In the most extreme cases of an early iceworld Mars that has been  hit by a major impact (of at least Argyre/Hellas/Utopia scale, or even the hypothesised Boreal basin impactor), then the majority of the polecaps will have been vaporised and a thick (~15 bar) CO2 atmosphere will have been generated. As the impact heat is dissipated to space over a timescale of (perhaps) decades, then that thick atmosphere will condense. Liquid CO2 rain will pour down on the planet and rivers of liquid CO2 will flow through erosoin channels to fill lakes and perhaps seas of liquid CO2!

This scenario would be complicated by the existance of large volumes of liquid water, too. Perhaps the water, being lighter, would float on top of the liquid CO2? However, I have reservations about this scenario because if it had ever occurred, then Carbonate production would have been rapid. If the atmosphere lasted more than a few decades after each impact, we might expect to see more extensive carbonate deposits. It is possible, but only if these episodes were very brief.
 

      Created: May 2002
      Last modified: May 2002
      Authorised by:  Head, Earth Sciences

      Maintained by: Nick Hoffman
      Email: nhoffman@unimelb.edu.au