Balmy Old Mars

Present-day Mars is bone dry and freezing cold, capable of sustaining neither water nor life. Global mean temperature on the Red Planet is 218 degrees Kelvin, or about -67 degrees Fahrenheit.

But close observations by spacecraft suggest a very different Mars. Specifically, say Penn State geoscientist James Kasting and Steven Squyres of Cornell, channels and valleys scarring the planet's landscape offer dramatic proof that water once flowed across its surface. In order to sustain this aqueous environment, Martian temperatures had to be substantially warmer than they are now. But how warm? And where did the water come from? Scientists have been asking these questions since the 1970s, the heyday of Mariner 9. In an August 5, 1994, article in Science, Kasting and Squyres reconsider the puzzle.

"Until recently," they write, "it was believed that early Mars could have been warmed substantially by the greenhouse effect of a dense CO2 atmosphere," one that retained the sun's heat. But this picture doesn't match with prevailing theories of solar evolution, which suggest that the young sun was fainter than it is now. "Mars receives only 43 percent as much sunlight as does Earth today and would have received even less in the past," they write. "Its atmosphere should therefore have been filled with clouds of CO2 ice," clouds which would have decreased temperatures on the surface by reflecting the sun's heat away from the planet. A Martian greenhouse effect would have required the abundant presence of other greenhouse gases in the paleoatmosphere.

Kasting and Squyres consider this possibility unlikely, given Mars' current atmospheric make-up. Instead, they propose a different scenario, one that depends more on geothermal heat than on climate.

Early Mars, they write, was frequently struck by meteors. The heat from these impacts, along with that left over from the planet's formation, would have made the interior of Mars plenty hot—hot enough to draw water to the surface by convection.

But convection alone could not account for the presence of those valleys, some of which stretch for 150 kilometers or more. In order for water to flow such distances across the planet's surface without freezing, they acknowledge, there must have been some degree of climatic warming.

But maybe not that much. Given other conditions their computer models can readily envision, "temperatures a few tens of degrees below freezing would have sufficed." Small amounts of atmospheric methane or ammonia in combination with modest increases in atmospheric CO2, they suggest, would have been enough to cause this warming.

James F. Kasting, Ph.D., is professor of geosciences and meteorology in the College of Earth and Mineral Sciences, 211 Deike Building, University Park PA 16802; 814-865-3207. Steven W. Squyres is affiliated with the Center for Radiophysics and Space Research at Cornell University in Ithaca, New York. This work was supported by NASA's Planetary Geosciences and Martian Surface and Atmosphere Through Time programs.

Last Updated March 01, 1995