MISSING SLAB C- HATMEHIT
On the day that Part 11 was posted, 23rd January 2015, a slew of papers were published on 67P in Science Magazine. They were authored by the scientists involved in the Rosetta mission. Two of these papers were concerned with the general morphology of the comet, both its surface features and its bi-lobed shape. Many, if not most of the puzzling features they recorded are explained by stretch theory and especially by the missing slabs that were torn from the areas they are observing. Their bemusement at what they were seeing was summed up by one of the authors:
“You’ve got to produce all of these diverse morphologies on the surface with that one energy source [the sun]—I find this tough.”
Links to the two papers are at the bottom of this post (full text available free).
Part 9 of this series, “The Missing Slabs”, would be the place to look for the bulk of the explanation for the strange features the scientists are witnessing. This is because much of the surprisingly varied terrain was due to the brittle fracturing along the the ‘arcs’ of the slabs (see header photo in Part 9).
TWO TYPES OF MISSING SLAB
However, there are two species of missing slab, the ones uplifted by the head as it rose from the body (described in Part 9) and another type which will now be described in this post. These are slabs which are missing from the top of the head and the base of the body. They would have departed under the influence of rotational forces alone. Here are annotated photos of the missing slab on the head. It comprises the entire Hatmehit region, no more, no less, which is telling in itself.
Key: orange dots- brittle fracture; red dots- flat, cleaved uplifted perimeter; blue dots- rotation plane.
The slab that’s missing from the base is the entire Imhotep region. That will be left until the next post, Part 13. Both slabs exposed exotically varied terrain as you might expect and prompted perhaps a third of the comments in the two Science papers, mostly regarding brittle fractures, scree debris, smooth plains and large amounts of “undercut” material and “mass wasting”. They also identified areas where huge volumes were now apparently missing but attributed it to outgassing and collapse on such a massive scale that it seemed to represent a paradigm shift in the theories on cometary sublimation. If there were any doubt about this, an accompanying piece in Science stated that the scientists involved say:
“the complexity of the comet today suggests that the comet-forming regions of the early solar system were more turbulent and chemically diverse than theorists have thought.”
Yet it would seem prudent to investigate all avenues before rewriting the science text books.
Being near the extremities of the long axis (y axis), the head and base slabs were shed by rotational forces alone whereas the ones in Part 9 had an initial helping hand due the the head lifting them at the shear line as it broke away and rose from the body. Those slabs would have been more sluggish in their departure or even remained, dumped to one side, were it not for that helping hand. This is because they were nearer the z axis of rotation and subject to less rotation force.
The loss of slabs from the head and base is described here as if it’s fact but of course, as stated in part 9, it is a working hypothesis albeit with much evidence going for it. The reason for this is that couching every proposition with conditionals and qualifications can become tedious. So no, it’s not proven yet.
Now that the comet’s regions have been named, it makes describing locations easier. As mentioned above, the slabs missing from head and base both comprise an entire region. They are Hatmehit (head) and Imhotep (base). They would have been of comparable size to the Part 9 slabs, and probably tens of metres thick as well. Their locations can be seen in the context of their surroundings in the ESA photo below.
MECHANISM OF HATMEHIT’S DEPARTURE
Hatmehit has all the same hallmarks of slab loss as those described in part 9: a brittle break at one end, forming a curved arc, debris at the base of the break, a smooth plain across the middle and no evidence of a brittle break on the opposite rim. In fact, quite the opposite: this end resembles clean, cleaved rock just like the Part 9 slabs do. The only difference in the case of Hatmehit is that, given the circular cross section of the head and its evenly layered strata, any brittle break in this area would likely form an arc anyway. In Part 9, the arc shapes were explained via the concentric arcs of equal force radiating from the head rim leverage points. On Hatmehit, if the location of the brittle break had to be curved anyway due to the shape of the head, it would automatically coincide quite well with the outermost leverage force arc. That would make the initial resistance along the brittle break somewhat less robust and the likelihood of slab uplift all the more plausible.
The slab on Hatmehit was cleaved away from the very defined fracture plane that now constitutes the base of the crater it left behind. This crater is dust-filled so you can’t see that plane from above but you can see it from the side in the photo below. It is defined by the uppermost pair of orange dots. There are three more pairs of dots delineating the ends of successive strata layers below the one that now forms the bottom of the crater. Half the rim of the crater remained after its slab departed. You can see it arcing round the back, constituting the very tip of the comet as seen in the photo. At the front of the crater, there is barely any rim left at all although in some high relief photos it’s apparent. That’s why the top pair of orange dots appear to be level with the base of the crater or, at best, a shallow dish sitting in front of the cliff at the back. Blue dots are the rotation plane (at right and across the top- zooming advised). The single red dot represents the entire curve of red dots in the other photos here because they would be exactly side-on in this photo. Similarly, the top left orange dot represents its curve of orange dots as well as the base of the crater. The viewpoint for this photo is from a similar angle to the OSIRIS header photo:
Orange- fracture planes
Blue- rotation plane in view across top and at right. It follows a line just behind the horizon on the right hand side of the head.
