Both the Mars Global Surveyor spacecraft, now orbiting the Red Planet, and Sojourner, the robotic explorer on the Martian Surface, clearly confirm that in the distant past Mars was a warmer, wetter place. Photos from the spacecraft show the dri ed-up river valleys, lake beds, and boulders apparently tumbled by running water. But while the pictures tell one story, physics tell a different one: 3.8billion years ago, when all this water was supposed to be flowing, the sun was about 25 percent fainter than it is today. How could such a feeble ancient sun make Mars warm and wet?

There is, however, a sticking point to this theory. Before carbon dioxide levels on Mars get high enough to warm it past the freezing point of water, the carbon dioxide starts forming clouds. Once the clouds form, carbon dioxide snow starts to fall, deprieve the atmosphere of this greenhouse component.

Two planetary scientists, Raymond Pierrehumbert of the University Of Chicago and Francois Forget of the Laboratory for Dynamic Meteorology in Paris, have modeled the behavior of carbon dioxide ice particles and found that the carbon dioxide clouds on Mars would not behaved like water vapor clouds.

"Water clouds act like a conventional greenhouse and absorb infrared," Pierrehumbert says."But solid carbon dioxide ice crystals in clouds don't absorb infrared."

Thus carbon dioxide tend not to warm up but reflect heat back to the surface. Also, because the clouds do not heat up, they don't break up as easily as clouds made up of water, so they could have kept a vaporous lid on Mars long enough to heat the surface above 32 degrees.

Unfortunately, it still takes a lot of carbon dioxide to sustain this heat mirror, but the warm conditions could easily have lasted more than 500 million years. "Where is all that water bow? " After the 'big freeze'," says Pierrehumbert, "there's no good way to lose the water to space, so most of it should still be on Mars in the form of permafrost. It's still waiting to be found."