THE REASON FOR THE SHEAR LINE DIRECTION AT HAPI AND ITS IMPLICATIONS FOR STRETCH VECTORS
Wavy terracotta line- this is the classic shear line along Hapi. It also runs off into the distance round to the back of Anuket and in doing so, it runs through the four coloured anchors which are also matches to the head lobe (Part 24). The mauve match has both of its ‘ears’ marked. The dark green match is practically out of sight behind the neck.
Straight terracotta line- a suggested initial shear in a lower onion layer, which is along the stretch vector line and from which the layer above (Site A and Babi) were shunted slightly.
The ESA regions map is photo 5 at the bottom of this post.
It should be noted that the shear line now includes the front rim of site A, that is, the lip of the flat area. This hasn’t been included before because the missing slab above it was what matched to the head rim but that was no longer in place. That meant the Site A rim couldn’t logically match to the head rim. However, the Site A rim shows sufficient matching to a line just below the head rim. That evidence won’t be presented for a while due to a backlog of stretch vector posts. But we have to move ahead with related phenomena and this added section across Site A helps to inform the arguments for those phenomena. It’s the last 1-kilometre section in an already well-matched 7 to 8-kilometre shear line.
The straight terracotta line is a line that links the outer lips of the two obvious, lower sections of onion layer in Hapi. The line is then extended towards us to what appears to be part of the same lower, sheared layer running in line with the other two sections. That section stops at the marked curve in the classic shear line in the foreground. The curve is the exact site of the fifth and last gull wing delamination, with one wing kissing the curve (although the gull wings are more like a hump with two pimples at the fifth delamination). As we shall see in an upcoming post, the other end of this straight line stops dead at the point of the furthest delamination in the opposite direction. This delamination and the fifth gull wing set are almost equidistant from the north pole and exactly equidistant from the paleo pole adjustment point, insofar as the paleo adjustment can be said to be accurate (see Part 37). This is strong evidence that the straight line was an inner shear line brought about by the long axis stretch vector. The line is parallel to the paleo y axis (adjusted from the current y axis) and the paleo y axis runs through the paleo rotation plane. Thus, the straight line is exactly along the long axis stretch vector when looking from above (but not quite in line from the side view in upright duck mode- see below). This alignment should be no surprise because the straight line is clearly parallel to the line along which all the crust delaminations occurred at the Hapi shear line. The lower layer preserves the straightness of that line better than the classic shear line presumably because it was somewhat pinned under the onion layer above it (Site A and Babi). It would have been pinned even though Site A and Babi were themselves sliding back, as one layer and and parallel, on that pinned layer.
The idea of Site A and Babi sliding albeit just a short distance, is a new proposition in this blog just in case you thought you’d missed it. The slide was short and translational for the full length of the straight line, not radial like the top layer crust delaminations. You can see that from the fact that the classic shear line and the newly proposed straight tear are close and parallel.
That short slide was a shunt back of a middle onion layer, which is actually site A and Babi. Although that ‘middle’ layer is today’s top layer of crust, we know from previous parts that there was another layer on top in the form of the two missing slabs. Those slabs are missing slabs A and B (Part 9) and were the former top layer. They were said to have escaped in Part 9. However, given the amount of radial sliding signatures that we’ve been discussing in recent posts, it appears that slabs A and B didn’t escape but slid and bunched up dramatically. It was the delamination of the main sink hole on the body into three (Part 32) that made me realise there might be slide signatures for the whole site A area and even Babi. And indeed those signatures are there (see the “implications for slabs A and B” heading further down). So we now have three levels of crust or onion layers when considering the shear line and crust sliding at Hapi.
THE THREE LEVELS OF ONION LAYER CRUST INVOLVED IN THE STRETCH ALONG HAPI AND SLIDE AWAY FROM HAPI.
The three levels are described separately below. Levels 1 and 2 are quite long. Level 2 has a photo too.
This is the classic top layer sliding radially as described in Parts 32 to 38. It includes the delamination of the three sink holes on the body at site A (Part 32); the Ash slide (Part 32); the monolithic slide out of the horseshoe (Part 33); the Babi slide (to be presented in Part 40). Also, the red triangle from Part 26. The result of this sliding back of the level one onion layer was that it revealed the level 2 onion layer beneath.
