From: g_d_pusch_remove_underscores@xnet.com   
      
   cray74@hotmail.com (Mike Miller) writes:   
      
   > If an Earth-like planet was struck by a 1000km-diameter planetoid (a   
   > mini-version of the Mars-sized rock that might have hit Earth and   
   > formed the moon), would it:   
   >   
   > 1) Pump enough heat into the environment to boil off the oceans?   
      
   It would not only boil the oceans, it would not only melt but volatize   
   significant amounts of both itself and the Earth.   
      
      
   > 2) Shatter the crust around the planet, or just a local continent?   
      
   There would not be _any_ crust left --- just a global layer of magma.   
      
      
   > 3) What time scale do the effects take place over?   
      
   For a straight-on impact, on the order of 1.5 minutes --- the time it takes   
   for the planetoid to move through its own diameter at escape velocity.   
      
   For a glancing impact, on the order of 90 minutes --- the time it takes for   
   the debris to orbit the Earth once.   
      
      
   > Would it take weeks or hours for the crust to sustain its damage?   
      
   The crust would be demolished in on the order of hours --- the time it   
   takes for a seismic shock wave to propagate through the Earth.   
      
      
   > Weeks or hours for the impact heat to suffuse the environment?   
      
   The heat will "suffuse" the atmosphere and biosphere in the time   
   that it takes the debris from the impact to re-impact the earth ---   
   on the order of 90 minutes or so. However, it will take on the order   
   of geological timescales for things to settle down and thermally   
   re-equilibrate.   
      
      
   -- Gordon D. Pusch   
      
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