I haven’t yet read any of the Edgar Rice Burroughs, so my knowledge of John Carter’s setting is close to null. What I do know is that John Carter begins his adventures still on Earth and that he goes back to Earth accidentaly once or twice. Does the John Carter of Mars RPG takes into account the possibility of world travelling, or is it a strictly Barsoom based game?
The games takes place on Barsoom aka Mars.
Hullo, MigRib,
As Katowice said, the game takes place on Barsoom (Mars). John Carter is transported through mystical means to Barsoom in the first story, his physical body still left behind in the cave where it happens. He returns to Earth (Jasoom) at the end of the first novel, and it is ten years before he is able to return to Barsoom once more. Given the source material and the nature of the game itself, I don’t really see a need or possibility for world travelling, other than to bring Earthborn to Barsoom.
Hope that helps. 
JohnK