To find dragons, search in deep, dark caves 🐉
Was looking through the Comet 67P/Churyumov-Gerasimenko image archives ahead of the tenth anniversary of the landing Philae probe landing on 12 November ☄️🛰️
Discovered this nice wide & narrow-angle pair from 22 May 2016 from 6.3km, centred on the 220m wide, 185m deep Seth_01 pit 📷
Zoom in, crank up the brightness, & look at that lovely scaly texture 🐲🙂👍
Credit: ESA, Rosetta, MPS for OSIRIS team, Mark McCaughrean
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There are many such pits on the surface of Comet 67P/C-G, but they're not due to external impacts.
Rather, they're akin to sinkholes, where volatile material inside the comet, close to the dusty surface is heated & escapes into space, leaving a large void underneath which can then collapse.
https://sci.esa.int/web/rosetta/-/56121-comet-pit-formation
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The pits were found to be active, with jets of material coming out of them.
https://www.esa.int/Science_Exploration/Space_Science/Rosetta/Comet_sinkholes_generate_jets
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And the interiors of some of the pits were seen to have a peculiar semi-regular texture, with lumps & bumps about 3 metres in size.
Similar structures were found in exposed locations elsewhere on the comet & were christened "goosebumps".
The theory was that these were perhaps the characteristic-sized primordial lumps of material that agglomerated to form the comet in the first place, billions of years ago.
https://sci.esa.int/web/rosetta/-/55305-comet-goosebumps
https://sci.esa.int/web/rosetta/-/55295-getting-to-know-rosetta-s-comet
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However, later work looking at the wider array of structures & fractures on the network emphasised the importance of thermal processing on material with water in it, as Comet 67P/C-G has plenty of.
In this model, a kind of freeze-thaw [edit: badly worded: a thermal expansion & contraction of a solid – no liquid phase] cycle takes place, causing material to crack & fracture on characteristic length scales.
Similar structures are well-known on Earth & Mars.
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015GL064500
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Work was done by Atree et al. a couple of years after the Rosetta mission ended on figuring out how this thermal fracturing process would work in detail on comets like 67P/C-G.
https://www.aanda.org/articles/aa/full_html/2018/02/aa31937-17/aa31937-17.html
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And a full observational paper was written around the same time by Auger et al., titled:
"Meter-scale thermal contraction crack polygons on the nucleus of comet 67P/Churyumov-Gerasimenko"
https://www.sciencedirect.com/science/article/abs/pii/S0019103516300410
Unfortunately this is behind a paywall at Icarus (boo!), so you likely won't be able to read it, although you can get an impression of the graphics here:
https://ui.adsabs.harvard.edu/abs/2018Icar..301..173A/graphics
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So, once a sinkhole forms on a comet, like this one in the Seth region near the neck between the two lobes, the exposed interior material gets processed by sunlight & forms this characteristic scale "crazy paving".
Later, like the rest of the comet, it gets covered in dust as material is lifted off the comet, the gases escape, & some of the dust falls back down.
This cycle repeats each time the comet gets near the sun, changing the colour of the surface.
https://www.esa.int/Science_Exploration/Space_Science/Rosetta/The_colour-changing_comet
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The Filacchione et al. paper on the way that Comet 67P/C-G changes its colour during its orbit & periodic journeys into the inner solar system can be found here, but is also paywalled, unfortunately:
https://www.nature.com/articles/s41586-020-1960-2
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Back to the pits & the dragon scales or "thermal contraction polygons", I'm not aware of an image which shows them quite as clearly as the one at the end of that movie sequence.
It was taken by the OSIRIS narrow-angle camera at just 6.3km distance, resulting in a resolution of about 12 centimetres per pixel, giving a nice view of the 3 metre-sized scales.
For reference, the original image name is:
N20160522T221253020ID4FF22
& the wide-field one is:
W20160522T221252757ID4FF12
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These & other Rosetta OSIRIS images can be downloaded in full-resolution & dynamic range (the latter being particularly important in this case, to see inside the 185m deep Seth pit) either from the OSIRIS gallery or ESA's Planetary Science Archive:
https://rosetta-osiris.eu
https://psa.esa.int
There are doubtless other treasures still to be found in there, but just be careful not to wake any sleeping dragons 🐉😬✌️
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And thanks to both Nick Thomas & Ramy El-Maarry, both heavily involved in Rosetta science & with many other planetary missions, for taking a look at this movie yesterday & sending kind comments.
I mean, just because I hadn't seen 67P/C-G dragon-scale images this good before didn't mean that they weren't out there in the literature, but it appears that this is among the best yet found.
Mark that as a win for the curious & dynamic-range-hungry astronomer 🙂👍
12/12
@nick_attree Ah ha – excellent, Nick 🙂
I was surprised when I dug into the OSIRIS images yesterday – I don't think I've seen many if any images that show the polygons this clearly& from this close up.
@barrygoldman1 Woah, woah, woah – that’s a case of me being too loose there. I shouldn’t have said “freeze-thaw”, which indeed invokes a liquid phase.
More properly, I think it’s a stress-related phenomena as solid material (ie the ice-dust matrix here) expands & contracts as the solar insolation changes, & also linked to desiccation as the volatiles in the matrix sublime off.
@nick_attree is the expert: perhaps he can comment.
https://www.aanda.org/articles/aa/full_html/2018/02/aa31937-17/aa31937-17.html
damm no chance of pressure building up under warming ice to create conditions for even a LITTLE water condensation under there?
i WANT commets to be chemistry couldrons!
@barrygoldman1 @nick_attree David Brin & Greg Benford have you covered 🙂
@markmccaughrean @nick_attree haha
but seriously, one thing the last few decades has taught us is that there is a LOT more liquid (water and methane) out there than we had imagined. so i'm hopeful. even convecting nitrogen paste on pluto!
@barrygoldman1 @nick_attree Indeed, there are many places with liquid water & hydrocarbons, but I strongly suspect that comets will not be one of them. I'm not an expert, but combining the considerable porosity of a comet with its really low gravity, I can really see how pressures & temperatures could ever get to a place where liquid water will exist.
That Brin & Benford book is pretty good though, IIRC, even if it's almost 40 years old now.
Mark McCaughrean
Dr. Nick Attree on Mastadon
Barry Goldman
Bleyddyn