67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 18.



Blue dots- rock D perimeter.

Red dots (very small, zooming required)- probable line of the rock’s bottom perimeter, judging by its shadow. The dots are small so as not to compromise the reader’s own judgment in estimating the line.

Photo 2- rock D in context with its rotation plane and rock C.  

Pair of bright blue dots- these show each end of rock D (top pair) and each end of its suggested seating position (bottom pair).

Dark blue- rock D’s  rotation plane corridor.

Yellow- rock C (middle-right of frame) and its seating position next to rock D’s seating.

Red- square feature on rock C’s seating that will serve for comparison and orientation with another photo later on.

Mauve- companion rocks to rock D, with similar rectangular features. They’re in line with the rock D rotation plane corridor and so are likely to have come from the same place. The two nearer companions are fuzzy in all photos so haven’t been given designations just in case they are proven to be fixed features and not rocks. The farthest one was discovered, just before posting, as being clearly detached so it should really be called rock E. Its position is significant, suggesting it should be twinned with rock D.


In part 14 we analysed how Rock A with a mass of around 30 million kilograms was detached from its original seating position and drifted 170 metres across the crater known as Landing Site A. The features on the detached rock matched those on the seating position. The mini crater that was actually sliced in two and shared between rock and seating made that match definitive. Rock B was dealt with in the same post, its seating position being slightly less definitive but its uplift probably collateral to rock A’s uprooting. And of course, the uprooting happened when the head lobe rim sheared away from this exact point and rose to its current position. Rock C was covered in Part 15. It was the biggest of the three rocks and whilst it didn’t have a sliced mini crater, two of its matches made its seating position and consequent 500-metre excursion virtually unequivocal. 

They’re all dubbed ‘rocks’ for the sake of convenience. We know they look like rocks but have a very different composition. As usual, scenarios involving rock D are described as if they are fact so as to save on peppering the prose with endless conditionals and qualifications. There are already plenty enough qualified statements in this post, just in order to avoid possible ambiguities as we twist and turn in three dimensions between strata.

So, unlike rocks A and C, rock D’s seating position and subsequent excursion is a hypothesis that can’t be proven definitively. However, in the light of the evidence presented here, the seating, detachment and rotation plane travel scenario is quite plausible. When looked at in the light of the accumulated mass of corroborating evidence for stretch presented in Parts 1-17 it’s quite probable. 


It would be easier to understand the references to the ‘rotation plane corridor’ and ‘missing slab A’ if you’ve already read Parts 14 and 15 but here’s a recap. Missing slab A was about 1 km across (see header photo, Part 9, for its shape and position). It was pulled up by the head lobe too, like yanking a bollard (the head lobe) violently out of a slabbed walkway and watching the surrounding slabs lift with it, each one lifting at one end and hinging at the other. Being near the rotation axis, slab A probably wouldn’t have escaped due to ‘centrifugal’ force alone. But the sudden shearing would have released pent-up tensile forces to do work, lifting the head and slab at an acceleration that was greater than slab A could otherwise muster. This pent-up tensile force would be commensurate with that needed to eject the head lobe at or near 1m/sec which is the comet’s escape velocity, implying spin-up to a circa 3.5 hour rotation period. The head lobe’s escape was prevented by having to drag the neck up with it but not before slab A had detached from the head (and body) at 1m/sec or more and escaped. 

The rotation plane corridor is the path that a detached rock would follow, backwards along the comet’s rotation plane, if it were detached from the surface. It would travel backwards along this line before being caught up by the body lobe rotating into it from behind. This would happen due to the rock being in a suborbital trajectory because the rocks in this vicinity, being near the rotation axis, were not subject to the same ‘centrifugal’ force that sheared the head lobe from the body. So, like slab A, they needed a helping hand from the uplifting head but they broke away from both head and slab early on in the upheaval process. They were therefore floating free at suborbital speeds.


Rock D is a more tentative match than the definitive matches of rocks A and C in terms of an exact mirror image of features between the rock and its seating position. However, the evidence for its former position being at that point or within a very small distance of it is compelling. 