Although level 1 slid back radially it also delaminated along with the gull wings prior to the radial slide. It had to do this in order to be loosened enough to slide radially. The level 2 gull wing delamination was in the long axis stretch vector direction which is between 30° and 90° to the various level 1 radial slide vectors around the pole. Level 1 behaved exactly like level 2 below it as they delaminated together along the long axis. Only then did level 1 slide radially.
So it would seem the only thing that distinguishes level 1 from level 2 is the very fact that level 1 was loosened enough to slide radially. The deeper layers were less loosened, being deeper into the comet and those, by definition became level 2. The level 2 surface is the present-day surface at Babi (including gull wings) and the flat part of Site A. This is why Babi looks riven, as if scalped, and it’s probably why the fourth and fifth gull wing sets are smooth bumps with pairs of pimples, which gives the impression of material having been flayed from above them.
That said, it seems likely that the dividing line between being loosened enough to slide, or not, was governed by the onion layers. Babi is quite smooth and Site A very smooth. That suggests a decoupling along the fracture line between two onion layers served as the weak spot between loosened enough and not loosened enough crust. It would also explain the ease with which the loosened top crust slid if it slid on its own smooth onion layer fracture plane.
The idea of delaminating one way and then sliding off in a completely different direction isn’t pure speculation. There are signatures of this all round the rims of Site A and Babi. The card pack sliding analogy fits perfectly. The stack was slid one way (long axis delamination) with a slight bunching of cards at each gull wing. Then the whole delaminated stack slid radially and up to 90° from the initial card slide. It would be nominally a line of cards moving outwards but arcing under the radial influence. It would still have cards that were very much overlapping but nevertheless clearly delaminated. When the line of cards had slid and arced out to a new, higher radius from the rotation axis (further from the pole) it stopped again, but not before the topmost cards had slid a little further radially, jutting out further over the rims of both Site A and Babi. That signature is actually there at the rims and, once you see it, it’s very obvious indeed. The configuration even retains the initial long axis delamination signature in the form of radial lines across the two arced lines of stacked card. That would be one radial line across the arc for each gull wing in the case of Babi. The lines are there too at the back of Site A but not currently traceable to the shear line at Site A where there are no gull wing type delaminations. Those may be on the head lobe. Close up photos of the two arced rims of cards, one for Babi, one for Site A, will be shown in Part 40 and beyond.
Seeing as levels 1 and 2 may have been separate onion layers we shall assume that to be the case for ease of explanation and they will be referred to as such as well as levels 1 and 2. It doesn’t make any difference to the sliding mechanisms if they were actually one onion layer that sheared into levels 1 and 2. In both cases it involves the layer or layers delaminating into many finer layers (the ‘cards’) in one direction then sliding radially in another direction. Level 3 seems even more distinct as a separate onion layer. It appears to be about the same thickness as the other two, which is an indication that they are indeed separate.
This is the middle layer that was predominantly delaminating along the long axis. That was the gull wing delamination along with their light blue ridges in Part 39. But this onion layer also shunted back a little way in sympathy with the radial slide of the top layer, level 1. The short shunt of level 2 is depicted in photo 2, a stunningly symmetrical illustration of the radial vectors around the pole and their close relationship with the straight section of shear line.
Photo 2- radial stretch vectors around the pole. This is a top-down view with the head lobe in the foreground.
(the red arrow should be ignored. It’s the current x axis which is irrelevant here).
Wavy terracotta- the shear line, which is the rim of level 2. Level 2 was originally the middle onion layer at Site A and Babi but is today’s top layer after level 1 slid back radially.
Straight terracotta- these smaller dots depict the straight line tear of the lower level, level 3.
Red dots- radial stretch vectors. Progressing clockwise: the bright green ridge (Parts 22-29); the three delaminated sink holes on Site A; The Ash slide matches (slightly angled to radial); the monolithic slide; two of four Babi radial stretch vectors, due in Part 40. The sink hole line and monolithic slide line are extended to the comet’s horizon in this view so as to make the radial stretch vector pattern more evident. The other four actually do extend to the horizon. The sink hole line may extend that far anyway if the large, isolated crater on Ash is related to the sink hole. That’s somewhat speculative but it did slide almost that far, a fact that’s betrayed by being entwined with the Ash slide matches (see Part 32).
