This is the second part in a mini-series regarding the long-axis delamination of pits along the shear line. Stretch theory is not a mainstream theory and is not entertained by the Rosetta mission scientists. Therefore, the pit delaminations described below are at odds with the current thinking on pit morphological evolution. If you are interested only in peer-reviewed literature this will save you further reading.
The two header photos are reproduced further below with their keys and originals. ESA’s regional map is at the bottom.
Everything applying to the long-axis stretch and delamination of pits along the Hapi/Babi rim (Part 62) also applies to pit delamination along the Hapi/Seth rim at Aswan. These two sections of the head lobe shear line underwent the same process in mirror fashion. It will be some time before this Aswan section is written up in full hence this overview to show how the Ma’at 01, 02 and 03 pit delaminations at Hapi/Babi were consistent with the overall picture across this north pole side of the comet.
Three pits delaminated along the Hapi/Seth rim in a mirror image to the delamination of Ma’at 01, 02 and 03 along the Hapi/Babi rim. That’s why there were two half-pits identified along the Aswan rim at Hapi/Seth in the original sinkhole paper by Vincent et al. (July 2015).
The large pit in the Seth region is the mirror image of the Ma’at 03 source described in Part 62. That source is the third mini-volcano and it was itself once an outgassing pit on the body. It has the largest source diameter of the three Ma’at pit sources. It’s that majestic, undercut curve overhanging Hapi. That diameter is about the same as the main Seth pit. And the Seth pit is the largest of the three pits that delaminated in the mirror image of Ma’at 01, 02, 03. The other two are the half-pits on the Aswan rim.
The Seth pit and the Ma’at 03 source pit lie at either end of the very straight long-axis stretch line along Hapi that will be described in the next part. Both pits are larger than any of the others because they share a unique morphological evolution as directed by the long-axis tensile forces of stretch.
For both pits, this evolution proceeded in an identical and mirrored fashion. Both pits are sited where the long-axis stretch vector turned to make its way in a straight line along the side of the herniating head lobe. The stretch vector did this in an attempt to make its way around the herniating head lobe. This ‘line of least resistance’ behaviour is patently clear, stamped on the comet’s surface in the form of the red triangle (Part 26, signature 6) and is the raison d’être for Serqet and Nut. So both pits are sited at the exact point where the long-axis tensile force vector joined the herniating head lobe rim.
Both pits opened up as a result of 200-metre-wide rifts careening up the long axis tensile force vector for 1.6 kilometres and into the future head lobe rim, passing underneath it before the rim sheared from the body. For the Seth rift see Parts 48 and 49. The two points at which the two rifts went under the head lobe opened up into a 200-metre-wide notional square with the Hapi rim acting as a third side and seated Hathor matrix as the fourth side. The head lobe acted as a loose lid.
So the Seth pit and Ma’at 03 pit are sited where there was both head lobe shear and a 200-metre rift opening up. Only the Hapi rim stayed (relatively) still while the three-way movement of two rift perimeters and head rim induced furious outgassing. This is why the Seth pit is by far the largest identified pit on the comet and the same 200 metres width as the rift it sat inside. The Ma’at 03 pit is the same diameter and is effectively an unidentified, dormant half-pit like the two along the Aswan rim.
Both pits, Seth and Ma’at 03 source pit, then delaminated away from the shear line after head lobe shear. The Ma’at 03 pit suffered a single delamination across Babi (Part 40) while its original Ma’at 03 source pit stayed put at the shear line as you might expect a hole to do.
However, the Aswan pit actually slid on the onion layer below it. To repeat, this hole did actually slide: a sliding hole within its sliding matrix which was the entire Aswan basin onion layer. This is why the Seth pit is flat at its base. The base is the next onion layer down, looking up at us and it’s the layer the Seth pit slid on. It’s also why the base of the pit is at exactly the same level as the lower lip in Hapi at the base of the Aswan cliff. That’s the layer over which Aswan along with its pit slid. This sliding scenario is absolutely dictated by the conclusions drawn from photo 7 in Part 49. There’s a line of boulders on this lower lip that is a translational match to the base of the Aswan cliff line. This is strong corroborative evidence for the Aswan-plus-pit slide but photo 7 in Part 49 is even more compelling:
Larger Red- the 200-metre-wide rift
Bright green- the extension of the rift that went under the head rim before the head sheared.
Light Blue- the Seth pit and the quasi square location where it first opened up before sliding (between the bright green lines).
Small red- red line at right: the Aswan cliff line (including its line in the shadow from other photos); at middle: a red line showing a boulder deposition line that matches to the cliff base; at left: the original seating of the cliff base. The Aswan cliff slide is almost the same distance as the Seth pit slide but certainly apparently less. In reality it has to be the same because the cliff is joined to the pit. The difference will be explained in a future part because the full extent of the sliding behaviour in this small area is beyond the scope of this post. What we see annotated and explained in this photo is 90% of the full picture anyway.
Slate blue- at top: a pair showing the upper half pit (or second half-pit if equating it to its mirror image Ma’at 02 source pit) along with a very obvious match to its seating next to the blue quasi-square; at bottom: the first half pit which equates to Ma’at 01’s source pit.
After the Seth pit slid, it delaminated into three across Aswan (Part 32).
There’s much more to say on the Aswan delaminations, along with copious evidence in the form of translational matches on the body and head and mirrored matches to the head rim underside. Some of this evidence has been noted in passing in Parts 37, the 48/49 twins and Part 50. The unavoidable conclusion is that all these pits that are today on Aswan, Seth, Babi and Ma’at were once delaminating their way in opposite directions along the Hapi shear line.
Most tellingly of all, this mirrored, long-axis delamination behaviour along the Hapi rim is symmetrical about the north pole point in Hapi. Since the north pole is where the rotation axis pierces the comet’s surface, this is a significant signature that the mirrored delamination process was driven by spin-up of the comet:
Photo 2- the mirror-image delamination of the holes across Seth/Hapi.
Brown- this is the paleo rotation axis adjusted up the comet’s x axis by 700 metres so as to make a comparison with today’s rotation axis (dark blue). The paleo rotation axis held sway before head shear. Only then did the axis precess 12°-15° to today’s axis location. The paleo axis is derived by taking the line that’s orthogonal to the inferred paleo rotation plane (see the Paleo Rotation Plane Adjustment page).
Notice how the paleo axis runs down the middle of those two horseshoe craters and also directly between Aswan and Babi. It’s the line of symmetry between them. It’s also the line of symmetry between the two red rifts running from the original Seth pit seating and from the Ma’at 03 source.
And most importantly of all, it represents an almost perfect line of symmetry between the two sets of three delaminating pits (light blue). You can see the three Ma’at pit sources delaminating one way (left) along the long axis from the brown line. And you can see the three Seth pits delaminating (and in their case, tearing and stretching) to the right along the same long-axis line at Seth.
The paleo pole is the larger brown end dot. It’s equidistant from the original Seth pit seating and the Ma’at 03 source pit. The original Seth pit seating is where the symmetry of tensile forces originally held sway before the pit slid. The paleo pole is therefore the centre point origin along the symmetry line for all the symmetrical features described above. This is strong evidence that spin-up of the comet was the mechanism by which this mirror-symmetrical pattern arose and the consequent long-axis delamination at Hapi occurred.
Photo 3- the ESA regions
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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All dotted annotations by A. Cooper.
Copyright: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/A.COOPER