Light blue- point where y axis pierces the body from right to left.
Red: cleaved perimeter of Hatmehit slab
As the slab was lifted, it opened like a circular trap door on the top of the comet, hinging at one end. The hinged end was where the brittle fracture is seen today. The reason it hinged is because the soon to be ejected slab was not at the exact extremity of the long axis when the head was still attached to the base. So under spin-up there was a tendency for the slab to want to slide forward on its fracture plane to the end of the axis before lifting off ‘vertically’ from the end of the axis via ‘centrifugal’ force. Those two words are placed in single quotes because they aren’t quite correct terms but are nevertheless perfect proxies for what was actually happening.
However, in its attempt to slide forward, the slab encountered some resilience in the form of a solid lump of material. This lump is the very solid-looking vertical cliff face that constitutes most of the brittle fracture, although it should be noted here that this cliff curves round some way to the side as well. The part where the brittle break occured is evidenced by boulders and scree. As a result of encountering resitance to its sliding tendency, the slab experienced a greater uplifting strain on the opposite rim to the hinge just prior to uplift, which caused it to shear at that end, cleave away cleanly from the well-defined fracture plane and and start rising. All through the lift, the other end of the slab remained wedged in against the cliff as it tore and ground against it. Eventually, the trap door passed through the vertical axis above the hinge and escaped, flipping end-over-end as it departed.
The process described above will be familiar to anyone who has read Part 10. It’s the same scenario that explained the uplifting of the entire head itself from the body. The only difference is that the head didn’t escape, partly due to experiencing less centrifugal force and partly due to tensile resistance in the neck. The slab that escaped from Hatmehit represents the same process in microcosm but it concluded with the slab escaping.
So the slab hinged open on the head while at the same time the head itself hinged open, so to speak, from the body. This is because both were simultaneously under the influence of rotational spin-up. Together, they were opening up like the cover and frontispiece of a book flapping open in a strong wind, albeit with a rather chunky frontispiece.
So far, so simple, but Hatmehit has to satisfy a much stricter constraint than just a casual observation that it has a cliff at one end with some rubble at the bottom of it. If a slab is to be flung out like a trap door via rotation alone, the hinge and therefore the rotation plane of the opening lid have to be in line with the comet’s rotation plane. Moreover, the trap door couldn’t flap open in either of the two directions along that rotation plane. It has to have happened in the manner described above, which means that the hinge was at the most forward position as the comet rotated and the smooth, cleaved area was at the most rearward position. The annotated photos above show that all these conditions are satisfied.
One fly in the ointment with the Hatmehit slab is that there shouldn’t have been any scree and boulders left over. Since it was shed via rotational forces alone, everything else should have departed with it. The reason debris could remain on the slab A and B sites was that they were closer to the rotation axis and so fragments could drop back down slowly or even just slide around. The initial kick from the head uplift pulled the slabs away giving the slabs themselves the last bit of uplift they needed to escape. Rotation alone probably wouldn’t have been quite enough, especially for slab A, right on the rotation axis.
So perhaps it’s not correct to cite the debris at the bottom of the cliff as strong evidence for the brittle break. That said, one could put it down to fragments that were loosened within the cliff face and were subsequently worked loose by sublimation and temperature stresses. In fact, it would be reasonable to suppose that in the chaos of the hinging up there would be entire monoliths detached by a few centimetres and effectively loose but wedged in just a few key places by the surrounding cliff. Those would then erode more easily via sublimation and temperature stress, shedding boulders over a long
period after slab departure. So maybe the debris we see is indeed an artifact of the hinging and the brittle break. This would mean that they weren’t the actual fragments that dropped out at the time but are there as an indirect result of the hinging process and so betray its position. It’s a factor that’s worthy of debate though.
Even if the scree debris is discounted, we still have a fractured outcrop on that side of the crater, a smooth plane in the middle and cleaved, bare strata at the opposite end- all aligned the correct way round along the rotation plane. Most of the evidence is still intact.
Science Mag articles on the morphological diversity of 67P:
Sierks et al (23rd January 2015)
Research Article On the nucleus structure and activity of comet 67P/Churyumov-Gerasimenko
Thomas et al (23rd January 2015)
Research Article The morphological diversity of comet 67P/Churyumov-Gerasimenko
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO