Part 72- The North and South Long-Axis Tensile Force Lines

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

Red- the two long-axis tensile forces. They’re almost perfectly mirrored, passing either side of the head lobe and from long-axis tip to long-axis tip. This symmetrical mirroring is helped by the fact that the shape of 67P itself is symmetrical about its long axis. 

Bright green- Apis at the long axis tip. It looks offset from the red triangle tip (the sharp vertex where the two lines join) from this view. But if we were to drift round so we were looking straight down on the tip, it would point straight at the central bright green dot of the five. This is because Apis is at the long-axis tip and the stretching of 67P, before the head lobe sheared, was directed from the centre of Apis to the centre of Khepry along the red lines. The symmetry of the tensile forces dictates that they have to join at the centre of Apis. In reality, they join just above Apis but the join is exactly in line with the Apis centre.

The stretching of 67P was due to spin-up to a 2- to 3-hour rotation rate and the spin-up torque would have been from random, asymmetrical outgassing. 

The two lines follow the same mirrored crustal features on either side of their centreline which itself traces the long axis of the body. This symmetry both of the features and the lines is because the lines are tensile force lines which had a shear gradient across them when 67P was stretching as a single body. The shear gradient sheared the crust along the tensile force lines, thus forever leaving their stamp on the comet’s surface. Since the forces were arranged symmetrically either side of the long axis centreline it follows that the crustal patterns they created (rifts and delaminations) are also symmetrical about the long-axis centreline. It’s a mirrored symmetry with the centreline being the reflection line. 

The centreline/reflection line is contiguous with the paleo equator for the two sections of the two lines running from Hapi to where they join just above Apis (see the Paleo Rotation Plane Adjustment page in the menu bar). 

At Hapi, the centreline runs through the centre of the neck, longways. So it’s actually ~400m below Hapi and at a level between where the northern and southern tensile force lines run i.e. in the same plane that’s spread between them. 

At the other end of the neck, the centreline emerges at Bastet/Aker. It then drops over the centre of the V-shaped Aker and traces the central ‘prow’ of Aker. The prow is the aforementioned V-shape translated down Aker into 3D. It runs down the centre of Aker and is also contiguous with the paleo equator like the centreline of the red triangle at the opposite end of the neck. We’ll see in later photos that the two red lines run parallel to each other down either side of Aker while remaining parallel to and equidistant from the prow. In other words the two tensile force lines maintain their symmetry past the end of the neck and down Aker. This also applies to Khepry as the two red lines run down its two outside edges which continue on from the Aker edges. By the time we reach the other long-axis tip where Khepry bends round sharply to the base of 67P the two red lines have maintained their symmetry for 4.5 kilometres, from long-axis tip to long-axis tip (Apis to Khepry). 

The apparent flaccidity of the red line through Hapi is due to the slide of the Babi/Hapi cliff line which occurred to a slightly greater extent than it did at Aswan/Hapi. The slide of the whole Hapi cliff line across Hapi was presented in Part 47. The extra movement of the Babi portion (radially away from the north pole as always) hasn’t been blogged yet. However, anyone who’s familiar with the dark green ‘gull wings’ (the classic third set of wings) and the fuchsia ‘India shapes’ nearby would realise this entire cliff line has to have shunted in sympathy with their shunting because they sit on its rim. This extra shunt southwards, which is essentially a shunting of the entire Babi layer, is the reason for that stunning dog-leg in the ridge running down from Hapi to the Cliffs of Aten. That ridge was formerly dead straight, under tension, and pointing directly at the north pole. It went flaccid when the tension was released by the Babi layer shunt. 


Photo 2- header reproduced.

Photo 3- a view from the other side. The southerly line is nearest to us here.
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0/A.COOPER

Photo 4- the shape model. You can see the sharp red vertex pointing at the centre of Apis here. 

Photo 5- view from the other end of the body. 

Red- the two tensile force lines. The right hand one is the northern one, running through Hapi, along the line of the boulders. You can see the largest boulders on the horizon. These are at the beginning of the boulder line so you can match these to the above photos. 

Bright green- the long-axis tip of the body at Khepry. 

The ‘prow’ at Aker is the faint central ridge running down from the top which borders the neck. The prow is faintly shadowed and has three boulders chipped away from its bottom end. Its top end starts in the middle between where the two red lines turn sharply over the edge of the top rim of Aker to follow its two rugged side perimeters. The prow defines the paleo equator. Today’s equator runs about 400 metres to its north, down the right hand side of Aker and Khepry as viewed here. 

