Sulfur Can’t Keep Mars Warm

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.

AbGradCon Ho!

My abstract on “Warming Early Mars with NO2” has been accepted for a talk at this summer’s Astrobiology Graduate Conference! I’ll discuss a possible mechanism by which early Mars (3 billion years ago) could have sustained above-freezing temperatures as well as liquid water with a carbon dioxide/water vapor/nitrogen dioxide greenhouse effect. We are currently putting the finishing touches on our manuscript before submitting it to Earth & Planetary Science Letters in a week or so.

The organizers of AbGradCon09 have done a great job of putting together an excellent conference, including a field trip to Mt. St. Helens. Additionally, the conference will be broadcast live on Second Life, so if you can’t travel to Seattle you can still participate!