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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@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

@nick_attree @markmccaughrean

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 🙂

https://en.wikipedia.org/wiki/Heart_of_the_Comet

@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.