role of magnetic field
I may have a false assumption here, but it is my understanding that it was the solidification of Mars' liquid core (due to relative Mantle:core size, abundance of radio isotopes, etc) that shut down it's magneetic dynamo, which "dropped it's shields", forever, and then any proto- plate techtonics (in this case basaltic differentiation) stopped, the atmosphere was largely ripped away, and the majority of the water (if there were major bodies of surficial water) boiled off. (would that leave evaporites..hum...)
I need to read more, I think.
But wouldn't that mean that any attempt to terraform Mars would have to deal with magnetic shielding on a huge scale? Or SPF 200K sunscreen?