67P/ Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 13

(Key below)




Dark blue- rotation plane.
Bright green- direction in which Imhotep is rotating, towards right of frame.
Orange- cliff face or ‘hinge’.
Red- perimeter of cleaved stratum layer.
Mauve (on shape model photo)- x axis. This axis sweeps along the rotation plane and is at 90 degrees to the z axis ‘axle’.
Light blue- point at which x axis pierces body, proving that the dark blue dots are on the rotation plane.


Part 12 was concerned with the missing slab from the top of the head, slab C. This slab comprised the entire region of Hatmehit. The exposed surface left behind was understandably very different from its undisturbed surroundings. Seeing as the regions on 67P are categorised primarily according to type of terrain, it’s no surprise that the perimeter of the missing Hatmehit slab and the perimeter of the Hatmehit region are contiguous. The same reasoning applies to missing slab D, Imhotep.

It was proposed that the Hatmehit slab was lost via rotational forces alone because it didn’t have a helping hand from the uplifting head lobe as did missing slabs A and B. It was mentioned that Imhotep was lost via the same process and that it would be the subject of this post. It should be easier to explain the mechanism of the loss of Imhotep because it is essentially the same as that of Hatmehit. I’ll go into less detail here. The Part 12 subheading, “The Mechanism of Hatmehit’s Departure” has more information. As usual, the missing slab is presented here as fact to spare the reader a multitude of conditionals and qualifications.


Imhotep exhibits all the same traits as the other three missing slabs described so far in Parts 9 and 12: a brittle break at one end with accompanying debris, at or near its base; a smooth, flat plain in the middle; and consolidated material resembling cleaved rock at the other end.

It is proposed that under spin-up of the comet via asymmetrical outgassing, the slab attempted to slide across its fracture plane in the direction of rotation before lifting off via centrifugal force. This is because it was near to but not on the rotational axis extremity. It then encountered resilient material at its forward end (as regards rotation direction of the comet). That material was the cliff edges we see today around one end of Imhotep’s perimeter. This resistance led to greater tensile, ‘lifting’ stress at the opposite end and across the middle of the slab. This caused the trailing end of the slab to cleave cleanly and vertically from its substratum layer, leaving that relatively flat, clean layer visible today.

As the slab lifted like an opening trap door, it hinged against the resilient cliffs the other end, grinding and tearing against them before reaching the vertical point above the hinge. Then it departed, flipping end-over-end. It may have fractured in the middle as it hinged open, being one of the largest of the missing slabs. But once it had broken away cleanly from its substratum, it had to have risen only a metre to be lost forever and its disintegration wouldn’t have affected the characteristic ‘missing slab’ signature it left behind.

However, as with the Hatmehit slab, Imhotep has to satisfy a much stricter criterion than the simple observation of the so-called missing slab signature. Those features, the cliff, the flat plain and the cleaved strata could be aligned in any direction across the base of the comet. For them to qualify as a true signature of a slab that’s missing due to rotation forces alone, they have to align along the rotation plane. Moreover, they can’t align in either of the two directions along that plane, it has to be the cliff in the forward position and the cleaved strata taking up the rear. This is because the trap door could open only in one direction due due to rotation and that direction is with the hinge and therefore the cliff in the forward position. As you can see from the annotated photos, Imhotep satisfies those constraints but with one anomaly: the cliff extends further round to the top of the frame than at the bottom. This isn’t quite as neatly symmetrical as Hatmehit. However, there are two very solid-looking crater rims in the Ash region where this extension lies. It would be reasonable to suggest that the slab would tear against these as it tried to lift up cleanly from its base.

Apart from this anomaly, the cliff and the cleaved strata form arcs in the expected positions or perhaps five to ten degrees off the line of the rotation plane. Interestingly, the Hatmehit slab signature is also skewed anticlockwise by the same amount. One wouldn’t expect both signatures to be exactly aligned with the rotation plane, down to the last degree, due to structural anomalies affecting the fracture lines along the cliff and cleaved plane. Yet both signatures appear to be almost exactly aligned with each other through the x axis of the comet.


-The cleaved fracture plane probably extends under the dust layer in the middle of Imhotep.

-It would be reasonable to argue that if the slab was lost due to rotational forces alone then all the detritus should have been flung off too, leaving a scrubbed surface. That objection is dealt with at the bottom of Part 12.

-Thomas et al (January 2015; link below) mentions “mass wasting” on several occasions when alluding to the cliff features. In reference to one accompanying photo (figure 7) it is proposed that a stratum of several metres’ depth is missing. On inspection of the photo and its 50-metre scale bar, this depth appears to be more like several tens of metres. Furthermore, the paper mentions the expulsion of very large volumes of material without much or any residual talus. This is evidently a puzzling observation because the material has simply vanished from the comet including, presumably, the non volatile component. A tentative proposal is made that gas pressure build-up might expel much larger lumps but acknowledges that the porosity of the comet may well militate against such a scenario. Seeing as the proposed missing slabs are by definition the thickness of the 30-50 metre cliffs they left behind, this neatly solves the paradox of the vanished material.

Thomas et al (23rd January 2015)
Research Article The morphological diversity of comet 67P/Churyumov-Gerasimenko


Photo credits:

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

67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 12.


This spectacular photo from the OSIRIS camera graces this week’s Science Magazine cover. It is the site of missing slab C, comprising the entire Hatmehit region.

On the day that Part 11 was posted, 23rd January 2015, a slew of papers were published on 67P in Science Magazine. They were authored by the scientists involved in the Rosetta mission. Two of these papers were concerned with the general morphology of the comet, both its surface features and its bi-lobed shape. Many, if not most of the puzzling features they recorded are explained by stretch theory and especially by the missing slabs that were torn from the areas they are observing. Their bemusement at what they were seeing was summed up by one of the authors:

“You’ve got to produce all of these diverse morphologies on the surface with that one energy source [the sun]—I find this tough.”

Links to the two papers are at the bottom of this post (full text available free).

Part 9 of this series, “The Missing Slabs”, would be the place to look for the bulk of the explanation for the strange features the scientists are witnessing. This is because much of the surprisingly varied terrain was due to the brittle fracturing along the the ‘arcs’ of the slabs (see header photo in Part 9).


However, there are two species of missing slab, the ones uplifted by the head as it rose from the body (described in Part 9) and another type which will now be described in this post. These are slabs which are missing from the top of the head and the base of the body. They would have departed under the influence of rotational forces alone. Here are annotated photos of the missing slab on the head. It comprises the entire Hatmehit region, no more, no less, which is telling in itself.

Key: orange dots- brittle fracture; red dots- flat, cleaved uplifted perimeter; blue dots- rotation plane.




