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- Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
Long comment follows. Since this was my attempt at visualising the minutiae of matching 3d surfaces from a written explanation from A Cooper, and his scute blog has done that rigorously, this is the explanations from the original ESA comment, that has been already expanded on. Short of ESA releasing names for all these features, we are stuck with wordy descriptions, coloured points and numbers.
Marco
Here are all the things that I've been meaning to say for the last few days. I should've copied the original comment and reposted but this comment has more thoughts and questions anyway so here goes.
The numbered line you have (1-10) is pretty accurate. The only place it went off slightly is in places where I'd made a mistake myself or it was not explained very well. I shall adjust them one by one below with my reasoning. Most are correct or very close.
1) correct
2) correct
3) correct - spot on. The other side of the lump it's next to is where the last dot of the line in my Part 2 is positioned, the fuchsia dot.
4) correct
5) correct
6) not quite. My fault. I can see you are following the nicely demarcated apparent shear line as I did. Although it was where the real rupturing happened, it took the adjoining rectangle with it. The perimeter of this rectangle is:
from the correctly positioned '5' hug your way round the mound next to it with the two pimples as far as the longest prong of the three-pronged fork. Travel straight up the fork. Follow the wavy line above it into the shadow. Just as a check- part of that wavy line looks, by coincidence, like a faded '5' just to the right of the pointed shadow. The left hand edge of that faded 5 is one tip of the seagull wing and the '6' is over the other tip. So 6 is on the line after all but only by chance and out of sequence.
7) needs to be shunted over to a point in line between the 6 and the 8.
8) correct. The 8 is actually positioned on the very edge of the head in the foreground but the shear line does run under the head at that point from this camera position's point of view.
9) correct. But I'm now wondering if this should be a bit further up the little ridge. I can match those little dips in the ridge to the line of dips at the right of the cove in the head but the head ones go further and tip over. Then I realised that the two strange curved rocks sitting on site A had almost certainly come from the continuation of that ridge just off frame to right. One appears to fit into a divot in the side of the ridge and is in keeping with the shape of other formations on the ridge as seen from above. Once in place though, I'm not sure it stretches far enough into this photo to marry to the ridge above. Of course, this is all fine detail. The two ridges almost match and this area around the 9 is getting close to where things get very unclear because just beyond it there should be no matches if a giant slab went missing. Before this point the matches are so evident that the missing slabs they were married to must have been thin. Site A looks deeper and much more catastrophic which is why....
10) I don't think the head sat quite here. Although this edge that the 10 marks is clearly a shear line, I think the head sat in a line that would mean shoving the 10 to the left till it's just under the actual head in this image. So if the 10 is marking an obvious shear line but isn't where the head sat it means that the giant slab took with it part of the material from this section of the trench, attached to its underside (that's if you buy into the slab theory. I don't expect you to but I seem to remember some positive feedback).
I may be wrong about the 10. But from most angles I can't quite stretch the head rim back to the crater rim in my minds eye so as to seat it down. It always comes up short, cutting across the low trench of the neck before meeting the ridge by the craters for its next likely match. This of course is evidence for a thick missing slab. All the matches are on another chunk flying round on the same orbit! Then again, your picture with the yellow line looks plausible at this point on the perimeter.
I think the yellow line is perfect where it zigzags at the right. At the left end where it almost appears to kiss the head, it needs to be erased a short distance to where the curve starts and turned to run along the dark strip to the end. Then sharp right at the end of the dark strip, over another dark strip and out of view.
The yellow line goes through the rectangle but your yellow dots straddle it at an angle and end up in the trench. If you look at the picture in my Part 5 that puts a green dot on each corner, you should be able to relate it to your picture because the lighting and distance is similar (although it's viewed from a different point.
The green line is in the trench. You may be thinking of the scalloped triangle edge which would be where the gull wing profile is. That is to be found under the end of the longer yellow line in the yellow V. This denotes one end of tbe rectangle.
