UPDATE: Marco Parigi summarised this post. I did a much more detailed version of the above photo for him. Link:
The two photos in the header are side-on profiles of two specific points on the head and body of comet 67P/C-G and used to be sandwiched together. Their end points are marked with red dots and they are in the order of a hundred metres long. They have virtually identical profiles and exhibit matching fine detail along their length such as small lumps and similar angled turns at similar points. The most striking of these details is perhaps the white triangular feature on the left tip of both profiles, directly to the left of each red dot. On the body photo it very roughly resembles the corner of a napkin folding round into view, with its tip torn slightly. In the head photo it is integral to the upper surface and comprises the triangular corner tip. Its profile is the same in both pictures despite the two sections being over a kilometre apart: the extreme tip curves round to the right in both photos and there is a small, kinked-down protrusion (just above the red dot on the head lobe and right under the red dot on the body lobe- due to my clumsy placement of the dots).
The small extent to which the profiles do differ is mostly down to a slight foreshortening of the head lobe photo and its partially shaded portion. This shaded portion is shown to curve, as expected, in other photos and if you zoom right into this photo you can just about see the shaded portion anyway.
The process of finding and meticulously checking this match is detailed below.
To recap, this is the fifth part in a series on matching features between the head and body lobes of 67P/C-G. It confirms the match already made in Part 1. That match was made in ‘plan’ view i.e. looking down from above the head. This analysis is going to look at it sideways-on to see if those two sections from Part 1 nest together with the same ‘vertical’ profile. If the two sections were never sandwiched together in the past and had only appeared to match in plan view, then the chances of such a vertical match would be essentially zero.
The match made in Part 1 was the wavy line that ran along the underside edge of the cliff on the head lobe, which matched the wavy line along the long edge of a rectangular feature on the body. Reading Part 1 before continuing would probably help but this post reconfirms those plan-view matches in the process of finding the end-on matches so it’s probably not essential except maybe a quick glance at the intro and the first two photos. Incidentally, I’ve noticed that it isn’t referred to as a rectangle in Part 1 because the lighting happened to make it appear as two triangles, tip-to-tip. But in most cases the lighting does show it as a rectangle, albeit with a scalloped triangle integrated at one end. So it will be referred to as a rectangle for this post.
I reasoned that if I could find two side-view images, one of the rectangle, end-on, and one of the portion of the head that’s supposed to sit on it, they should show an end-on match in their vertical relief. By ‘vertical’, I mean their profile at right angles to the ‘horizontal’ or plan view from above the head.
So I looked back through the Rosetta mission photos. I was looking specifically for an end-on profile of the scalloped triangle that sits at the end of the rectangle as well as an end-on profile of the corresponding rectangle shape on the head above. If that should show itself to be similarly fluted so that it nests perfectly inside the scallop we would have a solid match.
So I was restricting myself to two specific points on the comet, which under random conditions would have no chance at all of a match in this third dimension. I simply suspected they might, due to their very close match when viewed from above.
My first find was the photo from the Rosetta blog post, “Cometwatch 9th December”:
I didn’t use this photo for the paired image at the top of this post but it was highly instructive in terms of characterising the shapes I would go on to look for in other photos. It also confirms the plan view match using a different photo. It shows the rectangle not exactly end-on but in about 45 degree profile so it was enough to discern the rough shape of its end-on profile. The rectangle is at the left hand edge of the photo. It’s marked by four green dots (zoom right in!), the nearer pair adjoining red dots. The green line isn’t significant except as a location guide for the dots. Between the green dots along the long back edge, you can see the wavy line that matches the underside of the head. This was annotated in a different body photo in Part 1. It’s somewhat whited out here but most of the line is still visible.
The ‘rectangle’ looks less like a rectangle in this picture because the ‘horseshoe’ mentioned in Part 1 takes a big chunk out of the near side. The rectangle incorporates the scalloped triangle that makes up one end, the end nearer the viewer. The end-on profile of this triangle is therefore the end nearest to the viewer as well. It’s marked by red dots at either side in the same manner as the paired photos at the top of this post. It casts a shadow so the end-on profile is in fact the top of the shadow. It shows a distinctive dip with two humps either side and if you trace the exact edge, it would resemble the profile of a child’s schematic drawing of a seagull, with the right hand wing (from our viewpoint) arcing over slightly more prominently than the left.