The slab that’s missing from the base is the entire Imhotep region. That will be left until the next post, Part 13. Both slabs exposed exotically varied terrain as you might expect and prompted perhaps a third of the comments in the two Science papers, mostly regarding brittle fractures, scree debris, smooth plains and large amounts of “undercut” material and “mass wasting”. They also identified areas where huge volumes were now apparently missing but attributed it to outgassing and collapse on such a massive scale that it seemed to represent a paradigm shift in the theories on cometary sublimation. If there were any doubt about this, an accompanying piece in Science stated that the scientists involved say:

“the complexity of the comet today suggests that the comet-forming regions of the early solar system were more turbulent and chemically diverse than theorists have thought.”

Yet it would seem prudent to investigate all avenues before rewriting the science text books.

Being near the extremities of the long axis (y axis), the head and base slabs were shed by rotational forces alone whereas the ones in Part 9 had an initial helping hand due the the head lifting them at the shear line as it broke away and rose from the body. Those slabs would have been more sluggish in their departure or even remained, dumped to one side, were it not for that helping hand. This is because they were nearer the z axis of rotation and subject to less rotation force.

The loss of slabs from the head and base is described here as if it’s fact but of course, as stated in part 9, it is a working hypothesis albeit with much evidence going for it. The reason for this is that couching every proposition with conditionals and qualifications can become tedious. So no, it’s not proven yet.

Now that the comet’s regions have been named, it makes describing locations easier. As mentioned above, the slabs missing from head and base both comprise an entire region. They are Hatmehit (head) and Imhotep (base). They would have been of comparable size to the Part 9 slabs, and probably tens of metres thick as well. Their locations can be seen in the context of their surroundings in the ESA photo below.



Hatmehit has all the same hallmarks of slab loss as those described in part 9: a brittle break at one end, forming a curved arc, debris at the base of the break, a smooth plain across the middle and no evidence of a brittle break on the opposite rim. In fact, quite the opposite: this end resembles clean, cleaved rock just like the Part 9 slabs do. The only difference in the case of Hatmehit is that, given the circular cross section of the head and its evenly layered strata, any brittle break in this area would likely form an arc anyway. In Part 9, the arc shapes were explained via the concentric arcs of equal force radiating from the head rim leverage points. On Hatmehit, if the location of the brittle break had to be curved anyway due to the shape of the head, it would automatically coincide quite well with the outermost leverage force arc. That would make the initial resistance along the brittle break somewhat less robust and the likelihood of slab uplift all the more plausible.

The slab on Hatmehit was cleaved away from the very defined fracture plane that now constitutes the base of the crater it left behind. This crater is dust-filled so you can’t see that plane from above but you can see it from the side in the photo below. It is defined by the uppermost pair of orange dots. There are three more pairs of dots delineating the ends of successive strata layers below the one that now forms the bottom of the crater. Half the rim of the crater remained after its slab departed. You can see it arcing round the back, constituting the very tip of the comet as seen in the photo. At the front of the crater, there is barely any rim left at all although in some high relief photos it’s apparent. That’s why the top pair of orange dots appear to be level with the base of the crater or, at best, a shallow dish sitting in front of the cliff at the back. Blue dots are the rotation plane (at right and across the top- zooming advised). The single red dot represents the entire curve of red dots in the other photos here because they would be exactly side-on in this photo. Similarly, the top left orange dot represents its curve of orange dots as well as the base of the crater. The viewpoint for this photo is from a similar angle to the OSIRIS header photo:

Orange- fracture planes
Blue- rotation plane in view across top and at right. It follows a line just behind the horizon on the right hand side of the head.
Light blue- point where y axis pierces the body from right to left.
Red: cleaved perimeter of Hatmehit slab

As the slab was lifted, it opened like a circular trap door on the top of the comet, hinging at one end. The hinged end was where the brittle fracture is seen today. The reason it hinged is because the soon to be ejected slab was not at the exact extremity of the long axis when the head was still attached to the base. So under spin-up there was a tendency for the slab to want to slide forward on its fracture plane to the end of the axis before lifting off ‘vertically’ from the end of the axis via ‘centrifugal’ force. Those two words are placed in single quotes because they aren’t quite correct terms but are nevertheless perfect proxies for what was actually happening.

However, in its attempt to slide forward, the slab encountered some resilience in the form of a solid lump of material. This lump is the very solid-looking vertical cliff face that constitutes most of the brittle fracture, although it should be noted here that this cliff curves round some way to the side as well. The part where the brittle break occured is evidenced by boulders and scree. As a result of encountering resitance to its sliding tendency, the slab experienced a greater uplifting strain on the opposite rim to the hinge just prior to uplift, which caused it to shear at that end, cleave away cleanly from the well-defined fracture plane and and start rising. All through the lift, the other end of the slab remained wedged in against the cliff as it tore and ground against it. Eventually, the trap door passed through the vertical axis above the hinge and escaped, flipping end-over-end as it departed.

The process described above will be familiar to anyone who has read Part 10. It’s the same scenario that explained the uplifting of the entire head itself from the body. The only difference is that the head didn’t escape, partly due to experiencing less centrifugal force and partly due to tensile resistance in the neck. The slab that escaped from Hatmehit represents the same process in microcosm but it concluded with the slab escaping.

So the slab hinged open on the head while at the same time the head itself hinged open, so to speak, from the body. This is because both were simultaneously under the influence of rotational spin-up. Together, they were opening up like the cover and frontispiece of a book flapping open in a strong wind, albeit with a rather chunky frontispiece.

So far, so simple, but Hatmehit has to satisfy a much stricter constraint than just a casual observation that it has a cliff at one end with some rubble at the bottom of it. If a slab is to be flung out like a trap door via rotation alone, the hinge and therefore the rotation plane of the opening lid have to be in line with the comet’s rotation plane. Moreover, the trap door couldn’t flap open in either of the two directions along that rotation plane. It has to have happened in the manner described above, which means that the hinge was at the most forward position as the comet rotated and the smooth, cleaved area was at the most rearward position. The annotated photos above show that all these conditions are satisfied.

One fly in the ointment with the Hatmehit slab is that there shouldn’t have been any scree and boulders left over. Since it was shed via rotational forces alone, everything else should have departed with it. The reason debris could remain on the slab A and B sites was that they were closer to the rotation axis and so fragments could drop back down slowly or even just slide around. The initial kick from the head uplift pulled the slabs away giving the slabs themselves the last bit of uplift they needed to escape. Rotation alone probably wouldn’t have been quite enough, especially for slab A, right on the rotation axis.

So perhaps it’s not correct to cite the debris at the bottom of the cliff as strong evidence for the brittle break. That said, one could put it down to fragments that were loosened within the cliff face and were subsequently worked loose by sublimation and temperature stresses. In fact, it would be reasonable to suppose that in the chaos of the hinging up there would be entire monoliths detached by a few centimetres and effectively loose but wedged in just a few key places by the surrounding cliff. Those would then erode more easily via sublimation and temperature stress, shedding boulders over a long
period after slab departure. So maybe the debris we see is indeed an artifact of the hinging and the brittle break. This would mean that they weren’t the actual fragments that dropped out at the time but are there as an indirect result of the hinging process and so betray its position. It’s a factor that’s worthy of debate though.