Rock D is the last large rock on Site A that can be safely discerned as being a detached entity like rocks A to C and not a fixed stratum feature that has the appearance of being a detached rock. Doubtless, other true rocks will become discernible as the OSIRIS images are released. There are probably two more just behind rock D but they’re a bit fuzzy.

[UPDATE: after this post was largely completed a third companion was discovered. Like rock D, it’s discernible as a detached entity. It’s strewn in-line with rock D and the other two, along the rotation plane corridor, but landed much further down. It also exhibits similar surface features. So it should be dubbed rock E and borne in mind as being an equally valid twin to rock D for the rest of this post. Being in-line constitutes a signature- it would have come from the same vicinity and been subject to the same detachment and displacement process as rock D. It was added to the photo annotations at the last minute. It’s the isolated rock, dotted mauve and sitting close to the Site A crater rim].

Rock D is smaller than rocks A and C, and a little bigger than rock B. Its mass is still tens of millions of kilograms. Like the others, it’s sitting on Landing Site A, known informally as the amphitheatre. Whilst rocks A,B and C are towards one end of the crater, rock D is at the other end. It’s much further from its suggested seating position than the other three are from theirs. Readers of Parts 14 and 15 may discern a potential problem with this but it’s addressed lower down.

It can be seen from photo 2 that the suggested seating position for rock D is right next to that of rock C but more on the head lobe side of the shear line i.e. extending down the narrow, sloping ridge that falls away from the ‘horizontal’ seating area of all the other rocks. This ridge resembles a hollow-block wall, exhibiting square or rectangular dips along its top and two vertical ‘skins’ either side. The ‘wall’ is very straight and has the appearance of stunted remains, having been demolished almost to its sloping base. As we’ll see, the quasi rectangular hollows can be discerned to a greater or lesser degree in different photos and the two outside skins stick up slightly higher than the main body that contains the hollows. Rock D exhibits the same characteristic hollows at one end and has an S-shape that vaguely resembles the line of the sloping top of the wall, whichever way up it’s swiveled to make the fit.

The following photo, photo 3, is a close-up from the one used for the Part 15 header. It shows rock C’s seating position, including the red-dotted square that was mentioned in that part. The sloping ‘hollow-block wall’ remains are shown with two rows of bright blue dots delineating each of its top edges where rock D is thought to have been seated. The upper three pairs of dots enclose two square hollows. These look like one shadowed rectangle in most other photos. Two more square dimples extend below the two rows of dots as the hollow-block wall extends further down from the rock D position. These are out of focus in this photo but visible in photo 4. Being S-shaped it’s possible that rock D peeped over the top edge of the ridge and actually kissed rock C as well as rock B (not shown).

Photo 3  
Bright blue- suggested rock D seating position.

Yellow- rock C seating position.

Red square- this is retained here for comparison with photo 2 to allow for orientation. This is because the hollow-block ridge doesn’t look the same in both photos. It’s overexposed in photo 2 (witness rock A, 30 million kg, apparently turned to dust in the middle of Site A). The rock C seating position and its red square proves we’re looking at the same hollow-block ridge.

Orange dot- the pointed end of rock A’s seating position.

Photo 4 is taken from almost the same angle as photo 3 but with subtly different lighting conditions. It shows the other two square dimples along with the pair on the suggested seating position (very small yellow dots). So there are at least two pairs of squares, totaling four. Seeing as two squares weren’t apparent in photo 3 but are apparent here under different lighting, there may be more sitting before us but out of focus or overexposed. Four more rectangles that are further down in the same ridge are annotated as well, to show that rectangles are a common feature along this line.

Photo 4  
Red- each end of the hollow-block ridge as depicted in photo 3.

Yellow- two pairs of squares on the straight section and four more where the ridge curves round towards us slightly (below the bottom red dot).

Photo 5 shows this ridge in plan view in order to demonstrate how straight it is. This observation will be important for later discussion in this post. The photo is the only plan view of the ridge and shows its fairly long slope as being a bit stubby because it’s foreshortened.

