Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0/A.COOPER
Green- the Aswan terrace cliff base (furthest away) and its original seating (nearest). The middle line is a line of boulders that also mirrors the shape of the cliff base.
Mauve- a feature set into the cliff face and its seating nearer to us.
Arrows- direction of the 200-metre slide. Note the small arrow off to the right on a nearby section that experienced the same degree of sliding. The arrow is smaller only because the head lobe is in the way.
The header photos show the Aswan terrace, which is the main flat area. The front edge, nearest to the viewer, drops away in a cliff that’s about 100 metres or so high. It drops down to another remarkably flat terrace. That terrace is smaller and although it’s officially part of the Seth region, it looks rather as if it’s sitting in the Hapi region with Hapi dust skirting round its rectangular base.
The bright green lines trace the bottom of the ~100 metre Aswan terrace cliff as well as its original seating along the front of the smaller terrace below. The cliff base slid 200 metres across the lower terrace. This means the entire Aswan terrace slid 200 metres because the cliff is the front edge of the terrace. The slide was brought about by the centrifugal forces induced by spin-up of the comet to a 2 to 3 hour rotation period.
This should come as no surprise to regular readers. There are longer slides elsewhere on the comet and they involve larger slabs of the same thickness (e.g. Part 43, the ‘red slide’ at Imhotep). Moreover, Part 32 described the delamination of the main sink hole into the three holes we see today. The three holes are next door to this slide and they went in the same direction with a small radial difference in keeping with the radial nature of all the slides around the north pole (the radial pattern is shown in Part 37).
This slide of the entire Aswan terrace layer is one layer lower than than the sink hole delamination. The two extra sink holes essentially slid (i.e. delaminated from the main hole) across the top of the Aswan terrace layer. That’s why the base of the second sink hole is at exactly the same level as the Aswan terrace. So it was the next layer up, sitting on the Aswan layer, that was clamped around the main sink hole, lost shear resistance at its base and slid back. The main Aswan layer in this part succumbed to the same loss of shear resistance. The loss of shear resistance was due to centrifugal forces.
There’s an intermediate green line between the cliff base and its seating. That line traces a line of boulders. The boulder line mirrors the line of both the cliff base and its original seating. This phenomenon of leaving parallel lines of boulders in the wake of a slide is most obvious at Imhotep in one of the green slides. It’s shown in the Part 42 overview of Imhotep slides. The Imhotep green slides are not fully blogged yet. That Imhotep slide has left multiple lines of boulders that are parallel to the particular cliff base that slid. This suggests a stop-start component to the slide and supports Marco Parigi’s hypothesis that the spin-up of the comet is ‘pumped’ with several bouts of spin-up leading to several bouts of sliding along the same vector. Each time the slide starts anew, it involves a sudden loss of shear resistance at the base of the layer. The jerking motion as the layer sets off again is apt to dislodge boulders from the cliff face. The dislodged boulders of course trace the shape of the cliff base where they fall because that’s all they can do. This explains the green line of boulders in this part. They mirror the shape of the cliff base perfectly despite being 150 metres from it. Vincent et al. (2015) couldn’t explain this line of boulders as resulting from erosion of the Aswan cliff. This was because it seemed unlikely that these large boulders could travel over 100 metres from the cliff without breaking up. Link to the paper:
The full title of the paper is at the bottom of this post with the link repeated.
There’s also a curved feature in the header, marked in mauve, along with its seating. It’s set into the cliff wall at its base. It’s fairly rectangular as well as being curved. It resembles a cave entrance or fireplace though in reality it probably has little depth into the cliff face. It will be referred to as ‘the fireplace’ at times in this post and in future. This blog is replete with “features” of all shapes and sizes but unless they have a pithy, unofficial name, it’s difficult to remind the reader 40 parts into the future.
It’s difficult to tell how recessed the fireplace is but it certainly has some depth into the cliff. It’s rather dingy in that concave part of the cliff base, being in shadow, but this is one of the best photos for seeing into this recess. We can see that the curved base of the fireplace matches to the curve of its seating.
Yellow- the crack, directly above the fireplace.
