Part 73- The 4.5-Kilometre-Long Rift From The Northern Long-Axis Tensile Force Line


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

The headers are reproduced below with their keys and explanations. The ESA regions map is at the end of this post for those who are unfamiliar with the region names and locations. 


In Part 72, we saw how dominant the northern and southern long-axis tensile force vectors were. This part continues that theme but dwells on the fact that the northern tensile force line gave rise to a rift running the whole length of the body lobe. It should be regarded as the entire area of Seth, Babi and Ash, below the bottom red line, shunting away en masse from the northern, long-axis tensile force line. It involves at least two onion layer thickness. 


Photo 1- the long-axis tensile force lines. This is reproduced from Part 72.

Red- the northern and southern long-axis tensile force lines. The northern one is the nearer one. They run from long-axis tip to long-axis tip on the body. They passed either side of the proto-head lobe before it sheared from the body. They now pass either side of the neck which is itself elongated along the long axis for the very reason that these two force vectors were stretching it along that vector. 

Bright green- the Apis region at the long-axis tip. The two tensile force lines join just above Apis and in line with its centre, the exact long-axis tip. 

Photo 2- the lower onion layers that rifted from the northern long-axis tensile force line. In Hapi, it’s the third layer down from the paleo surface. The paleo surface is the original surface of the single body-  see Part 41, scroll to ‘the three levels’. This third layer down appears as the second layer down today because the old top layer slid to the back of Aswan (the terraced cliff) and Babi (the Cliffs of Aten). The two lines angled away, either side of Hapi, are today’s top layer rifted from the same northern tensile force line. This corresponds to the second paleo layer down except near Apis where it’s also the top paleo layer (see the Ash recoil, below). This completes the rift running the whole length of the body lobe.  

This is essentially a rift that runs from one end of the body lobe to the other. It runs between the two long-axis tips because the tensile force line it rifted from also runs between the two long-axis tips. It should be visualised as being the entire section of surface crust incorporating Seth, Ash and Babi doing a one-off shunt away from the tensile force line. The shunt was between 150 metres (at Aker/Babi) and 400 metres (central Hapi). It incorporates the 1.6km x 200m rift (Parts 48 and 49) running through Seth and Ash. It involves at least two layers. The upper layer slid even further on as described in Parts 32, 33, 40.

The wider, Hapi section of the rift corresponds to the shunt of the Hapi cliff line from the line of boulders along Hapi. This was presented in Part 47. It could be possible that, to some extent, it was the boulder line that rifted away from the Hapi cliff line when the head lobe sheared and drew neck material up with it and out of Hapi. The boulder line would in that case have been drawn across Hapi in a translational movement from the cliff whilst maintaining the shape of the cliff line along its length. Part 47 shows how that translational match across 350 to 400 metres is still discernible today. This sweeping up of neck material and dragging the boulder line back from the cliff in the process would explain the rift being wider at this point. This was touched on in the previous part. The principle of the head lobe drawing up neck material with it as it rose on the incipient neck after shearing is dealt with in Part 25. 

What is new in this part, regarding this much longer rift, is that the two rifts from Parts 48/9 and Part 47 have been linked as being one long rift. This was achievable because the line from which they rifted, the northern tensile force line, was identified as continuing along the Hapi boulder line from the southern perimeter of the 1.6km x 200m rift (Part 72). And in order to join those two lines, the southern perimeter of the 1.6km x 200m rift had to be identified as extending further, past the mauve anchor and into Hapi. That identification was done via the four mauve features that delaminated along the northern tensile force line, thereby betraying its existence in Hapi. That proved that the northern tensile force line continues from the mauve anchor, right up to the beginning of the boulder line. This discovery was presented in Part 71 and so it links the southern perimeter of the 1.6km x 200m rift to the Hapi boulder line. This means the rift runs from Apis to at least the other end of Hapi. 

The final piece in the puzzle is that the Babi slide (Part 40) incorporates a 150-metre-wide rift along the border of Aker and Khepry. Since this rift runs from the end of the boulder line in Hapi to the other long-axis tip, it completes the rift running the entire length of the body lobe, as depicted above. This rift hasn’t been blogged yet but it was responsible for getting the slide track of the fourth Babi cuboid wrong in the original Part 40 post. It was the discovery of the cuboid’s true track (and matched seating at the end of Hapi) that betrayed the rift. See the update at the end of Part 40 showing this seating match and slide track in detail on an OSIRIS image. The 150m rift is implied by the new track but wasn’t explicitly pointed out. So the rift now runs the entire length of the body lobe from long-axis tip to long-axis tip as shown in the photos above and photo 3 below. 

