The Martian atmosphere: old versus new.

The Martina atmosphere: old versus new

Many planetary scientists believe the atmosphere of Mars, less than 1 percent as dense as Earth's, was once much thicker than it is today. It is now too thin for liquid water to form on the Martian surface, but photos taken by the Viking spacecraft have shown widespread networks of "fluvial" valleys, apparently carved by flowing liquid. These are often cited as support for the idea of an initially thicker Martian atmosphere.

This week, however, at an international Mars conference in Tucson, Ariz., two scientists contended that such features alone do not prove the point.

More than mere quibbling over details, the question touches fundamental issues of the planet's evolution, ranging from its water content to the extent of its volcanic activity. It is of growing interest as the U.S. and Soviet space programs focus on planned returns to Mars. The U.S. Mars Observer mission is to take off in 1992, and two unmanned Soviet craft will soon arrive to study Mars and its moon Phobos (though one is apparently unusable due to a ground controller's error, and problems have been reported with the second).

The uncertainty about how substantial the atmosphere of Mars used to be arises from an area known as Alba Patera in the Martian northern hemisphere, according to Virginia C. Gulick and Victor R. Baker of the University of Arizona in Tucson. Like other parts of the surface, Alba Patera shows a number of "fluvial" features--details such as branching, dry "stream beds" or other channels apparently created by flowing liquid, presumably water.

Unlike most such regions, however, Alba appears to have been formed as recently as 1 billion or 2 billion years ago, judging from the number of meteorite impact craters on its surface, the researchers say. It is relatively smooth, suggesting the surface was somehow made over after the numerous impacts thought to have taken place in the solar system's early history, when meteorites were presumably more common. This period of "heavy bombardment" may have ended about 3.9 billion or 3.8 billion years ago, Baker says, and features such as branching channel networks exist in several places on the ancient, heavily cratered terrain.

Channel networks are scarce on the younger regions, however, making Alba a rarity and supporting the idea that, over time, much of the atmosphere escaped into space or was chemically bound in the rock, leaving too little surface prssure for liquid water.

In that case, the Arizona scientists ask, where did all the water needed to form the Alba valleys come from? According to Gulick and Baker, the "drainage densities" calculated for the Alba valleys "are among the highest estimated on any Martian surface," greater than the ancient terrain and more like those of fluvial features on Earth such as the flanks of Hawaii's volcanic peak Mauna Kea. Earth's atmosphere was thick enough to sustain liquid water, but how did liquid last long enough on Alba to sculpt the terrain?

Basaltic-lava flows typically harden and crack, so that surface water is absorbed rather than flowing along the cut channels. Some volcanoes, however, explode with ash flows whose chunks are then weathered into finer particles that pack more closely together, almost, says Gulick, like clay--or the surface of Alba. On Mars, water beneath the surface might have been released during volcanic explosions, or heated from beneath into hydrothermal systems that emerged to flow across the ground. With Alba lacking the permeability of the ancient basaltic flows, the researchers ask, was a thick atmosphere really necessary?