67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 33

THE MONOLITHIC SLIDE THAT OPENED UP THE CRATER AND COVE  

 

Red arrow- direction of slide. The tip of the arrow is touching the back rim of the small crater that sits under the head cove. The cove used to sit on this crater. Although some upper portions of the crater side rims also moved backwards, only the back rim slid in its entirety from rim tip to crater floor. In subsequent photos it looks more like a crater, or rather, a horseshoe remnant of a crater than it does here. In this photo, the nearside rim appears to merge with the floor. The tips of the red V kiss the ends of the horseshoe. The back rim is about 80-100 metres high from crater floor to rim tip.

Blue dot- the north pole.

Terracotta and yellow- the shear line where the head lobe sheared from the body. The yellow portion is the section of the shear line where the cove on the head lobe above fits to the body. The visible part of the head rim shear line is also shown on the head at the left of the photo. The small, yellow section fits to the near end of the yellow section on the body because it’s part of the cove. However, it’s not the true match- it’s a thin layer that’s delaminated upwards from the true match which is hidden below it. That’s why it doesn’t follow the ‘gull wing’ shape on the body. The hidden, true match follows the gull wing faithfully (see Part 5 and below). The terracotta portion peters out beyond the yellow portion due to body matches being lost on missing slab A. 

Photo 2- ESA regions map for orientation in this post. 

 

INTRODUCTION

This part could be considered as describing a mirror image of the three sink holes sliding back in Part 32, except this slide actually opened up a single hole rather than delaminating an existing one into three. Today it isn’t a hole but a horseshoe crater. It was a full hole or crater when the head lobe was seated on the body across the end of the horseshoe. This slide is a mirror image of the three sink holes because the two slides occurred at opposite rim tips of the Site A crater. 

Part 34 will rely heavily on this part and some evidence for this part will be presented in Part 34, such as the ‘gull wing’ slide.

All references to up, down, left and right are from the ‘upright duck’ viewpoint unless specifically stated. The upright duck viewpoint is with the head lobe of the duck shape positioned directly above the body lobe. 

BACKGROUND

In Part 32 we looked at the three sink holes, showing that the two smaller ones had delaminated from the large one by sliding back across Site A. Site A is the crater left by missing slab A and is called that because it was one of the candidate sites for the Philae lander. The tell-tale sign for the sink hole slide-back wasn’t only seen in the slid, terrace-like layers in the immediate surroundings of the three sink holes but also a long way down Ash, where layers had ridden up just like on the head lobe (Part 29). Perhaps we should say they are ridden-down layers because they slid down the body. That means it was in the opposite direction to the riding up on the head lobe and one would expect that to be the case seeing as Ash is on the opposite side of the shear line.

It was shown in Part 32 that the tensile forces across Site A and the slab A extension were radial in nature. That means the sliding and riding down of layers was radial too. The force vectors were centred on a point which was close to the large sink hole and also near the north pole of rotation. This strongly implicates stretch by centrifugal spin-up as being the cause of those forces. If that’s the case, the logical thing to do would be to carry on tracking round west towards and across Babi. We could then see if the same radial pattern is apparent. If it is, it would have to be fanning out in the required direction, away from the north pole or a point fairly close to it. 

We had already seen in Part 32 that there was a flaccid-looking ridden down layer in Ash that matches to the Site A crater rim. There are mini matches for this layer but we’ll leave those to another time. The average tensile force vector (stretch vector) of this ridden down layer was away from a point just to the right of the current north pole, complementing the other radial vectors.

The left hand side of the Site A crater rim is quite straight and points along a line that runs just to the left of the pole. It’s probably straight because it stretched backwards towards Ash along that line. It did so in sympathy with the flaccid, ridden down layer behind it. And one would expect this behaviour anyway because the three sink holes on the opposite rim delaminated and slid back in the same manner.

So the yanking back of this left hand side of the Site A rim was itself directed away from the current north pole, which is promising for our radial stretch vector studies. At the tip of the left hand Site A rim, there’s a small crater that fits to the so-called cove on the head lobe above it (matched in Part 3). And now it appears the small crater itself is more affiliated to the Site A rim than previously thought. This involves its relationship with rock C. Regular readers will know all about the rock C seating at the back of the small crater:

Photo 3- the header photo from Part 15, rock C. 