The two tensile force lines running down either side of Aker and Khepry had a shear gradient too, just like at the red triangle. This sheared the crust via slip-shear, thus actually creating the perimeters of Aker and Khepry. This shearing left Anhur on the south side and Babi on the north side, free and loosened from Aker. They were therefore now free to slide radially across the surface from their respective poles. They slid to a higher radius due to the high spin rate on shearing of the head lobe (2- to 3-hour rotation rate. See Spin Up Calcs in the menu bar). The slip-shear on the Babi side left a discernible rift of around 150 metres wide running from Hapi to the Cliffs of Aten. This is the corollary to the 1.6km x 200m rift at the opposite end of the body, caused by the same tensile force line. 

If you look from head-on in front of Aker/Khepry (or from above) you can see the symmetry of the Babi and Anhur slides either side. 

For more context on the morphological evolution at this end of the body, see the Paleo Rotation Plane Adjustment page in the menu bar (description after photo 7 on that page). Also Part 51 which matches the two Bastet pancakes to the depressions either side of the prow. And Part 61, which has close-up gifs for the same pancake/Aker match. 

Photo 6- south pole shot showing the same Aker and Khepry end on the right as in photo 5. Also the path of the southerly tensile force line along the southern side of the neck at Sobek. Few close-ups exist of this line at the moment.
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0/A.COOPER

The Sobek path of the southerly tensile force line is the mirror image of the force line in Hapi following the boulder line. It keeps up this mirror image faithfully along the top of Geb (Geb/Sobek) and part of Anhur but becomes blurred for a few hundred metres across the Anhur slide. After that it comes into view as in photo 5, above. 


The two tensile force lines are directly or indirectly responsible for the greater part of the morphological diversity on 67P. Luckily, these different areas exhibiting the different morphologies were noted and given names in the sub-series (Parts 22-29). This was before the tensile force lines were fully understood. It was simply noted that these lines seemed to divide off the areas. 

However, the shear gradient across the lines at the two red triangle long sides was noted in Part 26. This was the ‘wind-tail’ analogy i.e. the red triangle being protected by the proto-head and later, the neck, from the full force of stretch. This caused the shear gradient across both tensile force lines.

The shear gradient was an increase in tensile force from a small value inside the triangle to a much larger value outside the triangle. The shear gradient was steep, over just a ~20m width. So it was like a lot of parallel ropes under tension along the length of the triangle sides and across a band 20 metres wide. The ropes would be under greater tension on the outside of the band than on the inside, thus causing shear. That explains the inevitable slip-shearing of the crust along the tensile force lines. This caused the 1.6km x 200m rift (Parts 48 and 49) on one side of the red triangle and the Anubis tear and slide on the other side (a less neat and obvious rift). These rifts occurred outside the relative calm that prevailed inside the triangle whose shape actually represents the lee from the tensile forces sitting behind the neck. So the triangle is a visible representation of the of the ‘wind tail’, stamped onto the comet’s surface. 

The following photos show the areas noted in the sub-series, Parts 22-29, and they’re culled from those parts. They apply only to the Seth/Anubis end of the comet and how the two tensile force lines divided up the different morphologies at this end. The two tensile force lines also sheared and rifted the other end at Aker and Khepry. They therefore actually formed those two regions by tearing them away from Babi and Anhur either side of them. However, this dividing up of Aker, Khepry, Babi and Anhur by the tensile force lines at that end of the comet was explained amply above, along with the photos and parts suggested for further reading. 

Photo 7- the areas at the Seth/Anubis end of the comet that are named in this blog and are related to the two tensile force lines.
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0/A.COOPER

Yellow- Aswan, formerly known as site A. This doesn’t kiss the northern line today but used to (to be blogged soon).

Bright green- the slab A extension. So named because it appeared to be related to site A (and its missing slab). It looked related by virtue of sharing the same curved back as Site A and also by adjoining site A. Its southern perimeter is contiguous with the northern tensile force line in this post. That would be the upper-right line, the very straight one leading up to the mauve dot. It follows the tensile force line because the tensile force line sheared the crust along this line. This caused the 1.6km x 200m rift which explains most of the Slab A extension’s ‘flayed’ look. So the southern perimeter of the 1.6km x 200m rift is the southern perimeter of the slab A extension and the rift sits wholly within the extension. 

Incidentally, the subsequent discoveries that do indeed relate the slab A extension to Site A are beyond the scope of this post but are to be found in Part 32 (the Ash recoil), also 37 and 69.

