I completely agree that it is vital to put as many degrees of separation as possible between earth and off-world colonies to prevent contagion.

I completely disagree that Mars has anything to recommend it. A low gravity icy body has all the qualifiers, not Mars. It is the old misconception that Mars is somehow a sweet spot when it is not. It seems “just close enough” and because it is a planet it seems more hospitable. In reality it is not close enough to colonize with chemical propulsion and as a big rock with a deep gravity well it is not a good place to try and land and excavate.
Ceres is second base IMO

A few questions here. How long do we wait before determining that there is no bacterial life on Mars? How many aquifers must we test before so determining?

Alsi, what if we do find bacterial life on Mars? Should we hold off on human colonization there? If so, how long and what criteria before we start human colonization? Would it matter if the bacteria were similar or quite different than Earth’s life?

Perhaps most importantly, why the presumption that exposing Martian life to Earth bacteria would cause the extinction of Martian life? Has there ever been a recorded instance of the extinction of bacterial life here on Earth due to human impact? Wouldn’t Martian bacteria be more fit for the Martian environment and so it would be the Earth bacteria that would have a hard time of it and have difficulty maintaining a foothold. For example, I understand that the surface of Mars has chemical species which tends Ti sterilize life — an environment unlike Earth’s.

I am for colonizing the Moon first because I believe it to be the cheapest, fastest, safest place to do so. Time is of the essence here. But I think that, just as soon as we establish a small self-sustaining colony there, then we should quickly take those lessons learned and establish another colony further out. The Moon is pretty safe. But on the off chance that Earth is suddenly hit by a comet large enough to kill all human life, then it would probably be big enough to eject debris far enough to pelt the Moon putting and nascent colony in jeopardy. Much more likely is that self-replicating tech contaminate the lunar colony but this can be mitigated using continuous quarantine measures. Mars has many things to recommend it as a place for a long- term colony. We shouldn’t neglect it.

As for the topic of the establishment of a Moonbase as a launchpad for interplanetary travel — that is a related but different topic, and one I would agree with John also. I was not suggesting an alternative to this, but an additional longer-term strategy.

“that in some million years from now Mars may be flourishing with vegetation — evolved to suit the terrain — that a colony there could live off”.