Even if the scree debris is discounted, we still have a fractured outcrop on that side of the crater, a smooth plane in the middle and cleaved, bare strata at the opposite end- all aligned the correct way round along the rotation plane. Most of the evidence is still intact.

Science Mag articles on the morphological diversity of 67P:

Sierks et al (23rd January 2015)
Research Article On the nucleus structure and activity of comet 67P/Churyumov-Gerasimenko

Thomas et al (23rd January 2015)
Research Article The morphological diversity of comet 67P/Churyumov-Gerasimenko

Photo credits:

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

67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 11.


“The Collisional Problem”. I might have dreamt that up as an apt subtitle for this post because by the time we’re done it will surely be merited but, too bad, I was beaten to it. Those words were already taken as the subtitle for a poster accompanying a talk on 67P’s status as a contact binary. The venue was the American Geophysical Union (AGU) Fall Meeting in December 2014 and the verdict on that binary status remains equivocal.

The abstract for the presentation was entitled,”The Nucleus of Comet 67P/Churyumov-Gerasimenko : a New Case of Contact Binary?”


It acknowledged that:

“A contact-binary among the Jupiter family comets (JFC) such as 67P would have profound implications since it must be primordial and the comet must have survived a possible history of collisions in the Kuiper belt. The present cumulative distribution function of size of nuclei of JFC comets indeed suggests a collisionally-relaxed population.”

In other words, there is countervailing evidence that militates against the existence of contact binaries in such a “collisionaly relaxed” population.

The collisional problem is, however, just the first of many for the CB theorists. They might be able to attempt an explanation of the rotation plane head-tip and fractures but not without jumping through flaming hoops in the process. They seem up to the task though. The AGU abstract considered the “unlikely…scenario of a re-accumulated body following a catastrophic collision” (because it really ought to have been blown to smithereens by now), as if it was a last resort to explain the two-lobed shape as a contact binary.

True to say, everything should be considered. Asymmetrical erosion (carving out the lobes via lopsided outgassing) was given a fair hearing too. But it seems the notion of ditching both theories altogether and contemplating stretch theory was just one step too far.

At least one of those authors presented again two weeks later, at the American Astronomical Society’s 225th meeting (AAS 225, 4th to 8th January 2015), still referring to 67P as a contact binary or possibly a single, eroded body. Again, the phrase “a new case of contact binary?” was used, along with the poster, still reminding us of the collisional problem.

But the answer to the collisional problem was on the poster itself! It had a picture of the comet, looking straight down the z axis of rotation. The visible perimeter, the horizon, was therefore the xy rotation plane and the lower-right horizon was where the fractures happen to straddle the neck at 90 degrees. Furthermore, the uppermost tipping of the head kisses that horizon line too, right above the fractures.

That poster was what made me twig the relationship between rotation plane, spin-up, head tip and fractures that lead to this twinned pair of posts. So the answer to the collisional problem is what it was telling us all along- that 67P can’t be a contact binary. But you had to let go of CB theory fully, if only momentarily so, in order to see the hallmarks of stretch theory depicted on that poster. They are crying out at us: rotation plane, head-tip, fractures.

Here’s a tweet of “the collisional problem” poster:


Not much AAS 225 or AGU 14 material is available so we had to rely on tweets. I believe that at least two tweeters quoted here were present at the talks and the third, possibly so, or an astronomer with a live feed of some sort. I can’t be certain of this but they all seemed informed and genuine.

Respondents to the above tweet and elsewhere on the #AGU14 hashtag, some from respected institutions, were happy to support CB theory. One suggested tidal friction leading to tidal locking (of head and body) and then settling against each other. Another supported the asymmetrical erosion tack but no one considered stretching.

Incidentally, I can’t imagine a secondary with one side as flat as that shown in the photo below, offering that side up to the primary in an attempt to lock tidally. It would be inherently unstable. If it ever locked at all it would only be stable if it was the other way round with the topmost ‘crater’ facing into the neck. The reason it’s stable in stretch theory is because it always had a supporting neck to attenuate excessive tipping via compressive resistance to any downward vector on any side. This would even have been the case when it was ‘weightless’ during the stretch because the ‘locking’ referred to above is in two of the three rotational axes of the head, not its translational stretch. There are signs this compressive resistance did indeed happen because the head tip only went so far. Here’s the photo:

2015/01/img_22231.jpgPhoto from Part 10. Blue dots: xy rotation plane; yellow dots: ends of one fracture among several.

Of course, this remarkably flat underside of the head can now be seen for what it is. It’s simply the extension of the now well-documented cliff, reemerging on the other side of the neck. This was the first photo to come out that showed it, proving the head lobe has a flat underside all the way across and overhangs the neck evenly on both sides. The entire plane seats itself back down neatly all round the currently visible portion of the comet body and will doubtless prove to do so on the dark side when it comes into view.

At another AAS 225 presentation, 67P was presented as a contact binary, plain and simple. The presenter pointed out that while several other comets had rocks joined end-to-end, this one had one on top of the other so it looked like a sphinx. That’s actually a far from fatuous distinction, though I’m not so sure anyone was aware of the fact. It should immediately point to the reason the head is tipped up at the back and lead on to the underlying mechanism at play: spin-up, leading to head-tip, stretch and transverse fractures. The head was thrown forward during spin-up due to the very fact that it’s “on top” and not at the axis extremity. But this fact becomes clear only if the rotation plane is considered.

So, yes it does look like a sphinx but the contact binary assumption made on the way to that conclusion is troubling. The “on top” reference was unwittingly portentous but it became just an interesting observation in the absence of any willingness to loosen the grip on CB theory. Here are the relevant tweets:


The presenter went on to say that the fractures in the neck were due to the head rocking against the neck. That’s true up to a point. It’s due to the head tipping forward during the stretch, among other things, but “rocking against” (the tweeter’s words) suggests random movement with no known cause. Here’s the tweet:


As if this succession of respected presentations wasn’t enough to leave stretch theory buried for good another American Geophysical Union presentation, on December 18th 2015, managed to take all the evidence as presented in Part 10, which proves the comet stretched and use it, in error, to prove the exact opposite: that 67P’s shape resulted from the head lobe crashing in from the opposite direction. A sure case of a contact binary. Here’s an article from Wired magazine that reported on that presentation:


The reasoning went that, seeing as the strata in the head lobe and the body lobe don’t align, they can’t be related and so they must be two different bodies that drifted together. But the only reason they don’t align is the simple fact that the head has tipped up, taking them out of alignment. The Wired article explained that using new images taken with Rosetta’s OSIRIS camera, the presenter and his colleagues found “terrace-like layers” on the comet’s body and that:

“The layering aligns perfectly with parallel lines seen on the opposite side of the body, suggesting that these layers extend through the body as part of its internal structure. Although the head also has layers, they don’t align with those in the body, which implies that the two lobes were once two separate pieces. If the head and body were made from one piece, the layers should extend through both lobes in the same direction.”

