This page was written in response to the Rosetta blog post entitled, “The Changing Comet- Call For Contributions”. Here’s the link to that post:
Groussin et al. 2015 describes changes on the smooth terrain of Imhotep. In a nutshell, there were five areas of subsidence, named A to E, which suddenly appeared between 3rd June and 11th July 2015, just before perihelion. They all started from specific points (which I shall call foci) and grew rapidly into wide quasi circles. The circles eventually merged, covering 40% of the 0.8km squared smooth terrain. The depth of the subsidence was 5 metres +\- 2 metres.
This page scrutinises the photos from the Groussin et al. 2015 paper and finds that area B had started by the June 5th photo, eight days before the June 13th photo in which it’s first noted by Groussin et al.. The area is tiny and looks like part of the rocky perimeter along the western scarp of the smooth terrain. This is compounded by the fuzzy photo quality due to the great distance of the orbiter from the comet at that time. It was keeping its distance because of dust issues around the coming perihelion date.
The fact that the erosion has been found to have been underway on the 5th June is secondary to the fact that the focus for area B is now constrained to a 50-metre stretch along the rocky perimeter. This will be used as an input for the next menu bar page. That page will characterise post-perihelion erosion in the accumulation basin on the other side of the rocky perimeter. Indeed, it was the erosion on the other side that pointed to this being the focus for both episodes. This is what prompted me to revisit the Groussin et al photos.
Long, narrative photo keys always have ‘/////’ at the end to show they’ve concluded.
THE GROUSSIN ET AL PHOTOS
The photo montage below shows the sequence of events from 3rd June to 11th July as the five areas, A to E, started from their respective foci and grew in curved, quasi-circular fronts. It actually starts with a May 24th photo as a control for comparison and to show that no erosion had taken place before that date. The coloured arrows denote the separate, quasi-circular areas of erosion and of course, the colours run in chronological sequence from A to E. The letters themselves are shown in an extended version of this photo montage in the paper but this cropped one is copied from the accompanying Rosetta blog post. So for the sake of clarity, the key to the colours is below the photo montage in case you need to make a quick reference back to this photo when further on in the post.
Photo 2- the chronological montage for the erosion of 3rd June to 11th July.
Since it’s area B we’re concerned with here, it is denoted by the yellow arrow and that arrow appears first on the 13th June element in the montage. Here’s the original:
Photo 3- The 13th June element
Red- this is area A for orientation purposes only. Otherwise it’s irrelevant to this post. But it may also be a good idea to do a quick scan over the montage to see how the other four areas evolved around area B.
Yellow- three dots showing the extent of the growing area B on June 13th. They’re just outside its perimeter so you can judge it for yourself.
Blue- two rocks 100m apart (centre to centre) which act as a useful measuring tool.
The ‘two-rocks’ ruler indicates that area B is about 130m long, left to right, rim to rim. The incidence angle (viewing angle to surface) in this photo is 40° +\- 5° according to the paper. The apparent width of area B is 28m using our ‘ruler’. That’s the raw measurement upwards across its width on the photo. It comes with some error due to fuzziness of the rocky perimeter of course. Rosetta was moving far from the comet due to the expected dust enhancement around perihelion hence the fuzziness. If the angle is 40°, the true width is 1/sin 40 x 28= 44 metres. That’s the true width of what’s visible within the error margins of measuring such a fuzzy rocky perimeter. However, I took the measurement across the average width. There’s a small alcove in the rocky perimeter and a slightly bulbous extra curve in the eroding front. So using these and an intimate knowledge of this little cove from months of scrutiny, I would say area B was 60 metres wide at its widest point on June 13th. And 130 metres wide.
The June 13th element in the montage is the first element to get a yellow arrow, denoting area B which, as we’ve seen, was well underway. The Groussin paper also says, “On 13 June 2015, a second roundish feature (B) appeared and also expanded.”
However, area B had already started in the previous photo from June 5th.
