67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 14.


This boulder drifted 170 metres after breaking free at the shear line. It then toppled 90° onto its side on landing, like an overturned lorry.

Yellow- matching points of boulder to its seating position.

Terracotta- a matching mini crater, sliced in two and shared between boulder and its seating position.

Blue- lines joining the end points of the boulder to its original position. These are in line with (parallel to) the comet’s rotation plane.

Bright green- direction of comet rotation is towards this end of the blue dotted line.


One of the most familiar and intriguing features on 67P is the flat ‘crater’ known as Landing Site A or, more informally, the amphitheatre. Regular readers will know that in Part 9 it was suggested that this area is the site of a giant missing slab, dubbed slab A. This slab would have been yanked up along one edge by the head lobe as it rose during the stretch. And, of course, the stretch was due either to spin-up of the comet or a Roche pass at Jupiter (see Part 1 and later posts). Here’s the Part 9 header photo reproduced because it shows slab A and gives context for the area that’s discussed in this post.

Yellow- missing slab A

Orange- missing slab B

Terracotta- shear line where head lobe tore away from the body.

Red- boulder discussed in this post.


It would be reasonable to think that there should be some evidence of detritus associated with the breaking away of the head lobe from the body and taking slab A with it. One might assume this debris would be lying around near the ‘shear line’ where the rim of the head used to sit before it sheared away. However, due to the very low gravity on the comet (1/100,000th that of the Earth), any fragments that detached from the slab or the head lobe during the lift would continue rising with them or at least remain hovering above the comet. They would either escape with the slab or eventually drop back to the surface after as much as an hour or more.

But if some fragments did drop back to the surface, why is the shear line devoid of any such evidence? Why didn’t they just fall back down to where they came from in the first place?

They couldn’t do this because as soon as they were detached from the surface, they were travelling in a suborbital, parabolic trajectory above the comet and no longer on the circular rotational path they had been on just prior to detachment. The parabolic path would always require a longer distance for the fragment to travel, in order to stay above its former home, than if it was still attached and travelling in a circle. This means that the only possible outcome was for these fragments to witness the comet slowly rotating beneath them as they ‘tried’ in vain to keep up. This should in turn mean that every fragment that broke off along the shear line, and didn’t escape, must have been recaptured up range along the comet’s rotation line.

And this is indeed what we see in plain view in the middle of Site A. There are two gigantic boulders lying isolated with a debris field strewn up range along the rotation line (which, rather confusingly, is down range along their path of travel relative to the surface i.e. in the opposite direction to the rotation). If you trace a line from these boulders in the forward direction of rotation, it crosses the crater rim of Site A and kisses the shear line where the very corner of missing slab A once sat. Slab A would itself have sheared from the crater rim on one side of that corner and from the shear line on the other due to head lobe uplift.

We’ll concentrate predominantly on the larger of the two boulders because it’s easier to match back to its former position. Let’s just call it rock A for the sake of brevity even though its composition isn’t the same as a terrestrial rock. Judging by the dimensions of Site A, rock A is 170 metres long, about 20 metres high and 20 metres wide. Thats 68,000 cubic metres. Plugging in the estimated density of 470kg per cubic metre gives it a mass of 32 million kg. And it drifted 170 metres from where it broke away. The smaller boulder, which we can call rock B would have behaved in an almost identical manner in its detachment, drift and landing. A tentative pinpointing of its seating position is given in the annotated photos below, along with some discussion, but only in passing as we dwell more on rock A.

On closer inspection of the Site A crater rim from above, a particular formation can be seen, still intact, consisting of a straight section and a curved section of material adjoining it. The small area between those two sections and the shear line is flatish and unremarkable. It’s missing any such formation but that area is in line with rocks A and B i.e. down range of them along the comet’s rotation plane. Its footprint is also the same shape and size as the footprint of the two rocks as they are now configured out on the flat crater floor. The length of rock A and its seating position are within 5% of each other, depending on whether the pointed tip marries exactly to the apparent tip of the seating formation or whether it stops a little way short. Of course, the very tip may have chipped off. Also, part of that 5% will be due to slight differences in foreshortening and the limitations of discerning the end points in fuzzy NAVCAM close-ups. Here’s an annotated photo of what’s described above:

Pale green- similar formation that’s still in place.

Pale blue- denotes rocks A and B and their seating positions which are between the pale green formation and the shear line.

Terracotta- the shear line where head lobe used to sit.

Dark blue- lines joining the end points of rock A to the end points of its seating position. These lines are parallel to the comet’s rotation plane and direction of rotation is from bottom right to top left.

Rocks A and B out on the crater floor bear a resemblance to the formations that are still fixed in place, one curved and one straight. They are also orientated in the same direction as the fixed ones, at least along the length of their longitudinal axes.

On even closer inspection it can be seen that if the curve of rock A were to fit to the crater rim in a similar way to the one still affixed, it must have been knocked onto its side when it landed in its current position. The five yellow matching points also betray this roughly 90° tipping over as well as the mini crater that’s sliced in two and shared between boulder and seating position.

Red- the perimeter of rock A as it would have been seen from above when fixed in its former position.