Brown lines- the paleo y axis and paleo z rotation axis. They’re adjusted from the green and blue lines respectively because the rotation plane precessed by this amount. Notice how the paleo y axis is exactly parallel to the straight line tear. The paleo y axis is closely related to the paleo long axis since they are both in the same plane, at right angles to the paleo rotation axis and stacked almost one above the other when looking straight down from the top of the head.
Large brown- this is the paleo pole preliminary adjustment (see Part 37). The paleo rotation axis runs through it when viewed from above, as in this view. But when the head was on the body, it would have in fact run in line with it but below it. See part 37 for the paleo pole adjustments. You can see that the brown dot is at the exact mid point along the straight terracotta tear line and central to the whole symmetrical picture of radial vectors.
Photo 2 shows how (a) the radial force vectors of level 1 are focussed on the north pole or just beyond it and even more accurately so on the paleo adjustment point (b) their average direction tugged level 2 (with the classic shear line at its rim) at 90° away from the straight line tear on level 3, the lowest level. The result was that the shear line ended up parallel to the straight tear even though it became wavy at a finer scale due to vagaries of the local force vectors in the tug. It’s also wavy due to the local matrix weaknesses dictating the shear line tear path around stronger ridges like the horseshoe.
Although the shear line shunted in sympathy with the radial sliding of the top layer it didn’t exhibit any radial behaviour or at least, almost none. Its shunt was a short translational slide at 90° to the straight line tear of the layer below. That left it parallel to the straight tear. This probably happened because it was presumably tugged by the average radial vector fanning symmetrically around the pole (about 120° for the two radial stretch vectors at either end of the straight line tear). That average force vector shunted the level 2 shear line away at 90° to the original straight tear, leaving it parallel to it. The average force vector was also at 90° to the paleo y axis and therefore it acted along a line parallel to the paleo rotation axis. This scenario is especially likely for two reasons. Firstly, the length of the shunted section extended along a path that straddled the pole symmetrically and so this layer was tugged symmetrically by the 120° of radial vectors. Secondly, the extent of the line along which the shunt happened is exactly the same line as the straight tear and that line in turn comprises the delaminating layers and radial vectors emanating from it. With the exception of the red triangle, a separate entity as we’ll eventually see, there are no radial vectors, delaminations or straight tearing beyond the two ends of the straight terracotta line (the other end is Aker which is virtually unstretched crust riding on the tip of the stretching diamond shape). The parallel shunt of level 2 sits wholly inside this highly constrained geometric setup and extends along its full length, no more, no less.
Level 2 includes the gull wings, which delaminated along their long axis stretch vector line. It was along the long axis because it was directed by the comet’s core stretching underneath. That vector line is at 90° to the outward layer shunt away from Hapi. It also includes the entire flat area of Site A. As for the Site A portion of this second level, it doesn’t look as though it delaminated like the gull wings but that flat area appears to be under a lot of dust, possibly disguising signatures. However, the front rim of Site A doesn’t betray delaminations either whereas its mirror counterpart on the other side of the pole (and along the opposite half of the shunted section of shear line) exhibits the gull wing delaminations on its rim. That said, Site A may have simply stretched and that would be a neat explanation for its uncanny flatness. There are reasons for thinking it performed such a stretch and those reasons will be presented in a future post.
The third layer is the deepest of the three onion layers, peeping out from under the shunted middle layer, level 2. It exhibits the straight line tear along its rim that faces Hapi. The straight line is along the tensile stretch force vector. Although the three sections that make it up have gaps between them, making this level look like a sporadic assortment of jutting out chunks, at least two match to each other. They are the quasi rectangular section below the Site A rim and the section below the horseshoe crater. Their matching faces are the fuchsia V shapes in Parts 35-37. When joined, they would have made one long strip on one side of the pole. Their tearing and sliding apart along the straight line shouldn’t be a surprise because much or all of the second layer delaminated in the same direction along Hapi. After all, if the comet stretched along that straight line in Hapi before actually causing this massive shear stress fracture, level 3 would be expected to have either delaminated or torn into sections. It seems that these two sections tore. They are the furthest two sections along the straight terracotta tear line in the header photo. That tear left the matchable V-shaped gap between these two sections.
The other half of level 3 along the straight terracotta line seems to have delaminated instead of tearing in two. That’s the two fuchsia India shapes (Parts 35-37) and the stepped feature extending along to the fifth gull wing set.
