Transactions on either planet could happen in either Terracoin or Marscoin. It would be silly for people on Mars to conduct local transactions in Earth's Bitcoin (and vice versa), but it would be technically possible.
I see no need for dedicated edge nodes or anything similar. Today's Bitcoin could handle this situation (unless I'm missing something big).
I considered that there would be perhaps completely independent networks on other planets if this situation would happen, with some sort of "exchange rate" between the two currencies and some sort of gateway (exchange) between the two that could take care of interplanetary exchanges.
Here is another analogous situation: I happen to live in a mountain valley in a city with a bit over 100k people (including the corporate HQ for a fortune 500 company and a major internationally noted research university.... to note network bandwidth involved here). In a fit of horrible planning, it turns out that the communications into this valley were 100% dependent upon a single fiber optic cable bundle that was accidentally cut one day when some idiot on a road construction project ran a backhoe through the bundle. Yes, it was buried about 20' underground in a very deep trench and other precautions, but sometimes even that can't be helped. For the 2-3 days that it took to get the cable repaired again, all outside network and even telephone communications outside of the valley simply stopped. Internal communications were just fine and I could talk to the university, other companies, and even other residential customers.... but it was as if the entire outside world ceased to exist.
How does Bitcoins deal with a situation of that nature where perhaps some of the clients could talk to each other and set up a temporary sub-net (perhaps even creating bitcoins and certainly other transactions)? When the communications is restored to the larger network, I would imagine all generated coins would be essentially ignored, but I think it would still mess up transactions from the sub-net from previously generated coins (coins created prior to the disruptions and acknowledged by the network).
So what happens to transactions that include mixed "new" and "old" bitcoins in a disruption like this? The "new" bitcoins are the ones that were generated during the isolation and then spent, but the "old" coins are those which are recognized by the network as a whole.
