MARS isn’t Barsoom, but licensed property or not you can make it work with a bit of mangling.
My concern here isn’t for balance, but rather to present some ideas for rules that are representative of the setting, a synthesis of the new movie version and the original book version.
Earthmen
Whether by mysterious gases, astral projection, cosmic telegraph or good old-fashioned rocket ships. Men of Earth can sometimes reach Barsoom. Barsoom’s low gravity means that the men of Earth are approximately three times stronger than they otherwise would be and are able to leap three times as far and exert three times a much proportional strength.
Step up an Earth character’s Strength Dice twice while they are on Barsoom. A d6 becomes d10 and so forth. If this takes them over a d12, each step counts as +2. EG: A character that started with d12 Strength would now have d12+4 Strength.
Step down an Earth character’s Agility by one step due to their clumsiness in low gravity. A d6 becomes a D4, for example. If their Agility is already D4 then it becomes D4-2.
This can be overcome by taking the Martian Agility edge, which offsets the Agility penalty.
Green Men
The Green Martians wield enormous spears which do Str +3 damage, have a weight of 20 and a Reach of 3.
Radium rifles will serve as Green Martian rifles and can be found in the main MARS book.
Green Martians stand up to fifteen feet tall (twelve for the females). They have green skin that darkens as they age from a yellow-green to a deep olive (females retain a lighter skin). Their ears are small, directional antennae that come out just above the eyes and males and females alike bear tusks though, in defiance to Earth expectations, the females have the larger horns. Both genders have six limbs, true hands, true legs and intermediary limbs that can be used to run faster or to grasp weapons.
Martians reproduce by laying eggs and most of the time have no idea what young are individually theirs. The horde is all and all children belong to the horde. The eggs are tested those that do not hatch or which contain deformed spawn are destroyed.
The Green Martians are divided into rival hordes and these include the ascendent Thark, the Thurd, the Torqas and the Warhoon.
Green Martians wear no clothing save a simple harness upon which tokens and trophies may be hung. Their children are raised only to fight and survive, without love or compassion. All that is taught is language and combat. The only skills a true Green Martian should need.
Green Martians loathe labour, preferring to scour, fight and take rather than to make, though they do take a pride in their weapons. Especially their long spears and rifles which are crafted and maintained by their women.
As a result of their harsh lives and upbringings, their humour is crude and cruel. They find torture, pain and defeat in others amusing.