This line of thinking presupposes that the only alternative to contact binary theory is asymmetrical erosion gouging a scoop out of a single rock. The only way the presenter could envisage the strata lining up through both lobes was in the scenario where a single body started out with its strata intact all the way across and then this scoop was removed, leaving the two separate lobes but with their strata still in alignment.

That assumption is what steered the presenter away from the most obvious solution- that the head had tipped up, taking the strata out of alignment. If it was tipped back down by 30-40 degrees and seated onto the body, the strata lines would align perfectly, as demonstrated in part 6 of this series.

But it was the head-tip itself that led him to believe that that strata had always been out of alignment so this crucial piece of evidence that so strongly points to stretch theory was used unwittingly and in error to ‘prove’ instead that contact binary theory is correct.

That is a perfect example of the adherence to a cherished theory blocking out even the very thought processes that might lead to the correct theory- a case of fitting new data to an old way of thinking, rather than letting new information speak for itself.

It is also of note that this hypothesing around the finer points of CB theory is based on the highest resolution photos from the OSIRIS camera on the Rosetta orbiter. Just a few scientists are privy to this data at the time of writing (including those with the collisional problem) and it was my honest opinion that they could not help but alight on stretch theory with the abundance of evidence before them, much of it still to come in the next few parts of this series.

However, the Wired article went still further:

“The neck of the comet also shows signs of a collision between the head and the body. The region is covered in big fractures, [the ones straddling the xy rotation plane in the photo above] which would have been created by shockwaves that blasted through the comet during a crash. Some of the fractures are also misaligned, suggesting that they belonged to what were separate, smaller chunks that were floating around when the head merged with the body.”

I have never seen a compressive force cause fractures in a concave surface without it resulting in an explosive shearing event. There’s simply nowhere else for the material to go. Witness compression-testing of concrete pillars that aren’t even concave. They explode dramatically. If further compression of the comet’s neck material is invoked to counter that claim, it is self-contradictory because if it can compress still further it won’t fracture. Indeed, with a porosity of 74%, the neck would have plenty of give in compression and at most it would simply bulge out in folds. But it would not fracture under compression unless it sheared violently as well.

The fractures have clearly resulted from tensile, flexion and torsional forces none of which are compressive (except where flexion compresses the opposite side of the neck).

Incidentally, the mention that the fractures are ‘misaligned’ is interesting. Their average direction is distinctly at right angles to the rotation plane and in successive parallel lines. Yes, they do make noticeable excursions from that simple large-scale picture, presumably due to structural anomalies, but the overall impression is as presented in the photo above and the others in Part 10 (reproduced below).

So, according to this second AGU presentation, as well as the head rocking against the neck (on the sphinx), it had first of all collided with the neck, which was apparently ready-formed, protruding into empty space and pointing in exactly the right direction to make the catch. Once captured, the head ended up perfectly centred with its flat plane conveniently facing downwards, allowing it to overhanging evenly all the way round the neck. That’s quite an impressive claim, provoking at least four tough questions.

Moreover, even if the idea of the colliding head causing the fractures appeared to have some merit at first glance, it wouldn’t explain why those fractures are clustered in parallel lines, at one end of the comet, exactly straddling the rotation plane, at 90 degrees to it, sitting right under the most tipped-up part of the head, and found nowhere else on the neck.

That’s six more really tough questions, ten in total, for the CB adherents to answer regarding the collision of head and body- and that’s the real collisonal problem here, the one that truly merits the subtitle.

The simple answer is that this is where the neck stretched the most and the head tipped forward. Stretch theory would not only answer those 10 questions with ease, it would also predict all ten outcomes as being highly likely.

If they did somehow manage to jump through all ten hoops, the CB theorists would then have to go on to explain why the plan-view matches between head and body were irrelevant- along with their corroborating 3D matches (Parts 1-5). Then explain away the ridges that straddle head and body, followed by the matching strata layers (Part 6). After that, the 30-metre uplifted ‘gull wings’ and slurry piles arising, apparently, from gentle sublimation (Part 7). Then the dykes (Part 8), and the missing slabs (Part 9) -of which there are several more to come- and at least three more as yet unpublished pieces of compelling evidence. That’s eleven more hoops, twenty-one in total to date and counting.

Sorry, twenty-two, I forgot the the original “collisional problem”.

In conclusion, stretch theory answers a multitude of questions that contact binary theory cannot hope to address. Yet on January 8th 2015, the day that AAS 225 closed for business, and astronomers and reporters tweeted views of Seattle en route to the airport, stretch theory had yet to see the light of day.

Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
Here are the other two photos from part 10:



67P/ Churyumov-Gerasimenko. A Single Body That’s Been Stretched. Part 10.

(This post is twinned with Part 11)

2015/01/img_2220.jpgPhoto 1: Still from an ESA video. Key: blue dots- rotation plane; yellow- the most prominent of several parallel fractures; mauve- the comet’s y axis piercing the body at the light blue dot.

2015/01/img_2223.jpgPhoto 2: Close up of the rotation plane and fractures with the same annotations and key as above. Yellow dots are at each end of the fracture.

2015/01/img_2225-0.jpgPhoto 3: A closer view. Notice the most obvious fracture of all, running above the yellow line fracture. Also other fainter fractures are visible in line with the fourth blue dot from the top and just below the fifth blue dot. All are roughly parallel and at 90 degrees to the rotation plane.

At the end of Part 7 I mentioned how the forward position of the future head lobe, when still attached to the body, would lead to a greater tensile stress at the back of its neck. I was using the rubber duck analogy where the head is naturally set towards the front of the body. But I was describing it in its pre-stretched form with the head tucked onto the body like a dome. This would mean that the future neck area, the back of the dome where it attached to the body, was under the most strain under spin-up. It follows that this would be the most likely place to fail and tear away from the body first. It would also very likely become the most tipped-up portion of the head once the stretch was completed.

This scenario would make sense only if the head and body were aligned along the rotation plane, meaning the duck was continuously doing forward somersaults. That’s the only way the head could experience the tipping-forward strain prior to its shearing away. This is because there would be a tendency for the head (the dome) to slide to the end of the rotation axis before lifting away. If the head/dome were sitting on a lubricated plane, it really would slide forward to the axis extremity. But of course, it would have been attached to the body with some degree of shear resistance to the sliding force. The only other way it could succumb to the force, in the absence of a slide, was to tear along the back of the dome/neck and tip forward.