The fireplace is sitting directly below a 80-metre crack that runs along the top of the Aswan terrace cliff. The crack is within the terrace dust and notionally parallel to the cliff edge, 10 metres from it and arced to the cliff edge at either end. It was noted in Vincent et al. (2015) and it was hypothesised that the 10-metre strip might crumble away as part of ongoing cliff erosion. It may well do so but its location directly above the fireplace, and being the same length, suggests a structural weakness running up the cliff that’s an extension of the fireplace structure. This, coupled with the stresses of a 200-metre slide across the lower terrace would very likely give rise to the 80-metre crack. I therefore hypothesise that this crack is yet another artefact of the stretching process, is a one-off structural failing and is not part of an ongoing erosion process brought about by sublimation.
It’s acknowledged here, as always, that sublimation is happening and leading to erosion to some very small extent. It isn’t responsible for the gaping chasms across the 67P landscape though. They were caused by delamination, rifting and sliding.
Red- the slide track of the fireplace from seating to its present day position. The track shunts left (as viewed when looking directly towards the cliff face).
Yellow- the 80-metre crack sitting directly above the fireplace.
Bright green- As in the header (note the very end of the cliff base line peeping out of the shadow). The head lobe rim obscures part of the original seating line but it’s visible up to the central ‘nose’.
You may notice that the mauve seating is closer to the central ‘nose’ of the cliff seating as compared with the actual fireplace and cliff nose that are further apart. This is explicable via tracing the path of the fireplace, which really means tracing the path of the section of cliff that contains the fireplace as it executed its 200-metre slide. There’s a track line joining the right hand end of the fireplace to the right hand end of its seating (from the viewpoint in the header, facing the cliff). These track lines have been documented elsewhere on this blog especially for the ‘orange slide’ at Imhotep (Parts 44 and 45). This particular track line takes a dive to the left at a certain point.
The sideways shunt is in keeping with the long-axis stretch vector which is evidenced elsewhere along the Seth/Hapi rim (see next Part). The long-axis stretch has been documented in Parts 38-41 and various parts thereafter.
The stretching of the cliff itself might seem far-fetched but it will be seen in the next post that components nearby did indeed stretch along the long axis instead of undergoing the usual delaminating and rifting in response to the tensile forces of spin-up.
THE PART 49 PRECEDENT FOR THE ASWAN SLIDE
Photo 4- Part 49’s 200m rift photo: (a) as originally shown without the Aswan slide (b) with the Aswan slide and slide vector arrows for the slide and rift (c) Unannotated original.
ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/JB VINCENT ET AL. (2015)/A.COOPER
Part 49 presented the evidence for the 1.6 km x 200m rift across Seth and Ash. The 200m slide of the entire Aswan terrace is really just the large section that had to slide in order to open up that rift. That’s why both the rift and the slide are about the same distance of 200 metres. The Aswan portion is a bit less in photo 4- perhaps it overhung the seating. More likely, it dragged the supposedly stationary layer below it just a tad. We’ll eventually come to see that this lower layer did indeed slide as well and has its own seating.
So the Aswan slide is very closely related to the 1.6km x 200m rift. The additional information in this post is:
1) the areal extent of the slid layer on one side of the rift. That area is the main Aswan terrace. The sliding of the main Aswan terrace automatically implicates the wide, terraced cliff above the main terrace as sliding along with it.
2) the layer on which the Aswan terrace slid which is the small, lower terrace with the bright green lines on it.
3) the thickness of the layer that slid as implied by (1) and (2) i.e. the thickness of the main Aswan terrace.
4) the fireplace identification and the fact that it may inform the evolution of the crack directly above it.
5) the jerk to the left of the fireplace slide, implying a stretching of the cliff face and, by extension, the actual terrace area (long-axis stretch in keeping with the Babi/Hapi delaminations in Parts 38 and 39). This is further evidenced by the fact that the bright green boulder line is offset slightly from the straight-line translational symmetry between the cliff base line and seating line in photo 4 and the previous photos. This implies a shunt to the right of the central ‘nose’ from its seating while the fireplace shunted to the left.
6) the deposition of the boulder line mirrors the cliff base line, which would be due, perhaps, to a stop-start sliding history. This implies a possible ‘pumped’ sliding and therefore intermittent spin-up to the rotation period necessary for shear resistance failure.
THE J.B. VINCENT PAPER
Are fractured cliffs the source of cometary dust jets ? insights from OSIRIS/Rosetta at 67P
JB Vincent et al. (2015)
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/JB VINCENT ET AL. (2015)/A.COOPER