[netcan]

I don't think you can really "justify" extraterrestrial colonization on critical, legible, practical grounds. There is no roi.

The real, immediate reasons are inspiration related. Some of that gets very practical, though it's always at least speculative. Technological spillover, for example, can be justification in itself. Earthen solidarity is maybe another, and I do actually think that the existence of a few of us elsewhere helps form the concept of "us." Both of these are "on mission" in the sense that they might be key to human survival.

Anyway... Mars colonization is a horizon goal. It's something to focus our minds. Practical activities are dictated by minor goals (eg visit Mars, generate energy locally, etc). A self sustainable mars colony that can survive earths vogon destruction is so far ahead that it's more of a symbol than anything.

We didn't have true practical reasons for going to the moon, or for the ISS either. The main reason is (imo) that space faring is a human mission, for its own sake.

No matter what though, I dont think space faring is something you could sell to your conservative money manager. It's imagination dependant (and inspiring)... a job for da Vinci, not Medici.