Even though Mars is currently too cold and has too little pressure to prevent water from freezing, the planet had liquid water on its surface in the distant past. That could happen if the ancient Martian atmosphere, thicker than its present-day counterpart, had enough greenhouse gasses to keep the planet warm and the water liquid. So over the past decades of our observations and robotic visitations of Mars, researchers have been looking for evidence for Mars’ past carbon dioxide levels.

The evidence we’ve gathered indicates there was some CO₂ present but not nearly enough to keep water liquid, especially given that the early Sun was less active than it is at present. So far, these estimates contained large uncertainties, so it remained possible that there was enough carbon in the atmosphere to allow the ancient water to flow.

A team of researchers has created a new estimate of Mars’ ancient carbon levels using data collected by the Curiosity rover. They’ve also concluded that there was nowhere near enough CO₂ to warm the planet to the point where water on the surface would remain liquid.