Then I analysed the portion of the head lobe that supposedly sits in the scalloped section of the rectangle. This is exactly edge-on in the photo so it’s showing its end-on profile. It’s the barely-lit, spidery strip that appears to hang down from the top-left tip of the head. It’s below the green dots which demarcate the cliff perimeter that corresponds to the green dots on the rectangle.
Incidentally, this means that the frilly edge of the cliff would have to have a low, bullnose ridge at the back of the rectangle to curve over. That’s exactly what is there so we already have a vertical profile match on our way to the main one.
Again, red dots mark each tip of the spidery end section. It’s in a triangular shadow which is cast by what is in fact a strange-looking overhang that’s been remarked on before in the Rosetta blog posts. The spidery strip is the actual section that would have sat on the scalloped triangle. You can see it is v-shaped and extends upwards and over in a wing shape so that’s promising. It extends downwards in a wing shape too. You can’t see the downwards section in this photo due to shading so I searched for another. I found one with the whole fluted/seagull wing profile, similarly end-on, from the Rosetta blog post “Cometwatch – focus on the neck” on 8th October 2014. This is the photo I used for the head lobe in the paired photo. The blog post is linked below and the hi-res photo link is below that (needs rotating 180 degrees):
Before continuing with the analysis of this photo, it would be useful to include a link to a photo of the underside of the head so you can see the fluted v-shape from below.
The annotated version of this underside photo clearly shows a fluted section even when at 90 degrees to the end-on view. It’s the photo used in Part 1 for the plan-view matching of this section. The fluted part is the section at the right hand end of the wavy line between two pink dots and casts a shadow over the deep, coved section of the head lobe. Incidentally, some other matches while we’re on this photo:
1) As mentioned above, the wavy line matches the line on the farthest long edge of the rectangle on the body between the green dots (in the Cometwatch 9th December image, above). This underside of the head image was marked with 11 different-colored dots that matched the line in another photo in Part 1. I’ve now added more dots below the wavy line to illustrate points 3 and 4, below.
2) that wavy line runs along the underside of the frill seen in the other photo above. The entire edge is kinked upwards although it’s difficult to see in this photo. It foreshortens the curves in the wavy line- they are deeper than they look. As mentioned above, the profile appears to fit the bullnose edge of the rectangle.
3) there is the finger of protruding rock that fits into to the horseshoe hollow carved out of the rectangle. This finger is below the wavy line and demarcated with red dots. In another photo (see Part 1) this finger has two strata for the two stepped strata in the horseshoe.
4) and lastly, there are the clear-cut nested features that fit to the curved steps mentioned in Part 1. These are marked here with faint yellow dots around their perimeter and with terracotta dots at their centres. These terracotta dots correspond to their counterparts in the top-down view in Part 1. There are erosion factors regarding this match- the body lobe steps have eroded to their current depth and the flatter nested features on the head must have married with the steps when they were less eroded.
So there is a wealth of plan-view matching points just in this one small stretch. But we must now return to the side-on or vertical profile of this fluted section and the scallop it’s supposed to sit in.
As for the 8th December ‘Focus on the Neck’ photo, the annotated version above shows the head lobe to the right, a small gap and then a very small portion of the body on the left. The head portion shows the deep cove referred to above and the underside of the head (the cliff) in shadow. The body shows part of the scallop but not its end-on profile which is in shadow.
At the far end of the cove in the head lobe, just under the cliff edge and in shadow, you can see the same distinctive ‘v’ shape which is turned 90 degrees on its side. It’s dotted at each end and is next to its indicating red line. Only its front edge is catching the sun. That front edge would have sat in the scalloped triangle below. If you zoom in, you can see that the v shape is only part of that front edge and it extends in a curve at both ends- at one end it goes to the very edge of the cliff and curls over as part of the frill- the very tip corresponds to the ‘napkin’ on the scallop below. At the the other end, it goes through a short stretch of shadow and kinks back up a tiny bit at the tip. Peering carefully into the shadowed portion, you can still trace the curve. So the entire end-on profile of this head section also resembles a child’s schematic drawing of a seagull.
I still hadn’t found a really good end-on profile of the scalloped triangle on the body where the fIuted section once sat. I finally found what I was looking for in the following photo from ‘Cometwatch- 26km on 26th September’, linked here with the hi-res photo linked below that (needs rotating 180 degrees):
In the annotated version, the seagull shape is below its indicating red line and dotted at each tip in the usual manner.
