Red- the three scallops that form the so-called cove in the centre of the head lobe, directly above the north pole in Hapi. The cove was first mentioned in Part 3. This is a blurred photo but it shows the three scallops quite well. In most photos, the top one is less obvious due to being shallower and more prone to partial whiting out. And in some other photos there can be a perspective issue, making the bottom scallop appear to merge with the second.
Light blue- three holes that have delaminated along with the three scallops.
Photo 2- ESA regions map for orientation in this post.
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 sitting directly above the body lobe.
Some keys to the photos are narrative and quite long so their conclusion is denoted with ‘/////’.
THE COVE AND HORSESHOE CRATER FORMATION
Photo 3- the cove and horseshoe crater formation.
(Checking the unannotated photos will be useful here due to dots obscuring features).
Upper red arrow- the direction of delamination for the three scallops in the cove.
Lower red arrow- the direction of the monolithic slide that opened up the horseshoe-shaped crater (Part 33). The horseshoe opening is the flat rim towards Hapi, not the other way round. In other words, it shouldn’t be confused with the yellow-dotted horseshoe of the cove, which is facing the other way. Indeed, the two opposite facing horseshoes, overlapping along their sides are what made this half-crater into a whole crater when the head was seated on the body. Both red arrows point in exactly opposite directions and away from the north pole of rotation situated between them (dark blue dot). They are therefore betraying the radial, tensile forces of comet spin-up. The degree to which the arrows appear not to be in exact opposite directions is due to the head lobe tip-up at the right. When the head was seated, they were in exactly opposite radial directions.
Pale green- the cove perimeter. The three scallops within it haven’t been annotated due to the reasons cited above. However, the arrow tip is pointing to a quasi-rectangular section that is a rib dividing the middle scallop from the top one. The bottom of the arrow is sitting on the rib between the first and second scallops.
Light blue- three sink holes that delaminated from one sink hole. They delaminated along with the three scallops in the cove. They serve as a guide as to where the scallops are when they’re whited out or in shadow.
Dark blue dot- (single dot in Hapi). This is the north pole of rotation. It’s where the z-axis of rotation emerges at the comet’s surface. The z-axis is the axle around which the comet rotates. If the axis were produced further out, you could imagine holding it and spinning the comet round on it, as if on a skewer or a spit. If produced out into space beyond the comet surface, the axis would come towards us but pass slightly below us and somewhat to the left. This means the rotation of the comet in this view is downwards at top left and upwards at bottom right.
Terracotta- the rest of the shear line that’s visible in this photo.
Small green dots- rock C (on Site A) and its seating (against the back rim of the horseshoe crater). Rock C was introduced and matched to the seating in Part 15. In Part 33 it was shown to have been part of the monolithic slide and to have been attached to the gull wings (Part 5) before the slide.
Fuchsia- these two dots mark the tips of twin shapes that have delaminated from each other under the tensile forces of stretch, i.e. centrifugal force. They’re essentially triangular but with kinks that make them look a bit like an upside-down India shape. If you draw a line between them and produce it, it skims right past the north pole blue dot. This again betrays the direction of the delamination slide to be radial, away from the north pole. That’s why they are said to have delaminated under centrifugal force. The distance of the slide is the same length as the diameters of the three sink holes on the head. They sat on top of this small section of the shear line (already matched in Part 3). Evidently, the slide of the India shapes opened a new fissure or a newly widened section of the shear line. Gases spewed out and through the layers draped above. That created one hole which then delaminated into the three we see on the head.
Single yellow dot- this denotes the small horseshoe crater and is in its centre.
Single orange dot- this flat, white area is an upper-level crater that the cove-end slid across in sympathy with the same force vector that delaminated the India shapes (but also with a tug towards Ash due to the radial nature of the vectors). In fact, the cove slide actually created this upper level, with the Babi onion layer sliding back en masse to accommodate the opening.
Yellow- on the head lobe. This is the cove seating that sat on the horseshoe crater below.
Yellow- on the body. This is the seating of the original cove position directly before the monolithic slide. It was bent tight around the horseshoe crater with the gull wings attached to rock C when rock C was inside the crater (Part 33). The cove then tore from the left rim, slid to the left, and formed the larger crater. It probably happened at exactly the same time as the monolithic slide i.e. from the moment the gull wings sheared from rock C. Indeed, there’s a gull wing seating remnant that was kicked down and away and left marooned as if not knowing whether to follow rock C or the gull wings. That’s a signature of the two slides happening together and also a subtle sign of a radial force vector radiating from the pole and between the two main slide vectors.
Light yellow- this is the line to which the cove slid and stretched to. It defines the final seating of the cove before head lobe lift off and is the line that has always been depicted in previous posts for the cove seating. The gull wings correspond to the last four dots. The fourth from the end is attached to the centre of the India shape. The centre of the other India shape is on the horseshoe crater rim. The gull wings slid with the India shapes and that’s why they were said to be attached to rock C at the rim of the horseshoe (Part 33). The gull wing shape of rock C confirmed this.
This post is concerned with the tearing open of the cove and horseshoe crater formation which then divided into the cove on the head lobe and the crater on the body lobe. The dividing of the two was due to the shearing of the head away from the body. The cove ended up on one side of the shear line, remaining with the head. The crater, which the cove had surrounded like a rim around one side, remained on the body. When seated, the cove surrounding one side of the crater, formed a hole. That hole then became the horseshoe-shaped crater we see today after the cove tore away from it. It’s horseshoe-shaped because the cove used to clamp across one end of it. The cove used to wrap round the sides too but only from halfway up. That’s why, when viewed from above, the crater looks intact on three sides with one open end. That is the case up to a point but the the upper sides were torn away somewhat too by the curving-round cove. The only part of the crater to escape the cove-ripping was the back rim, towards Ash. The back rim performed a monolithic slide-back from that straight front lip of the crater to its current position (Part 33). It slid across the crater floor en masse, with the two side walls acting like rails. It was a very neat rift.
