67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 43

THE ‘RED’ SLIDE ON IMHOTEP 

    

Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/A.COOPER

INTRODUCTION

Part 42 gave an overview of the general sliding behaviour of the crust layers on Imhotep. They slid due to the comet stretching by at least 700 metres, perhaps a kilometre, along its long axis due to spin-up. The slides were divided into four colours based on their sliding direction and which layer, of two, that they involved. 

To be clear, these aren’t mirrored matches like they were for the head lobe shearing from the body lobe. Those head-body matches were positive and negative shapes like a rubber stamp and the stamped paper. The Imhotep crust slides are translational matches like tearing a piece of paper roughly and then offering the wavy lines of the torn halves up to each other. 

Furthermore, in some places there are ‘bled’ matches to one side of the main tear line. This is where the torn crust slopes instead of being a neat vertical tear and we see features that match vertically through the layers. Sometimes the seating layer itself bleeds right through the entire thickness of crust so that’s another way of matching crust to seating. The bled matches are translational too and reinforce the ordinary tear matches. We are starting with the red slide. 

USING THE BLUE SLIDE TO FIND A START POINT ON THE RED SLIDE

It may seem strange to start with the blue slide. However, you may remember that in Part 42 it was shown that the red slide and blue slide crust originally kissed along a short length of about 250 metres. The blue slide is easier to match to that shared line than the red slide. The location of the shared section of line is right next to the low point of Imhotep which has been identified as the kissing point for all the crust slides. The kissing point (a single point as opposed to a line) is a bright green dot in Part 42 and is located in the so-called depression. It sits on the intersection of the paleo rotation plane (paleo equator) and the body lobe’s short axis. That means it sits at the centre of the diamond-shaped base of the body. That would be the place where you’d expect the crust pieces to tear away from each other and indeed they did tear from that point. 

To be exact, the four coloured slides kissed within about 250 metres of the kissing point and the point itself is based on the rough averaging (to within a hundred metres or so) of the slightly staggered crust tears. So it’s at the centre of the original, fairly focussed array of cracks that appeared in the depression when the crust pieces sheared from this area. Since this averaged point seemed to be virtually indistinguishable from the Imhotep low point, as defined by A.T. Auger et al in the blog link below, the low point was chosen as the kissing point. That’s because, as stated in Part 42, the lowest point sitting in the depression full of catastrophic outgassing, suggests it was the site of highest activity and therefore where the initial crack appeared. Moreover, when all the crust pieces are eventually matched back together, we’ll see that there’s a gap where the low point is. 

So the red and blue slides shared a small portion of their original seating perimeters as a single line, with red crust on one side and blue crust on the other. The line was 250 metres long, perhaps 300 metres or so with all the wiggles. Its southern end stopped about 200 metres from the kissing point in the depression. And it ran notionally along the north-south short axis line, which is telling. This also means that red and blue were originally attached along this line and sheared from each other.

Since the blue slide crust perimeter is easier to fit to the seating line, it seems sensible to do that in order to find the red slide seating point that’s the same 250-metre line. Then we can skate 700 metres west across the Imhotep plain to find the matching shape on the red slide. That shape is pretty easy to see anyway but there are a couple of issues that make it initially a little harder. Once you see it you can’t ‘unsee’ it. 

A NOTE ON MATCH VISUALISATION TECHNIQUES 

Although hi-res OSIRIS pictures are used in this post, they don’t show the whole of Imhotep. However, the following Rosetta blog post link has a mosaic of six OSIRIS photos showing the whole area. It would be useful to have this alongside you as an extra guide since it gives a better overview. It’s also lit differently and enhanced slightly so it adds information when used as a supplement to the annotated photos. 

http://blogs.esa.int/rosetta/2015/07/20/inside-imhotep-2/

It’s also highly recommended that you use the annotated photo versions only as an initial guide for each match or mini match and then move on to the harder work of discerning the matches unaided. That’s why the unannotated versions are supplied alongside. Toggling between the two is a good intermediate step before flying solo with just the unannotated version. I have a feeling that relying too much on the dots means that some people think they can see the matches when all they really see is the dots matching. Conversely, others jump too early to the unannotated versions and say the matches don’t exist. It requires some patience but the matches are undeniably there. 

Even within the unannotated versions it can be useful to toggle rapidly between the slide and its seating. This is especially the case with the finer mini matches. The faster the toggle the more detail gets juggled in the visual memory- down to as little as 2 seconds between toggles. Toggling would be either by the eye, scanning across a stationery photo or scrolling the photo up and down quickly under the line of sight (or a combination of the two). 

Astronomers will know that this is taking advantage of the ‘blink comparator’ effect to see changes flicking on and off in each image. In this case, it’s used slightly differently to build up a memory of many mini matches within a larger match and even tiny differences within mini matches. You can’t retain that information in your memory so well by looking at the crust for a minute and then looking at the seating for a minute and then moving on, unimpressed. 

