Red planet water wars waged before Mars odyssey
(CNN) -- Planetary geologists have assaulted increasingly popular theories that water shaped features small and great on Mars, days before a NASA spacecraft begins an odyssey to search the red planet for signs of the life-making elixir.
Scientists last June proposed that a high-resolution satellite picture of Mars showed evidence that water had recently seeped down deep gullies on cliffs and crater edges.
The announcement sent a buzz through the scientific community, bolstering speculation that life could exist in underground water aquifers near the surface.
But puzzling questions remained about the unusual features, which had formed in the least likely places liquid water would exist. Most were in the southern highlands, the coldest part of the planet, or on slopes facing the poles, which receive little or no sunlight.
By gully, it's carbon dioxide
University of Arizona scientists published a study this week proposing an alternative explanation for the gully locations -- that an exotic form of carbon dioxide rather than water shaped them.
"Why don't we see the gullies in other places? If you have water cutting these gullies, you should see that everywhere, not just at these specific location," said Don Musselwhite, lead author of the report in the April 1 issue of Geophysical Letters.
The most compelling evidence -- gullies consistently begin about 100 meters (330 feet) below cliff tops. At that depth, the pressure of the overhanging rock is just enough to stabilize liquid carbon dioxide, Musselwhite said.
The liquid carbon dioxide builds up behind a barrier of dry ice, then bursts forth into the thin, cold Martian atmosphere, vaporizing into carbon dioxide snow, which mixes with gas and debris into a rushing slurry strong enough to erode rock, the authors suggest.
"What's coming out is liquid carbon dioxide that suddenly vaporizes," Musselwhite said. "It doesn't take very much liquid each time to add to gully formation."
Report: ocean theory all wet
In a separate report this week, two planetary geologists dismissed conjecture that an ancient ocean covered much of Mars.
Brown University scientists in December 1999, also looking at Mars satellite pictures, suggested that ridges and terraces on the northern plains were remnants of the coastline along an ocean that expired hundreds of millions of years ago.
But the candidate shorelines "were more likely to have formed by tectonic rather than oceanic processes," concluded Paul Withers of the University of Arizona and Gregory Neumann of the Massachusetts Institute of Technology.
After studying data from the red planet satellite, they determined that most of the ridges were wrinkles left on the surface as tectonic plates collided, or appeared related to geologic stress centers like volcanoes or impact basins.
The duo, reporting their findings in the April 5 issue of the journal Nature, said they could not explain the youth and smoothness of the northern plains, the flattest known surface in the solar system.
Watery Mars hopes kept afloat
For those envisioning Mars as a watery world, there is hope. The abundance of a gray-hued variety of iron oxide near the equator raises the possibility that hot springs were once active on the planet.
"We believe that the gray hematite is very strong evidence that water was once present in the area," said Victoria Hamilton, a planetary geologist at Arizona State University. The deposit, perhaps several hundred million years old, is becoming exposed by wind erosion, she said.
Hematite is a mineral cousin of common rust, which gives Mars its characteristic red color. Hematite on Earth can precipitate in hot springs or in standing pools of water.
The Mars 2001 Odyssey, scheduled for launch on Saturday, should improve the ability of scientists to investigate the hematite site and other locations offering tantalizing evidence of past or present water activity.
Slated to arrive in the Mars system later this year, the Odyssey orbiter is equipped with an infrared imaging camera that can distinguish the mineral content of geologic features only 100 meter (110 yards) across, compared to 3 km (1.9 miles) for a similar instrument on the Mars Global Surveyor, which started orbiting Mars September 12, 1997.
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