Mars Views Hint at Early Land of Lakes.

For 30 years, ever since spacecraft recorded what appeared to be dried-up channels on Mars, scientists have sought to determine whether water ever flowed on the Red Planet. High-resolution images unveiled this week offer more evidence that parts of ancient Mars resembled a land of lakes. They also point out prime locations to look for fossils that would indicate life once existed on the planet.

The images, taken by the Mars Global Surveyor spacecraft, show in unprecedented detail that massive structures of layered rock, like Earth's Grand Canyon, are widespread on Mars. On Earth, such structures are made of sediments deposited over eons. The layers often sandwich fossils between them. Common at the site of dried-up lakes, these structures preserve our planet's ancient history.

In the Dec. 8 SCIENCE, Michael C. Malin and Kenneth S. Edgett of Malin Space Science Systems in San Diego report their analysis of the Surveyor images and argue that the Martian features also stem from water deposition.

Other mechanisms, such as fierce wind from a once-thick Martian atmosphere, could have sculpted the deposits, they caution. The team notes, however, that most of the layered topography resides in or near craters, which could have provided basins for standing water.

The locations of the structures "may be a good argument for water-laid sediments," agrees Harry Y. McSween Jr. of the University of Tennessee in Knoxville.

Although the layering of Martian rocks "has been well recognized since the days of Mariner 9" in 1969, the new images "have honed our information about where we should look next," notes Jim Garvin, NASA's Washington, D.C.-based project scientist for Mars exploration. He finds the idea of landing on the layered terrains to search for signs of the earliest Martian life "incredibly provocative."

Scattered across the planet, the layered formations are common in such places as the impact craters of western Arabia Terra, the intercrater plains of northern Terra Meridiani, and the chasms of the giant canyon Valles Marineris. Malin and Edgett estimate that many of the deposits date back more than 3.5 billion years.

The images reveal three types of structures. So-called layered units consist of thin bands of rock, some only a few meters thick. More massive structures appear as a single, bulky layer, up to several kilometers deep, that sometimes sits atop bands of rock like frosting on a layer cake. Thin mesa formations typically coat eroded layered or massive units and can be smooth or heavily grooved or pitted.

Malin and Edgett point out a possible glitch in their story line. No sources of water or travel routes for that water, such as channels, lie near the layered regions. The researchers suggest that these may have eroded away.

Waterborne deposits leave behind telltale minerals, notes David J. Des Marais of NASA's Ames Research Center in Moffett Field, Calif. Using this criterion, a high-resolution infrared spectrometer may help reveal whether or not the layered features have a fluid origin. Scientists hope to include such a device on a Mars-orbiting spacecraft set for launch in 2005.

Christopher E Chyba of the SETI Institute in Mountain View, Calif., notes that it may be more interesting for craft to visit polar regions on the Red Planet where, according to previous Surveyor images, water may have flowed much more recently (SN: 7/1/00, p. 5). These areas, however, have more rugged terrain that would be difficult for landings. Planetary scientists soon will debate whether to first explore the layered regions or these polar areas, Chyba predicts. Not a bad predicament to be in.