The eroded canals and floodplains of Mars suggest that water once flowed freely on the red planet. Scientists have been stumped, however, about how the atmosphere could have ever retained enough heat to thaw ice on the planet's surface. Now, an analysis in today's Science  says that carbon dioxide crystals, like the CO2 gas surrounding Earth, could have raised temperatures in the martian atmosphere eons ago. Experts say that this greenhouse effect could have made Mars more hospitable to life--boosting the odds of finding life outside our solar system.
Planets reflect much of the sun's rays, which radiate back into space unless bounced toward the surface again by atmospheric molecules such as water or carbon dioxide. Six years ago, James Kasting, a planetary scientist at Penn State University, University Park, estimated that the amount of carbon dioxide needed to insulate Mars would create so much pressure that the gas would condense to a liquid and fall out of the clouds.
But in a new model of the martian atmosphere, Ray Pierrehumbert of the University of Chicago and François Forget of Curie University in Paris suggest that frozen crystals of carbon dioxide could have warmed the planet. These crystals allow the visible wavelengths of sunlight to pass through them to the surface, but they would reflect the longer wavelengths of light rebounding off of Mars. The pair estimates that 60% of this radiation would be reflected back to Mars, warming the planet enough to melt frozen water some 3.5 billion years ago.
The model is "good news" for those searching for extraterrestrial life, says Kasting, because it suggests that more planets could exist that might contain liquid water. But what ever happened to this great atmosphere Mars once may have had? One theory is that most of the carbon dioxide reacted with the water and became trapped as carbonate in the planet's rocks.