Photo 5  
Red- one edge of the hollow-block ridge (zooming required). Its two pairs of squares aren’t shown here.

Yellow- three of the four extra rectangles mentioned in photo 4. At this point, the ridge starts curving away from the main, straight section that we’ll be focussing on but the existence of more rectangles proves that these are a common feature on the ridge.

Photo 6 is a close-up of photo 2. Again, it shows how straight the hollow-block ridge is, albeit from a profile viewpoint.   
Red- either end of the straight section of the hollow-block ridge.

Yellow dots on ridge- denote two shaded areas each of which accommodates a pair of squares, as mentioned in photos 3 and 4.

Whatever relationship rock D may have had with the hollow-block ridge, the main piece of evidence that suggests it came from this seating position is that the rock and its seating are sitting at either end of the rock D rotation plane corridor. The corridor can only be aligned at one angle from the rock where it sits today and that line goes through the seating position. Here’s a plan view of the corridor.

Photo 7  
Bright blue- rock D. Its seating is at the other end of the corridor, specifically, between the red and yellow dots.

Dark blue- rock D rotation plane corridor. Note that the bottom (outer) perimeter line almost touches rock A and its seating. This will be further discussed below in photo 8.

Terracotta- shear line.

Yellow- continuation of shear line around the crater that mirrors the ‘cove’ in the head above. This has often been dotted yellow by convention in former posts.

Mauve- rock D’s 2 blurry companion rocks and rock E (isolated to the right).

The rock D corridor line is all the more accurate because we can piggyback on the rock A corridor which relates to an unequivocal 170-metres of rock travel from seating to present day position (photo 8, below).

The suggested seating position also happens to be where the corridor intersects the shear line. In fact, this intersection is plumb on the three-way join of head, body and the perimeter of missing slab A. This is where the most upheaval was likely to have happened. 

There’s much evidence for this. As well as kissing the shear line and rock C’s seating position, rock D is likely to have kissed rock B. Rock B was almost certainly lifted at the same time as rock A because their configuration as they are arranged today on the flat crater floor of Site A is almost identical to their seating configuration but for the fact they tipped on their sides when they landed. Rock B’s seating is contiguous with rock C’s for its entire length, completing a suite of connections and actual movements that furnish strong evidence that all four rocks were detached in the same event. That event can only be explained by something sudden and catastrophic- the head lobe shearing and lifting. The join between rocks B and C also constitutes part of the perimeter of missing slab A, i.e. the rim of the residual crater we see. So rock D came from the exact point of the three-way join of head, body and missing slab. 

Rock A’s line of travel back across the crater floor is mappable to a very accurate degree by drawing a line between its pointed tip and the corresponding pointed end of its seating position. This line can then be used for the rock D rotation plane corridor line which would have been parallel. In fact, in order for it to be plotted parallel to rock A’s corridor, its outside line has to be adjusted so that it’s virtually contiguous with rock A’s inside line. Any other line isn’t parallel. 

Photo 8 (culled from Part 14 and reannotated- apologies for having three different types of blue).  
Bright blue- rock D seating at the top-left end of its corridor (needs zoom). Rock D itself is off-frame.

Dark blue- shows both the rock A corridor and the rock D corridor. Rock A’s corridor is solidly derived from drawing straight lines from the two end points of the rock to the end points of its seating. Rock D’s corridor is then constrained by having to be parallel to this line while also embracing rock D where it sits today. This constrains rock D’s seating to being somewhere along this line. 

Pastel blue- shows both rocks A and B plus their seating positions. Note how they retained their seating configuration even after the 170-metre drift. The dots depicting their seating positions show only the right hand edge for each rock in this plan view. Rock B’s is a little more debatable but it uses the shadow of the low ridge (mentioned below) for guidance. If this is correct, its other long edge was contiguous with rock C. So if it kissed rock D as well, it might have represented a continuation of the hollow-block ridge formation.