Once fully broken away, this tipping-forward force would act in concert with the ‘lifting’ force (the so-called centrifugal force) to lift the head directly away from the body and continue tipping it forward as it lifted. The centrifugal force would provide the lift. The tipping force would reduce somewhat as the head lifted away because the fulcrum for the tipping (front of the neck) would have broken free and be rising as well.

It should be said here that in reality the tipping force is actually a vector component of the centrifugal force but it’s beyond the scope of this post to talk about vector summing of forces.

As you can see from the header photo, the head lobe is indeed tipped up exactly along the rotation plane. The blue dots show the comet longitude line that corresponds to that plane. The rotation plane is the xy plane of the comet, which is at 90 degrees to the rotation axis, the z axis. You can see the x axis pointing downwards and, together with the y axis (mauve dots), it forms a cross shape that can only sit in one plane and that is the rotation plane. The z axis points to the left, out of frame. It is the spinning axis, the axle if you like and is by definition at 90 degrees to the xy rotation plane.

The lighter blue dot in the middle of the blue dotted line is where the y axis pierces the body and continues through the core to the centre of gravity and out the other side of the comet. This proves that the blue dotted line is on the xy rotation plane.

The header photo is a still taken from a video posted on the Rosetta blog. You can see the original video here, which shows all the axes labelled as the comet spins:


I tilted the header photo so that it looked very roughly similar to how it would look if you sat on the ecliptic plane to observe the comet, i.e. with the z axis of rotation at 41.6 degrees to the ecliptic. In the video it’s spinning on its side with the z axis straight up which would be ecliptic north and the xy rotation plane would therefore be at one with the ecliptic plane. I think that was done so that all parts of the comet would be presented as it rotated.

So we have a head that’s tilted up at the back and exactly in line with the xy rotation plane. This is what you would expect from stretch theory, in fact it’s almost a prerequisite. However, in addition to this compelling evidence there’s the fact that the fractures exactly straddle the rotation plane and do so at near enough 90 degrees. This has all the hallmarks of the stretching process putting too much strain on the material to yield plastically all the way through the stretch. So it fractured across several weak spots at the same time as it stretched. Those fractures were naturally going to appear at 90 degrees to the direction of the stretching force vector.

The head-tipping would probably have contributed to the fractures too. We know it swivelled anticlockwise by about 5 degrees (looking from above; mentioned in Part 3 with photo) and that must have been after detachment. That means tipping probably occurred as well, despite having a less solid fulcrum at the front.

Moreover, the quasi-circular cross sectional profile of this strange portion of the neck and its pinched appearance is indicative of the pinching you see when two plastic lobes are pulled apart, like pulling apart bread dough. You could take the analogy further and say that if there’s not enough water in the dough to stretch plastically, fractures will appear at 90 degrees to the stretch, curving around the cross sectional profile.

Parts 7 and 8 explain how a supposedly brittle comet might stretch like dough, and this link is a further piece of evidence that backs up that hypothesis. It’s wholly relevant to 67P and mentions the authors’ prediction for the fine-grain structure of 67P- ice pebbles (paywalled but abstract is long and informative).


Part 11 will see how contact binary theory fares at explaining the head tip and fractures being exactly along the rotation plane.

Photo and video still credits:

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

67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 9



Throughout the first eight parts of this series I’ve made passing references to “missing slabs” that were broken from the flanks of the body lobe of the comet and lost to deep space when the head lifted away from the body. Just like the outgassing in part 7 and the Dykes in part 8, this is a working hypothesis which needs more proof. However, as is the case with those two other posts, the existing evidence for this is favourable.

In the header photo, there are two distinct arcs on the body which curve down and up along its flank and roughly parallel to the ‘shear line’ where the rim of the head broke free from the body. One arc comprises the whole of site A the very flat crater. This is denoted by yellow dots. Let’s call that slab site A, which is to say, this is the area where the lost slab A once sat. The other is less pronounced in this photo but more pronounced in the photo below. Let’s call that slab site B (orange dots). The terracotta dots run along the shear line for all photos in this post.

Notice how slab site B’s floor is quite flat, rather like slab site A’s but without the dust covering. It’s somewhat riven but essentially flat with an appearance reminiscent of cleaved rock as if there was once a vast slab, some tens of metres thick, sitting above this stratum layer. In stark contrast to this, the appearance of the curved perimeter couldn’t be more different. It has a crystalline quality reminiscent of fractured marble. That’s not so noticeable in the header photo but obvious in others, below. Taken together, the cleaved stratum running down the flank of the body from the shear line and the crystalline, fractured arc suggest that if there indeed was a slab there, it was levered up along the shear line and broke with a brittle fracture along the arc. Rather like the process of quarrying rock on the Earth, it appears there was little adhesive resistance between the two layers along the cleavage but the lifting or snapping forces along the arc were across the stratum layers of the slab, hence the brittle fracture.

The same principle applies for slab site A. Although you can’t see a cleaved surface due to the dust layer it is very flat so it’s in keeping with the same cleavage scenario as suggested for slab site B. It also exhibits the same crystalline fracture along the arc.

Here’s an additional photo showing slab sites A and B in profile:


And a photo showing the crystalline nature of the break around the arc of slab site B with its key below:



This photo is culled from Part 7 with added annotation. These are the annotations relevant to this post and are all down the right hand side of the photo:

Bright yellow: slab site A perimeter

Bright orange: slab site B perimeter

Light blue: the visible part of the crystalline appearance of the arc (but only about a third of the arc is visible here).

Notice the three orange dots in the bottom right corner showing the arc perimeter coming back into ‘view’- it’s actually a tiny bit further right but the three dots serve to orientate the viewer. I know the visualization is tricky here. Other annotations are with the photo in Part 7 but the terracotta dots showing the shear line are worthy of mention here as well, especially if you try to relate them to the terracotta dots in the header photo. The head lobe obscures the shear line in the upper part before it resumes at the flat-edged crater next to slab site A (that crater was viewed in detail from the opposite side in part 8)

Additional evidence for the brittle nature of the fracturing around the arcs is listed below:

1) The sloping nature of the fracture. This is common when levering cleaved slabs of rock on the earth or breaking highly stratified slabs of slate or sandstone.

2) That said, when the slab is almost free at the last moment, the resistance along the very top edge of the rim of the arc is near to zero and so the fracture can become vertical. We see this very low, vertical ridge around parts of the very top rim of slab site A. (Not very easy to discern in the photos for this post).

