Evidence of Extensive Outgassing Along The Shear Line
These two photos are mirror images of each other. The top photo is a view of the underside of the head lobe of the comet. The bottom photo is the view of the body lobe. The rectangle defined by the four green dots on the head used to sit on the same rectangle defined by green dots on the body. All the other dots and dotted lines mirror each other as well. These photos will be used to show evidence of extensive outgassing along the shear line between head and body before the head was significantly displaced above the body.
Key to coloured dots:
Green- denote corners of the rectangle
Red- this line traces the perimeter of the finger of protruding rock on the head photo. This finger fits the horseshoe-shaped intrusion into the rectangle on the body.
Light orange- this is the second stratum in the finger which fits the second step-down in the horseshoe. This is more evident in the second pair of photos, below. It protrudes further out from the neck corresponding to a deeper stratum layer in the body.
Purple- suggested pathway for high-pressure sublimating gas and slurry. Where it fans into four paths there is some evidence of conduits. Where it is depicted as a single track it is a conjectured pathway based on the apparent scouring around this side of the finger where it sat on the body.
Light blue- apparent scouring out of detritus which contributed to the semicircular slurry piles (pile perimeters are delineated with light yellow dots). This detritus also contributed to the slurry ridge along the back of the rectangle on the body (overlain by the wavy line between the two green dots).
Light yellow- this denotes the perimeter of the two slurry piles. These appear to have been formed mainly by slurry emerging from just under the rectangle and not so much from the purple slurry paths depicted on the top surface of the rectangle. These two piles have pushed up the two ‘gull wings’ on the body (see Part 5). The slurry paths that are depicted by the purple dots may also have contributed a little to these piles but the top two fans seem to have fed a wider, flatter continuation of the slurry ridge along the back of the rectangle. The existence of these piles show that at least two fracture planes were oozing gas and slurry: one above and one below the rectangle surface we see today. The gull wings on the body match those on the head (Part 5) and there appears to be a third layer of wings on the head when viewed from above. All three were layered like puff pastry before rising and separating. The top layer exhibits a clover-leaf shape at its corner that is mirrored on the body in some less exposed photos (see last photo in this post) but is also just visible in this and the next body photo. It’s beyond the slurry ridge, next to the green dot and purple fan.
Here are two more photos from different viewpoints. They depict the same features as above and use the same colour coding. In the body photo, the finger’s former position is depicted floating in ‘thin air’ to the right of the horseshoe. This is where material has since been eroded away. The dark blue dot represents the drop from the the last elevated red dot to the actual neck.
Explanation of Extensive Outgassing Along the Shear Line
At the end of Part 5, I noted the evidence regarding what appears to have been massive outgassing and slurry flows under the rim of the head lobe of the comet before it detached from the body lobe. This consists primarily the two slurry piles sitting squarely under the two pushed-up ‘gull wings’ on the base, suggesting that they were pushed up 20 metres or more by the emerging slurry and gas. And it also includes the frill along the edge of the cliff top on the head, which used to sit neatly onto what appears to be an elongated slurry pile on the body. This slurry pile, a low, bulbous ridge, runs along the back edge of the rectangle and defines the shear line where the current cliff edge used to sit. It also contains the wavy line that matches its mirror image on the underside of the cliff edge (Part 1). There is evidence of scouring just before the exit points on the body, along the wavy line, although the resolution in the Rosetta NAVCAM images means it is hard to make these out for sure. If they are there, it chimes with the notion of very high-pressure outgassing eroding the fracture plane directly under the head lobe when it was still sitting on the body. This detritus would presumably be in a slurry-like state and be deposited at the outlet apertures along the back of the rectangle. By that process, an elongated slurry pile would be formed, pushing up the frill before the head was lifted off its base.
To be clear, at no point am I suggesting that such explosive outgassing actually caused the head to split from the body. The only energy input large enough to achieve that and the subsequent 1km separation would be the differential g forces on a close approach with Jupiter (a Roche pass at around 120,000 km altitude) or the stretching force from a spin-up to 90-120 minutes per rotation of the comet. The latter would probably need to rely on asymmetrical outgassing over hundreds of years or more but that type of slow, continuous impetus bears no relation to this short-lived, high-pressure event along the shear line. The only ‘uplift’ I’m entertaining is the bending up of thin layers around the rim of the head, i.e. the gull wings and also the edge of the cliff as it is now but at a time when it sat firmly on its base. The only other way in which this outgassing may have aided uplift was in that it seems to have scoured out parts of the fracture plane that the head was sitting on, leaving fewer contact points and less adherence.
