Evolution of Mars' Atmosphere and Climate

Carbon dioxide is the primary component of Mars' atmosphere at present. It is conventional wisdom that CO2 was the dominant atmospheric gas from the time of earliest degassing of the Martian interior. The dominant degassed volatile on/near the Martian surface was (and probably is) water. The evidence for very large volumes of water in Mars' distant past is indisputable. Greenhouse gas (usually considered to be CO2) concentrations in the Martian atmosphere at present are obviously insufficient to either produce a warm enough climate or high enough atmospheric pressure to sustain liquid water for any significant length of time. Considering the increase in luminosity of the sun over the last 4.5 BY, the presence of liquid water on Mars' surface in the distant past would require even higher concentrations of greenhouse gases at that time. To get from such an early warm wet climate, with a faint sun and high atmospheric CO2 levels, to the present cold climate seems to require loss of huge quantities of CO2 and perhaps water (hydrogen). Recent results suggest that atmospheric loss mechanisms are not sufficient to the task. Even worse, surface robotic missions provide little or no evidence for the carbonate minerals that should be abundantly produced by a water and CO2-rich surface environment. Clearly we have a problem. The solution is simple. The primary carbon species degassed from Mars' interior was methane, probably accompanied by ammonia, and certainly by a much larger amount of water. Methane is a much more potent greenhouse gas than CO2. Although atmospheric photochemistry destroys methane by producing various polymerized and oxidized organic species, deposition of these molecules on the Martian surface and in the early Martian ocean provides a large reduced carbon reservoir from which methane can be regenerated through hydrothermal reactions driven by volcanism. Equable surface climates (and flowing water) can be maintained as long as hydrothermal activity produces enough methane to offset photochemical destruction, at sufficient levels for greenhouse warming. Over time (and continuing until the present) the loss of hydrogen to space leads to oxidation of the surface and the production of CO2. Because of Mars relatively small size, the loss of internal heat sources eventually reduces volcanism and hydrothermal activity below the point at which methane can be regenerated to maintain greenhouse warming, even with the increasing luminosity of the sun. At this point Mars lapses into the cold regime of the present day. The transition may well have been punctuated by occasional outbursts of methane to temporarily restore mild conditions. With secular cooling the water of the Martian ocean froze, along with its residual reservoir of carbonaceous compounds. The oxidized surface we see may be a thin veneer produced and maintained by hydrogen loss (oxygen production) from the small amount of water released into the atmosphere over time.