 

Rock C is sitting upside-down in relation to its seating. It flipped over when it drifted across Site A. This is covered in detail in Part 15.

The apparently solid block of the rock C seating used to take up the volume of the small crater before sliding back. You can see the crater just below the rock C seating in the photo. The seating (the two closer yellow dots) is kissing the rim of the crater so rock C was kissing the rim when it was seated there. 

The seating should, for our purposes, include that square that the lower left yellow dot is touching. That’s because the rock C perimeter went round the back of it and so the rock was the size of that entire isosceles triangle you can see but minus the square. The lower left yellow dot is where the front edge of rock C ended against the crater rim so its position is technically correct but a tad misleading in terms of depicting the true width of the rock. Once you get past the square, the width is the same as the back rim of the crater. It’s best to visualise rock C as that entire isosceles triangle with a square gouge at its thick end. That gouge will be seen to be quite important.

The combination of rock C, and its seating used to take up the volume of the crater. So rock C and the crater rim it kisses used to be slid right forward to that straight, front lip of today’s crater, filling the horseshoe shape completely. The front lip marks the drop-away to Hapi and is where the shear line runs. When rock C, plus seating, slid back towards Ash (away from the current north pole) they automatically created the small crater as we see it today. The sides of today’s crater remained in place while just the back rim slid back. Rock C was really just part of the rim at that time so it slid back as well. 

We shall see in the next part (34) that the gull wings were originally attached to the left rim of the small crater. The gull wings were first mentioned in Part 5. They detached at the same time that rock C and its seating slid back. They recoiled to the left (looking from the upright duck viewpoint) along a line that’s also radial from the current north pole, creating the upper level crater as they did so. That’s the wider, shallower crater that surrounds the left side of the small crater. That gull wing slide dragged the cove (Part 3) out to the same spot because the cove was still attached to the body at that time. 

This means the gull wings we see today on the body must have originally fitted to rock C. That would be when rock C was sitting on its seating and forming the topmost part of the filled crater volume- and when the gull wings were clamped to the side of the crater. That in turn means that today’s rock C, displaced 300 metres away on Site A, should be an upside-down gull wing shape:

Photo 4- Rock C. 

 
photo 5- the gull wings. 
 

Photo 4 depicts the gull wing shape on rock C in dark green. 

Photo 5 is the head-body match of the gull wings from Part 5. Multiple colours map across the various individual mini matches. Full zoom is recommended.

That dark green perimeter of rock C would have attached to the gull wings. Since rock C is flipped over in relation to its seating, it needs to be flipped back again to make the match. That would flip the upside-down gull wings in the photo to an upright position and facing away from us, which is the correct orientation for the match. At the time of posting Part 15 it wasn’t known that rock C and its seating had slid back with the Site A rim to open up the small crater. Nor was it known that gull wings were clamped to the left side of the small crater. So rock C wasn’t perceived as being an upside-down gull wing. 

One additional point. The sides of the small crater have strong-looking, straight ridges that run vertically, like walls, to the bottom of the crater or horseshoe shape. It seems that these were strong enough to resist the tensile forces of spin-up. But it appears they acted as perfect side walls for the rock C and seating combination to slide along and between. In photo 3, the upper right yellow dot under the ‘e’ in ‘here’ is at the crater rim, abutting one side wall. Lower down, this wall’s vertical face is in shadow. The other side wall is abutting that small square we discussed above and has a dark shadowed side facing towards us as well. That’s the ‘hollow block wall’ that fits to the cove above in Part 18. That’s very straight, looking from above. In upright duck mode, the hollow block wall is on the right of the crater or horseshoe and the other side wall is on the left. 

THE PALEO POLE 

The paleo north pole may still have been slightly below the small crater (towards Ash) at the time of the rock C seating slide, thereby complicating the issue. That’s because it had to be lower down the body than the current north pole when the head lobe was seated on the body lobe. That would be in keeping with the lowered centre of gravity. Subsequent events show that the head was seated during the slide and these events will be laid out in an upcoming post. If the pole was indeed lower than the small crater, the rock C/seating slide should theoretically be the opposite way. However, if the Site A rim acted as one solid chunk as it seems to have done, then rock C and its seating on the ‘wrong’ side of the pole would have been dragged back by the vast amount of mass on the ‘correct’ side of the pole. It would just mean that the shear line opened up very close to the paleo pole but chose to shear at the nearest available weak point. That weak point happened to be a little beyond the pole. Also, the radial force vectors near the pole itself are very weak due to the short radius of rotation. The apparently strong radial force signatures right next to the pole we’ll see in the next post are probably more to do with the bigger masses at bigger radii tugging on crust material at the pole. 

