What do we mean when we say an environment is “habitable”? When referring to exoplanets, the term “habitability” is usually equated to whether or not liquid water can exist on the planet’s surface. But that doesn’t always answer the question of whether humans can inhabit a given environment. After all, Earth’s South Pole doesn’t have liquid water on the surface. Neither does low-Earth orbit. Yet resourceful humans have been inhabiting both locations for decades.

What about Mars? Mars is on the outer boundary of our solar system’s habitable zone, and we know what looks like briny, liquid water can exist on the surface for short periods of time. But does that really make Mars habitable? From a practical standpoint, the answer depends on what technologies we bring there to create our own artificial habitable zones on the surface.

Long-term habitation on Mars will require us to master the conversion of raw Martian materials into resources we can use to survive. Fortunately, Mars has a wealth of these materials, making it arguably the most human-habitable place in the solar system, other than the Earth itself.

In a 2014 conference at the NASA Ames Research Center, Dr. Chris McKay, a planetary scientist and founding member of The Mars Society, presented a list of Mars’ most important resources that early Martian colonists would exploit to make the planet habitable. Here’s a look at each of those resources, in the order of ease McKay predicted they would be accessed:

Atmospheric CO 2

Mars’ atmosphere is its most easily accessible resource, providing feedstock for manufacturing methane propellant. The chemistry involved in separating it is simple, low power, and has been employed on Earth for more than a century.