Red- the red triangle. The red triangle includes the four bright green dots running up to the mauve dot. They’re only green so as to show the perimeter of the slab A extension above, which is contiguous with the red triangle. It’s contiguous because the northern tensile force line sheared the crust, thereby separating the red triangle from the slab A extension. It did so by causing the 1.6km x 200m rift. So what used to be attached to the red triangle is now 200m away and parallel to it (the opposite rift perimeter). That’s why the slab A extension looks flayed, being the floor of the rift. The red triangle extends beyond the slab A extension by three red dots, the last one being at the tip. This is an old photo- the last two red dots at the bottom, placed in shadow, are too low. They should be raised to kiss the edge of the shadow and thus form a sharper triangle tip. They should follow the feature with two dark eyes and a bright droopy nose. This one place on 67P where recognising a face in the rocks is useful. It holds up well at almost all angles and is a boon for locating the red triangle tip.

Orange- the ‘missing’ Babi slab which is now known not to be missing. It slid radially from the north pole and concertinaed up forming the Cliffs of Aten (Part 40). That’s why the ‘dog-leg’ ridge was described above as being under tension. The first Babi cuboid that had slid 800 metres was pulling it tight before the Babi layer dislodged to ease the tension. The ridge is attached to the cuboid to this day and the cuboid is unequivocally matched to the Hapi shear line via its jet source and slide tracks (Part 52).

Photo 8- shows the Anubis slide which was sheared from the red triangle by the very straight southern tensile force line. 
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0/A.COOPER

T- red triangle (swamped by other colours).

Fuchsia- this is the perimeter of what was supposed as being the missing Anubis slab in Part 23. It has since been established that even if some slab material was flung from the comet, much of it slid instead. This was established in Part 54 and other translational slide matches have since been found but not blogged as of the date of this part. The main point for our purposes in this part is that there was material that used to be attached to the red triangle but was sheared away from it along that very straight, southern tensile force line. You can see it here except it’s dotted fuchsia because it’s a Part 23 photo. The traditional red triangle southern, long side runs from the dark green dot to the last small red dot at the sharp end of the triangle. There are five fuchsia dots running between them along that line, including the one kissing the dark green dot. The second and third fuchsia dots to the left of dark green are on the really straight part of the tensile force line, betraying the fact that it is indeed a tensile force line with a very steep shear gradient. 

We now know from Parts 70 and 71 that the red triangle wasn’t quite as undisturbed as originally thought and contains delaminated layers. Those strange floppy bits overhanging Anubis are the Part 71 layers that were sheared across their widths by the southern tensile force line and are now drooping over under the influence of gravity. They can be translationally matched to Atum, 600 metres away. The delaminated layer lines are depicted in Part 71 with red dots. The lines meander across the width of the red triangle and three of them arrive at one or other end of each floppy piece. This proves that the floppy pieces are just the delaminations sliced across their widths like lasagne strips. 

This concludes the relationship between the two tensile force lines and the morphologically distinct areas they created that were identified in the sub-series: the red triangle; the slab A extension; the Anubis slide/rift; Aswan. All these areas were caused by the slip-shearing along the two tensile force lines and the subsequent rifting from or sliding along the lines.

The two lines caused more rifting and delamination as they dropped down into Hapi and did so on both the northern and southern sides of the neck. Both force lines were responsible for the first and second delaminated layers in Hapi as described in Part 70 and the first and second mauve features as described in Part 71. They were responsible in the sense that their shear component slip-sheared the layers, allowing them to delaminate. Then the tensile component of the force lines delaminated them. This created the red triangle extension as described in Part 70. 

Inside the red triangle extension, the mauve features delaminated along the northern tensile force line, kissing it faithfully all the way and directly outside the line everything slid away from it at 90°. This was already the case for the classic red triangle with the 1.6km x 200m rift opening up at 90° and parallel to the triangle long side. But it also occurred in Hapi with the Aswan slide of Part 69. This will be elaborated on soon because Part 69 didn’t ever show Aswan attached to the northern tensile force line. But ultimately it was, and Part 69 just shows the last stage of the slide. 

This takes us to the beginning of the boulders in Hapi which run along Hapi’s length. They define the northern tensile force line between the #1 mauve delamination (on layer #1) and the point where the line dives down the front, northern side of Aker. We know from Part 47 that the Hapi cliff rim recoiled or slid from the boulder line and is a translational match to the line. It may conversely be the case that the boulder line slid from the Hapi rim as the neck was extruded from the body (see Part 25). But the translational match is sound in both cases, whichever way round they slid, and so one or other had to be the true tensile force line. 

Apart from the Sobek morphology along the south pole side of the neck, this completes the description of how the two tensile force lines divided up the comet into morphologically distinct areas. Sobek will have to wait until better photos come along. 



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. 




One thought on “Part 72- The North and South Long-Axis Tensile Force Lines

  1. Pingback: Part 73- The 4.5-Kilometre-Long Rift From The Northern Long-Axis Tensile Force Line | 67P/Churyumov-Gerasimenko- A Single Body That's Been Stretched

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