rwg: Steel? Ice? I think we're taking too mechanistic a view. At these scales and energy densities, everything (air, water, crust, impactor) is just fluids of various densities.

--to the extent that is valid, you are already dead. More precisely, your microbes are already dead. Only way you microbes survive is if they are inside a chunk of rock, ice, or whatever, which stays cold and stays solid and act like normal ice, rock acts the whole time.