The answer, says a University of Chicago climatologist and his French colleague, is reflective carbon-dioxide ice clouds that retain thermal radiation near the planet's surface. The scientists' theory is published in the Friday, Nov. 14, issue of the journal Science.

"This is a problem that has perplexed scientists ever since the '70s, when Viking provided the first detailed images of Mars," said Raymond Pierrehumbert, University of Chicago Professor of Geophysical Sciences. "How can you account for Mars being warm enough to have flowing water, especially when the sun was actually fainter early in Mars' evolution?"

Pierrehumbert collaborated with French climatologist Francois Forget, from the Laboratoire de Meteorologie Dynamique du CNRS in Paris.

Previous models of the atmosphere of ancient Mars have incorporated carbon dioxide in the atmosphere to use effects similar to global warming to heat the planet. "The problem was," said Pierrehumbert, "when you try to put enough CO2 in the atmosphere to warm it sufficiently, the carbon dioxide condenses out. It was thought that the thick clouds that form as a result would reflect sunlight back to space and actually cool the planet.

"When we re-examined this, we found that this dry-ice `blanket' actually warms the planet because it reflects infrared light back to the surface more than it reflects solar radiation outward."

The curious property of carbon dioxide ice clouds, as opposed to the water ice clouds found on Earth, is that the particles are large enough to scatter infrared light more effectively than visible light coming from the sun. Ordinary, Earth-type clouds absorb heat from the planet's surface and re-emit it both back to the surface and to outer space, losing half of the heat in the process.

"But the carbon dioxide clouds act like a one-way mirror, and, although not a lot of sunlight gets through to the planet's surface, what does reach the planet is converted to heat, which the clouds then reflect back to the surface," said Pierrehumbert. "This mechanism produces a large enough effect that it can, in fact, warm the planet to the point where it is possible to have liquid water."

Pierrehumbert said this climate model provides some clues as to the types of life forms that might have evolved on Mars. "If we're going to be looking for analogues of terrestrial life forms on Mars," he said, "then we should be looking for the kinds of organisms that might evolve in extreme environments, like the bottoms of oceans or in caves.

"The conditions on early Mars -- some four billion years ago -- were a little more like the conditions at the bottom of the ocean than like a rainforest. It would have been dark, warm enough for liquid water, but without a large energy source for photosynthesis," he said.

Pierrehumbert and Forget's model also extends the habitable zone on extrasolar planets and increases the likelihood that life exists outside our solar system. Previously, scientists thought that only planets orbiting within 1.37 astronomical units (one AU is the distance between Earth and the Sun) of a star could have water above the freezing point. But if the planets have carbon-dioxide ice clouds, they could have liquid water as far away as 2.4 AU. Mars is 1.52 AU from the Sun.

Similarly, carbon-dioxide ice clouds could have played a role in warming Earth when the Sun was fainter than it is today, preventing a global freeze that could have kept Earth locked forever in ice. If the Earth had ever cooled to the point where its oceans had all frozen, it would never have warmed up again because too much solar radiation would have been reflected back to space by all of the surface ice.

Pierrehumbert and Forget say their model fits well with a theory proposed by Carl Sagan and Christopher Chyba, and published in Science earlier this year, that a methane and ammonia atmosphere warmed early Mars. "The problem with methane," said Pierrehumbert, "is that it breaks down very quickly when exposed to sunlight, so you need a biological engine -- life on Mars -- to feed the atmosphere as the methane is depleted. Our model provides the starting conditions under which life could have evolved and started the production of methane gas. And once the gas forms, the carbon dioxide ice clouds actually shield the methane from sunlight and keep it from breaking down as quickly."

Pierrehumbert and Forget next plan to tackle the problem of what weather might have been like on early Mars, including the possibility of carbon dioxide blizzards and carbon dioxide-ice glaciers.