WASHINGTON — A blanket of icy clouds may have kept ancient Mars warm enough to let water flow on its surface and may have encouraged life in its caves and oceans, scientists in Paris and Chicago reported Thursday.
Since the 1970s, astronomers have seen channels on Mars they thought to be the remnants of flowing liquid water, but since the planet is now far too cold to allow that, they wondered how the water got warm enough to flow.
Current temperatures on Mars, as measured by the Mars Pathfinder, are a chilly minus 100 degrees Fahrenheit.
Now researchers at the University of Chicago and the Laboratoire de Meteorologie Dynamique du CNRS in Paris believe that a thick layer of icy carbon dioxide clouds warmed Mars through a kind of Martian greenhouse effect, trapping infrared light and reflecting it back to the planetary surface.
"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," University of Chicago professor Raymond Pierrehumbert said.
Unlike water vapor clouds on Earth, carbon dioxide ice clouds are made up of particles large enough to scatter infrared light more effectively than the visible light coming from the sun.
By contrast, earthly clouds generally absorb heat from the planet and send it both back to the planet's surface and into space, losing half the heat in the process.
"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," Pierrehumbert said.
This mechanism could have worked well enough to make liquid water possible, Pierrehumbert and his fellow researchers reported in Science magazine.
This kind of Martian climate could have supported life in caves or at the bottom of oceans, he said.
"The conditions on early Mars, some 4 billion years ago, were a little more like the conditions at the bottom of the ocean than like a rain forest," Pierrehumbert said. "It would have been dark, warm enough for liquid water, but without a large energy source for photosynthesis."
The theory of warming ice clouds on Mars means that life may be possible on planets farther from the sun than previously thought.
Until now, scientists believed life could exist only on planets within 1.37 astronomical units of a star like the sun. An astronomical unit is about 93 million miles, the distance between Earth and the sun.
But the ice cloud theory could extend that range to as far away as 2.4 astronomical units, the scientists said. Mars is 1.52 astronomical units away.