Photos 4 and 5- the June 5th element from the montage and the 24th May element to check against as a control.
Copyright ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/A.COOPER
Yellow (for 5th June photo at top)- the perimeter of two small, joined circles which are right up against the rocky perimeter. Also, the future June 13th perimeter superimposed (based on measurements above and calibrated to our rock ruler).
Yellow (for the May 24th control photo at bottom)- two yellow dots denote just the centre points of the yet-to-appear circles. You can see from the control that these two circles didn’t exist on 24th May and had appeared on the dust of the the smooth terrain by 5th of June.
Photos 6 and 7- these are the same as photos 4 and 5 but left unannotated. The dots are good for locating features but it’s best to fly solo once familiarised because you see more without the dots.
HEADING OFF APPARENTLY CONFOUNDING ISSUES
There are also three jagged, rocky circles which curve round and kiss our two genuine erosion circles on their left (north). These rocky ones are more visible in the control photo and rather elusive in the 5th June photo. You have to superimpose them with your mind’s eye in order to catch them masquerading as erosion circles and reclassify them as rocky. That’s why the yellow dots going round the genuine circles stop abruptly on the left when they bump into the middle rocky circle.
You may also notice that the control has a shark-tooth point on the perimeter above the three rocky circles. It’s all but disappeared in the 5th June photo. That’s just the lighting. It’s a hill that can’t have disappeared, although the very tip is formed from a triangular arrangement of boulders. This is why, in photo 4, the superimposed line of the 13th June perimeter appears to stop before it reaches the rocky perimeter. It has in fact reached the shark-tooth tip.
EXTRA PROOF THAT THE TWO LITTLE ERODED CIRCLES DIDN’T EXIST EARLIER IN 2015
Photos 8 and 9, below, were taken from a much closer, orbit, from very different angles and with different lighting. One is from the Valentine’s Day flyby and the other is an OSIRIS photo from late 2014. They illustrate three things:
1) the non-existence of the two circles at that time.
2) the three rocky circles, looking very ragged and rather squarer due to higher resolution.
3) the shark-tooth tip of boulders at the bottom of the triangular hill.
Key for both photos
Yellow- the location of the centres of the two circles which had yet to appear when these photos were taken. Since these are the centres, the actual circumferences kissed all the way along the two little bays next to the yellow dots. The two bays are about 50 metres long hence the area B focus is constrained to this exact 50-metre line on the rocky perimeter. There’s the faintest hint of a curve on the Valentine’s Day close-up suggesting a shadow of the future, deeper circle. However it’s so faint that it cannot be invoked as a proto-erosion circle. And it’s not visible on photo 8.
Red- the three ragged, rocky circles.
Green- the tip of the shark tooth at the tip of the triangular hill.
This is the Rosetta blog post with the original Valentine’s Day photo:
The incidence angle for 24th May is 80° and for 5th June it’s 60°. The measurement and comparison of features between the two photos is little affected. The left-right dimensions are almost the same and the up-down measurements are about 14% less for June 5th. This has no bearing on the two circles- they really are there, sitting in the dust on 5th June and aren’t there on May 24th.
RELATIVE AREAS OF THE GROWING AREA B
Photo 10- the larger, 13th June extent of area B superimposed on the 5th June photo with its two (real and extant) circles.
Copyright ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/A.COOPER
The larger area was carefully calibrated against the 100-metre, ‘two-rock ruler’ using the calculated 130m x 60m dimensions. The two little circles are no more than 50 x 20 or 25 metres so the area ratio is around 7:1. From the dotted photo it looks as if it’s about 5:1. However, our eye is drawn to the outside of the yellow perimeter.
The most important aspect of finding these two incipient circles, is that we’ve now located the exact focus of area B along the rocky perimeter. It can now be used as an input for the next page (not a post) which is entitled “Post-Perihelion Erosion and Subsidence in Western Imhotep”. It will be adduced as evidence for erosion advancing in the opposite direction on the other side of the rocky perimeter.