3) The very fact that the breaks are arced suggests the slabs were levered up from the opposite side, along the shear line. If levered from a point source, the leverage force is the same along concentric circles from the source. If levered up in sympathy with the rim of the uplifting head, as is proposed, the leverage points (along the lifting head rim) would be spread along various straightish lines as opposed to being point sources. The forces would still fan out in concentric circles but they would be slightly flatter, more oval-shaped. Many other factors to do with the comet structure will muddy this ideal shape (ridges, slab thickness etc.) but these flattened arcs are what we see and are consistent with the leverage force coming from the shear line.

4) Exposed subsurface showing pock marks of lighter material. This is apparent across one side of the slope of the brittle fracture in slab site A. It was remarked upon in one of the Rosetta blog posts before Christmas 2014, which suggested it was newly exposed material. This is of course evidence in its own right for a slab to have been overlying that area. But the pock-marked nature of the whiter material is also suggestive of a brittle break. Here is the photo from that Rosetta blog post:

2015/01/img_2137.png Continue reading

67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 8.

Are These The Dykes That Channeled Gases From The Core?


In Part 7 a working hypothesis for the first stage of the head shearing away from the body was presented. This included the proposition that a very deep, narrow chasm opened up between the head and the body as they initially separated, allowing internal heating, consequent large-scale sublimation and high-pressure outgassing at the shear line.

The next thing to look for would be evidence of gas/slurry conduits that might have formed spontaneously in that narrow crevice as it opened and found itself immediately pressurised with escaping gas. This would be streaming from the heated core where the incipient neck was just starting to stretch. These conduits might form spontaneously and be spaced fairly evenly apart in the same way that convection cells form spontaneously in almost even columns across a heat flux. They would radiate out from the centre to the rim in straight lines.

That is exactly what we see in the underside of the head lobe photos. These radial streaks going straight up the cliff have thus far been a complete mystery to everyone. Could they be the conduits?

The conduits shown in the header photo are just some of these radial lines. It just happens to be the only ESA photo that shows them fairly clearly as they exit at the rim but there are others streaming up all over the cliff. The original photo (zoomed out) and two other photos at the bottom of this post show the whole cliff with these few lines from the header photo denoted, again in mauve.

High-pressure conduits might well have been long and narrow, acting more like high-pressure kimberlite dykes than normal, low-pressure vents or fissures. This is because the initial gap between head and body would have been 500-700 metres deep and perhaps only centimetres wide. The gases would all have to flow one way, towards the shear line, while yet more heating and sublimation was pushing out gas relentlessly in their wake. The dykes would pressurise then force and scour their way out radially, come-what-may.

The lines we see do scour their way over and through the various lumps and bumps on the cliff and do so in very straight lines. They even straddle the large lump protruding from the bottom of the cliff. They also exhibit a number of apparent holes along their length which might be extra, sideways tributaries. What’s more, they run at right angles to any discernible strata so they can’t be strata lines.

Perhaps the best test to see if they were dykes is to see if these radial lines are trumpet-shaped at their outlets, like miniature versions of kimberlite pipes on the Earth. Real kimberlite pipes are long and thin and end in 1-2km long funnels which result from the explosive behaviour of the magma near the surface. This is similarly due to pressure drop.

It’s difficult to tell if the outlets are trumpet-shaped, looking at the NAVCAM photos, but there are several depicted in the header close-up that appear to be. These are around the rim of the cove on the head. Their accompanying dykes, if that is what they are, scour their way up the cliff, cutting through any protrusions on their way to their outlets. There are three outlets on the straight part of the cove rim, each with a small dot above them (which may be rocks or bleed-across outlets). The central outlet appears not to have its own dyke but may have been fed from the two either side. There is a fourth outlet just into the shadow of the step-up beyond the three rocks/outlets. And a fifth in the sunlight just beyond the step-up.

When seated on the body, the straight portion of the cove with the first four outlets would have been at the bottom rim of a straight-edged crater of the same width (see below). The emerging gases would have had no choice but to travel up from the crater floor as they emerged. The fifth outlet, beyond the step-up is actually emerging from a face that was formerly shunted against the body from the side (this small area being at roughly 90 degrees to the cliff). Its corresponding seating position on the body has what appears to be a gouged-out hole where the emerging gasses slammed straight into a wall before turning upwards and exiting. Three of the other four dyke outlets show some correlation with the scouring on the body although due to the very nature of scouring they aren’t perfect matches. To save boring readers with yet more head-body matches, the photos with the usual dotted annotations for the above matches have been relegated to the bottom of this post.

Regarding the three rocks or sub-outlets in the cove, the shadowing suggests two are rocks and one is a dip. However, they are fairly well aligned with the main outlets so one would deduce that they are all rocks or all sub-outlets. Whilst contemplating how to bring other evidence to bear on this, I realised that what I had seen as little strings of rocks impossibly aligned along the head lobe strata lines are probably very small dyke outlets. This would imply that all the stratum layers were under tension at one time (as we would expect in a spin-up or Roche pass at Jupiter) and were exhibiting the same behaviour as described above but with less gas, narrower dykes and no more than a minuscule amount of uplift. One of them, the weakest, had to give and that is the one we see yawning open before us.


The photos are annotated with dots in the same manner as posts 1-7, with the key below them. The non-annotated versions are at the bottom of this post and are unzoomed for context.




Mauve: these show the dykes on the head lobe photo and their supposed pathway traced in ‘mid-air’ over the neck region on the body lobe. Material has since been eroded from this part of the body. Two of the body lobe tracks trace the path they would have if you could see them behind the head lobe (i.e. the two paths running into the black area).

Bright yellow: outlets from dykes that are discernible on the head lobe and their supposed position on the body.

Bright green: outlets that are less obvious on the head lobe one in shadow, the other supposedly fed by dykes on either side.

light blue: a matching ridge that used to step up from the crater on the body and from there the shear line curved round to the ‘gull wings’ discussed in Part 5. (This curving round is also matched when viewed from above in Part 4).

Dark green: the gull wings (see header photo, Part 5). The head lobe wings are fully visible but the body wings barely so. However, there are three dots depicting the very tip of one wing on the body. The end dot of the three corresponds to the top green dot on the head lobe photo. The slurry pile (one of two described in Part 7) is to be seen just below the wing tip on the body. The base perimeter is outlined with light yellow and each of three slumping tiers is dotted with terracotta. These tiers can be seen as being tightly constrained by the shape of the wing in the Part 5 header photo, suggesting it pushed up the wing some 20-40 metres.

Light yellow: see above.

Terracotta: see above.

Orange: a ridge set back from the cliff edge which curves round. This roughly matches features on the body that are set back from the course of the shear line which is the straight cut across the crater. Notice how the portion of the head cove (the true cliff rim) where the four adjacent outlets are, is very straight as well. This corresponds to the straight break across the crater.

Red: two diagonal, straight lines that seem to jump sideways from one dyke to another. Possibly bleed-across channels? They emerge near the corner of the crater where you might expect them to if seeking a path of least resistance. Whatever they are, they are quite well matched from head to body. On the body photo, only the ends are indicated to save cluttering with dots in that area.

