I love just how many bitcoiners actually spend time thinking about full-scale economies on other planets. I guess we're all optimists.

There's a few ways the economy on Mars would work, given a BTC-based earth economy:

1. For a long time, a Mars colony (or any other distant colony) is essentially run by one organization, or a very small set of them. Just like today's astronauts, the colonists don't need a currency system while up there. Even if they run a local bitcoin node, because their link back to earth is completely controlled by the guys paying them, they can't know if they've been shunted to a different branch or not.

2. If a small Mars economy develops, they can free ride on bitcoin's chain for a long time. That way prices are already known, the infrastructure already exists, and you don't need to run power-hungry mining rigs in a resource-strapped context. The downsides are having to rely on zero-conf being reasonably hard to double spend against, and having to wait an extra 8-48 minutes when actually waiting for confirmations.

3. Once those constraints are too problematic there may be a need to start a mars altcoin, or figure out some clever way to have multiple chains within bitcoin, one for each sufficiently distant system. We have a lot of time to figure that out though.