My vote
A thermal contraction crack due to obliquity (and hence temperature) change. But don't get excited, we're still talking -40C rather than -60!
(If you look at MOC images and Viking, you see a nice pattern of polygonal cracks at km-scale)
Apologies for typos. I'm in Tokyo Narita airport and conditions are less than ideal, but at least I don't have to eat any more raw fish!