Alternatively, the whole crust in this vicinity might simply have continued to slide south just after the head lobe sheared and was rising apace. The paleo pole would be creeping up fast from its lower position to its upper position while the comet was slowing from a ~2-hour rotation to a ~5-hour rotation. There was still a fair bit of ‘centrifugal’ force on the site A and Ash area during this period even if it was only strong enough to induce a continuation of southward sliding of already loosened crust. It didn’t have to be anything like the centrifugal forces needed for ejection to orbit or escape. And we know from Part 32 that Ash was dragging elements of Site A with it a long way down the side of the comet. 

A NEW ROCK C MATCH TO THE CRATER RIM

Another rock C match that wasn’t presented in Part 15 is the two mini-scallops at the thick end of the rock. They’re dubbed ‘mini-scallops’ because we’ll be referring to the larger scallops in the head cove in the next part and need to differentiate between the two types.

The rock C mini-scallops are presented below in photos 6 and 7 because we’re in the process of showing all the available evidence for rock C kissing the small crater edge. Having done that, we are showing that the seated rock C and the back of the crater it matched to both slid back as one mass of ‘rock’, thus opening up the small crater. Before we look at the photos, here’s the reasoning as to why rock C might have these mini-scallops running up its thick end. 

It would have been quite a spectacular ‘open sesame’ style slide: those well-defined crater side walls stayed in place, like rails, and only the back portion, including rock C, slid back along the flat floor of the crater. Rock C and its seating simply represented the tip of the Site A rim, sliding back in its entirety. The slide was towards Ash just like the three delaminated sink holes on the opposite side of the Site A rim. The slide would have caused that very straight tear across the front rim of the small crater. But at that time, the head lobe would have been pressed against that straight edge with gases spewing up through the fissure between crater and head. Indeed, the head formed that side of the crater at that time. The straight crater rim is the shear line where the head lobe sheared from the body. It’s of course likely that a developing fissure would be responsible for weakening the rock C/seating combination and working it loose from the head lobe. That would allow it to detach and slide back under the influence of centrifugal forces. 

The shear line runs away from both ends of that crater rim and on, in opposite directions right round the comet. They meet up again at the south pole in a similar scalloped area of apparently violent outgassing (Part 30).

This means that the rock C/seating monolith tore from the head lobe at the front rim of the small crater and this was therefore a premature unzipping of the shear line before the sections either side of the crater unzipped. It also means that the rock C mini-scallops were right on the shear line. That explains why there was so much outgassing scouring up past rock C. It was one of the the most prone areas on the comet for catastrophic outgassing. And the reason this section failed before the rest of the shear line either side is due to the radial nature of the tensile force vectors. The spin-up of the comet was causing the crust to start sliding away from the pole under the influence of centrifugal force. And at the pole itself, the forces were all pulling radially around one point- the small crater. 

Photos 6 and 7, rock C mini-scallops and their matches around the back of the small crater. Remember, you have to flip rock C over in your mind’s eye so the gull wings turn to face away from you and the two mini-scallops are sitting further away from you with their gas ejection arrows pointing up, not down. More explanation below.  

   

The crater photo shows the same mini-scallop annotations as the rock C photo but projected into ‘thin air’ above its seating. In other words, they’re following the lines of the mini-scallops as if rock C was sitting there (flipped right-side-up) in its former home. You can see the two (physically present) mini-scallops at the back of the small crater that feed seamlessly into the two mini-scallops of rock C above them.