Yellow- rock C seating position (rock C is off frame).

Terracotta- shear line.

It can be seen from photo 8 that it’s likely that rock D kissed rock B when originally seated. This is because rock B was right next to rock A and protruded a fraction closer to the shear line. In fact, rock B’s shape and seating suggest it may have been a continuation of the hollow-block wall. It left behind a low ridge that appears to be a continuation of one of the skins of the wall that accommodated rock D. However there are no hollows visible along that line and the relevant facet on rock B is always either in shadow or too fuzzy to make out. 

So there’s some strong evidence that rock D came from this seating position but its actual seating position, as proposed, doesn’t have the sort of definitive matches that rocks A and C had. However, there’s more.

Photo 9 shows the head lobe seating position for rock D. The long line of rectangles between the two yellow dots shows what is in effect the seating position for the entire straight section of hollow-block ridge sitting below on the body. But with one proviso- they wouldn’t quite meet because mass went missing from between them in the form of rock D and other ejected rocks. The top facet of rock D would have been seated against this matching head lobe feature. It probably would have been a little way down the line from the ‘top’ rectangle, as viewed in this photo, and would have taken up around half of its total length. 

This very straight line of rectangles faithfully hugs a straight section of the oft-mentioned ‘cove’ on the head rim. The rectangles are just a fraction away from the perimeter itself, running along the underside of the head lobe. The whole line constitutes one side of the cove so the ‘top’ rectangle is at one end of the cove.

This straight section has been documented before as being the match to the sloping hollow-block ridge below it by virtue of the cove on the head matching the crater on the body. So it’s already irrevocably linked to rock D’s suggested seating position (whether rock D sat between them or not). Such a line of rectangles is to be found only here, positioned directly above the line of rectangles on the body ridge, and nowhere else on the head lobe rim. 

You can even see the two parallel skins of the ‘wall’ that were noted on the body’s hollow-block ridge. Notice also how the cove perimeter turns at a sharp 90° when it reaches the base of the line of rectangles. This will be of some significance in the discussions below and in future posts.

Photo 10 is a profile view of the line of rectangles. It shows the cove in context within a larger view of the head lobe.

Photo 10  
So the character and shape of the hollow-block ridge on the body isn’t the only evidence for rock D’s seating position. The seating position on the head lobe, now 1 km ‘vertically’ above, exhibits the same feature in great detail. The head lobe seating position would have been attached to the upper facet of rock D or possibly to a larger section of ‘wall’ that fragmented into rock D, rock E and their fuzzy companions. In other words, it would have sandwiched Rock D between head and body. It would follow that rock D was part of a seamless continuation of the hollow-block wall formation from head to body. Rock E and the two fuzzy companions do appear to be dimpled with squares or rectangles and are about the same width as rock D and the ‘wall’. They’re also perfectly positioned to have been part of the collateral damage.

The fact that the head lobe ridge hugs the perimeter of the cove means that, in structural terms, the perimeter is actually defined by the hollow-block wall feature, not vice versa. This means that there’s what might be considered to be a retaining wall running up this side of the cove, with mass on one side and the void of the cove on the other. This behaviour is mirrored on the body by the sloping hollow-block wall feature. It too acts, or rather, acted as a demarcation line with mass on one side and a void on the other. The mass has largely been ripped away but was once sitting there in the form of slab A. The void is in the form of the crater that mirrors the cove. The detailed matches around the curve of crater and cove were documented in Part 8. So the hollow-block ridge acts as a demarcation line between mass on one side and void on the other, in exactly the same way on both head and body.

This matching line of rectangles on the head lobe wasn’t found as an afterthought. In other words, there was no trawling around under the head lobe rim after a sudden realisation there might be something up there to match the rock D body seating. The head lobe line was already constrained to match the body seating by all the other matches along the shear line that were presented in Parts 1 to 8. Each match dictated the exact point of the next match, head-to-body, in lock-step all along the shear line. This was the penultimate match in the line before the matches disappear at the edge of missing slab A. These two hollow-block ridges were paired up 3 months and 15 parts ago, in Part 3. That’s why the match is said to be previously documented and irrevocable. It was just the hollow-block aspect that wasn’t mentioned before.

