MISSING SLAB D- IMHOTEP
Dark blue- rotation plane.
Bright green- direction in which Imhotep is rotating, towards right of frame.
Orange- cliff face or ‘hinge’.
Red- perimeter of cleaved stratum layer.
Mauve (on shape model photo)- x axis. This axis sweeps along the rotation plane and is at 90 degrees to the z axis ‘axle’.
Light blue- point at which x axis pierces body, proving that the dark blue dots are on the rotation plane.
Part 12 was concerned with the missing slab from the top of the head, slab C. This slab comprised the entire region of Hatmehit. The exposed surface left behind was understandably very different from its undisturbed surroundings. Seeing as the regions on 67P are categorised primarily according to type of terrain, it’s no surprise that the perimeter of the missing Hatmehit slab and the perimeter of the Hatmehit region are contiguous. The same reasoning applies to missing slab D, Imhotep.
It was proposed that the Hatmehit slab was lost via rotational forces alone because it didn’t have a helping hand from the uplifting head lobe as did missing slabs A and B. It was mentioned that Imhotep was lost via the same process and that it would be the subject of this post. It should be easier to explain the mechanism of the loss of Imhotep because it is essentially the same as that of Hatmehit. I’ll go into less detail here. The Part 12 subheading, “The Mechanism of Hatmehit’s Departure” has more information. As usual, the missing slab is presented here as fact to spare the reader a multitude of conditionals and qualifications.
MECHANISM OF IMHOTEP’S DEPARTURE
Imhotep exhibits all the same traits as the other three missing slabs described so far in Parts 9 and 12: a brittle break at one end with accompanying debris, at or near its base; a smooth, flat plain in the middle; and consolidated material resembling cleaved rock at the other end.
It is proposed that under spin-up of the comet via asymmetrical outgassing, the slab attempted to slide across its fracture plane in the direction of rotation before lifting off via centrifugal force. This is because it was near to but not on the rotational axis extremity. It then encountered resilient material at its forward end (as regards rotation direction of the comet). That material was the cliff edges we see today around one end of Imhotep’s perimeter. This resistance led to greater tensile, ‘lifting’ stress at the opposite end and across the middle of the slab. This caused the trailing end of the slab to cleave cleanly and vertically from its substratum layer, leaving that relatively flat, clean layer visible today.
As the slab lifted like an opening trap door, it hinged against the resilient cliffs the other end, grinding and tearing against them before reaching the vertical point above the hinge. Then it departed, flipping end-over-end. It may have fractured in the middle as it hinged open, being one of the largest of the missing slabs. But once it had broken away cleanly from its substratum, it had to have risen only a metre to be lost forever and its disintegration wouldn’t have affected the characteristic ‘missing slab’ signature it left behind.
However, as with the Hatmehit slab, Imhotep has to satisfy a much stricter criterion than the simple observation of the so-called missing slab signature. Those features, the cliff, the flat plain and the cleaved strata could be aligned in any direction across the base of the comet. For them to qualify as a true signature of a slab that’s missing due to rotation forces alone, they have to align along the rotation plane. Moreover, they can’t align in either of the two directions along that plane, it has to be the cliff in the forward position and the cleaved strata taking up the rear. This is because the trap door could open only in one direction due due to rotation and that direction is with the hinge and therefore the cliff in the forward position. As you can see from the annotated photos, Imhotep satisfies those constraints but with one anomaly: the cliff extends further round to the top of the frame than at the bottom. This isn’t quite as neatly symmetrical as Hatmehit. However, there are two very solid-looking crater rims in the Ash region where this extension lies. It would be reasonable to suggest that the slab would tear against these as it tried to lift up cleanly from its base.
Apart from this anomaly, the cliff and the cleaved strata form arcs in the expected positions or perhaps five to ten degrees off the line of the rotation plane. Interestingly, the Hatmehit slab signature is also skewed anticlockwise by the same amount. One wouldn’t expect both signatures to be exactly aligned with the rotation plane, down to the last degree, due to structural anomalies affecting the fracture lines along the cliff and cleaved plane. Yet both signatures appear to be almost exactly aligned with each other through the x axis of the comet.
-The cleaved fracture plane probably extends under the dust layer in the middle of Imhotep.
-It would be reasonable to argue that if the slab was lost due to rotational forces alone then all the detritus should have been flung off too, leaving a scrubbed surface. That objection is dealt with at the bottom of Part 12.
-Thomas et al (January 2015; link below) mentions “mass wasting” on several occasions when alluding to the cliff features. In reference to one accompanying photo (figure 7) it is proposed that a stratum of several metres’ depth is missing. On inspection of the photo and its 50-metre scale bar, this depth appears to be more like several tens of metres. Furthermore, the paper mentions the expulsion of very large volumes of material without much or any residual talus. This is evidently a puzzling observation because the material has simply vanished from the comet including, presumably, the non volatile component. A tentative proposal is made that gas pressure build-up might expel much larger lumps but acknowledges that the porosity of the comet may well militate against such a scenario. Seeing as the proposed missing slabs are by definition the thickness of the 30-50 metre cliffs they left behind, this neatly solves the paradox of the vanished material.
Thomas et al (23rd January 2015)
Research Article The morphological diversity of comet 67P/Churyumov-Gerasimenko
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0