It’s interesting that the pole was the dividing point between tearing on one half of the straight line tear and delamination on the other half. I have no explanation for that yet, but it will probably inform us in due course regarding the flatness of Site A.
That concludes the description of the three onion layers, levels 1-3.
THE STRAIGHT LINE STRETCH ALONG HAPI MAY GIVE SOME INDICATION OF THE PRE-STRETCH SHAPE OF 67P
Looked at in the round, it looks as if the cometary matrix along the straight tear line of the third level onion layer used to take up something approaching half the length of the straight line. And that’s why it now consists of sparsely spaced chunks. It had to develop gaps in order to stretch along Hapi and along the long axis vector. If it originally took up half the length, it would mean the comet was originally shorter along its long axis to the tune of about 1km. That mass would have to go somewhere if we reversed the stretch of the comet. The short axis widening to accommodate it is the obvious candidate. This is especially likely seeing as the existing shape of the body is an elegant, elongated diamond that got that way due to stretch. The converse is that it had to have stretched from a stubbier diamond, even a former square. And seeing as stretch tensile vectors likely caused the appearance of the straight sides of the diamond (formerly a stubby diamond) it probably used to be more rounded. It was probably just a stubby potato shape with a faintly protruding proto head lobe lump that was ripe for future herniation on spin-up.
OTHER EVIDENCE FOR THE LONG AXIS STRETCH ALONG HAPI
The direction of the shear line along Hapi is predominantly along the long axis direction. The line also happens to be running away radially from the pole in both directions. It’s actually straddling the pole diametrically along Hapi, which is why it can be both straight and radial from the pole at the same time. It’s no coincidence that the shear line along Hapi is parallel, or nearly parallel, to the long axis. It’s also at 90° to the rotation axis. The main stretching of the comet’s core under the surface crust was along the long axis due to that axis being the most susceptible axis to stretch under spin-up. That’s why the delamination of the gull wings happened along the that line. The crust couldn’t stretch by over a kilometre in the long axis direction to accommodate the core stretch, so it delaminated (or performed straightforward tears) in that direction instead. It was the core’s long axis stretching that was causing the gull wings’ direction of delamination along the soon-to-be shear line. That presumably caused shear stresses in the long axis direction as well i.e. causing stress fractures in that direction and not in some other random direction. That would eventually become the shear line. Or perhaps it was just one gigantic stress fracture, the shear line itself succumbing as the weakest point at a very early stage, and without any collateral stress fractures nearby. That’s why the Hapi shear line runs in the long axis direction. It’s also why the neck’s cross section is comparatively long and thin. It’s somewhat longer in the long axis direction.
Obviously, to have a head shearing away, the shear line had to curve round at both ends which would be across the long axis direction. That might seem to contradict what’s laid out above. However, the point being made here is that the shear line on the north pole side and, it seems, on the south pole side, was longer and straighter than the two curved ends and that the length and straightness were due to the stretch vector. The south pole pictures released so far seem to support this.
The fact that the shear line crosses the long axis where it does at either end of the neck would be due to random factors to do with head herniation from the body. Those factors would be the initial potato shape and the relative tensile strengths of different areas of core and crust. These would dictate the line along which the head started herniating, front and back. They would also dictate up to a point where it started herniating at the sides. But those side lines would also be strongly affected by the stretch vector. They would be moderated into longer, straighter lines as the comet stretched whereas the two curving end tears would remain the same width, or even narrow slightly under stretch. They would also remain curved or become slightly more curved under stretch. This all follows from the fact that any initial, quasi circular shear line will be stretched into an oval or, as it appears to be on 67P, a stubby rectangle with very curved ends. That would be the cross sectional profile of the neck if we sliced through it to take a look. And the straightening of the oval’s sides would be due to the overwhelming influence of the stretch vector along the long axis.
Incidentally, the influence of the same force that straightened the sides of the shear line may have been so strong as to slice the vertical wall onion layer on either side to create the vertical wall itself (Part 27). That would explain why it symmetrically straddles the paleo rotation plane and is comparable to the width of the neck. However, this aside is a work in progress because the width of the neck and vertical wall isn’t the same thing as the width of the shear line oval due to the fact that the neck was extruded and narrowed as the lobes separated after shear.
