I'd have to agree with Tom here.

If you're saying that at the stage of having a small (<1000) colony on Mars that colony would be totally self-sufficient in the case of the Earth being wiped out, I doubt it. I doubt very much they'd have every resource, technical capability, etc. to make it totally on their own. They'd certainly be very vulnerable to epidemics (either among people or affecting food stuffs), system failures, etc.

By the time we have a full-fledged civilization on Mars, we probably have the capability to handle most situations. The exception would be a comet in on visit 1 from the Oort Cloud, but even then, I suspect that a fully self-sustaining Mars civilization will take longer than it will to have an adequate system of sensors, high delta-v propulsion, and whopping big bombs to deal with the threat.

It'll take a long, long time to have a Mars civilization that needs nothing from Earth, unless you posit such big technological advances that the impact threat is taken care of too.