Below are the original versions of all the photos in this post. Photo credits:

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

Header photo has additional processing by Stuart Atkinson at


This processed version was featured on NASA’s APOD site:


ESA Original

Stuart Atkinson version

Body match photo original

Two more cliff photos for context of where the dykes are. Zooming advised.



67P/Churyumov-Gerasimenko- A Single Body That’s Been Stretched- Part 7.

Evidence of Extensive Outgassing Along The Shear Line



These two photos are mirror images of each other. The top photo is a view of the underside of the head lobe of the comet. The bottom photo is the view of the body lobe. The rectangle defined by the four green dots on the head used to sit on the same rectangle defined by green dots on the body. All the other dots and dotted lines mirror each other as well. These photos will be used to show evidence of extensive outgassing along the shear line between head and body before the head was significantly displaced above the body.

Key to coloured dots:

Green- denote corners of the rectangle

Red- this line traces the perimeter of the finger of protruding rock on the head photo. This finger fits the horseshoe-shaped intrusion into the rectangle on the body.

Light orange- this is the second stratum in the finger which fits the second step-down in the horseshoe. This is more evident in the second pair of photos, below. It protrudes further out from the neck corresponding to a deeper stratum layer in the body.

Purple- suggested pathway for high-pressure sublimating gas and slurry. Where it fans into four paths there is some evidence of conduits. Where it is depicted as a single track it is a conjectured pathway based on the apparent scouring around this side of the finger where it sat on the body.

Light blue- apparent scouring out of detritus which contributed to the semicircular slurry piles (pile perimeters are delineated with light yellow dots). This detritus also contributed to the slurry ridge along the back of the rectangle on the body (overlain by the wavy line between the two green dots).

Light yellow- this denotes the perimeter of the two slurry piles. These appear to have been formed mainly by slurry emerging from just under the rectangle and not so much from the purple slurry paths depicted on the top surface of the rectangle. These two piles have pushed up the two ‘gull wings’ on the body (see Part 5). The slurry paths that are depicted by the purple dots may also have contributed a little to these piles but the top two fans seem to have fed a wider, flatter continuation of the slurry ridge along the back of the rectangle. The existence of these piles show that at least two fracture planes were oozing gas and slurry: one above and one below the rectangle surface we see today. The gull wings on the body match those on the head (Part 5) and there appears to be a third layer of wings on the head when viewed from above. All three were layered like puff pastry before rising and separating. The top layer exhibits a clover-leaf shape at its corner that is mirrored on the body in some less exposed photos (see last photo in this post) but is also just visible in this and the next body photo. It’s beyond the slurry ridge, next to the green dot and purple fan.

Here are two more photos from different viewpoints. They depict the same features as above and use the same colour coding. In the body photo, the finger’s former position is depicted floating in ‘thin air’ to the right of the horseshoe. This is where material has since been eroded away. The dark blue dot represents the drop from the the last elevated red dot to the actual neck.



Explanation of Extensive Outgassing Along the Shear Line

At the end of Part 5, I noted the evidence regarding what appears to have been massive outgassing and slurry flows under the rim of the head lobe of the comet before it detached from the body lobe. This consists primarily the two slurry piles sitting squarely under the two pushed-up ‘gull wings’ on the base, suggesting that they were pushed up 20 metres or more by the emerging slurry and gas. And it also includes the frill along the edge of the cliff top on the head, which used to sit neatly onto what appears to be an elongated slurry pile on the body. This slurry pile, a low, bulbous ridge, runs along the back edge of the rectangle and defines the shear line where the current cliff edge used to sit. It also contains the wavy line that matches its mirror image on the underside of the cliff edge (Part 1). There is evidence of scouring just before the exit points on the body, along the wavy line, although the resolution in the Rosetta NAVCAM images means it is hard to make these out for sure. If they are there, it chimes with the notion of very high-pressure outgassing eroding the fracture plane directly under the head lobe when it was still sitting on the body. This detritus would presumably be in a slurry-like state and be deposited at the outlet apertures along the back of the rectangle. By that process, an elongated slurry pile would be formed, pushing up the frill before the head was lifted off its base.

To be clear, at no point am I suggesting that such explosive outgassing actually caused the head to split from the body. The only energy input large enough to achieve that and the subsequent 1km separation would be the differential g forces on a close approach with Jupiter (a Roche pass at around 120,000 km altitude) or the stretching force from a spin-up to 90-120 minutes per rotation of the comet. The latter would probably need to rely on asymmetrical outgassing over hundreds of years or more but that type of slow, continuous impetus bears no relation to this short-lived, high-pressure event along the shear line. The only ‘uplift’ I’m entertaining is the bending up of thin layers around the rim of the head, i.e. the gull wings and also the edge of the cliff as it is now but at a time when it sat firmly on its base. The only other way in which this outgassing may have aided uplift was in that it seems to have scoured out parts of the fracture plane that the head was sitting on, leaving fewer contact points and less adherence.

I had mentioned in Part 5 that I thought the uplift of the gull wings could have happened over time, prior to uplift, but I think this was premature. I now believe it’s possible that the initial lifting of the head due to the Roche pass or spin-up would have been in the form of a sudden faliure of the brittle, sintered casing of the comet. Prior to that faliure, the stretching forces would initially be resisted via the tensile strength of the comet including the casing and no stretching movement would be apparent, despite the stress. But once the casing failed along the shear line there would be an instantaneous transfer of tensile stress to the central area (the incipient neck between the lobes). Stretching would commence instantaneously as well but immediately self correct to a much slower rate when the forces rebalanced. This sudden initial stretch would have caused massive instantaneous heating in the interior of the comet due to longitudinal shearing and result in a one-off outpouring of explosive, sublimating gases. Perhaps this can be further explained by including an excerpt from a comment I made to Marco on the subject (comments, Part 5).

“…we have made it clear in several comments that any sublimation, whether a hypothetical explosive excess or simple background, is not the cause but an effect of the split. In other words, any excess or even explosive generation of gases over the background level would be as a result of the heat (and internal pressure drop) generated by pulling the lobes apart. The only mechanisms suggested for that pulling force are spin-up or Roche pass delta g forces. As for the background levels, they may have served to weaken the the fracture plane between the strata leaving them looser prior to detachment but not lifted them and certainly not propelled them 1 km apart.

“Where I think I did make a mistake is that in my initial observation of explosive outgassing around the rectangle/scallop area (end of Part 5) I was quick to cite it as a more exaggerated version of this fracture-weakening process and assumed that it was *prior* to separation via spin-up or Roche forces, not due to it. That’s because I couldn’t deny what I was seeing in the photos and made the knee-jerk assumption that it was simply a very enhanced spike over and above background levels. (This still wouldn’t mean the gases were pushing the head up from the body, just the gull wings and frills around the edges).