You can see the two semicircular formations peeping out from under the yellow line, which sit against each gull wing. I believe this is dried slurry that was being forced through the stratum layer and pushed up the gull wings. They obviously continue underneath if they fit perfectly at the exit aperture.
In fact, I believe that all those lines that form the V and the rectangle and the s shape up to your first yellow dot are slurry residue from the exit fissures along a slab that is long gone. This is because they all resemble the one along the back of the rectangle which has very good evidence of being a slurry residue line (it pushed up the frilly edge of the cliff).
Also, I noticed that the only real sublimation activity on the head seems to be in the five craters just above this point. I'm wondering whether this was where the shearing started and the tear worked its way round the perimeter from here. This is typical of many failures like exploding gas canisters. They tear more than explode like a bomb. But there is a momentary explosive rupture before the tear starts. This is what we may be seeing here in this small area.
I left a comment on the 14th December Rosetta post in reply to Robin Sherman regarding all this. I was suggesting a scenario involving explosive sublimation but seeing as I know nothing about sublimation explosive or otherwise, feel free to take a look and correct me!
Seeing as I've nearly got as far round as site A, have you got any ideas why it's so flat? I know one can say it's thicker dust or if there was a slab there then it was fracked by gases underneath and it's a smooth stratum. But it's so absolutely flat it puzzles me especially when there's evidence of chaos all around it.
I'm sure there's evidence staring right at us and we're staring right at it and yet not seeing it. It took me 3 months to realise the gull wings were pushed up but I'd puzzled over those two little semicircular splodges every time I passed over them on my way somewhere else. Same thing with the rocks on site A and the boulder at the back. Now that I fully accept the lobes were joined I'm seeing more things which follow from that, things which you couldn't notice if you were stuck in contact binary mode.
I tweeted Phil Plait the 'Bad Astronomer' blogger. He sounded very interested but has now gone silent.
Regards
Andrew (A.Cooper)
8 comments:
As far as the flatness of the site A "strata", it belies a sense of order when a lot of the comet gives the impression of haphazard disorder. Stratifications are a bit of a stalking horse for EU theories saying it is "rock", that is planetary origin. I perceive, however, that the layers are somewhat concentric to the original spheroid shape, like onion rings. Thus, however strange it seems, they must be deposited from activity of the comet itself some time in the past. Additionally, the outermost layer (near the shear lines, for instance) has stayed static at least since the time stretch occured. Most near surface sublimation models have the outer surface ablating or collapsing from mass loss inherent in cometary activity, which would destroy most evidence of stretch. Clearly, the models have this wrong, I think the flatness is related to the process of internal stratification.
One thing in which my views differ from yours is that I perceive the stretching mechanism to be a step by step process happening over many hundreds of years. Some of the steps are *relatively* violent and sudden which accounts for the leverage of slabs, etc. but I feel we should see some lengthening in the order of centimetres to metres through the alignment of centrifugal forces and tidal forces overcoming gravity in a pulsating fashion come May 2015.
'Andrew Cooper said:
Marco
I'm going to try and prove that all the fracture planes to the left of site A are parallel to site A. "To the left" means as you look towards the neck. This then leads smoothly into yet more independent corroborating matches, this time regarding ridge lines between head and body.
Thanks for your thoughts on site A. Its fracture plane might be in a line with the apparent fracture planes at the bottom of the head in this photo:
http://www.esa.int/spaceinimages/Images/2014/08/Comet_on_19_August_2014_-_NavCam
This would mean that all fracture planes to the left of site A in this pic are roughly parallel to it. For guidance in following these planes, there are some very obvious straight and parallel lines at the top of the head which slope up to the right and then two at the bottom which slope down to the right. The two at the bottom already coincide with ridge lines on the body when viewed from above the head (really compelling evidence which I've not even got around to mentioning yet). This is evident in the top-down photo in Part 1 of my series:
https://scute1133site.files.wordpress.com/2014/12/img_1853.png
For guidance as to the features and fitting together for this photo, described below, you may want to glance at another (annotated) photo which matches the head cove to the body (yellow dots), along with ridges (blue dots) and point-to-point matches (red dots). And perhaps keep it to one side to refer back to during the explanation.
