Uh, I leave the details to someone who knows better :) - it is a while since I checked, and it might indeed be underspecified right now.
To the best of my knowledge, there is only one widely used coordinate system for each Mars and Titan. I might be wrong. But in the worst case we would need to specify the default system for either.
I am not saying that the whole thing is not a problem - I am just saying that the data model, as spec'ed and implemented, has a space for solving it. It is obvious that without support in the UI the whole thing is slightly moot anyway.