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. Sandia has done some finite element simulations<http://www.sandia.gov/media/comethit.htm>, but seems to have (re?)moved them. The blobbiness is puzzling. They even did Tunguska<http://www.youtube.com/watch?v=Gr7i33UhmC8>. I think if we look around, we'll find F.E. video of an escape speed "rebound jet" poking clean through the atmosphere. --rwg In a Tunguska article think I saw mention of a modest suspected ice impact crater. Maybe we should drill into it? Maybe with Andromeda Strain precautions?