https://scute1133site.files.wordpress.com/2014/12/img_18852.jpg
Returning to the non-annotated top-down photo, you can see a shadowed ridge at top right that looks like an elongated question mark. It has some very straight ridge lines cutting across it and below it, going from top right towards bottom left. Since they are ridges where the head and missing slabs tore away, the areas between them are the the fracture planes in plan view and these planes seem roughly the same as eachother and also parallel to site A's plane, even if they are a few substrata above or below site A. One ridge line runs contiguous to the bottom edge of the scalloped triangle (which is half in shadow at the tip of the question mark). You can also see the three-pronged fork to the right of the scallop and the wavy line running between them, as was shown in close-up in my Part one photo. I believe that the question mark ridge and the three pronged fork are possibly ridges of deposited slurry along fracture lines and so the true fracture plane was at one time quite smooth. This would explain why site A is so flat. Its missing slab appears to have been quite thick and so its fracture plane was deep and not subjected to sublimation and the ingress of slurry. However, that doesn't explain how it came to be flat in the first place. All it does point to is that all the fracture planes in this area are relatively smooth and parallel, lending credence to your observation that it could be cometary processes that formed them. If you add a deeper layer of dust to site A due to being near the neck it might mean that underneath, it's about as flat as the surrounding areas or slightly flatter due to no slurry ridges.
Continuing:
The ridge that runs along the edge of the scallop also runs contiguous to the shear line from the scallop, arcing very slightly as it progresses leftward towards the crater next to site A, that is, the crater that's snugly against it with a short, 45 deg shadow making the crater appear triangular. That 45 deg shadow is arguably right on the other, upper ridge line that extends across the elongated question mark. At the bottom-centre of this triangular-looking crater, the shear line jumps to the next parallel ridge (upwards in the photo). In fact, the reason for the slight arcing I mentioned above is that it's already starting to jump, or tear, from the straight and true ridge line it had been following. Once it has made the full jump to the next ridge, that line is followed only for a tiny distance before the missing slab on site A confounds any matches. But it does follow it for long enough to make two matches back to the head lobe. These are the point-to-point red dots.
Now, when you look at the head in the foreground (bottom right of photo) you see the cove. It has two fairly obvious lines that, when rotated anticlockwise by a few degrees would fit to those two ridge lines below. I believe this is because these really are one and the same, the same ridge lines and were
married against the ones below. Since the head is tipped upwards and needs tipping back as well as rotating, it is the *bottom* line on the head lobe that fits to the *upper* ridge line on the body. This line on the head runs across the bottom right corner of the photo, kissing the small circle, the top of the dark shadow and continuing off-frame half way along the bottom. Tight against the shadow to its left are two unremarkable but similar looking mini-curves, both shallow and ragged. These correspond to the two red dots on the head in the annotated image above. The right-hand one fits to the site A ridge that protrudes into the 'triangular' crater below. The left hand one doesn't match exactly in the 3rd dimension but does in this plan view. Its actual seating point is missing whereas the right hand one of the two fits very well. This would correspond to the left hand one of the two red dots in the annotated version due to viewing from the opposite direction.
The bottom of the cove on the head (the head shear line) is fairly foreshortened but that lends itself to giving away the ridge line it corresponds to. The right-hand portion of the curve corresponds to the ridge line below between the end of the scallop triangle and the middle-bottom of the 'triangular' crater. The left hand portion of the curve, which arcs up towards us into the deep shadow, corresponds to the jump from one ridge to the next. It ends at the red-dotted mini curve in an exact mirror image to the shear line below that jumps around the left hand side of the crater and up to the site A ridge. The tip of that ridge is the home of the lower left red dot in the annotated photo.
