Each planet will have its own locally-mined cryptocurrency (Marscoin, etc), which each have a floating exchange rate to the original Earth-based Bitcoin. Marscoins and bitcoins can be directly traded for each other in order to settle interplanetary trade balances, the only issue is the speed-of-light delay for confirmations (which is unavoidable no matter what). Naturally Bitcoin can't be mined on Mars and Marscoin can't be mined on Earth, but that's not important.