Mars, may be planned, but apparently there's currently no work being done on it. Destructoid spoke with Spiders CEO Jehanne Rousseau, who said that the company sorta needs a publisher to fund the project first.
Rousseau explained that the developer is currently shopping for a publisher and will give a "final answer" on an Xbox 360 version when it has a contract in hand -- or lack thereof. Bucking the trend, Spiders is developing Mars for PS3 because it's a "very complicated platform." Hopefully, the extra work will put it in the company of Naughty Dog and Sucker Punch -- and not that of the ill-fated Factor 5 and Free Radical.