MARS primarily is a social game of character inter-action. It is set in the hard science-fiction setting of a human exploratory colony on the planet Mars. The game is a rules-light role-playing system. Characters have a Mission Specialty of Scientific, Technical, or Surface Ops, which should direct the focus of their training, education, and experience. Characters should have a pre-Mars background. Characters have six Assets, which are a type of narrative tag. Two Assets must come from the Mission Specialty and the remaining Assets come from either the Mission Specialty or Personal Development areas. Sample Assets include Persuasion, Polar Survival, Drive Rover, Psychology, and Radio Ops. Characters are Expert in two Assets and Good in their remaining four Assets. They are assumed to be Poor in any other Asset activity. Finally, characters should have a Reputation, a secret Motivation, and a secret Ally Group defined.
Tasks are resolved using a five-step Ladder: Superb, Expert, Good, Poor/Fail, and Terrible. The GM determines which level must be achieved, and the character rolls 1d6; 1s have negative effects, 6s have positive effects, and anything else means the character performs the task at their Asset level.
Most of the rules detail the setting, which is comprised not only of the alien environment of Mars, but also of several organizations with financial and scientific interests and pursuits. There is also extensive advice on how to run a campaign that, most likely, will not involve combat or looting activities.