67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 37


This post is one long narrative key to the header photo. It summarises the stretch vectors and crust sliding/delamination presented in Parts 31 to 36.
References to up, down, left and right are with respect to the ‘upright duck’ configuration as shown in the header photo i.e. with the head lobe on top and body lobe at the bottom. 

Blue dot- the north pole. The north pole is where the rotation axis pierces the comet’s surface. It pierces the other side at the south pole. You could imagine extending it out both ways and twiddling it in your fingers to rotate the comet. In this view the extended axis passes a little to our left and somewhat below us. Rotation is the head at top-left downwards and body at bottom right upwards. 

Upper brown dot (next to the blue dot)- an adjustment of the current north pole. It’s a point that’s swivelled by the same angle from the blue dot as the paleo rotation plane is swivelled from today’s rotation plane. See the appendix at the bottom of this post for the workings to arrive at this point. This doesn’t give us the exact paleo pole position for when the head was on the body but it constrains its possible original position to being somewhere on a line from this point, down the rim of the Site A crater. That’s because the crater rim is in the same plane that the head lifted, which was almost vertically above the horseshoe. This means that the head dragged the centre of gravity up through this plane as it lifted. That in turn means that the z-axis of the comet’s rotation rose up within this plane. This axis lift was almost a pure, single dimensional, translational lift upwards with very little swivelling about the centre of gravity. It would be like lifting a barbell (rotation axis) using one hand gripping the middle (the centre of gravity) and successfully raising it while keeping it horizontal- and with no swivelling about. The extent to which the rotation axis did swivel is betrayed by that short distance between the blue dot and brown dot. The swivel was a precession about the long axis of the comet, which was probably brought about by the head lobe slice (Part 20) which in turn caused the head lobe itself to precess anticlockwise about the long axis by about 15°. 

Lower brown dot- this is an estimated position of the actual paleo north pole. That would be the north pole when the head was seated on the body but starting to herniate from it. Seeing as the pole is the point where the rotation axis pierces the comet, it means that when the axis was dragged up vertically in its translational lift, the paleo pole, where the axis pierced the surface was also dragged upwards. If you reverse the head lift and seat it back on the body, the current north pole dot would initially drift left towards the top brown dot (as the precession that happened during head lift was reversed) and then drift down the Site A rim to its estimated original position. In fact, it would drift down as well as left even in the initial stages of reversal because both precession reversal and axis drop would be happening at once. So it wouldn’t actually drift through the upper brown dot but left and under it until it was on the line between the two brown dots. And then it would be travelling straight down along that line which is the crater rim. 

Red arrows- the tensile stretch vectors. They’re pointing predominantly radially, away from the cove/horseshoe formation. That’s the same as saying that they point radially away from the current north pole because the north pole is right next to the horseshoe crater. And the paleo pole would have passed through or very close to the horseshoe crater during head lobe lift so wherever it was located before or during head lobe lift, it was close to the central focus of the radial vectors. However, during herniation, the paleo pole wasn’t quite the central focus. That might be explained by the ‘pie-shape effect’ explained in Part 36. That was where the big outer masses of crust attached to inner portions would rip those inner portions out regardless of their being slightly off the radial centre. In other words, these larger attached chunks would dominate over the neat geometry of everything sliding out like pieces of cake with their tips all kissing the north pole. That would be too much to expect. Instead, localised random fractures around the pole would succumb to the forces exerted from the larger, attached masses at the extremities, which were also under a greater centrifugal force. There are some missing arrows, off-frame to the right across Seth that were presented as part of the lattice in Part 31. Seeing as a similar number are missing to the left, off-frame and yet to be presented, this photo is showing just those central vectors that are clustered closer to the cove/ horseshoe crater. This picture will be widened and updated once Babi is dealt with to show all the stretch vectors across the body. And eventually the whole of Ma’at on the head will be included.

Fuchsia- the two India shapes that delaminated, one from the other. This slide opened up the curved fissure which fed gases to the 3 holes that sat on this spot (and are now on the head lobe). Also, there are two fuchsia V shapes to the right. The first is a delamination, the second is a straightforward tear of the lower onion layer. They are both thought to be related to the India shapes. This photo doesn’t do them justice so better photos will be presented in future. 

