67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 41

THE RECOILED HEAD LOBE LAYERS THAT MATCH THE CLIFFS OF ATEN CUBOIDS

   

Orange- delaminated layers that rode up the head lobe due to crustal tensile stress after shearing from the body lobe.

Light blue- the four ridges that correspond to the delaminated ‘gull wings’ on the body.

Photo 2- a different view showing the four cuboids that match to the light blue ridges. The cuboid slide was described in Part 40. 

   

Light blue- as for photo 1.

Orange- as for photo 1 on the head lobe but orange shows the Aten perimeter on the body. 

Fuchsia- four quadrilaterals that are the faces of the four cuboids. 

Red- cuboid slide lines. The red kinked sections directly behind the quadrilaterals were yellow in Part 40, depicting the sloping backs of the cuboids. Although these kinked sections retain lines that are a signature of the slide they are strictly speaking the signature of the delaminated ‘Venetian’ bands that bunched up behind the quadrilaterals to give them their bulk. But their direction is very nearly along the slide vector anyway. The other sections of the red lines leading to the green gull wings are along the true slide vector. The kink is largely due to the sloping backs of the cuboids. 

Green- the gull wing delaminations.

Dark blue- north pole.

Brown- the paleo pole preliminary adjustment (Parts 37 and 40).

Photo 3 -another view. The isolated orange dot is a rogue dot. 

  

CLOSE LINK OF THE TEAR LINE WITH THE LINE OF ‘SUNSET JETS’

The upper, slid/torn layer corresponds exactly to this line of prolific sunset jets as described by the OSIRIS team on the Rosetta blog:

http://blogs.esa.int/rosetta/2015/06/08/sunset-jets/

These jets are partially if not fully explained by the tearing of the layer at the shear line.

INTRODUCTION

Apologies for being a bit repetitive in this post. It’s because we’re dealing with two delamination processes that happened at roughly 90° to each other and then looking at the signatures of both in one area on the head. One delamination process was the long axis, core-directed delamination and the other was the radial sliding of crust up the head lobe once the crust had been loosened enough to slide radially. Both involve the same chunk of crust obeying the long axis delamination first and then sliding (riding up the head) into two delaminated layers. This is actually no different from the ‘Venetian blind’ layers delaminating across Babi along the long axis and then sliding radially to form the cuboids (Part 40). But the signature is harder to see on the head lobe. And it’s especially difficult to visualise what is effectively a delamination of a delamination. 

All references to up, down, left and right are in the ‘upright duck’ mode with the head lobe directly above the body lobe.

Long, narrative keys to photos are concluded with ‘/////’.

Here’s the ESA regions map for navigation purposes.

Photo 4- ESA regions 

 

BACKGROUND

In Part 39 we looked at three onion layer levels at Babi and designated them as levels 1,2 and 3. Level 1 was the top layer that slid radially from the north pole. Level 2 was the present-day Babi surface containing the shear line at its rim, overlooking Hapi. Level 2 looks almost like riven rock as if it’s an exposed fracture plane and if level 1 slid from above it that must indeed be what we see today. Level three was the next onion layer down, in Hapi, peeping out from under level 2.

Photo 5- This is the Part 39 header, reproduced and showing the three levels. 


 

Terracotta- three lines of dots show the front lip of each of the onion layers, levels 1,2 and 3. The two short lines of terracotta at right are the back edge of the radially recoiled level 1, where visible. It’s largely visible around the back of Site A. It’s also just visible in the bottom-right corner where it traces the bottom edge of the cuboids at Aten. The level 2 line continues on into the distance past Site A because it’s the classic shear line. The other two lines for levels 1 and 3 could be extended too because there are level 1 and 3 onion layers beyond Site A as well but it’s not relevant to this post. Those layers have been dealt with in the past: level 1 is the red triangle recoil; and the fracture plane below the four coloured anchors is level 3 (both in Part 26). They just weren’t called levels 1 and 3 at that time.

The two sets of numbers in photo 3 denote the same three layers, but the farther set traces them across Site A and the nearer set traces them across Babi. 

Light blue- delaminated ridges embedded in the surface of level 2. These delaminated predominantly along the long axis but succumbed to a small amount of radial tugging without breaking free and sliding like the level 1 crust above. The radial tugging is evident in the fan shapes. Their widest extent betrays the width of the properly delaminated ‘Venetian’ bands above level 2, i.e. the bands in level 1. And those four bands became the four cuboids perched over Aten by sliding radially and ‘scooping themselves up’ along their length. 

Dark green- the five sets of gull wings.

