67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 9

THE MISSING SLABS

2015/01/img_2148.jpg

Throughout the first eight parts of this series I’ve made passing references to “missing slabs” that were broken from the flanks of the body lobe of the comet and lost to deep space when the head lifted away from the body. Just like the outgassing in part 7 and the Dykes in part 8, this is a working hypothesis which needs more proof. However, as is the case with those two other posts, the existing evidence for this is favourable.

In the header photo, there are two distinct arcs on the body which curve down and up along its flank and roughly parallel to the ‘shear line’ where the rim of the head broke free from the body. One arc comprises the whole of site A the very flat crater. This is denoted by yellow dots. Let’s call that slab site A, which is to say, this is the area where the lost slab A once sat. The other is less pronounced in this photo but more pronounced in the photo below. Let’s call that slab site B (orange dots). The terracotta dots run along the shear line for all photos in this post.

Notice how slab site B’s floor is quite flat, rather like slab site A’s but without the dust covering. It’s somewhat riven but essentially flat with an appearance reminiscent of cleaved rock as if there was once a vast slab, some tens of metres thick, sitting above this stratum layer. In stark contrast to this, the appearance of the curved perimeter couldn’t be more different. It has a crystalline quality reminiscent of fractured marble. That’s not so noticeable in the header photo but obvious in others, below. Taken together, the cleaved stratum running down the flank of the body from the shear line and the crystalline, fractured arc suggest that if there indeed was a slab there, it was levered up along the shear line and broke with a brittle fracture along the arc. Rather like the process of quarrying rock on the Earth, it appears there was little adhesive resistance between the two layers along the cleavage but the lifting or snapping forces along the arc were across the stratum layers of the slab, hence the brittle fracture.

The same principle applies for slab site A. Although you can’t see a cleaved surface due to the dust layer it is very flat so it’s in keeping with the same cleavage scenario as suggested for slab site B. It also exhibits the same crystalline fracture along the arc.

Here’s an additional photo showing slab sites A and B in profile:

2015/01/img_2145.jpg

And a photo showing the crystalline nature of the break around the arc of slab site B with its key below:

2015/01/img_2150.jpg

Key

This photo is culled from Part 7 with added annotation. These are the annotations relevant to this post and are all down the right hand side of the photo:

Bright yellow: slab site A perimeter

Bright orange: slab site B perimeter

Light blue: the visible part of the crystalline appearance of the arc (but only about a third of the arc is visible here).

Notice the three orange dots in the bottom right corner showing the arc perimeter coming back into ‘view’- it’s actually a tiny bit further right but the three dots serve to orientate the viewer. I know the visualization is tricky here. Other annotations are with the photo in Part 7 but the terracotta dots showing the shear line are worthy of mention here as well, especially if you try to relate them to the terracotta dots in the header photo. The head lobe obscures the shear line in the upper part before it resumes at the flat-edged crater next to slab site A (that crater was viewed in detail from the opposite side in part 8)

Additional evidence for the brittle nature of the fracturing around the arcs is listed below:

1) The sloping nature of the fracture. This is common when levering cleaved slabs of rock on the earth or breaking highly stratified slabs of slate or sandstone.

2) That said, when the slab is almost free at the last moment, the resistance along the very top edge of the rim of the arc is near to zero and so the fracture can become vertical. We see this very low, vertical ridge around parts of the very top rim of slab site A. (Not very easy to discern in the photos for this post).

3) The very fact that the breaks are arced suggests the slabs were levered up from the opposite side, along the shear line. If levered from a point source, the leverage force is the same along concentric circles from the source. If levered up in sympathy with the rim of the uplifting head, as is proposed, the leverage points (along the lifting head rim) would be spread along various straightish lines as opposed to being point sources. The forces would still fan out in concentric circles but they would be slightly flatter, more oval-shaped. Many other factors to do with the comet structure will muddy this ideal shape (ridges, slab thickness etc.) but these flattened arcs are what we see and are consistent with the leverage force coming from the shear line.

4) Exposed subsurface showing pock marks of lighter material. This is apparent across one side of the slope of the brittle fracture in slab site A. It was remarked upon in one of the Rosetta blog posts before Christmas 2014, which suggested it was newly exposed material. This is of course evidence in its own right for a slab to have been overlying that area. But the pock-marked nature of the whiter material is also suggestive of a brittle break. Here is the photo from that Rosetta blog post:

2015/01/img_2137.png

5) Detritus on the slopes of both fractures. This would suggest they were brittle in nature. Whilst the slabs would have been lifted at one end by the uplifting head, much of the detritus from the brittle break wouldn’t have been disturbed in the sense of being lifted away from the centre of gravity with sufficient velocity to escape the gravity well. Some detritus would have been barely disturbed at all, perhaps snapping and grinding off the underside of the slab on the slope of the fracture and scraping along the surface a few metres, across the arc. That would be the debris we see today all around these two fracture arcs. Some debris however would have been lifted into suborbital trajectories and landed elsewhere on the comet. This could be the reason for the random distribution of so many boulders across all parts of the comet. Another group of rocks, however small it might have been, would have escaped the gravity well along with the slabs. A fourth group would have fallen between the suborbitals and the escapees, achieving orbital velocity but not escape velocity. There is circumstantial evidence for this. The American Astronomical Society 225th meeting in January 2015 hosted presentations on 67P. One mentioned that there were golf ball and baseball-sized rocks orbiting the comet. I would expect a few bigger ones too but perhaps they broke apart via spin-up or micrometeoroid stream encounters which are known to exist between the Earth and Jupiter.