The tearing open of the crater/cove formation was via tensile force vectors (stretch vectors) acting in a radial pattern surrounding the cove and crater before the head lobe sheared away. These radial forces were centred on or near the north pole, which is the north pole of rotation for the comet. In other words, the north pole is where the imaginary rotation axle pierces 67P. So these radial tensile forces appear to be centrifugal forces brought about by spin-up of the comet.
Whilst there are references in this part to the fact that the cove fits to the body, the matches were already dealt with in detail in Parts 1,3,4,5,6,8 and 18. So we’re not trying to prove the cove fitted to the crater. We’re describing how the cove/crater combination was ripped open and the cove delaminated into three large scallops. It’s true there are references to the matches (and even some refining of them) but that’s incidental to the thrust of the post, which is the ripping apart, the delamination and the radial forces that were responsible.
THE COVE-WIDENING AND DELAMINATION
The lateral cove-widening event (that created the upper, shallower crater) happened when the head was still attached to the body and most probably occurred at the same time as the monolithic slide. The vertical delamination of the cove into three scallops also happened when the head was seated and would presumably have occurred straight after the cove slide. The monolithic slide (rock C plus seating sliding back to open up the horseshoe crater) would have exposed the weakened, gas-scoured cove layers at one end. That exposed end would have been the part of the head lobe against which rock C and its seating (the monolith) had been attached before the monolithic slide. Except, it probably wasn’t attached by this time because that line is the shear line and we know from Part 33 that gases and slurry were scouring their way up through this fissure. But the monolith would have been against the head with the fissure of the shear line between them.
The tensile force vector that delaminated the cove was in the exact opposite radial direction to the monolithic slide direction. The north pole is situated almost between the two so the forces that were responsible for the cove delamination were directed away from the north pole and complement the radial force pattern being built up in Parts 32 and 33. This again strongly implicates centrifugal forces. Those forces would have been increasing steadily during comet spin-up due to asymmetrical outgassing. It appears that the cove succumbed to these forces near the time of head lobe shearing. It had to be before head shear because the moment the head lifted off the body, all stretch (centrifugal) forces were transferred to the incipient neck and were no longer available for delaminating the crust.
The rotation period at the point of head lobe shear had to be around two hours for the missing slabs to have escaped at the required escape velocity speed of 0.8 metres per second. That in turn means a tangential speed of 0.8 metres per second for the discarded slabs. So the delamination of the cove probably started at a juncture just before the two-hour rotation period was reached and continued right up to the very moment the head fully sheared.
However, the centrifugal forces close to the north pole, due to spin up, would be small due to the small radius of rotation, despite the fast, two-hour period. A small radius gives a slower tangential speed than a large radius at the same rotation rate. The large radius experiences a greater centrifugal force due to the faster tangential speed. That’s why adventurous children hang off the outer circumference of playground merry-go-rounds while cautious ones remain crouched in the middle, next to the axis of rotation. That suggests the delaminating cove and sliding monolith weren’t under enough centrifugal force to display such clear radial stretch vectors. This paradox is addressed below.
THE PIE-SHAPED CRUST EFFECT
The centrifugal forces close to the pole, or rotation axis, are small but the crustal features at the pole such as the delaminating cove and sliding monolith were attached to other parts of the crust at greater radii and under greater centrifugal force. Let’s imagine an idealised cometary crust model essentially made up of pie-shaped sections centred on the pole. Each pie shape would have its small, low-mass (and slow tangential speed) tip kissing the pole and its large-mass (and fast tangential speed) outer section far from the pole. Therefore, the pie tips (represented by the cove delamination and monolith sliding out of the horseshoe) were being yanked back from the pole by sections that were being pulled harder. And those sections were pulling harder because they were under far more centrifugal force due to their greater radius and mass.
So today’s cove and monolith slide-back are really just betraying the much greater radial forces further out that were pulling on them. The cove and monolith were attached to the crustal matter subject to those greater forces so they both slid as if under a greater force than their radius would suggest. In the merry-go-round analogy, it’s as if the child on the circumference let go of the hand rail keeping him on and grabbed the child in the centre. The child in the centre probably wouldn’t delaminate but he would certainly slide, despite being under little direct centrifugal force.
The sliding and delamination directions of cove and monolith therefore remain as strong tensile force direction signatures. Those directions are both radial and in opposite directions from where they kissed at the pole. So their current tensile force signatures preserve the radial nature of the centrifugal forces acting on them via their ‘pie-shaped’ hangers-on. And in reality, the hangers-on would be anything but pie-shaped but they would be bigger, more massive and at a greater radius, which is all that’s needed.
This part adds several tensile force vectors to those presented in Parts 32 and 33. They’re all radiating from the north pole, building up the overall radial pattern that implicates centrifugal force via spin-up as the culprit.
There will be more on the cove and horseshoe crater in due course. They are important features with far-reaching ramifications. These will be presented eventually but this post presents the most salient aspects of the cove/crater opening and the cove delamination into three scallops.
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All dotted annotations by Scute1133.