Only after the long axis, core-directed delamination of all three levels, 1 to 3, did the level 1 pieces of crust start sliding away from the north pole. Level 1 was the top layer and it was now loosened enough to slide radially in all directions down Seth, Ash and (as we shall see, Babi). Those were mostly very different directions from the initial long axis delamination stretch vector. In fact, the definition of level 1 is that it was the layer which was loosened enough to slide. At some depth it wouldn’t be loose enough to slide and that depth defines the top of level 2 which is today’s Site A and Babi.
The following photos, 3 and 4, serve to illustrate the geometry of the intimate relationship between the straight shear line at Hapi, the long axis and the rotation axis. Photo 3 is included to show that, from the side, the Hapi stretch vector isn’t actually quite in line with the long axis. However, it’s nearly as close as it can be because it’s fairly close to parallel when viewed from the side and exactly parallel when viewed from above. And although the stretch vector isn’t quite parallel overall, the straight tear in Hapi straddles the rotation axis (and pole) exactly. That’s strong evidence that it was centrifugal stretch forces along the long axis that brought about the straight shear. And that would be why it’s also almost in line with the long axis.
Photo 3- the long axis viewed from the side and back.
Dark orange- the current long axis.
Light orange- the paleo long axis, estimated for when the head sat on the body. It passes below the current rotation axis (blue) because the head was 1000 metres lower when seated. In the view from behind, this fact makes the light orange line appear to be ‘to far’ to the left at first glance. But it’s deeper into the comet than it appears to be. That’s because our eye is drawn to the blue rotation axis line and assumes it runs through the axis and not below it.
Terracotta- the shear line.
Photo 4- the long axis direction of the gull wing delamination. This also includes the two fuchsia V shapes from Part 37.
Brown line- the paleo rotation plane (see the relevant page in the menu bar and Part 26 onwards)
Dark blue line- (very small dots to the right of the brown line). The current rotation plane or equator of the comet.
Single dark blue dot- this larger blue dot is today’s north pole
Two brown dots- the right hand one of these two larger brown dots (next to the blue dot) is the preliminary adjustment of the paleo pole from the north pole (see Part 37). The other one is the estimated position of the actual paleo pole which has to be somewhere down the body along a line that is at 90° to the paleo rotation plane. It’s positioned very slightly differently from its position in Part 37 due to a better view of the paleo rotation plane here.
Dark green- the delaminated gull wing sets. Firstly, there are only three sets shown instead of five. That’s because the first set, nearest to us, is in fact the first three sets attached together at the horseshoe where they started. Their current position is skewed towards Ash from their original delamination vector due to the monolithic slide (Part 33). Secondly, due to the scale, there aren’t pairs of green dots because they’d be on top of each other. The three we see are kissing the shear line so they nominally denote the wing on the shear line side because the sets of wings themselves kiss the shear line.
Fuchsia- these two dots correspond to the fuchsia v shapes from Part 37. They aren’t the India shapes but the clean tear of the lower onion layer, level 3. The dots aren’t on the V shapes which are only important as matches. They’re still sited either side of the same gap but are nestled to the end of it, against the shear line because we’re looking at the direction of the delamination vector along the shear line. Seeing as the two fuchsia dots are the same distance from their matched V’s while still kissing the shear line we’re not fudging the force vector along the shear line here. This has all been ascertained from a multitude of close up NAVCAM photos from different angles, not from this grainy shape model.
Red- this is an old photo so the red dots aren’t significant for the stretch vectors. They’re interesting in that they show the well-defined diamond shape of the body, although only one end of the diamond was annotated in this case. That’s because it was used to show how the V- shapes on the head match to the body diamond shape which is itself, of course, a V at each end. You can even see the oft-mentioned 15° anticlockwise head lobe rotation clearly displayed here. It’s apparent in the difference between the V-shape orientation angles on head and body. It’s also very obvious in the path of the paleo rotation plane (brown). There’s a distinct angle between its path on the head and on the body. The paleo plane is the centreline of the V’s anyway but its path is defined by eleven stretch signatures running round the whole comet of which these V’s are only one.