Another reason for using the unannotated photo versions is that the dots can actually obscure the detail being described. This is especially the case in this post with hi-res OSIRIS photos and very detailed matches. That’s despite using very small dots in an attempt to avoid the obscuring issue. 

Photo 2- the blue slide match to its seating 

   

Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/A.COOPER

Blue- the perimeter of the tear on the blue slide (top-right) that fits to its seating point (bottom). It also fits to the red slide that’s a kilometre away, as we shall soon see. Much of the blue slide perimeter and the features within it are cut off in this view but our 250-metre line is in view and is the length that’s annotated blue. The sections that are off-frame have already been matched to their seatings but haven’t been published yet. Part 42 and the ‘Imhotep Sliding Crust Matches’ page show the blue slide very roughly but with no mini matches. The following matches constitute perhaps half of the mini matches found so far. 

Bright green- this curved feature is best seen in the top-right version, the actual slid crust. This is because the blue slide is top crust and so appears to retain the sharp edges of features better. Incidentally, this is a bled feature as are all those outlined below. They are described so as to nail down the blue slide as genuine instead of just presenting its blue perimeter and racing off to find the red slide. You can see that the blue perimeter line running round that 90° corner outside the bright green line is really part of the perimeter of this feature. It’s less clear on the seating line at the bottom. However, it will be seen more clearly on the red slide perimeter in due course. Seeing as we’ll be referring to it a lot, we’d better give it a name. It doesn’t really look like anything except a coal shovel so let’s call it the coal shovel. 

Terracotta- two lines extending from the curved end of the coal shovel described above.

Small yellow- five shadowed grooves (top-right) of apparently triangular cross-section. These appear to fit to the five long ridges (bottom/seating) of apparently triangular cross-section. There are small discrepancies between the gap distances especially between the right hand two ridges. The ridge on the very right has a dot that was faded by the dotting machine. It’s next to a blue dot and a shadow, and looks white. If the small ‘triangular’ shadows crust do fit to the ridges, it would mean that this edge of the blue crust snapped vertically, leaving 10-metre-high triangles. You really need to look at the unannotated version because the dots get in the way of the detail. 

Large yellow- these two dots denote the flattened tip of a triangle that will become quite important as a fiduciary point in several of the slides. The triangle is barely discernible at the bottom and only the tip is visible peeping out from the shadow under the coal shovel. The whole triangle will become visible in the NAVCAM photo below, which has different lighting. The upper version is really part of the blue slide because it went in the same direction. That’s despite being part of the lower ‘onion’ layer of crust- the blue slide is notionally defined as being the top layer of crust as well as sliding that way. It clearly dragged some lower crust with it.

Orange- the top perimeter of the shallow slope that has the curved base and the five ridges (it will be seen to be a slope in the blue slide post).

Large pale mauve dot- a smooth plateau that sits in the middle of a ‘crater’ that looks like the pastry top of a pie with a crinkly rim. Only half the pie is visible at top right and its seating at the bottom is just a faint outline of its full perimeter, the right hand side of which isn’t annotated here. The pie and its crinkly pastry rim will be seen to be a match in future so these larger-than-life analogies are carefully laid out in advance so that you’ll remember them easily when the relevant part comes out. 

Pale blue- this line is just above the mauve dot. It’s the bottom perimeter of the crinkly pastry rim (top-right) and its seating (bottom).

Large pale green dot- at top right of both sets of annotations. This is a flattish area of roughly the same shape but is cut off at the top.

Pale yellow- apologies, this is hard to see because it looks white and was made small so as not to overload the picture with dots. Unfortunately, redotting just one dot after saving degrades every dot in the picture. On the pie at the top, this dot is in the middle of a rectangle adjoining the bright green line and is exactly at the end of the pale blue line. Its raison d’être is to denote that rectangle. On the seating at the bottom, the rectangle is apparently sitting across the coal shovel and not next to it as it was for the pie. This is the only large anomaly. It may have something to do with the fact that the coal shovel and its seating area are quite steeply inclined and so the coal shovel on the pie that slid may have stretched, folded out or delaminated under the force of gravity now that it’s overhanging. That would extend the perimeter of the pie. The overall kissing perimeter is indeed longer on the pie and the extent of that surplus is the length of the rectangle. So that partially explains the anomaly (stretch, sag and/or perspective issues). It doesn’t entirely explain it but, for one thing, these anomalies have been subsequently explained in the past and for another, that rectangle is surrounded by matches. The linked Rosetta blog photo shows the coal shovel, its two terracotta ridges and the pie crust matching very well. And they are all neighbouring the rectangle so the anomaly doesn’t negate the match. 

Photo 3- matching the red slide to the blue (and red) seating line.  

   Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Red- this is a small section of the red slide. It’s the middle portion and it’s just the section that matches to the blue (and red) seating. The seating is currently still marked blue as it was in photo 2. You can see a gap in the red line. That’s because these piece of crust stretched and/or delaminated just in this section. You can see the stretch lines running in an arc between the known matching sections of perimeter line. So if we were to cut out the gap and join the two halves of the red line, they would match the blue seating line. We shall soon see that this same stretch caused a similar similar sized rift in the red line just a little way along. So rather than being a nuisance it’s a match signature because the second rift disrupts the line of matches in an identical manner. 

Other colours- All other colours are as for photo 2 but now we can see three sets of bled matches: one set for the blue slide (the pie including its green coal shovel); the seating set; and a new set for the red slide. The colours retained from photo 2 are only those that can be discerned on the red slide. The coal shovel is particularly well delineated.

Photo 4- extending the red slide line northwards (and a little way south) 

   

Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Red- the red crust slide line is extended northwards. It’s delineated in two lines because it appears that the northern extension past the end of the photo 3 line is the true tear, running along the top of the slope, and the detached line running along the bottom of the slope is the bottom of the tear. It seems that although the original tear line on the right of the plain (running north of the red/blue seating line) appears to have been notionally vertical, the slid crust on the left of the plain delaminated to the left a little. The NAVCAM photo below shows the top tear on the left much more clearly. It peters out in this photo and is annotated only as far as it can be traced. In the NAVCAM photo, it goes all the way to that uncannily even, rectangular ‘ski ramp’. It should be noted that the green slide (a long way from being published) tugged this area to the left anyway, which is in keeping with this small delamination. The small southern extension below the blue line is as far south as the line can be matched before hitting the depression. And of course, it matches to the similarly shaped bottom end of the actual slide on the left. That’s the portion below the sharp, 90° turn. 

Photo 5- tracing the red line further southwards. A NAVCAM photo is used because it has much more favourable shadowing for this section of the red slide and its seating.  

   

PHOTO CREDIT: NAVCAM/ full credit below. 

Orange- this is a single dot that’s sitting just next to the end of the traced line in photo 4.

Red- the small section to the left (south) of the orange dot is traced as far as the smooth, blocky protrusion. Notice how it’s a continuous, zig zagging line in and along the depression but there’s a small gap in the slid crust line. This gap is in keeping with the gap for the stretched section that was matched to the seating in photo 3. These two gaps when seen in unison and in context betray the fact that the marooned section, protruding into the Imhotep plain, decided to stay put and allow the rest of the red slide to carry on another hundred metres or so. The northern section is also shown as it was in photo 4 but you can see the top (delaminated) tear running right up to the base of the ski ramp. 

Bright green- this is the coal shovel. The lighting has given it a small tail that wasn’t so visible in the OSIRIS lighting. The tail is replicated in the seating and the tear. 

The terracotta ridges aren’t annotated because they’re visible only on the seating and not on the slid crust line in this photo. 

Photo 6- this photo 5 reproduced but with the blocky protrusion included. 

  

  Red- as for photo 5 but with the blocky protrusion included. It was left out of photo 5 because it doesn’t quite join to the end of the line at its seating point. There’s a 100-metre or so gap so that needs to be noted. However, seeing as there’s a huge delamination beyond the actual slid protrusion (the green slide in Part 42) and two mini delaminations too, it seems reasonable that this small gap that’s opened up along the same slide vector, is explicable via delamination at the seating point. Notice how the seating fits snugly against the line of roundish features in the depression. This neat fit is the first of the crust pieces that will fit like a jigsaw into the depression- but with long, thin gaps corresponding to the lines of roundish features. The roundish features were sitting at the bottom of deep cracks when the crust was seated in the depression and had just begun to crack open. 

Just to recap Part 42, all the pieces that kissed but tore away from the depression were lower level crust, that is, the lower onion layer of stratum. They went predominantly east and west to become the red and orange slides. The upper onion layer of the two layers sat fairly passively on top of all this cracking and wrenching. It’s the the large pancake ‘lid’ that fits to the depression and which eventually slid to the left (south). It’s sitting in plain view in the above photo and is the southern portion of the blue slide. You can see the exact matches in the Imhotep crust slides page in the menu bar. The northern and southern blue slides will be posted after the orange slide.

Photo 7- the OSIRIS photo with the southern section added. 

   

Photo 8- the OSIRIS photo with the full red slide (including northern extension to the ski ramp) and its seating. 

   

 NEXT POST

There is a 3D (sideways) match for the red slide. It’s not really compelling because it’s so fuzzy. It will get an airing but this 2D analysis is good enough for now so we’ll press on with the orange slide in Part 44. Perhaps some better pictures for the 3D match will come to light in the meantime. 

PHOTO CREDITS:

OSIRIS:

Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

NAVCAM:

Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

To view a copy of this licence please visit:

http://creativecommons.org/licenses/by-sa/3.0/igo/

All dotted annotations by Scute1133.

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