Last summer in Breckenridge, Colorado, a quiet scientific revolution took place. Many of the world's leading Mars researchers came together to address issues surrounding the red planet's formative years, known as the Noachian Age, an era that ended some 3.8 billion years ago. Based on the presence of channels and extensive valley networks, mainstream theorists have suggested a "wet" early Mars - - warmed by atmospheric greenhouse effects, bathed in thunder-showers, and dusted by snowstorms. However, in recent years there has been a slight paradigm shift.
"Up until just a couple years ago, warm wet Mars was popular among quite a few of us, including myself," said James F. Kasting, an atmospheric modeler at NASA's Ames Research Center. "It was based on an idea that you could warm Mars with a dense atmosphere of carbon dioxide |C|O.sub.2~~. There were actually some good theoretical reasons for thinking that; the paradigm was that if there was enough carbon around, C|O.sub.2~ would simply accumulate in the atmosphere until enough of a greenhouse effect built up, turning Mars into a moderately earthlike planet."
But new models demonstrate otherwise, according to Kasting. "We realized that there is a limit at which the greenhouse effect doesn't work anymore because the C|O.sub.2~ begins to condense out of the atmosphere." With the new models, Kasting saw that all the early Mars calculations were too optimistic about greenhouse warming. He now believes that early conditions on Mars were not so earthlike after all.
Steve W. Squyres, a planetary scientist at Cornell University, commented on a second change in thinking: "Thermal models for the interior of Mars that were done back in the early 1980s had Mars starting off cold. But we've come to realize that it's almost inescapable that the interior of Mars started off very hot." Squyres credits better models and new data from meteorites that have come from the Martian surface for the change in perspective. "The greenhouse effect only gets you partway to explaining ancient features on Mars. You need something else to help, and something else is there in the form of internal heating." Whether it was a warm climate, internal heating, or a combination that contributed to the present landscape of Mars, Squyres says most researchers agree that conditions on Mars during the Noachian Age were markedly different from those of today. "The atmosphere was significantly more dense; the global heat flow was greater; the rate of valley system formation was a lot higher back then, and overall rates of deposition and erosion were very much higher." The ill-fated Mars Observer was to have shed considerable light on the reasons for these differences. For now, Mars researchers must continue to ponder them from afar.