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AGU '95

Early Mars may have been more friendly to life than Earth

STANFORD -- Life was tough for organisms in the early solar system. Planetary orbits were crowded with comets and asteroids, and every few million years a massive chunk of rock or ice would smash into a planet with so much energy that it would boil the upper levels of the oceans and vaporize rocks.

Any living thing that survived to evolve again would have to be hiding deep in the ocean or deep underground. All other species would be instantly extinct.

In those days - 4.5 billion to 3.8 billion years ago - Mars would have been a more hospitable home to life than Earth, if geophysicist Norman Sleep's calculations about the history of the planets are correct.

Sleep, a professor of geophysics at Stanford, presented this hypothesis at a session titled "Exploration for a Martian Biosphere" during the American Geophysical Union meeting in San Francisco on Tuesday, Dec. 12. Sleep worked with planetary scientist Kevin Zahnle, of NASA Ames in Mountain View, to compare conditions on early Earth and early Mars, and calculate the energetics of collisions with huge space-borne objects.

They calculated the effects of three different impacts: from an object 70 kilometers in diameter, with a force explosive enough to boil away the water in all the lakes on Mars; an asteroid 200 kilometers in diameter; and a 400-kilometer object the size of the asteroid Vega, big enough to vaporize all the oceans on Earth.

Sleep said it was those oceans, the probable cradle of life, that made Earth less safe than Mars in the weeks or years after a big collision. Earth's oceans would have retained the heat generated by large impacts. A 200-kilometer asteroid, the size that struck Earth every 20 million years, would boil away the top 550 meters of ocean and vaporize a cloud of molten rock so thick that it would cover the planet to a thickness of 8 meters.

For another 500 years, the atmosphere of Earth would be a pressure cooker of hot steam, condensing into rain at the rate of only a meter a year. The lower levels of this
steam-bath would be scalding hot, causing the surface of the ocean to boil continuously, and churning the water, perhaps to great depths. The only living things to survive would have to hide away for many years in the deep subsurface or the deep ocean.

Martian bodies of water were mere lakes. It took only a 70-kilometer asteroid, the type that has struck Mars perhaps 100 times over its history, to vaporize the water and everything living in the lakes. It would take that water 25 years to condense and fall to the surface again. But because rock does not transfer heat well, the deep fissures beneath the Martian surface could harbor some microorganisms until the surface cooled and they could re-colonize the returning lakes.

"The surface of Mars was a dangerous place to live," said Sleep. "But the subsurface was safe even from large impacts." A 200-kilometer asteroid would bury the surface of Mars in a rain of molten rock 29 meters thick, but the small amount of water vapor would still only have taken 25 years to rain out. Organisms living well beneath the surface still would have moderate enough temperatures - and enough water- to survive.

The lakes of Mars are dry now, and the surface is truly hostile to life. But Sleep said if he were directing a Mars mission to look for signs of past life on Mars, he would look in the dry lakebeds where successions of living things may have survived catastrophes and evolved new forms.

The luckiest survivors on either planet might have been a few microorganisms clinging to the crevices of a rock tossed clear out of the planet's atmosphere during a catastrophic impact. Such ejected travelers might have stayed in space long enough so that the home planet was livable again by the time they fell back to ground. They even might have carried the seeds of life from one planet to the next.

Life on Earth clearly has survived many catastrophic impacts over the millennia. Sleep said that at some point, a relatively complex creature, sophisticated enough to depend on paired genes for reproduction, found a refuge that was safe during a catastrophic impact. This was not the first ancestor, but it was the last common ancestor of all terrestrial life, he said.

Sleep speculates that whatever that ancestor was, it was able to thrive in an overheated environment. He said it is quite possible that one of the periodic catastrophic impacts sterilized all of the environments on Earth where low-temperature organisms could survive. Everything now living on the planet thus may be descended from high-temperature-loving creatures that emerged from a deep-sea ocean vent or a deeply buried fissure in the rock.


This mosaic of Mars is composed of about 100 Viking Orbiter images. The images were acquired in 1980 during mid-northern summer on Mars. Crater Schiaparelli, left of center, is 461 km (277 mi) in diameter. The dark streaks with bright margins emanating from craters in the Oxia Palus region, in the upper left, are caused by erosion and deposition by the wind. Bright white areas to the south, including the Hellas impact basin at extreme lower right, are covered by carbon dioxide frost.



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