Yellow- the five matching points (same ones as in the header photo).

Pale orange- the side of rock A that is now facing upwards but was originally orientated vertically and married sideways to the structure of the crater rim. The two short, parallel lines in this colour denote a sharp step-up that’s visible in the header photo.
(the mini crater is not visible here- see header photo).

The photo above shows the curved top of rock A as it would have been seen from above the crater rim if it were fixed in its former postion. You would need to step out onto Site A and tip it over 90 degrees in order to give it the correct orientation (like righting a lorry that’s tipped onto its side). Then you’d have to translate it down range along the rotation plane to get it to that old position. No rotational adjustments would be required about the other two axes. Put another way, rock A exhibits a signature of its former orientation that was retained as it floated across Site A: it retained its orientation in 2 axes (or 4 degrees of rotational freedom out of the 6 possible degrees of rotational freedom).

There’s a good reason for it flipping over in the third axis. The only reason it flipped on its side like a lorry blown over in the wind is that it was travelling sideways as it landed and its massive inertia overcame the tiny gravitational force as it teetered along its long edge. That long edge (the one that’s not visible in these photos) hit the deck first and acted as a fulcrum to topple it.


It can be seen from the header photo that rock A exhibits an upturned end furthest from the viewer. This upturned section has a transverse line across it. In the latest ESA Rosetta blog photo it exhibits three of these transverse lines in parallel. Another photo, from last year, shows the original seating position of this upturned section when it was flipped back up 90° and married against the crater rim formation. That seating position shows vertical lines that appear to match the ones on the boulder in the more recent photo- although, it has to be emphasised that due to the viewing angles they can’t all be the actual matching lines to the boulder although one might be. The others can only match in their nature because the actual matches on the rim, if they exist, are out of view due to their being behind the crater rim from our viewpoint. The two photos are below. It can be seen that one is so overexposed that rocks A and B appear to have been incinerated where they sit so the yellow dots represent an estimate of the end points of rock A. The only useful piece of evidence in this photo is the seating position with the characteristic vertical lines.


Key for the seating position photo:

Yellow- end points of rock A and its seating position. Bottom-right pair of dots have the vertical lines below them at rock A’s seating position.

Terracotta- the shear line.

The recent ESA photo has yellow for the parallel lines on the boulder and their assumed seating positions in the shadows of the recess on the crater rim. The bottom two dots would actually be out of view behind the protrusion of the housing as it curves round to cup this end of rock A.

The final piece of evidence is the debris field that is strewn along the comet’s rotation plane and is roughly the same width as rock A is long. This debris probably slid off the top and flew on further as the rock was stopped in its tracks and flipped over.

If this is proof enough that rock A was detached from the corner where crater rim and shear line meet, then it follows that rock B, sitting next to it was also detached from that area. It would have been fixed in place alongside its larger companion in a manner that is similar to its current orientation on the floor of Site A. This can be corroborated albeit tentatively by the arrangement of the similarly shaped formation that is still in place on the rim and an adjoining length of outcrop between this formation and the shear line. That outcrop is the same length as rock B. But the NAVCAM photos aren’t clear enough to show a definitive match for rock B.

Incidentally, there’s another boulder that’s twice as big and twice as travelled as rock A. However, rock A was chosen first because it’s easier to describe. In part 15, the still larger boulder, rock C, will be given exactly the same analysis: break off; up range rotation plane displacement; capture; flipping. Perhaps readers will identify it before it gets posted. It’s fairly evident as you might expect but the more difficult part is identifying its former home. Both the rock and its home are in the rough vicinity of rocks A and B.


In Thomas et al (January 2015) it was observed that large pancake-shaped slabs in the Maftet region appeared to have been dislodged from the surface of the comet and dumped alongside their former position. The paper’s supplementary information shows a photo of a slab about 100 metres in diameter next to a hole of roughly the same dimensions. The authors suggested that this ejection might be due to the build-up of pressure from sublimating gases below the surface. The pressure would ultimately eject the slab. However, they acknowledged that the porosity of the comet should militate against such pressure build-ups. I haven’t been able to establish exactly where in Maftet these slabs are located.

Whether or not the Maftet slabs were ejected due to gas pressure, the Site A rocks are completely different both in their shape and their original siting. They aren’t pancake-shaped, quite the opposite. They have detached from a solid, otherwise flat surface, and from a narrow footprint, leaving no evidence of a hollow where gas could have built up. In the absence of such a gas build-up, there are only two other possibilities: a small meteor strike or shearing away due to the uplifting of the head lobe from the body and the shearing away of missing slab A. Any small meteor strike would have left damage at the site of impact and probably shattered the boulders as it liberated them. There is no damage apparent at all.

The only reasonable conclusion is that the position, orientation and rotation plane displacement of rocks A and B represent the signature of the head lobe uplifting and tearing fragments away from along the shear line. The fact that their former seating position actually kissed the shear line itself makes this conclusion all the more compelling.

And the clincher for a match of rock A to its former position is the fact that after identifying the five yellow-dotted matching points in the header photo to get an initial fix, along came the mini crater, sliced in two and shared between rock and seating position. That is a 100% exact match.

Photo credits:

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

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