Mauve, yellow, orange, green- the four anchors (Part 24) in the distance. These aren’t relevant to this post but are included for context. 

/////

The wiggly nature of the classic shear line is probably due to all the radial tugging of the crust above it, which was the level 1 onion layer. The level 1 crust slid away radially from the north pole point and bunched up further down the body at the back of Site A and Babi (Parts 32 and 40). One telling signature in Photo 3 is that the terracotta line defining the shear line (rim of level 2) is remarkably parallel to the level 1 line (rim of level 1). That’s despite the fact that both lines are quite wiggly. They stay parallel across both site A and Babi and even across the central divide between the two. That central divide is the short, dusty section of Ash and in fact the level 1 rim line traces that short section via the dividing line between dust and no dust. As for Babi, that’s a signature of the division between level 1 recoiled material and the bare (riven) level 2 surface it slid over.

It should be borne in mind that today’s head rim matches prove that the head was still attached to the level 2, classic shear line while all this radial tugging was happening on the layer above it. Both head and body had level 1 crust unzipping and riding around radially above the level 2 layer. Level one unzipped and so level 1 crust rode up the head in mirror fashion to the level 1 crust riding down the body. That exposed the head rim and shear line which we see today and tend to think of as the true surface of the comet. But the head rim and shear line used to be neatly covered by the level one layer that’s now split and shared between head and body- way up the head and way down the body. So even the head rim and a short section above it is level 2 as well. That’s because it was attached at the shear line. The short expanse between the head rim and first section of ridden-up layer 1 material on the head would be the short section of level 2 that was attached to today’s Babi just before shearing. 

Indeed, the level 1 crust on the body needed something to tear away from in the first place and we know it couldn’t have torn from today’s exact head rim line because that rim matches to today’s body shear line. Today’s shear line was originally buried under level 1 and hadn’t yet sheared when the level 1 crust was unzipping and sliding away radially above it. The head rim certainly doesn’t match to all that crumpled-up level 1 crust now concertinaed up at the back of Site A and Babi; it definitely matches the shear line directly below it. 

That concertinaed crust at the back of Site A and Babi actually tore from its counterpart crust on the head lobe when the head was still seated on the body and just before the head sheared away for good. The level 1 crust on the body simply continued on over the herniating head lobe when it was still seated. The level 1 crust tore along a line that was, yes, directly over today’s shear line but since the head hadn’t departed yet, today’s classic shear line underneath hadn’t quite opened up yet. After tearing, the two components of the level 1 crust on head and body both recoiled, one down the body to the back of Site A and Babi (plus the slab A extension) and the other up the head i.e. up Ma’at and towards Hatmehit. 

The preliminary tearing of the level 1 crust along the same line as the shear line could be considered as the precursor to the classic shear line at level 2 directly below it. It suggests that the classic shear line was already beginning to weaken and on the point of sudden failure followed immediately by head lobe release.

Since the head was herniating from the body, it could be visualised as being like an expanding balloon: as core material migrated from the future body lobe into to the interior of the head, it became more and more of a bell shape sitting on top of the body. If you coated a half-blown-up balloon with papier mâché layers and let them dry before blowing it up more, the layers of papier mâché would crack and move apart as it expanded. If the glue between the layers were still slightly wet, the layers would tear, possibly along one line, and slide away from each other in two halves under the released tension, rather than crack into many islands that move apart. If they did tear along one line, the layers would recoil in two directions at 90° to the tear. That’s what happened along the shear line. 

The recoil under tension in the case of the comet was due to the pent-up radial tension in the level 1 crust due to spin-up force. 

There are clear signatures of this sliding happening at Imhotep and the south pole as well (see the Imhotep slide page in the menu bar and Part 30 for the south pole slide). Both have crustal slides of the same order of magnitude as for Babi, Seth and Ma’at, around 800 metres).

So pent-up tensile stress in the crust and possibly lubrication could have facilitated the crust recoiling process. There certainly was a dramatic recoil of layers on the comet and there are slurry signatures all over Seth and Babi so perhaps there was some lubrication that explains the degree of sliding. But you still need the tensile force stretch vector (achieved via spin-up) in the first place so slurry would only be helping along the process of crustal sliding, not causing it. 

THE BABI CUBOIDS MATCH TO THE LIGHT BLUE RIDGES AT MA’AT

The ridden-up (recoiled) head layer component that matches to the cuboids was shown to be constrained between the light blue, vertical ridges in Part 40. However, if you recall, there are four light blue ridges enclosing three interstices. This means that it is the left hand three cuboids that match to the three interstices. The fourth cuboid was attached to a point somewhere in the cove. Since the left side of the cove is defined by the right hand member of the four light blue ridges, the right hand cuboid is outside the array of four ridges. So it doesn’t match to the three interstices, only its three companions to the the left do so. 