I had mentioned in Part 5 that I thought the uplift of the gull wings could have happened over time, prior to uplift, but I think this was premature. I now believe it’s possible that the initial lifting of the head due to the Roche pass or spin-up would have been in the form of a sudden faliure of the brittle, sintered casing of the comet. Prior to that faliure, the stretching forces would initially be resisted via the tensile strength of the comet including the casing and no stretching movement would be apparent, despite the stress. But once the casing failed along the shear line there would be an instantaneous transfer of tensile stress to the central area (the incipient neck between the lobes). Stretching would commence instantaneously as well but immediately self correct to a much slower rate when the forces rebalanced. This sudden initial stretch would have caused massive instantaneous heating in the interior of the comet due to longitudinal shearing and result in a one-off outpouring of explosive, sublimating gases. Perhaps this can be further explained by including an excerpt from a comment I made to Marco on the subject (comments, Part 5).
“…we have made it clear in several comments that any sublimation, whether a hypothetical explosive excess or simple background, is not the cause but an effect of the split. In other words, any excess or even explosive generation of gases over the background level would be as a result of the heat (and internal pressure drop) generated by pulling the lobes apart. The only mechanisms suggested for that pulling force are spin-up or Roche pass delta g forces. As for the background levels, they may have served to weaken the the fracture plane between the strata leaving them looser prior to detachment but not lifted them and certainly not propelled them 1 km apart.
“Where I think I did make a mistake is that in my initial observation of explosive outgassing around the rectangle/scallop area (end of Part 5) I was quick to cite it as a more exaggerated version of this fracture-weakening process and assumed that it was *prior* to separation via spin-up or Roche forces, not due to it. That’s because I couldn’t deny what I was seeing in the photos and made the knee-jerk assumption that it was simply a very enhanced spike over and above background levels. (This still wouldn’t mean the gases were pushing the head up from the body, just the gull wings and frills around the edges).
“I then corrected that idea of it being prior to stretching in a comment to Robin Sherman on the 14th December Rosetta blog post which also quoted the relevant section from Part 5. That comment said that I now realised it didn’t necessarily have to be a spike in normal activity, prior to the stretch. On the contrary, the vast amounts of energy that intitiated the stretch could be the very energy source required to fuel such a spectacular outgassing.
“In short, I hadn’t stitched together all the various ideas from the previous few days. I’d already made comments elsewhere on the Rosetta blog regarding what would be a sudden transfer of tensile stress to the interior, (the incipient neck), causing heat, possible ‘cryovolcanism’ and reduced tensile resistance. I just hadn’t gone the extra step in realising that this should cause excess sublimation with the gases streaming out along a fracture plane crevice that’s over 500 metres from source to outlet and possibly only a few cm wide (from head to body) on its initial fracturing. That scenario would be a one-off scenario on intial fracture and subside as soon as the crevice opened to a width that allowed reduced upstream pressures. That in turn would mean reduced outlet pressures resulting in no more gull-wing uplift and frilly edges. Those features would already have risen with the head lobe to a point where gasses could stream through a wide aperture underneath them. That head uplift would be entirely due to spin-up or Roche forces and not be due to the gasses.
“This is all conjecture, of course, but what I see in the form of the uplifted gull wings, slurry piles, slurry ridges and frilly cliff rims that sat over them speaks of outgassing on a massive scale, whatever the cause. And I think my dimension estimate of a 20-metre gull wing uplift may be low. It was a conservative guess- it could be 40 metres, the height of a small office block.”
[End of comment quote]
The evidence of this massive outgassing is there on both lobes. On the body lobe, sitting up against the seagull profile under its left wing, is the obvious semicircular structure which I mentioned above. In the paired photo of the head and body gull wings (the tiltle photo in Part 5) it has the same cross-sectional profile as the wing. In the photos above, it is roughly semicircular and the same width as the wing. Both these observations suggest it extends underneath that wing. The concentric semicircles within it resemble oozing ‘mud’ that slumped like thick custard after oozing between the layers and being ejected at the end of the rectangle. When it found an exit, it pushed up the edge as it emerged. The other wing next to it has a similar but less defined slurry pile structure.
