Satellites and rovers sent to Mars keep giving us compelling geologic evidence that liquid water flowed on the surface of the red planet in the distant past. Three billion years ago, when oceans may have existed on parts of Mars, the sun was about 30% fainter. Mars today is well below the freezing point of water, so any lakes or oceans would be frozen over. In the distant past, then, this problem is even more pronounced: how was Mars able to stay warm enough to sustain liquid water?

Many attempted resolutions have been proposed to this problem, but none has provided a complete solution for a warm, wet early Mars. In a recent paper published in Earth and Planetary Science Letters, on which I am a co-author, we argue that greenhouse warming by sulfur dioxide could not have kept early Mars warm enough. Sulfur dioxide has been suggested in the literature because it is an effective greenhouse gas, similar to carbon dioxide or methane. However, we show that atmospheric photochemistry with sulfur dioxide leads to the production of sulfate aerosols in the upper atmosphere that absorb incoming sunlight and cool the surface. Thus, sulfur dioxide may have caused net cooling on early Mars, rather than warming.

We’re still trying other mechanisms to explain a warm, wet early Mars. Most likely, it was some combination of processes, including several greenhouse gases and warming by clouds. A negative result for sulfur dioxide is not as exciting as a solution to the early Mars problem, but it’s still a small step forward.