Part 73A- Erosion in The Anubis Region on Comet 67P/Churyumov-Gerasimenko During Perihelion 2015. 

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

The gif above is composed of the two stills below it. The first still was taken by the Rosetta orbiter navigation camera (NAVCAM) several months before the perihelion of comet 67P in August 2015. The black area on this photo is due to swivelling the frame to align it for the gif. This was coloured grey in the gif to reduce distraction. 

The second still was taken several months after perihelion. You can see that the flat-bottomed depression has grown over the course of perihelion. 

The third photo is an image taken by the NAVCAM on arrival at 67P in August 2014. It’s presented here as a context image with an arrow pointing at the lip of the depression. This would be the bottom lip with respect to the orientation of the gif stills. You can just about see that it’s the pre-perihelion shape.

The two gif stills have been rotated and sized so that the two large boulders at upper-left and lower right are perfectly aligned. This introduces small inbuilt errors into the gif due to the slight parallax between the images. It’s as if the gif is trying to force them into alignment. These errors are small compared to the dimensions of the newly eroded area. However, they’re discussed in the appendix for completeness. 


The Anubis region is defined by the fact it’s composed almost entirely of smooth, dusty terrain. Anubis already had a small depression when Rosetta arrived at the comet in August 2014. It was roughly speaking a square, about 300m x 300m, with rounded corners (see the first still used for the gif). It incorporated three finger-like features at one end, within its depressed boundary line. It was flat-bottomed and exhibited a circa five-metre-deep scarp all the way round, defining the boundary. 

This depression grew noticeably during the perihelion of 2015. The growth is easy to make out when toggling between ‘before’ and ‘after’ photos and even clearer in the gif. This change on 67P hasn’t been documented yet in any scientific papers. 

The main area of new erosion is the stubby ‘thumb’ on the right that’s now been added to the three fingers. It now looks like a dinosaur footprint. The thumb has grown far enough to incorporate the lower-right boulder into the depression. The fingers have also grown in length and become more defined. The left finger has grown enough to kiss the upper-left boulder. 

Although the growth of the depression is obvious, the two boulders act as fiduciary points in both photos to prove the growth definitively: if they were outside the depression before perihelion and are now at least touching the edge after perihelion, the perimeter must have grown. 

The perimeter of the thumb and extended fingers has the same characteristic scarp as the rest of the depression. The scarp is also uncannily similar to the depressions that appeared in the Imhotep region during June and July 2015, just before perihelion. These are discussed in the next sub-heading. The similarity suggests that the erosion mechanism that brought about the Imhotep depressions is also responsible for the growth of the fingers and thumb at Anubis. 

It’s difficult to find photos showing the progress of the erosion because Rosetta was far from the comet at perihelion in order to avoid the greater flux of dust at that time. 


This short summary of the discoveries made in Groussin et al. (2015) allows the Anubis discovery to be put into some sort of context. The similar depressions they found at Imhotep serve as a precedent for the study of the Anubis depression. 

In early June 2015, as 67P was approaching perihelion, a large area of the smooth terrain in the Imhotep region began to erode in dramatic fashion. The whole event was captured by the OSIRIS Narrow Angle Camera (NAC) on the Rosetta orbiter. Five distinct depressions grew at a surprisingly fast rate. Each area was quasi-circular, flat-bottomed and exhibited an eroding scarp some five metres in depth. The circles grew as their circumferential scarps eroded their way across the smooth terrain. By mid-July, all five depressions had merged into one depression covering 40% of the smooth terrain, about 0.32 square kilometres. 

The photo below is of the first two quasi-circular depressions to appear at Imhotep. They’re shown as they were on 27th June 2015, arrowed red and yellow (NAC photo).
A detail from Groussin et al. (2015) Fig. 1

A&A 583, A36 (2015)
DOI: 10.1051/0004-6361/201527020 ⃝c ESO 2015 


The two depressions in the photo above are known as A and B in Groussin et al. (2015). They were already growing fast at this point with the other three about to start.

The data collected by the OSIRIS team was presented soon afterwards in Groussin et al. (2015). It wasn’t just photos, the various colour filters on the NAC enabled the detection of a probable ice signature in each depression. This suggests that sublimation of ice was responsible for the erosion, either H20 ice or CO2 ice or some mixture of the two. However, the extent to which ice loss contributed to the overall erosion is currently a subject of debate because Groussin et al. (2015) found that the volume of erosion was orders of magnitude greater than their ice sublimation models would suggest as being possible. Furthermore, very little dust was seen to be escaping from above the area in question at that time. The paper is here (not paywalled):


Although the Groussin et al. erosion event was by far the most obvious one during the few months around perihelion, there were at least two other small areas that exhibited the same apparent behaviour, that is, a scarp of some five metres in depth eroding its way across and through a section of smooth terrain. One area is the Anubis depression discussed above. The other one is a new depression measuring about 200m x 200m. It’s located in a small basin next to the much larger Imhotep depression described above. This area is somewhat harder to make out and is beyond the scope of this post. 


The ‘before’ and ‘after’ photos used for the gif appear to have been taken from a similar angle but they’re some 10° to 20° apart as measured from the centre of gravity of the comet. This translates to a 10° to 20° difference in the angle of incidence of the viewpoint line at the Anubis depression’s surface. By locking the gif components to the two large boulders at upper-left and lower-right we automatically stretch the more foreshortened one by up to 1.5% (for 10°) and 6.5% (for 20°). 

It can’t be the full 6.5% because the angle between the two viewing points isn’t subtended within a plane that could be coplanar with any plane projected by the line between the boulders. In other words, part of the angle difference, or drift, between the two views is across the boulder line and not in line with it. This is evident in the fact that there’s a clockwise/anti-clockwise wiggle between the two gif images. So the anomaly is shared between the component of the angle difference along the boulder line and the component across the boulder line. It’s probably something like 3% along the boulder line meaning that one image has been stretched by 3% so as to align the boulders. Whichever way, this small anomaly is swamped by the size of the newly eroded area. 
Andrew Cooper is a citizen scientist with an interest in NEO’s and comets, especially their orbital dynamics and spin behaviour. He has worked with Marco Parigi since August 2014, analysing the morphology of 67P. 



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

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All dotted annotations by A. Cooper. 

FOR OSIRIS NAC – Groussin et al. (2015) detail.