As for the fluted portion on the underside of the head lobe, there are three distinct channels and a possible fourth that add weight to this suspicion. These channels follow the same path as the ‘mud’ would have followed under the scallop on the base to form the semicircular slurry piles. The channels are narrow but flare out as they reach the two wing apexes- evidence that the escaping gases were under pressure and pushed the wings up. The channels look as though they may have been fed from a conduit on the right hand side of the finger as seen on the underside of the head or the left side on the body where there appears to be scouring around the left hand edge of the horseshoe.
All these head channel features are roughly mirrored in their seating position in the scallop (the triangular dip) at the end of the rectangle on the body: there is the apparent ablation of material to the left-hand and upper perimeter of the horseshoe followed by striations that lead to what would have been the underside of one of the head lobe gull wings. These gases would have been forcing their way through the next stratum fracture line, above the scallop.
This suggests that gases and their accompanying detritus were forcing their way through the very two layers that have now separated a kilometre apart. And in turn, that suggests that the shear line had already torn: the incipient neck was now stretching, colossal heat was being generated and gases were streaming through the the obvious escape route- the newly opened shear line. Hence the gull wings and the frill.
The following photo is difficult to follow because it’s from a different angle and in high relief due to shadowing. But it’s detailed and shows possible evidence of the actual tear.
The rectangle on the body is shown with the usual green dots. Its complementary rectangle on the head is out of sight but its outside edge runs roughly along the head perimeter, downwards from the four light yellow dots. The horseshoe, along with its second tier (red and light orange) is shown emerging from behind the head in the foreground. The shear line is depicted with slightly larger terracotta dots all the way down to the bottom of the photo. You can see what looks like evidence of tearing along the ridge just to the left of those dots. The top dot, just below the horseshoe isn’t strictly speaking on the shear line or tear line because we know the head lobe extended across the rectangle to the right of that dot. However the obvious tear that carries on past that dot to the horseshoe suggests that this was where the really catastrophic shearing took place and from where gases and slurry streamed. The gases would then have streamed under the rectangle, pushing up its edges just before it was taken up with the head lobe.
The light yellow dots are not slurry piles this time. There are four on the head and three on the body. They show the clover leaf shape, mentioned above, that is matched from head to body. It looks less like a clover leaf here than in the other photo above but it’s the same feature and matches an almost identical shape on the body. In fact, the shape match is even more faithful than I have outlined with the dots. The bottom dot of the four dots on the head should be lower so as to incorporate the line below it that runs right-left then angles up and left. The bottom body dot incorporates this line. The result is three lines, identically matched from head to body.
To be clear, this is not some random match of two parts of a photo that appear rather similar. On the contrary, all previous matches dictate that the only place for that part of the head to have sat is in the very place on the body where the clover leaf pattern is duplicated- at that corner of the rectangle. This isn’t a mirror image because we are looking down on both head and body. The pattern must have bled through these thin layers to cause a bloom on the surface. That surface layer has now risen a kilometre along with the head and is part of the overhang at the edge of the cliff.
Was This Where The Head Initially Ruptured?
Seeing as the rectangle is the only place along the shear line where such evidence of large scale outgassing is apparent, it may be the place where the tear started. It would then have progressed around the rim of the head in both directions from that point. This is also in keeping with the fact that this small area of activity is under the most tipped-up portion of the head. Again, that is not to say it gave any greater impetus to the separation force. It is just a sign that this was where the most tensile strain was in the first place so there was more lifting force at the point of rupture and more time to rise under that force.
Furthermore, it is to be expected that this point would sustain the most tensile strain whether through spin-up or a Roche pass. In the case of spin-up, the forward placement of the future head lobe on the body would mean a force vector pulling the head forward as well as up as it spun ever faster. That’s the same as saying the head was straining to detach at the back. This area of outgassing is roughly at that point.
With a Roche pass, the delta g forces are similarly aligned with the rotation so the same tipping vector would operate on the head with the same tipping strain at the back. However, this assumes a fairly slow pre-pass rotation that gets overwhelmed and realigned by the Roche pass delta g vectors. The present day ~10 degree anticlockwise twist in the head may be an artefact of its pre-pass rotation axis alignment.
One last piece of evidence for this catastrophic outgassing theory is that there is a ring of vent holes in the head lobe that are near to the fluted section and would’ve been near to the rectangle too when the head was seated on the body. There isn’t such a closely bunched circle of well defined holes anywhere else on the comet. This was clearly a little hive of high activity and pressurised gas escape at some point in the past.
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0