. The red triangle suffered a scaled-down version of what happened to its neighbours. 

Regarding the vertical wall signature, this recoil of the base of the red triangle could only have happened in response to the shearing away of the vertical wall. That sudden release of tension along its front edge allowed it to spring back under the tension that had built up along its long axis. That tensile force vector ran back along the body from the vertical wall towards Apis, hence the triangle’s recoil in that direction. And the movement was that of the triangle crust in relation to the core beneath it that refused to recoil at all or as much. One can only assume that this was due to plastic stretching of the core in contrast with a small amount of elasticity in the crust on which the craters sit. So the core sat passively under the body anchors as well as the red triangle as the red triangle slid back.

If the red triangle recoil can only be explained by a sudden release of tensile stress, it means it detached from the vertical wall very suddenly, in other words when the vertical wall itself detached suddenly from the four anchors. This, coupled with signature 5 (protection of the red triangle before shearing), means that the vertical wall held firm till the last moment because it’s clear that mayhem ensued either side of it before it sheared. And if it held firm, it means it was pinning the back of the head lobe down onto the body while the sides of the head lobe unzipped, slid forward and stretched around it. And since the sub series theme is concerned ultimately with head lobe stretch before shearing, the red triangle recoil signature is of prime importance. It brings the total of stretch signatures along the paleo rotation plane to 12. 

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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