It is estimated that a terraformed Mars would have an atmospheric pressure from 0.2 to 2 atmospheres in pressure based on worst case to best case estimates of its volatiles inventory. Since this pressure is decreased by a factor of 0.38 relative to the same atmospheric mass on Earth due to Mars' gravity, the mass above any unit of area on terraformed Mars would be roughly 0.5 to 5 times above an equivalent area on Earth.

According to Duncan Steel, the minimum penetrating size of an impactor is roughly proportional to the mass of air it must pass through. This means a terraformed Mars could actually afford better protection against small impactors than Earth and would at least be in a similar ballpark.

With regard to cosmic rays, in the absence of a magnetosphere the best protection is to put mass between yourself and the incoming rays. This is the basis for the living underground plans of dealing with this hazard. While a terraformed atmosphere would not provide the same level of protection from Solar flares and cosmic rays as Earth's Magnetospher, I believe it would provide a significant improvement over an airless world. I'd appreciate some more exact numbers from any of the real experts who post here.

In the worst case that terraforming is not possible, I am truly amazed that Grondine either appears to believe living underground is impossible or unaware that such simple solutions are possible.

Regards,
Mark