The red line on rock C, photo 6, follows the top perimeter of the 90° gouge that runs down the side of the rock. That gouge was briefly mentioned, above. The gouge itself is running down the other side of the rock, out of sight, in photo 6. The top perimeter is almost out of sight as well but nevertheless visible in high profile. The corresponding red lines on the seating aren’t in thin air like the blue lines. This is because if they are at the far, top side of the rock in photo 6, they must be at the near, bottom side in photo 7. That means they’re a conventional mirrored match, kissing each other. The match forms the base of a square chimney-shaped void that runs up the side of rock C (the gouge). That void, though out of sight in photo 6, is discussed, with photos, in Part 15. The chimney is a vertically sliced diagonal half of a chimney in the gouged rock but the square it fits to (in photo 7) suggests there was mass to the left, in that view, that completed the other two walls of the chimney. Further evidence of this is that the square base of the supposed chimney has two lines running down the back rim of the small crater from it. These appear to be related to the chimney because they’re the same width and parallel. So, just like the blue-dotted mini-scallops, the chimney had its own flattened conduit up the back of the small crater rim. The conduit would have been active when rock C was slid forward into the crater volume and pressed against the head lobe at the shear line. 

That completes the matching of rock C to the back rim of the crater. The matches for its perimeter seating above the rim are dealt with in Part 15. 

Although we can’t be sure yet, it does look as if there was a chimney with mass to one side and along the crater rim completing the third and fourth sides. If the rim was pressed against the head lobe when in the crater, that would create one more side. And the side of the cove on the head may well have formed the fourth side. We’ll see below that mini-scallops on the cove might have fitted to the gouged side of rock C when it was at the back of the crater. Even if the cove wasn’t sufficiently overlapped to do that, it still kissed the corner of the chimney base so we are discussing exquisitely small adjustments here. 

All the matching rocks and scallops and flattened conduits would originally have been right on the shear line at the sharp, straight lip of the small crater. They would have had the head lobe pressed against them causing the conduits to be flat, lower down, and scalloped, up high near the exit (i.e. fluted due to explosive gas escape with the sudden pressure drop). The two light blue-dotted scallops, and the red chimney which is a scallop of a sort, start at the base of rock C and that’s presumably why rock C became loosened at that point. 

Since the original gull wings from Part 5 will be seen to match to the side of the small crater (Part 34) then the rock C gull wings had to be inside the crater we see today in order to match to them. In other words, the dark green gull wing line depicted on rock C, above, had to kiss the small crater edge as well. That means rock C had to be sitting right in the crater when the gull wings were attached i.e. filling up the crater volume completely (along with its seating). 

The seated base of rock C represented the point at which the pressure drop caused a last burst of extra explosive gas and slurry escape (circa 40-50 metres depth). That caused the fluting within the rock C mini-scallops- they were scoured out. Eventually, after much outgassing and loosening, rock C and its seating slid back towards Ash and away from the shear line, thus opening the small crater. 

THE ROCK C MINI-SCALLOPS CONTINUED ROUND THE CRATER FROM THE HEAD COVE MINI-SCALLOPS. 

The edge point of the chimney was matched to the cove tip, 1 kilometre above it in Part 3 with red dots. Now we can join (not match) the actual mini-scallops of the cove to the mini-scallops of rock C. The scallops that run round the perimeter of the cove continued across the end of rock C when they were both clamped side by side and formed the top perimeter of the small crater rim.

Photos 8 and 9, below, show the cove mini-scallops and the rock C mini-scallops. They are the same photo. The first is a skeletal annotation showing the cove mini-scallops and rock C crater rim features. The second inserts context around them. 

As mentioned above, any overlapped match of the cove tip mini-scallop to align with the chimney is somewhat tentative. The main point here is that the cove tip did seat to the crater rim next to rock C and both cove and rock exhibit the same characteristic mini-scallops. These would have been running vertically up the crater side, both up the cove (which would have formed one side of the crater) and up the back side of the crater, including rock C when seated. Just to be clear, we’re not matching the cove mini-scallops to the rock C mini-scallops in the sense of marrying them in a conventional mirrored match. We’re simply showing that the rock C mini-scallops continue on round the crater rim from the cove mini-scallops when the cove was seated. And that they were similar in character. The full key is below. 

Photos 8 and 9- the rock C mini-scallops continued on round from the cove mini-scallops and are similar in character. (See also photo 6 for rock C close up). 