“I then corrected that idea of it being prior to stretching in a comment to Robin Sherman on the 14th December Rosetta blog post which also quoted the relevant section from Part 5. That comment said that I now realised it didn’t necessarily have to be a spike in normal activity, prior to the stretch. On the contrary, the vast amounts of energy that intitiated the stretch could be the very energy source required to fuel such a spectacular outgassing.

“In short, I hadn’t stitched together all the various ideas from the previous few days. I’d already made comments elsewhere on the Rosetta blog regarding what would be a sudden transfer of tensile stress to the interior, (the incipient neck), causing heat, possible ‘cryovolcanism’ and reduced tensile resistance. I just hadn’t gone the extra step in realising that this should cause excess sublimation with the gases streaming out along a fracture plane crevice that’s over 500 metres from source to outlet and possibly only a few cm wide (from head to body) on its initial fracturing. That scenario would be a one-off scenario on intial fracture and subside as soon as the crevice opened to a width that allowed reduced upstream pressures. That in turn would mean reduced outlet pressures resulting in no more gull-wing uplift and frilly edges. Those features would already have risen with the head lobe to a point where gasses could stream through a wide aperture underneath them. That head uplift would be entirely due to spin-up or Roche forces and not be due to the gasses.

“This is all conjecture, of course, but what I see in the form of the uplifted gull wings, slurry piles, slurry ridges and frilly cliff rims that sat over them speaks of outgassing on a massive scale, whatever the cause. And I think my dimension estimate of a 20-metre gull wing uplift may be low. It was a conservative guess- it could be 40 metres, the height of a small office block.”

[End of comment quote]

The evidence of this massive outgassing is there on both lobes. On the body lobe, sitting up against the seagull profile under its left wing, is the obvious semicircular structure which I mentioned above. In the paired photo of the head and body gull wings (the tiltle photo in Part 5) it has the same cross-sectional profile as the wing. In the photos above, it is roughly semicircular and the same width as the wing. Both these observations suggest it extends underneath that wing. The concentric semicircles within it resemble oozing ‘mud’ that slumped like thick custard after oozing between the layers and being ejected at the end of the rectangle. When it found an exit, it pushed up the edge as it emerged. The other wing next to it has a similar but less defined slurry pile structure.

As for the fluted portion on the underside of the head lobe, there are three distinct channels and a possible fourth that add weight to this suspicion. These channels follow the same path as the ‘mud’ would have followed under the scallop on the base to form the semicircular slurry piles. The channels are narrow but flare out as they reach the two wing apexes- evidence that the escaping gases were under pressure and pushed the wings up. The channels look as though they may have been fed from a conduit on the right hand side of the finger as seen on the underside of the head or the left side on the body where there appears to be scouring around the left hand edge of the horseshoe.

All these head channel features are roughly mirrored in their seating position in the scallop (the triangular dip) at the end of the rectangle on the body: there is the apparent ablation of material to the left-hand and upper perimeter of the horseshoe followed by striations that lead to what would have been the underside of one of the head lobe gull wings. These gases would have been forcing their way through the next stratum fracture line, above the scallop.

This suggests that gases and their accompanying detritus were forcing their way through the very two layers that have now separated a kilometre apart. And in turn, that suggests that the shear line had already torn: the incipient neck was now stretching, colossal heat was being generated and gases were streaming through the the obvious escape route- the newly opened shear line. Hence the gull wings and the frill.

The following photo is difficult to follow because it’s from a different angle and in high relief due to shadowing. But it’s detailed and shows possible evidence of the actual tear.


The rectangle on the body is shown with the usual green dots. Its complementary rectangle on the head is out of sight but its outside edge runs roughly along the head perimeter, downwards from the four light yellow dots. The horseshoe, along with its second tier (red and light orange) is shown emerging from behind the head in the foreground. The shear line is depicted with slightly larger terracotta dots all the way down to the bottom of the photo. You can see what looks like evidence of tearing along the ridge just to the left of those dots. The top dot, just below the horseshoe isn’t strictly speaking on the shear line or tear line because we know the head lobe extended across the rectangle to the right of that dot. However the obvious tear that carries on past that dot to the horseshoe suggests that this was where the really catastrophic shearing took place and from where gases and slurry streamed. The gases would then have streamed under the rectangle, pushing up its edges just before it was taken up with the head lobe.

The light yellow dots are not slurry piles this time. There are four on the head and three on the body. They show the clover leaf shape, mentioned above, that is matched from head to body. It looks less like a clover leaf here than in the other photo above but it’s the same feature and matches an almost identical shape on the body. In fact, the shape match is even more faithful than I have outlined with the dots. The bottom dot of the four dots on the head should be lower so as to incorporate the line below it that runs right-left then angles up and left. The bottom body dot incorporates this line. The result is three lines, identically matched from head to body.

To be clear, this is not some random match of two parts of a photo that appear rather similar. On the contrary, all previous matches dictate that the only place for that part of the head to have sat is in the very place on the body where the clover leaf pattern is duplicated- at that corner of the rectangle. This isn’t a mirror image because we are looking down on both head and body. The pattern must have bled through these thin layers to cause a bloom on the surface. That surface layer has now risen a kilometre along with the head and is part of the overhang at the edge of the cliff.

Was This Where The Head Initially Ruptured?

Seeing as the rectangle is the only place along the shear line where such evidence of large scale outgassing is apparent, it may be the place where the tear started. It would then have progressed around the rim of the head in both directions from that point. This is also in keeping with the fact that this small area of activity is under the most tipped-up portion of the head. Again, that is not to say it gave any greater impetus to the separation force. It is just a sign that this was where the most tensile strain was in the first place so there was more lifting force at the point of rupture and more time to rise under that force.

Furthermore, it is to be expected that this point would sustain the most tensile strain whether through spin-up or a Roche pass. In the case of spin-up, the forward placement of the future head lobe on the body would mean a force vector pulling the head forward as well as up as it spun ever faster. That’s the same as saying the head was straining to detach at the back. This area of outgassing is roughly at that point.

With a Roche pass, the delta g forces are similarly aligned with the rotation so the same tipping vector would operate on the head with the same tipping strain at the back. However, this assumes a fairly slow pre-pass rotation that gets overwhelmed and realigned by the Roche pass delta g vectors. The present day ~10 degree anticlockwise twist in the head may be an artefact of its pre-pass rotation axis alignment.

One last piece of evidence for this catastrophic outgassing theory is that there is a ring of vent holes in the head lobe that are near to the fluted section and would’ve been near to the rectangle too when the head was seated on the body. There isn’t such a closely bunched circle of well defined holes anywhere else on the comet. This was clearly a little hive of high activity and pressurised gas escape at some point in the past.

Photo credits:

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