Photo 3- the v’s show the direction of the rifting crust away from the northern long-axis tensile force line. The v’s are therefore slide vectors. 

Photo 4- this shows how there were two overall stretching and sliding vectors at play: (1) long-axis, core-directed stretch (running between and parallel to the two tensile force lines) and (2) radial sliding of crust (outside the tensile force lines). 

Although the long axis stretch and radial slide vectors appear somewhat schematic in photo 4, they are real slide vectors that have been identified via translational matches and were blogged long ago. See photos 7 to 11 which build on the above slides with several more arrow vectors. Photo 11 shows the various part numbers for each slide. 

The reason photo 4 has fewer arrows is because the intention is to make it appear schematic so as to emphasise the obviously different direction of the long arrow between the tensile force lines. That arrow is running along the long axis i.e. parallel to the tensile force lines while all the others are directed away from it in a radial pattern. Clearly, there were two different mechanisms at play either side of the northern tensile force line. We saw this very much in close-up in Part 71 with the mauve delaminations sliding along the length of the tensile force line, kissing one side of the line as they slid along it. Meanwhile, the Aswan slide rifted away from the other side of the line at 90°. The northern tensile force line is a very strong demarcation line between these two different slide vectors. This was also shown as far back as Part 50 with a really crisp depiction of the orthogonal nature of the slide vectors in the vicinity of the ‘blocky rectangle’, which is the #4 mauve delamination. That was a detailed OSIRIS photo.

Photo 5- this shows the continuation of the long-axis stretch vector going under the neck and along its centreline.

In photo 5, the head lobe is in the way of the neck so we’re looking through the head, and the neck as well, to the arrow running along the base of the neck. It has points at both ends, depicting the fact that the neck was stretching both ways, causing it to elongate along the long axis. In reality, all points along the long axis were stretching ‘both ways’ with an equal and opposite tension along the lines at any given point. However, the double-pointed arrow helps to show that equal and opposite tension averaged across the middle of the comet, while the two ends are intuitively seen as stretching away from each other in opposite directions. That’s why the short arrow at Aker (top-left) is pointing in the opposite direction to the ones on the red triangle that points at Apis. 

Now that all the long-axis stretch lines are in place, you can see that they are following the core-directed stretch vector which is completely different from the radial vector for the sliding surface crust. The only reason the surface crust slid radially is that it had been sheared by the shear gradient across the northern tensile force line. That meant the crust actually sheared along the length of the tensile force line. This was the initial stage for allowing the rift being described in this part to happen. The crust was now free to slide and it slid radially, en masse, to a higher radius because the comet was spinning so fast: a 2- to 3-hour rotation period on head shear. 

The shearing of the lower layer of crust by the northern tensile force line isn’t quite the same as the classic shear line itself as matched in the very early parts of the blog. The classic shear line was the exact body matches (to the head rim) on the next layer above. This rift of the lower onion layer seems to be a sympathetic shearing slightly further in and under the head lobe. However, the second layer up that sits on this deeper layer does host the true shear line. It’s slightly further back and must’ve dragged the head rim out with it or the two wouldn’t exhibit the matches we see today. 

This sympathetic inner/lower layer was dealt with in more detail in Parts 39 and 41 as part of the “three levels”. It’s level 3 which is this inner/lower level; level 2 is the main Aswan terrace and also the smooth, riven-looking area of Babi; and level 1, is the slid Babi cuboids (the Cliffs of Aten) and the stacked up cliff creating the rim around Aswan. The photo of these layers, annotated, is in Part 41. 

The three levels described above have nothing to do with the four layers on the other side of the tensile force line that are within the red triangle. Such is the strong demarcation line either side of the tensile force line. Three of the four layers in the red triangle probably do correspond to the three levels the other side because they were once attached prior to the northern tensile force line holding sway and shearing them apart. However, the sliding and delaminating processes either side of the line are so markedly different that it’s difficult to trace the layers across the gap of the rift. This will be a future project but is not considered worthwhile at the moment.  

