Mars H2O theory all wet, say UA scientists

Two recent studies by UA researchers may have NASA working harder to find evidence of water on Mars.

The studies suggest that gullies on crater walls probably were carved not by water but carbon dioxide, and that features thought to be shorelines were shaped not by ancient oceans but tectonic forces.

Don Musselwhite, Tim Swindle and Jonathan Lunine of the UA's Lunar and Planetary Laboratory published their hypothesis on the formation of martian gullies in the April 1 issue of Geophysical Research Letters.

Images from the Mars Surveyor spacecraft studied last June excited scientists because gullies seen below martian crater walls crosscut what appeared to be young features such as sand dunes. This gave rise to the notion that water had carved the gullies and had done so recently.

But that notion bothered Swindle "right away," he said.

"When I saw that, I went, 'Wait a minute,' " Swindle said. "What bothered me was that you have the very coldest places on the planet being touted as the very most recent places with recent liquid water."

The gullies are found near Mars' south pole, where temperatures range from about minus 80 to minus 130 degrees Fahrenheit, temperatures cold enough to actually freeze the carbon dioxide that prevails in the martian atmosphere, he said.

In the winter, its gets so cold that carbon dioxide goes directly from gas to dry ice and fills up pore spaces in rocks below the surface. At a depth of about 100 meters below these crater walls, there is enough pressure that when the temperatures rise in the spring, the pressure in the pores builds up enough to allow the dry ice - actually frozen carbon dioxide - to turn into a liquid.

"Generally, it just evaporates, but as the pressure is building and the walls are thinning, you have liquid CO2 inside a bottle of dry ice and eventually it pops the cork," Musselwhite said. "It vaporizes as it starts to flow and turns to a slurry of rock debris and dry ice snow."

On the other side of Mars, in what is the smoothest surface known in the solar system, an ancient ocean is thought to have formed shorelines.

Paul Withers, a UA doctoral student in planetary sciences, studied topographic data from the Mars Orbiter Laser Altimeter this summer with an MIT researcher at NASA's Goddard Space Flight Center.

The features at first glance were consistent with a shoreline - a series of downward-sloping terraces that could have been left by receding water.

But on further inspection, it became clear that the features had little ridges, Withers said. That would mean that receding water would have had to miraculously avoid a ridge while making the area behind it flat, he said.

Withers and his co-researcher realized that the wrinkled features looked more like they were caused by tectonic shifts resulting from volcanic activity.

Withers said Mars seems to lack the kind of plate tectonics that formed the Earth's continents, but that volcanic activity has indeed affected the red planet.

"In addition to the simple weight of volcanoes pushing downward, there are also horizontal forces many thousands of kilometers away," he said.

* Contact Thomas Stauffer at 573-4197 or stauffer@azstarnet.
com.