The four light blue ridges correspond to the gull wing delaminations directly below them on the body. The level 1 recoil component on the head corresponds to two thin layers that rose above the head rim and parallel to the rim. Those two layers constitute what was the head component of our supposed single level 1 onion layer. It might have been discernibly two layers or simply susceptible to delamination (as were the cuboid components). Those two ridden-up layers are dotted orange in all the photos in this post. 

The two ridden-up layers are at 90° to the blue ridges and that’s because they are the recoiled layers, recoiling only after they had delaminated along the long axis stretch vector. And the recoil vector, being radial, meant that it was at about 90° to the delamination vector. That’s why the orange lines in the photos cross the light blue lines at 90°. The light blue lines represent the vestiges of the delaminated layer fronts that spread out in bands along the long axis stretch vector. They were likened to the slats of a Venetian blind in part 40. That was for the body but in truth, they spread on up the soon-to-be head lobe from Babi and into the future Ma’at area. And the reason they succumbed to the long axis vector before sliding radially is that the core stretch was dominant, stealing core material from the comet’s short axis and donating it to the long axis. The crust had to keep up by delaminating and only once it was loosened by the delamination could the more feeble radial forces induce the radial slide at the surface. And the radial forces were able to act only after the delaminated level 1 had torn in half along a line directly above today’s classic shear line. That tear or upper shear line was across the delaminated layers and at 90° to them. That’s why the recoil, at 90° to the tear, bunched up the delaminated layers along their length as they rose up the head (as happened for the cuboids). That left the signature we see, the vestiges of the layer fronts. Those are the light blue ridges and they look slightly crumpled, probably due to the old layer fronts bunching up under recoil, becoming more flaccid and zig zagging a bit. 

The vestige of the long tear, unzipping just above the actual shear line, is the two orange layers running roughly parallel to the head rim. And they run parallel because of course they tore from a line that ran along the same line as to soon-to-tear head rim. The two layers then recoiled up Ma’at, above the rim line, just before the head rim itself was released (and once the rim was released, it allowed the head lobe to rise on the growing neck). 

The two ridden-up layers are actually extensions of the second and fourth ribs of the cove next to them to their right. That’s because the cove was subjected to exactly the same process of tear and radial recoil at or around same time. The two lines of the layers are even traceable right through the second and fourth cove ribs and on down to a point where they almost meet on the Ma’at perimeter to the right of the cove. This fact will be seen to be significant for the Site A slide in a future post. 

Photo 6- the two lines that comprise the two delaminated layers in this post as well as the second and fourth cove ribs and and two extensions to the head rim perimeter.  

   

Orange- the two layers in this post.

Yellow- the second and fourth cove ribs. 

Red- the two extensions along and down to the Ma’at perimeter. The Ma’at perimeter is the head rim. The bottom line extension actually reaches the head rim immediately after traversing the second cove rib. However its path between there and the bend in the head rim is about parallel to the upper extension and in that sense they are structurally linked. 

SUMMARY

The classic shear line along the level 2 onion layer at Babi and Seth is the catastrophic shearing of the entire head lobe. That shearing happened only after the level 1 layer components had torn along the same line as the shear line and recoiled from each other, up the head and down the body. Only then did the classic shear line actually shear, allowing the head lobe to rise on the growing neck with the matches perfectly preserved at the moment of shearing. The matches were preserved on the head rim and shear line. The level 1 crust, which tore into two halves, slid up the head and down the body under the influence of radial forces. One half ended up as the four cuboids in Babi, overhanging Aten and the other half ended up as two parallel layers that rode up Ma’at. Since both the head lobe shear and the level 1 tearing above it happened only after the long axis delamination into ‘Venetian’ bands, the slid components retain a crumpled signature of those bands. The signature is in the form of wiggly lines tracing the crumpled layer fronts. They are crumpled due to being flaccid after all tension was released after the tear. 

The same principle applies to Seth. The level 1 crust tore into two halves. The body half slid to the back of Site A and the slab A extension. The head lobe half is hiding in plain view on Ma’at. It will be weeks before it’s characterised in detail on this blog. Perhaps someone else will identify its location and slide vector (including a sideways split) before it gets posted.

PHOTO CREDITS:

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

To view a copy of this licence please visit:

http://creativecommons.org/licenses/by-sa/3.0/igo/

All dotted annotations by Scute1133.

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One thought on “67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 41

  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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