In the final analysis, it makes little difference distinguishing the line of the lower layer from the one above it with the shear line matches. This is because we’ll come to see that they were nested together at the northern tensile force line just prior to shear and the shear went through both layers along that line. The distinction between the line of the layers is however a useful concept because the second layer with the shear line matches slid back from the tensile force line somewhat further than its lower companion. The classic example of this is the Aswan slide in Part 69. The bright green matches in that part are etched onto the lower layer that didn’t slide as far. And when the Aswan layer is slid back to its seating on that lower layer, the lips or cliffs of the two layers would nest to form one big cliff. That cliff is the original tear line- and both were sheared together along the northern tensile force line when they were seated. Their location at the time of the shear was exactly between the end of the boulders and mauve feature #1. After shearing, they then slid together. Then Aswan slid on still further. This hasn’t been blogged yet but has been implied repeatedly in the last few parts. It will have its own post soon. 

Photo 6- this shows the slides and delaminations from Parts 69, 70 and 71

Photo 6 is shown for context so that you can start to see how all three parts, 69 to 71, rely very heavily on this major rift running from long-axis tip to long-axis tip of the body. This is despite the fact that the slide in Part 69 looks to be completely independent from the four layers when viewed in close-up. When all the layers are eventually slid back to the tensile force line in future Parts, we’ll see that the four layers and mauve features were nested and kissing the right hand end of the upper green wavy line. 


Small bright green wavy lines- the Part 69 translational matches that show the entire Aswan layer slid over this lower layer to where it is today. The front rim or cliff of Aswan therefore used to be nested to the front rim of this quasi rectangular lump of lower layer. This is the layer described above as nesting below Aswan and the two together being sheared along the tensile force line between the boulders and the #1 mauve feature. 

Large red- the northern and southern tensile force lines. 

Four short lines in smaller red dots- these run between the northern and southern tensile force lines and stop abruptly at the tensile force lines. They represent the four delaminated layers, #1 to #4, in Part 70. #1 is the farthest and lowest one of the four. They were sheared at either end by the tensile force lines leaving four short lengths of layers to delaminate towards Apis within the red triangle. They delaminated towards Apis because that was the direction of the long-axis stretch vector. 

Mauve dots- these are the four mauve features, #1 to #4, as described in Part 71. The farthest and lowest one is #1. Each mauve feature sits on its layer of the same number. 

Photo 7- the slide vectors. Basic version without additional annotations. 

Photo 8- same as photo 7 with additional annotations.

Added bright green curve- this is the Ash recoil, first described in Part 32. It’s curved because it’s betraying the radial nature of the layer slides. This is the main layer front in this vicinity and so it will be at 90° to the slide vectors. Since the slide vectors are radial, this line can’t help but be curved. 

The dusty Ash surface between the Ash recoil and Apis (the other green line) is both today’s top layer and the paleo top layer i.e. nothing slid away from above it. It did all the sliding itself beyond the Ash recoil line hence the recoil curve itself and the flaccid, blanket-like look of Ash beyond it.  The recoil curve is the loose edge of the blanket, curving according to the exigencies of the radial force vectors. 

Notice the recoil curve has a gap across the 200-metre width of the 1.6km x 200m rift before resuming within the red triangle. In reality, you can make out its line across the rift because it dragged material across the rift in its wake (see original). But it’s not very noticeable here. It is noticeable in the photos of Part 49 though. 

Photo 9- with extra slide vector arrows
Two more slide arrow vectors are added to photo 8. One is further up the red triangle and in line with the long axis stretch vector between the two tensile force lines. This vector represents the delamination vector for the four layers of Part 70. 

The other added arrow is at the end of the 1.6km x 200m rift. Although the rift is predominantly associated with rifting across its 200m width, it also stretched along its length. This is implied by the two stretch vectors either side of the rift which means the two perimeters and floor of the rift couldn’t escape the stretching along its length. The matches on opposite sides all along the rift show this assumption to be correct. 

Photo 10- with the mauve delaminations from part 71. 

Mauve- the mauve feature delaminations, #1 to #4. The two middle ones are slightly obscured by their red arrow. 

Dark blue- the north pole. 

Brown- the paleo north pole preliminary adjustment position (see Part 37). 

Photo 11- the fully annotated photo with yellow numbers showing the blog part that describes that particular slide vector arrow in that area. The parts show the relevant translational matches. 

Photo 12- 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 A. Cooper. 




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