Terracotta- these steps were first presented in Part 7. They must have stretched with the head because they match to similar steps on the underside of the head rim i.e. on the Hathor cliff. There are other close up photos that show them as being at least 5 very consistently matching steps. There are two well-defined ones here and two less well defined and it appears the one in the middle is whited out. But this is enough to show the direction of delamination betraying the stretch vectors. 

Pale yellow- a new, large-scale slide. 

Bright yellow- the lower line on the body is the classic line of the cove when it sat on the body. And the two lines on the head show how the middle scallop would collapse onto the bottom scallop if the stretching process were reversed and the head seated back how it used to be. 

Red- at the right hand end of the bright yellow lines. These simply denote the ends of the top and bottom ribs of the middle scallop in the cove and where they would have sat on the body. Notice how the bottom pair are closer together and the top pair wider apart. That’s because the head tipped up somewhat during the lift so the cove is angled up a little more towards us. Incidentally, the head tip wouldn’t run counter to the idea of the rotation axis staying horizontal and not swivelling. That’s because the head tipped (and swivelled) largely about its own centre of gravity during the rise and the rising rotation axis was only responding to the separating centres of gravity of head and body.

Larger dark green- on the body: the two end points of the ‘India shape’ slide which opened up the fissure and by doing so started opening up that shallower crater above the horseshoe. So the green dots mark the width of the fissure. On the head: these two green dots would sit on the two green dots either side of the fissure below. They also correspond to the width of the two delaminated sets of gull wings on the head (see Part 36). Since they’re the same width as the fissure they also mark the width of the conduit through which gases passed to the original hole that delaminated into the three on the head (Part 36 as well). And of course, the fissures on the body and head are the same width as the holes. The first hole in the bottom scallop is really just the scoured base of the hole before it delaminated into the two obvious holes above. However, in this picture, it always looks more like a hole due to the lighting. And now that we’ve established the actual scoured base location in Part 36, we know this apparent hole is actually the conduit leading to the scoured base. The base is actually tucked into the top right corner of this neat-looking ‘hole’ which is really the flat, scoured conduit. That’s how easily the lighting can fool us. The tiniest bit of relief shadow at top right corner tempts us into thinking the whole area is a hole. Foreshortening is another issue here because the scoured base is sitting in the bottom scallop and almost hiding under the middle scallop rim. 

Small green- these dots run along the gull wings on the body (contiguous with the end of the yellow line) and also along the seating from which they delaminated. That seating was originally next to the rim of the horseshoe because we established that the gull wings were attached to rock C when rock C was in the horseshoe. It appears that even the seating itself slid away from the pole in sympathy with the rock C slide that opened up the horseshoe (Part 33, the ‘monolithic slide’).


This appendix concerns the brown dots representing the paleo north pole in the header photo. It explains the adjustments that were made, using the current north pole and knowledge of the current and paleo rotation planes, to place them.

The adjustment was done by observing the distance between the current plane and paleo plane paths through the four coloured body matches facing Anuket (coloured matches: Part 24). The point of measurement was at a similar distance from the centre of gravity as the north pole point. And the centre of gravity is at the origin of the 3-axis reference frame. Seeing as the north pole swung by the same angle as the rotation plane, the distance of that swing is the same as the distance of the rotation plane swing. Or at least, that’s the case for points that are the same distance from the centre of gravity. The distance of the swing was estimated at 2-300 metres.

However, the paleo pole for the original body was somewhere further down towards the other brown dot or even slightly beyond it. That’s because the head lobe, rising after shear, moved it up towards the first brown dot and then across to the current blue dot when the rotation plane precessed. When the head was on the body, the centre of gravity had to be lower. Therefore the z-axis of rotation had to be lower. Therefore the point at which the z-axis emerged at the surface had to be lower. And the point at which it emerged is the paleo north pole for the single body paleo comet. The lower brown dot is a crude estimate of its location but it had to be straight down the site A rim because that’s in the same plane that the rotation axis rose as the head lobe rose.


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

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All dotted annotations by Scute1133.


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