I identified this as the end-on view of the scalloped triangle by relating the features around it to the top-down view in other photos. I was very careful to match the successive features along the base of the shear line and across the ridge in the foreground with their corresponding features as viewed from above:
1) The end-on view of the scallop is sitting just above and to the left of two small mini horseshoe shapes that sit between the main horseshoe protrusion into the rectangle and the viewer’s position.
2) It’s also in keeping with the even height of the rim of the small crater just in front of it and the extra sloping portion of residual (or second tier) rim just beyond it.
3) There’s a small rock just below and to the right of the seagull profile that matches the top-down image.
4) Finally, and most convincingly of all, you can just about see several tight strata lines tucked below the left wing (from our viewpoint) of the gull. These are the concentric circular lines that can be seen in the 9th December photo above.
So after some searching around, the two photos I used for the paired image at the top of this post were the following:
Head photo: Cometwatch 8/10/14 Focus on the Neck.
Body photo: Cometwatch 26km on 26th September (posted 2/10/14).
Of course, I used the hi-res quadrant photos for maximum clarity. I rotated the head photo of the gull shape round to the same angle as the body photo so that it was easier to see the matching end-on profiles. I also zoomed in and cropped both photos.
One might object to the fact that the head photo was rotated and in reality would have to tip over about 45 degrees before being seated down onto the rectangle. However this tipping up could easily happen as the head rose up from the body. It would be quite acceptable with the separate centres of gravity of head and body readjusting their relative orientation on drifting apart while still under the influence of each other’s delta g gravity fields. As for a Roche close approach with Jupiter (see below), this tipping back force on the head after rotating through bottom-dead-centre is a prerequisite.
The tip-up of the head is probably nearer to 30 degrees, the rest being the usual photographic illusions. It looks like 30 degrees or so from the side views of the comet. Part of the illusion is because the head is also rotated anticlockwise about its ‘vertical’ axis from above. It’s about 5-10 degrees as is consistently corroborated by many of the matches to date.
I think that this matching of the fluted head section to the scalloped triangle and the wealth of other matches cited above are irrefutable evidence that the head broke away from the body. That means the comet can’t be a contact binary or highly eroded single body. The two lobes were once joined as one single body and parted at some point, extruding the neck between them.
The only two scenarios that I can think of for such an extrusion are a close approach to Jupiter under the Roche limit and right through it’s inner ring system (115,000-135,000 km above jovicentre) or spin-up to a 90-120 minute rotation period via asymmetrical outgassing- and then a spin-down to its current 12.4-hour rotation.
H/T to commenter Marco Parigi on the Rosetta blog for the spin-up theory. These two scenarios are dealt with in a little more detail in Part 1.
[That concludes the 3D side-on match but the following is a bonus that arose from analysing all these photos] :
And finally, although this is shoved in at the bottom it really deserves a whole new post. There is evidence that the reason the fluted section broke away from the scallop at this particular stratum is that the entire rectangle was fracked, leaving it completely detached.
The two seagull profiles on the head lobe and the body lobe show evidence of escaping gases and their accompanying detritus being forced through the very same two separated strata that are now a kilometre apart. In fact, it’s the escape of these gases that probably caused the two uplifted wings in the first place. The evidence is there on both lobes. On the body lobe, sitting up against the seagull profile under its left wing is the very obvious circular structure with concentic circles, which I mentioned above. In the paired photo it has the same cross-sectional profile as the wing. In the 9th December photo it is roughly semicircular and the same width as the wing. Both these observations suggest it extends underneath that wing. The concentric circles 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 visible structure.
As for the fluted portion on the underside of the head lobe, there are two distinct channels 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 feature. 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 reservoir just below the light blue dot and to the right of the ‘finger’ in the head underside photo.
All these channel and reservoir features are roughly mirrored in their seating position in the scallop below: ablation of material to the left hand and upper perimeter of the horseshoe, leading to dips where the reservoir should be and then 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 apart the very two layers that have now separated a kilometre apart.
This would not have caused the dramatic escape of the head lobe but it would have left a large portion of its underside sitting detached and ready to escape. If this process had occurred in other areas such as Site A, it would explain the fracture line being where it is.
In addition, the frill itself is evidence of the escape of gases along a line of several hundred metres at the back of the rectangle. It is in effect a linear version of the pushed-up gull wings. This indeed suggests that the entire rectangle was fracked resulting in zero tensile resistance to any future uplifting force.
One other piece of evidence for this fracking theory is that there is a ring of holes in the head that are near to the fluted section and would’ve been near to the scallop too. This was clearly a little hive of sublimation activity and pressurised gas escape at some point in the past.
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0