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.

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!

In a paper published in the February issue of the Journal of the British Interplanetary Society titled “The Sustainability Solution to the Fermi Paradox”, we challenge this conclusion with the possibility that exponential growth is an unsustainable development. That is, even if an extraterrestrial civilization has colonized the galaxy, it would have done so through rapid unsustainable growth and collapsed upon reaching a physical resource limit. Not enough time has yet passed for a sustainable growth civilization to colonize the galaxy, so there is still promise in the search for extraterrestrial life. Furthermore, though the absence of extraterrestrial civilization does not imply the unsustainability of exponential growth, it does increase the probability that humanity should transition to sustainable development in order to prevent its collapse. A more detailed writeup is available on the Lifeboat Foundation blog.

In other news, NASA’s Kepler Mission successfully launched yesterday evening! Over the next three years, Kepler will observe 100,000 stars in a patch of the Milky Way in search of Earth sized planets. This is the first mission with the capability of detecting Earth at a distance, so with any luck we’ll soon have a better idea of just how common small rocky planets are in the galaxy.

We argue that the warm, ice-free climate of the early Earth (2.8 billion years ago) was maintained by a water vapor/carbon dioxide/methane/ethane atmospheric greenhouse effect that offset the ~20% reduction in solar luminosity from the faint young sun. Furthermore, a stabilizing feedback between life and the climate system may have resulted in a thin stratospheric organic haze that maintained above-freezing temperatures and shielded ultraviolet radiation. An excellent write-up of our work is available at The Planetologist.

I’ve given this talk several times over the past couple years, most recently on the Forum for Astrobiology Research (which should eventually be available as a podcast), and it feels good to finally see the paper come out.