Whether ancient Mars harbored life has tantalized scientists and the public for more than a century. Today the surface of Mars is cold and dry, inhospitable to life as we know it. The martian atmosphere today is thin (CO₂-dominated with 0.01 bar pressure, compared to Earth’s N₂-dominated with 1 bar pressure at sea level). However, there are strong indications from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission — which detects atmospheric loss rates and has found widespread evidence of fluvial activity (deltas and channels) — that the ancient martian atmosphere may have been much thicker, possibly composed of CO₂ and H₂. This mix of greenhouse gases might have created an Earth-like climate, allowing microbial life to flourish. The question is: if Mars once had a temperate climate and harbored life, what led to its catastrophic change in climate, with loss of atmosphere and habitability?

Boris Sauterey from the University of Arizona and colleagues used several models to investigate the habitability of the martian subsurface 4 billion years ago and determine a possible cause of Mars’ catastrophic climate change. These included climate-atmosphere models to determine surface temperatures and the composition of the atmosphere, elevation and hydrosphere models to determine where liquid water could exist, and ecological models to determine microbe population growth. Results of this work suggest that hydrogen-eating microbes could have flourished in the early martian subsurface that remained ice-free, as on Earth today. However, as these microbes scoured H₂ and CO₂ from the atmosphere, Mars’ climate would have been affected by the loss of these greenhouse gases, with surface temperatures plunging nearly 200°C. Any near-surface microbes would have been forced to bury deeper to survive the induced ice age. Ultimately, in this model, the microbes would have rendered any habitat on Mars uninhabitable for themselves. These results suggest that even simple life can alter the surface of a planet, perhaps irrevocably, and may commonly cause its demise. READ MORE