Mars Canyons Tied to Rains After Meteor Impacts
By KENNETH CHANG
Published: December 6, 2002
The vast canyons and river valleys of Mars may have been carved by brief bursts of near-boiling torrential rains that followed giant meteor impacts, scientists are reporting today.
The new research appears to rebut the common notion that Mars passed through an Earth-like warm, wet phase lasting hundreds of millions of years, with an ocean covering its northern hemisphere and steadily flowing rivers crisscrossing the southern highlands.
A rationale for the procession of NASA spacecraft sent to Mars has been that this early warm period would have been favorable to the advent of life, and that if life did arise, Martian microbes may yet survive deep in the interior of the planet. But in the journal Science today, researchers portray a much harsher environment for young Mars. The climate then, they say, was much as it is now -- frigid and dry -- but periodically punctured by cataclysms.
The researchers were from the University of Colorado, the National Aeronautics and Space Administration's Ames Research Center in Mountain View, Calif., and an organization dedicated to the search for extraterrestrial life, the SETI Institute, also in Mountain View.
In the early solar system, littered with debris from the formation of the planets, meteors 60 to 150 miles wide slammed into Mars every 10 million to 20 million years. Computer simulations by the researchers show that the giant impacts would have turned the polar ice caps to steam and melted vast amounts of ice below the surface. The largest meteors would have melted enough ice to inundate the Martian surface to a depth averaging more than 100 feet, the scientists said.
The computer simulations indicate that after each impact the steam in the atmosphere would have condensed in scalding downpours -- global rainfall at a rate of six feet per year -- that lasted years to decades.
Those events would have produced enough water to have left the Martian landscape as we see it, said Dr. Owen B. Toon, director of the atmospheric and oceanic science program at the University of Colorado and an author of the Science paper. Such an environment does not rule out the possibility of life on Mars, but, Dr. Toon said, ''it sure makes it harder.'' The other authors of the Science paper are Teresa L. Segura, a graduate student at Colorado; Dr. Anthony Colaprete, a scientist at the SETI Institute; and Dr. Kevin Zahnle of the Ames institute.
Starting in the 1970's, spacecraft orbiting Mars revealed networks of river valleys that resembled those on Earth. The presence of river valleys suggested that Mars once had rivers like those on Earth, and scientists looked for ways to explain how the surface of early Mars could have had above-freezing temperatures.
At first they suspected global warming caused by carbon dioxide. Just as artificial emissions of carbon dioxide are blamed for the rising temperatures on Earth and a carbon dioxide atmosphere keeps Venus's surface at 850 degrees Fahrenheit, scientists thought Mars might have been warmed by a blanket of carbon dioxide that later dissipated into space.
In the early 1990's, however, scientists realized that it would have had to be an implausibly thick blanket. Air pressure at Earth's surface is 14.7 pounds per square inch. To warm Mars above freezing, the pressure of carbon dioxide would have had to be at least five times as much, which seems unlikely. More recent data from Mars also shows no signs of the limestones that should have formed in an atmosphere rich in carbon dioxide.
''We've all realized there's a problem with the greenhouse explanation,'' said Dr. James Kasting, a professor of geosciences and meteorology at Pennsylvania State University.
That has led some scientists to look for other explanations. Five years ago, Dr. Raymond T. Pierrehumbert, a professor of geophysical sciences at the University of Chicago, proposed that carbon dioxide clouds could provide an insulating effect. But Dr. Toon said he and Dr. Colaprete had performed new calculations showing that while the clouds do warm the atmosphere, they dissipate as the air warms, limiting that effect.
Other scientists now suspect there was no warm period after all. Except for the carved river valleys and the inference of liquid water, ''there's no other evidence for it,'' Dr. Colaprete said. ''Either we're thinking about it wrong, or something else is happening.''
Some believe that the water came from underground ice melted by geothermal heat. Others believe that the valleys were eroded not by flowing water, but by wind, snow or frozen carbon dioxide. But proponents of those theories have not as yet convincingly shown how those could produce all of Mars's landscape features.
Dr. Toon said the new theory required no major assumptions or new geological processes. Cratered scars provide unequivocal evidence that at least 25 large meteors did hit Mars about 3.5 billion years ago, at the same time the river channels formed. The energy of the impacts would have vaporized the ice, and the steam would have condensed as hot rain.
Others agree that meteor-induced rainfall must have occurred, but are not sure the brief episodes could have eroded all of the valleys.
Dr. Kasting points to an image taken in 1998 by NASA's Mars Global Surveyor showing a wending canyon about a mile wide, called Nanedi Vallis, ''which looks just like the Colorado River running through the Grand Canyon,'' Dr. Kasting said. ''I would point at that thing and say I don't think that forms in a thousand years.''
Photo: Mars canyons like this one, called Nanedi Vallis, have been attributed to torrential rains. (NASA)