Photo 4 from Part 3 is reproduced below (photo 11 for this post). The two pairs of red dots next to the two yellow curves match from head to body. Rock D’s seating positions i.e. top-to-head lobe and bottom-to-body lobe are represented by the left hand dot of each pair, directly adjoining the beginning of each yellow curve. The bright blue dot on site A is rock D’s present position. 

Photo 11  
More specifically, the body seating suggested above would extend between the left hand red dot and first yellow dot; the head seating would extend downwards from the corresponding head dot or even starting a little below it, so it’s just out of sight. The right hand dot of each pair corresponds to the very last match before missing slab A took the rest of the matches with it along its top edge. The large scale cove-to-crater match is there too, denoted by the two yellow curves. 

So, as always, there’s simply no wiggle room in this current post to go casting around on the head rim for random ‘hollow-block’ matches in the vague vicinity of where they’re thought to be. The two points, body seating and head seating, are straitjacketed by the other matches along the shear line, which precludes any woolly match-making. We’re constrained to focus our gaze on one point and one point only on the head lobe rim whether there are hollow-block features there or not. And yet at that very point of focus we see the same hollow-block formation, set in the same very straight line, aligned at the same angle, holding mass back on side, open to the void on the other and found nowhere else on the head rim.

At present, the exact lengthwise matching of the two hollow-block lines is hampered by several factors, not least of all the several tens of metres of missing mass thickness. Lengthwise alignment refers to which rectangle on the head ridge sits above which rectangle on the body ridge to initiate the match, rather like sliding a set of two ladders a little way along their length and deciding whether to lock them on the second or third rung. The ‘straitjacket’ reference was to the ridges’ sideways alignment which is very exact but even the lengthwise alignment is straitjacketed by the other matches- it’s just harder to judge it exactly until someone somewhere cuts the neck out of their newly minted shape model and sits the head directly on the body.

This lengthwise alignment could be pinned down much more accurately using the more detailed ESA computer shape model and cutting the neck out virtually. The head to body match would also have to align features at 90° to the two ridges discussed in this post. Such a feature is the slurry pile at the back of the rectangle mentioned in parts 1-7. This would automatically lock the two ‘ladders’ on their correct rungs. The resolution would have to be at the current 5 metres at the very least and preferably 2 or 3 metres to get a really good picture of what’s going on. Clamping the head rim onto the shear line around the rest of the body would automatically show the two hollow-block ridges trying to clamp together but ending up parallel, one directly above the other and with a large gap between them. The distance between the tip of the head ridge and the scoured slab A crater rim below it would be the thickness of the slab A perimeter. It would be the same or thicker than rock D.

This exercise may produce the unsurprising conclusion that the the top tip of the head ridge protruded a little beyond rock D and up the side of rock B to its top surface. This would be an outcome that satisfies the fact that the head rim broke away from the true surface as it existed before slab A and the rocks were dispersed. After all, it was the head shearing from the actual top surface of slab A that lifted it and caused all this upheaval. That’s why I said that the seating for rock D might start just a little way down the head lobe line of rectangles from the exact tip. The actual tip would in that case reach to the top of rock B, or a bit higher, when it was seated. This obscure point wouldn’t have been mentioned but for the fact that it will crop up in future posts when grappling with the three-dimensional character of the shear line at this point.

That 3D nature is actually caused by the sloping ridge itself, coupled with the slab A thickness. And the underlying cause of all this weird sloping up of the shear line is due to it jumping from one stratum layer to another like tearing a piece of pleated paper at an angle to the pleats. The tear line jumps from pleat to pleat, running straight along each pleat for some distance before the next jump. If you put sticky tape across one of the pleats on both sides, the tear will very likely do a 90° turn at the tape and jump to the next pleat as a consequence. The shear line did exactly this because it couldn’t tear through the very ridge we’re discussing in this post. So it turned an abrupt 90° to go round it, jumping up to the next stratum layer in the process. That was indelibly recorded in that emphatic 90° turn at the base of the head lobe ridge of rectangles. There will be a post on this stratum jump but the evidence is all there in Part 6, either stated or implied. 

