An Explanation for Liquid Water on Ancient Mars
Temperatures on Mars today are well below the freezing point for water. Nonetheless, recent Mars missions, Mars Pathfinder and Mars Global Surveyor, have found evidence that once-liquid water was present in abundance. According to a new model, clouds of carbon dioxide fueled a massive greenhouse effect that raised temperatures in our red neighbor's early days to the point where life could have developed. Through photographs taken by Viking, Mars Pathfinder, and Mars Global Surveyor, there is ample evidence of deep channels on the Martian surface, thought to have been dug by flowing liquid water. How could Mars, in a freezing 40 oC below zero at Pathfinder's landing site, ever be warm enough for liquid water to form on its surface?
The answer, say a University of Chicago climatologist and his French colleague, is reflective carbon dioxide ice clouds that trap thermal radiation near the planet's surface (Science 15 November 1997).
This problem has puzzled scientists since the 1970s, when Viking radioed the first detailed images from Mars to Earth, said Raymond Pierrehumbert, a professor of geophysics at the University of Chicago. How can it be explained that Mars became warm enough for flowing water to form? Especially considering that the Sun was even dimmer early in Mars' evolution? Pierrehumbert worked with French climatologist Francois Forget of the Laboratoire de Meteorologie Dynamique du CNRS in Paris.
Previous models of the atmosphere of ancient Mars assumed that carbon dioxide in the atmosphere caused global warming of the planet. The problem was, said Pierrehumbert, that if you try to put enough CO2 into the atmosphere to warm it, carbon dioxide condenses out. It was thought that the thick clouds that would form reflected sunlight back into space, actually cooling the planet. When we reexamined this, we found that this dry ice blanket is warming the planet because it reflects infrared light back onto the surface to a greater extent than reflecting solar radiation outward.
Unlike the water ice clouds that exist on Earth, the particles in the carbon dioxide ice clouds are large enough to scatter infrared light more effectively than visible sunlight. Ordinary clouds, such as those found on Earth, absorb heat from the planet's surface and radiate it both toward the surface and into space. Half of the heat is lost in the process.
But the carbon dioxide clouds behave like a one-way mirror. Little sunlight penetrates the planet's surface. However, the sun's rays that reach the planet are converted into heat, which the clouds then reflect back onto the surface explained Pierrehumbert. This effect causes the planet to heat up enough for liquid water to form.
Pierrehumbert believes that this climate model provides clues about the kind of life that might have evolved on Mars: If we're looking for analogies on Mars with life forms on Earth, he said, then we should look focus on organisms that can develop under extreme conditions, e.g. on the seabed or in caves. Conditions on Mars about four billion years ago are more like conditions on the ocean floor than a rainforest. It was dark and warm enough for liquid water, but there was no energy source for photosynthesis.
Pierrehumbert and Forget's model also extends the habitable zone to planets outside our solar system, thus increasing the likelihood that life exists outside our solar system. Scientists used to believe that only planets orbiting a star within 1.37 astronomical units (one astronomical unit (AU) is the mean distance between Earth and the Sun) could have water above freezing. However, if the planets have carbon dioxide clouds, then they could have liquid water as far as 2.4 AU away. Mars is 1.52 AU from the Sun.
Carbon dioxide clouds may also have played a role in warming the earth when the sun was dimmer than it is today. It is possible that a global ice age was prevented, which would have kept the earth in its ice-cold stranglehold forever. If the earth had ever cooled down to the point where all the oceans were frozen, it could never have warmed up again. The ice surface would have reflected a large part of the solar radiation back into space.
Pierrehumbert and Forget believe their model is consistent with a hypothesis published in Science by Carl Sagan and Christopher Chyba earlier this year. It states that a methane and ammonia atmosphere warmed early Mars. The problem with methane, however, said Pierrehumbert, is that it breaks down very quickly when exposed to sunlight. Therefore, a biological engine is required - life on Mars - to fuel the atmosphere as methane dwindled. Our model provides the starting conditions under which life may have evolved and methane gas production began. As gas forms, the carbon dioxide clouds shield the methane from sunlight, preventing it from breaking down quickly.
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