Marco
Thanks for posting those two comments and for linking my blog on twitter. You mentioned in your second comment about where your views differ from mine in that you see it as a step by step process over many hundreds of years and that you feel we should see some lengthening next May in the order of centimetres to metres due to tidal forces aligning with centrifugal forces.
I did some calculations for the tidal stretch and then shelved them because I'm more used to the neater case of a net force pulling boulders away from the the tips and even shredding comets. I'm not so conversant with tiny stretching forces with all the attendant considerations due to plastic or elastic deformation (although I'm sure these could be roughly modeled). What the tidal effects of the sun do is effectively reduce the gravitational pull at the tips when the comet rotates into line with the sun. How the comet responds whether elastically or plastically is difficult to know. And if it keeps stretching on every rotation due to plastic stretch, it couldn't go on forever because the comet would be at 1.6AU. As long as it's not under the sun's Roche limit (around 2 million km for a loose body) this strectching would have to be limited or, more likely, result in a snapping of the neck leaving the two lobes orbiting each other. This doesn't cause any problems for your theory, I'm just trying to visualise how far it can stretch or how far apart the orbiting lobes can go while the gravitational force between them is very much greater than the tidal difference across the ~5km long axis.
Then, on rereading your comment I saw that you said "in the order of centimeters or metres". That's close to what I calculated so I thought I may as well give you the figures I had but with some caveats: 2mm stretch across the 5km long axis for each elastic stretch; 8mm for each half-rotation with plastic stretch. The plastic scenario would result in 0.96 metres of stretch over the two months of rotational alignment with the sun- or a bit less, 0.92m or more, accounting for the 15 degrees offset either side of perfect alignment. However, I wouldn't quote me on that. These are just raw figures that assume the tiny tidal force causes a differential acceleration to a completely loose rubble pile or perfectly elastic neck. I'm not sure that would automatically happen. It also assumes the mass is divided in two and concentrated at the two ends, 5km apart so it is an absolute maximum value. The two lobes are starting to behave somewhat as two concentrations of mass though.
The calcs assume 1.6AU in May-June where the acceleration due to gravity if the sun is 2.26mm/sec^2. It assumes a long axis of 5km and a quasi linear drop in the sun's gravitational force over that length. It also accounts for the sin wave nature of the force (maximum on alignment and zero at 90 deg to alignment) and is integrated over that sin wave. The elastic version will pulsate twice every rotation and never go above 2mm. The plastic version would be pumped twice every rotation and would presumably have to have some tensile integrity to lock in that stretch as the force vector moved out of alignment.
These values are around the sort of distances that might be detectable via radio doppler analysis but only if Philae's position is known and the rotation period of the comet is more accurate than the current 2 second error bar (Lamy et al Sept 2014). I think 2mm is probably too small but 1 metre is quite a lot. Even if it's half of my ersatz version with its 5km separated lobes, it might be within the capabilities of Doppler analysis.
Thank you so much for that! I can imagine that around half a meter of (mostly plastic) stretch is plausible to me given your rough calculations - and measurable only if we find Philae, and possibly need Philae to wake up to confirm second measurement. Other than that, measurements at stretch point if found (wasn't there a fissure or crack somewhere near the thinnest part of the neck?), or angular momentum disturbances may also give some information...
A definite point of data that could verify stretch.
I shall post your comment and reply also.
regards
A Cooper said
Ah yes, the neck cracks. Those are a great place to look for a stretch! I was just looking at the pic yesterday and noticing several more (that were always there but I hadn't noticed) and the original crack they pointed out is longer than I had originally noticed.
The cracks could be like stretch marks on humans. The surface could be firm but brittle(like aerogel), but could have the gooey candy centre (crude oil?) below the surface at the crack and throughout the neck.
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