The reason this head-to-body match wasn’t elaborated on before by highlighting its matching hollow-block ridges, was because it was clear that there was some missing mass between them. It wouldn’t have been correct to say that they matched in the sense of actually marrying together although it was clear they shared the same morphological structure that ran through both points in both lobes. Rock D and rock E are possibly part of that missing mass that was sandwiched between the two, perhaps along with their two fuzzy companions.

The missing mass between rock D’s head seating and body seating comes as no surprise in stretch theory because the thickness of the missing mass is also the thickness of missing slab A as its edge tapered up to its perimeter line at this point (it was thicker across the low, flat crater floor).

All four rocks, A to D, were at that 3-way join of head, body and missing slab perimeter, giving rise to the focus of mayhem at this point. Rocks A,B and C were more attached to missing slab A (but kissing the shear line) while rock D was right on the shear line or even on the head lobe side. They all represent part of that somewhat tapered thickness of the 50-70-metre stratum that’s missing. Rock D’s shape, current position and integral hollows would simply suggest it’s part of the missing piece of hollow-block wall that originally continued seamlessly from head to body. 


There are several apparent objections to rock D coming from this seating position:

1) It’s four times further down the rotation plane than rock A so that suggests it must have been dropped from a great height, possibly above 750 metres (see calculations for rock C in Part 15). But slab A was only about 750 metres high even when hinged to vertical.

2) It couldn’t be dropped from the underside of slab A at vertical anyway because it’s in line with its own rotation plane corridor. 

3) It couldn’t be dropped from slab A low down because it would’ve landed alongside rock A.

The answer is that it didn’t drop from slab A. It’s on shear line or even on the head side of the shear line. It must have risen with the head lobe and been dropped from a good height, yes, but from directly above its seating position. This height probably wouldn’t need to be 750 metres anyway because on release it would be under a significant influence from the head lobe gravity but still experiencing a net draw to the body. This would allow it to drop much more slowly than rock A did and so travel four times further but from a lower release height than if it had been released from slab A. Remember that 4 times further down the rotation plane means a 16 times higher drop height if slab A is involved (see Part 15). That’s higher than the dimensions of slab A so it must have been released from the head  lobe.

Dropping from the head, and from directly above its seating position would also have allowed rock D to stay within its rotation plane corridor. It would still be fairly close to the rotation axis of the comet, meaning the body lobe gravity would still be dominant but compromised by rock D’s proximity to the head lobe gravity field at the beginning of its fall. This would mean a slower, longer descent resulting in capture further down the rotation plane.

The head would continue to rise despite rock D dropping because its centre of gravity was rotating at around twice the radius of the release point on its underside so it would have twice the ‘centrifugal’ force. Its original ~1m/sec rise would have to have been attenuated before release of rock D, though. Otherwise rock D would have escaped too. However, after several hundred metres of stretching, there would have been considerable shearing and heat generation in the emerging neck, leading to slowing of the stretch and retention of the head lobe.


Despite the circumstantial evidence for the rough original siting of rock D, its exact seating position is less definitive than the unequivocal sitings for rocks A and C. However, whether it’s rock D, rock E, the fuzzy companions or even escaped rocks that sat in the void between head and body lobes, this post has finally firmed up the significant match along the shear line that was made in Part 3: the hollow-block wall ridge that ran as a single structure through both head and body. 

Photo credits:

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


New evidence for stretch theory is emerging all the time and there will be more posts to come. Recent photos of the newly illuminated south pole of the comet confirm the prediction made in Part 11 that the head lobe perimeter profile would match the body lobe perimeter on that side. They also show evidence of the initial tipping forward of the head as it lifted off. So the next post might focus on that, but the Anubis slab needs a post too as well as matches from Bastet to Aker.

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