Mars may have once been a blue and water-covered planet. According to a new study published by Kaveh Pahlevan and colleagues at Arizona State University, Mars’ earliest atmosphere was denser and more hydrogen-rich than its present thin, carbon dioxide-rich atmosphere.

To determine the composition of the ancient martian atmosphere, the team developed atmosphere evolutionary models that begin with the high temperatures associated with Mars’ early molten state and follow through to the formation of the first oceans and atmosphere. The models showed that the main gases emerging from the early magma ocean would be a mix of molecular hydrogen (H) and water vapor. Hydrogen is the lightest molecule, but in a dense atmosphere, it acts as a powerful greenhouse gas. The team’s calculations show that there was enough H to have facilitated warm oceans to be stable on the martian surface for millions of years.

The models suggest that, like modern Earth, water vapor in the martian atmosphere condensed in the lower atmosphere to form clouds, creating a “drier” upper atmosphere. Molecular hydrogen (H₂) did not condense and was the main constituent of the upper martian atmosphere, where it was gradually lost to space. These results explain the measurements of 3-billion-year-old clays made by the Curiosity rover. These clays have a hydrogen isotope ratio (D/H; ratio between deuterium (heavy hydrogen, ²H) and hydrogen (¹H)) three times higher than Earth’s modern oceans. This means that the surface water of Mars that reacted with rocks to produce these clays was rich in deuterium, and the only way to achieve this enrichment was to lose the light H gas to space. These findings have not been explained by any model until the present study. If early Mars had a hydrogen-rich atmosphere and water-rich surface, this has important implications for its potential habitability. READ MORE