Introduction

    Can we live on Mars? While a familiar topic in science fiction since time immemorial, serious speculation on the possible terraformation of Mars has been ongoing in the technical literature since the 1970s.  These early days saw the likes of Carl Sagan working out the difficulties while popularizing the idea amongst the scientific and lay community. The prospect seemed unlikely however, if not downright far-fetched. But with the recent realization that human industry can, and does, alter the Earth's climate on a global scale, the possibility of a terraformed Mars now seems less like science fiction and more like a reachable, albeit longterm, goal.

    It was realized early on that making a sterile planet habitable is not simply a matter of “bringing over” the necessary ingredients in the right proportions - to create an Earth-like 1 bar atmospheric pressure on Mars would require 4 x 10¹⁵ tons of imported gas! (McKay et al., 1991) Rather, terraforming necessitates the careful tweaking of environmental parameters in such a way as to influence the planet’s own chemical budget. If done so carefully, it may just be possible to create a stable, self-sustaining environment conducive to life.

   What is required in order for Mars to be habitable? Obviously, it depends on what variety of organism you’re discussing - terraforming is generally viewed as an incremental process, so at first we are interested only in creating the conditions necessary for sustaining the simplest microorganisms. This is called ecopoiesis: “...the fabrication of an uncontained, anaerobic biosphere on the surface of a sterile planet.” (Fogg, 1995) Simplistically, it requires that:

                1) the mean global surface temperature be raised by  ~ 60 K (up to the freezing point
                    of  water);
                2) the atmospheric mass be increased (to provide levels adequate for metabolic activity);
                3) liquid water be made available;
                4) the surface UV radiation and cosmic ray flux be substantially reduced.

Fortunately, these requirements are all interrelated by positive feedback. For instance, increasing atmospheric mass will shield the ground from UV and cosmic rays while simultaneously inducing greenhouse heating. The warmed planet will begin to free more atmospheric constituents, inducing further greenhouse heating, etc. Eventually liquid water will be made sustainable, and finally, hopefully, life.

    Ecopoiesis can be thought of as a “bare minimum” requirement - once it is achieved (ostensibly through the introduction of a suitable pioneering microorganism), there still remains a great deal before more complicated plant or animal organisms can thrive. Specifically, O₂ and N₂ partial pressures must be increased to within habitable bounds - oxygen because it's required for respiration, and nitrogen because it is the only constituent present on Mars in sufficient quantities to be a buffer gas. The table below, taken from McKay et al., lists reasonable bounds for temperatures and partial pressures on a terraformed mars.
 
 

    Full terraformation requires a large-scale chemical modification of the atmosphere. Such a task would most likely be achieved through planetary-scale biology over very long time periods: 10⁵ years for an average terrestrial ecosystem, but perhaps an order of magnitude less for a specially-engineered organism with a high O₂ production efficiency. Unfortunately, the necessity of a large available water supply, combined with subsequent substantial O₂ losses due to surface reoxidation, together make this process quite difficult, if not prohibative.

    Thus it would seem that, while warming the Martian surface through a positive feedback cycle of regolith and/or polar cap outgassing might be achievable within a reasonable (100 or so years) timeframe, making Mars “open-sky habitable” to humans is a considerably more difficult undertaking. Still, these processes are worth further exploration, as are various alternatives to full-scale terraformation. One of these examples of "paraterraforming" is the Worldhouse, described below.

    Michael Carr, of the US Geological Survey, said the following of Mars:

    "It's the only planet out there where one can reasonably expect that humans could go and
    establish themselves permanently, for whatever reason. People can't go and live on Jupiter, but
    they can go and live on Mars. I think just because it's available - we almost have the capacity
    to go there already - that it's inevitable that we will go."

Taking this inevitability at face value, the importance of understanding terraformation processes becomes apparent. The problems outlined on these pages may be the key, not only to the success of an eventual Martian settlement, but to any future space exploration at all.