As for evidence that the slabs were levered up from a fracture plane with little adhesive resistance, the following evidence is offered:

1) There is evidence along the shear line that sublimated gases preferentially followed the very obvious fracture planes of the comet (Part 7). Indeed the head broke away from these fracture planes (Part 6). Whatever the deep history of these planes and their strata is, it remains to be discovered but they are undeniably there in the photos and were also mentioned at AAS 225 in January 2015. If gases were finding their way through these fracture planes quite easily it suggests those planes were already weak due to whatever ancient process laid them down. It would follow that they were therefore very susceptible to any uplift.

2) In several of the previous parts I’ve referred to the 300-metre long slurry pile that pushed up the frilly rim of the cliff when it once sat along that line. I proposed that this slurry pile was deposited by gases exiting at what was once a long fissure across the back perimeter of the oft-mentioned rectangle and that this fissure constituted the shear line. This pile now sits marooned like a piece of rope laid across the fracture plane. It even curves round the back of the hillock mentioned in Part 2 (which sits at 2 o’clock from ‘B’ in this post’s header photo) and pretty well traces the shear line for another half a kilometre after that. There are other elongated ridges of a very similar appearance in the same area of slab site B. They also have a rope-like look to them, low and rounded (to the right of the ‘B’ in the header photo). If these were all slurry piles they are the fingerprints of what were once longitudinal fissures, just like the well-documented one in the rectangle. Today, they are sitting pretty atop the fracture plane but if they do represent ancient fissures then, by definition, a slab albeit cracked by these fissures, was sitting on slab site B and these ‘ropes’ were at the bottom of those fissures.

There will be more posts on missing slabs including one site that is appearing slowly from behind the terminator as the comet approaches the sun. As things stand at this date, it looks to have very similar characteristics to slab site B. Its presence is unsurprising, given that it is sited below the most tipped-up portion of the head lobe (so possibly experiencing the biggest uplift force) and is in the most favorable spot on the rotation plane to fling its slab free.

Apropos the above paragraph, I mentioned in a recent post that the cove was around the most tipped up part of the head because it was around the central portion of the cliff but I wasn’t taking into consideration the more dominant plane of tipping which means the cove is really around half way along the slope of the tip.

Photo credits:

Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO

Advertisements

5 thoughts on “67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 9

  1. Hi Andrew,
    This corollary neatly and solidly explains the following:

    A) why the matching shear line suddenly stops matching at site A, after a good kilometre of matches.

    B) the distribution, the size distribution and the location of boulders. It also discounts the proposition that they could be protrusions from below, rather than loose detritus.

    C) the nature of subsurface “mantle” is layered, hard and brittle, to the depth of the slab sites. There is evidence from Philae that the surface crust is brittle and hard.

    D) How the craters are formed. Craters are obviously not from impacts, but this corollary also discounts the possibility that they are formed from collapse or other ablation reasons.

    E) The flatness and geometric shapes of the craters.

    F) The different surface textures and boundaries between them. This is only even discernible in some of the NavCam pictures, but will be extremely obvious in the OSIRIS images. This entry neatly explains a mystery that hasn’t even been discussed with the general public, or even other scientists yet.

    I have more points, but believe me, This is BIG. I am glad you gave this post priority.

    Like

  2. So, turns out there are indeed chunks up to two meters orbiting 67p: see http://blogs.esa.int/rosetta/2015/01/22/giadas-dust-measurements-3-7-3-4-au/, a couple paragraphs in:

    “The space density of bound grains is at least 100 times lower than that of out-flowing grains and, in general bound grains are much bigger than out-flowing grains. Indeed, based on the observed brightness range, we infer that the bound grains varied from 4 cm to 2 m*, …”

    which confirms your sense of what you say you would expect — debris thrown right off the comet.

    “A fourth group would have fallen between the suborbitals and the escapees, achieving orbital velocity but not escape velocity. There is circumstantial evidence for this. The American Astronomical Society 225th meeting in January 2015 hosted presentations on 67P. One mentioned that there were golf ball and baseball-sized rocks orbiting the comet. I would expect a few bigger ones too but perhaps they broke apart via spin-up or micrometeoroid stream encounters which are known to exist between the Earth and Jupiter.”

    Not to say that I think the chunks observed are *proof* of your hypothesis, but there they are, and I’m glad, because I’m finding your observations compelling.

    Like

    • Judy
      Thanks for your comment. I had only heard of the “baseball” sized fragments so far. Seeing as this update was on 22/1/15, I wonder if it was written as a primer for the Science Magazine paper on the dust, which came out the next day. In that case, it would be new information.

      It’s interesting that they assume all the bound material was ejected at the last perihelion passage. A few of the larger fragments could be from ancient orbit capture events or, of course, from missing slabs. That said, I think it could be that many of them really were ejected at the last perihelion, via explosions of pent-up gases. One of the other seven papers released on the 23rd (Thomas et al) mentions this possibility for a huge slab that looks as if it popped out like a cork from a bottle and is now resting at the side of the hole it made. Though it wasn’t ejected into orbit (being too massive for the gas flux to accelerate it much) I would have thought that smaller fragments round the edge might have experienced an anomalously higher gas flux just at the apoint of rupture.

      However, there’s a caveat: Thomas says that pent-up gases would pose questions as to how the porosity of the comet would allow them to build up. I was aware of this in Part 7 which was why I thought the evidence of catastrophic outgassing had to mean a truly short, sharp event i.e. head shear, uplift, heat generation, then massive outgassing from the core.

      I’m just writing Part 12, the post on the ‘pole feature’ as promised- the one you mentioned in the Rosetta comments. It should be up tomorrow or Monday.

      Andrew

      Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s