The main point of photo 4 is to show that the line of the three green and two fuchsia dots and their associated straight terracotta line are all parallel to the brown rotation plane line. The rotation plane line has the long axis running between its two furthest extremities (at Apis and the Hatmehit/Bastet border) so it’s closely related to our stretch vector studies. But it doesn’t look quite as convincing as the top-down photo above. However, it has to be remembered that the brown line on the body is going ‘uphill’ all the time and never gets a chance to arc over to direct itself along the level of Hapi. Instead, it starts to arc over a bit and then gets flipped upwards at the neck base. So it looks more off than it really might be. If it could be arced right over and directed through the neck, staying within its plane and following a line that was the same ‘level’ as the straight terracotta line, it would indeed be parallel to that terracotta line. That’s the assumption because the top-down photo is showing it to be so.
Since the long axis also runs through the rotation plane it means that the five delaminations depicted are nearly parallel to the long axis stretch vector. And this jumps to seven delaminations when the nested triad at the horseshoe rim is included. All seven were in a roughly straight line along the long axis stretch vector. Two of them (gull wing sets 1 and 2) subsequently got yanked back by the monolithic slide which was radial from the north pole and towards Ash rather than instigated by the long axis stretch vector. See the next section for an explanation of this difference between the gull wing delamination vector and subsequent radial crust-sliding vectors.
CORE VERSUS CRUST RADIAL VECTORS
The surface crust-sliding was radial, away from the pole, as we know. The long axis stretching of the core was also radial because the long axis runs through the z-axis of rotation. Except there’s a difference. Really, the core stretch was in the two long axis directions that are opposite to each other and away from the z axis of rotation, the axle of the comet. That’s because the core is inside the comet, not sitting around on the surface near the pole like the three levels of crust we’re looking at here. So core stretch was, strictly speaking, not away from the pole itself- each chunk of buried core was stretching away from whichever part of the rotation axis it was rotating around. The whole core was stretching away from the whole rotation axis.
Since the core was wholly dominant in the stretching process, the crust had to do whatever it could to keep up. It was too brittle to stretch but it was clearly composed of onion layers so those layers delaminated in line with the predominant stretch force vector, which was along the long axis and *not* radially away from the north pole. Hence, any actual layer delamination was in the long axis direction. Only after that did the now-loosened crust (level 1) respond to the radial vectors on the surface of the comet. Only that layer (and not the core or lower layers) was sitting loosened on the surface and so it was the only layer that could slide radially. Before delamination, the integrity of the onion layers was perfectly adequate to resist these radial forces on the surface. But after delamination, the top level slid away radially from the pole. And crucially, it wasn’t just the actual gull wings that delaminated, it was the blue ridges they were a part of (Part 38) that slid across the Babi long axis direction as far as Aker. The whole area of Babi was delaminated in the long axis direction. The width of the delamination was from Aten to the shear line. The gull wings were just the most obvious features, perched on the ends of the delaminated layers at the shear line. The layers delaminated in the long axis direction but as we shall see, the loosened crust then immediately slid radially.
You may be wondering why we’re talking about crust sliding radially when it’s clear from the Part 38 annotations that the light blue ridges prove the delaminated layers are still spread out along the long axis direction. However, as stated above, the current Babi surface is in fact level 2. Level 1 was above this layer, was loosened, and is now missing. Babi is known to be a gravitationally low region in comparison to Ash and Seth. That was stated in one of the OSIRIS morphology papers and it’s fairly obvious just looking at it. That’s why it was always cited on this blog as being the crater left by missing slab B. Described in Part 9, slab B was said to have departed the comet at escape velocity. And it had to have had a kick from the detaching head lobe to do so because its radial velocity close to the pole wasn’t enough to eject it from the comet at escape velocity (0.8 metres per second). If there’s a missing slab it means by definition that there was more crust sitting on top of the present day Babi region. It follows that the annotated blue ridges in Part 38 were originally sitting well below the surface and did delaminate but weren’t loosened enough to start sliding radially from the pole. There was a depth limit at which the delaminating crust was loosened enough to slide radially and that depth limit was the present-day Babi surface. In fact, you’d expect some sympathetic attempt by this layer to slide radially as well, along with its companion layers above and we do see that. It’s apparent in the fanning of three of the delaminated layers, which means by definition that two at least were arcing round to the radial vector towards Aten. And the first two sets of gull wings drifted back in sympathy with the monolithic slide towards Ash. These current surface layers on Babi, as annotated in Part 38, were the first long axis delaminated layers that were deep enough to survive the subsequent radial slide. But they still eased their way in that direction a tad. Their direction of movement betrays the radial direction of the full-blown level 1 slide above them.
