QUESTION:
Under what conditions can CO2 act as a liquid?

ANSWER from Bruce Jakosky on January 6, 1998:
CO2 can be a liquid if it is compressed to high enough pressures.  The
triple point of CO2, at which the solid, liquid, and gas all co-exist
together, occurs at a temperature of about 220 K and a pressure of about 5
bars.  Pressures this high do not occur at the Martian surface, but they
would occur at a depth of only a few kilometers.  Thus, for example, if the
polar ice caps contained CO2 throughout their entire depth, the CO2 might
liquify at a depth of a few kilometers.  Or, if there had been a thick CO2
greenhouse atmosphere early in Martian history, then it might have been
possible for liquid CO2 to exist if the pressure were about 5 bars.