   

Light blue (full zoom needed)- on the left end of the head lobe cove, this depicts four ribs delineating three mini-scallops. On the body, it’s just two very small light blue dots marking the centres of the rock C feeder mini-scallops. They are not the ‘thin air’ matches. They’re physically there in the crater rim side and directly below the rock C ‘thin air’ matches. They’re the two ‘feeder’ mini-scallops for rock C. They’re more obvious in photo 7.

Terracotta and yellow- these mark the shear line and cove. As with the header photo, the yellow line is simply the section of the shear line that matches the cove on the head to the crater and gull wings on the body.

Red- on the body, this is the base of the hypothesised chimney that is thought to have had a square cross section. Only two sides of the base are marked because those are the perimeter of rock C and therefore depict the gouge that runs up the side of rock C. The gouge represents half the chimney, sliced diagonally down the middle. The other half would be made complete if we could find the other two sides or chimney walls that would sit on the two undotted sides of the square base. One wall would be the head lobe when rock C was shunted against it. The other may be that thin scallop at the very tip of the head cove. That might seem rather tentative but that cove tip has always been matched to the front corner of the square chimney base. If we shunt the match one chimney width away from us (in this photo) towards Ash, then that thin scallop on the tip of the cove becomes the missing side of the chimney. And of course, it’s the correct width. So that scallop is dotted red as well, along its base, in recognition of this possible chimney match. 

Orange- rock C seating perimeter and approximate position of the present-day rock C. The seating perimeter includes the red, 90° gouge of the supposed chimney. The tip of the rock C seating is sharply bent up. On the actual rock it’s bent down because the rock flipped over.

ROCK C LOOSENING SCENARIO

The fact that the pressure drop and explosive scouring of the the rock C mini-scallops started at the base of rock C probably isn’t a coincidence. That demarcation line may have been the very reason rock C was detached at that particular depth. The base of the scouring betrays the threshold of the explosive pressure drop. The base of the explosive pressure drop is where the rising gases and slurry might find their way in between fracture planes with less mass and therefore weight above them than lower down. That would explain all the flat-bottomed square trays in the rock C seating. They have pooled slurry in them. 

Photo 10- the flat, square trays in the rock C seating. 

 

Slurry was mentioned above because there are signs of slurry deposits all round this small area of violent tearing and sliding (Parts 7 and 8). In addition, the small crater has a mirror-flat floor, perpendicular to the gravity vector. It may have a thin coating of dust but it’s almost certainly the hardened slurry underneath that caused the flatness. It’s just like pouring thick oil paint into a tray and seeing it find its own level. The upper tier crater extension running across to the gull wings is almost mirror-flat too. 

AN ADDITIONAL MATCH TO PROVE THE ROCK C SLIDE

Since we now know that the three sink holes on the other side of Site A delaminated with a multiple slide-back of terraced layers, it would be reasonable to look for signs of the same behaviour on the rock C side. That means looking for delaminated, slid-back layers behind the rock C seating and towards Ash. If they could be matched to their former seating, all the better. 

In Part 14, rocks A and B, currently out on site A, were matched to a seating location on the crater rim that is contiguous with the rock C seating. That rock A,B seating has an identical shape just behind it, towards Ash, in the same direction as the rock C/seating slide. So that shape was originally part of the rock A,B formation and slid off the top of rocks A and B at the same time that rock C and its seating slid back to form the crater. The distance of the rock A,B match slide is the same distance as the rock C/seating slide.

Photo 11- the rock A,B match slide. This is a screenshot of the Part 14 post, rather than just the photo on its own. That’s because it contains the first line of the key showing a naive recognition of the match without realising it had slid back.  

 

Terracotta- the shear line. 

Pale blue- rocks A and B out on Site A to the right, and their seating to the left. 

Dark blue- the rotation plane corridor along which they drifted from their seating. 

Pale green- (not simply a similar shape!) the rock A,B match that slid back from its original position. The slide was a delamination event just like the multiple delaminations that caused the numerous terraced layers around the three sink holes. This rock A,B match was originally seated directly on top of rocks A and B. Since it’s a single delamination identification and the three sink holes sport multiple delaminations, it would be reasonable to suppose that all it’s surroundings have also delaminated from a position nearer the shear line. That suggests there are multiple delamination matches yet to be identified around this match.

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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