As for that level 1 slide which involved the rest of the crust sitting loosened above the gull wings and their associated delaminated ridges, it slid in the most spectacular fashion and in its entirety across Babi and towards Aten. It never did escape, after all. It became the cliffs of Aten: four cuboid lumps corresponding to the gull wing delaminations
THE REASON FOR RADIAL SLIDING OF LOOSENED CRUST
Technically speaking, the loosened sections of crust were also trying to move in a line that was away from the actual axis of rotation and not the pole. And if there was zero gravity and cohesion they would have left in such a radial direction (notwithstanding an initial tangential component if detaching and escaping). But there was significant enough gravity and cohesion, which meant that the net effect of these sections of crust trying to fly away from the rotation axis actually meant sliding away radially from the north pole. That was due to the gravitational and cohesive forces keeping the pieces of crust stuck to the comet but nevertheless sliding up to a larger radius of rotation. This was especially the case for all this crust we’re looking at around the north pole. This crust, near the axis of spin, was less susceptible to being flung away outright into free space, from centrifugal force alone (like the Imhotep and Hatmehit slabs were). So the next best option was to slide to a larger radius. And for a piece of crust consigned to staying stuck to the comet, the quickest path to a larger radius is radially, away from the pole. That’s why all the crust sliding and delamination vectors across Seth, Ash and Babi are radial, away from the pole. Beyond this area of sliding but not escaping is Imhotep and the Imhotep slab escaped because it’s at the longest radius from the axis.
IMPLICATIONS FOR MISSING SLABS A AND B.
This clearly has implications for missing slabs A and B (Part 9). Those are the famous (in this blog) missing slabs on Site A and Babi. Site A and Babi are close to the north pole and it was always known that the centrifugal force of a two-hour spin-up was nowhere near enough to eject them from the comet. So it was suggested that they were flipped up by the head shearing, “like yanking a bollard out of a slab pavement and watching the slabs getting flipped upwards around it”. That wasn’t all that satisfactory though, because the head itself probably never quite reached escape velocity. It obviously didn’t escape but could conceivably have sheared at just above escape velocity before being attenuated by the neck. However, the tensile strength of the neck is paltry compared with the force per square metre exerted by the head lobe on the neck even when departing at just below escape velocity (270 pascals exerted vs 50 pascals of tensile attenuation- a calculation but with no more than informed guesses for head lobe mass and neck cross-sectional area). That 50 pascals leaves a very small margin for being just above escape velocity on shear but getting attenuated enough not to escape. It’s also an absolute maximum as ascertained by various scientific papers on 67P and other comets. Moreover, it’s tricky to get the head to flip the slabs to escape velocity from one side only.
And yet, there are clearly two missing slabs on this side of the comet. Well, they’ve now been found. Slab A didn’t escape- it’s all that bunched-up material at the back of Site A. It slid radially to the back of the crater.
And the missing Babi slab is the crust described above that was delaminated on the long axis vector but by doing so, was loosened enough to slide radially. The whole lot slid the best part of a kilometre across Babi to a satisfactorily higher radius and is now sits as four cuboid lumps teetering over the Aten depression.
The evidence for the radial Babi slide, with photos, will be presented in Part 40.
So Part 9 gets full marks for noticing the slabs were missing but zero for thinking they’d escaped. It’s taken over a year to garner enough information about the head shear, sliding layers, head stretch before shearing, stretch vectors, and much more, to be able to realise where the missing material went. And it’s been right under our noses for all that time, just like every other discovery and doubtless, more to come. Still, 39 steps forward, one step back, can’t be bad.
MARCO PARIGI’S BLOG
Marco Parigi has a stretch theory blog as well. It describes many of the aspects of stretch in more concise terms than here and with annotated photos. It then links to the relevant posts in this blog for those readers who want the full, in-depth analysis:
Marco thought of stretch theory, as it would be applied to comets in general, and did so long before Rosetta arrived at 67P-CG. My first realisation regarding the possibility of stretch was on seeing the first published close-up of 67P on August 6th 2014.
Photo 5- The ESA regions.
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
To view a copy of this licence please visit:
All dotted annotations by Scute1133.