What became of the water on Mars? This January, a cluster of spacecraft will converge on the Red Planet, probing for clues to the mysterious but unmistakable role of water in its past

Natural History, Dec, 2003 by Michael H. Carr

In spite of Lowell's claim to the contrary, little can be seen of Mars's surface features through a telescope; the planet is just too small and too far away. The sightings of the canals proved to be imaginary, the result of too much striving to make out features at the limits of telescopic resolution. Scientific interpretation of the martian surface did not realistically begin before observations could be made from spacecraft. And for those hoping to confirm Lowell's ideas, the first such images, obtained in the 1960s by NASA's Mariner 4, were deeply disappointing. The small areas photographed showed no canals, no oceans, no oases.

But water on Mars still seemed a real possibility. The Mariner 9 spacecraft revealed a complex surface geology: volcanoes, canyons, dry valleys, lava plains, and, most intriguingly, flood channels. The discovery of the flood channels led to tantalizing visions of running water--and it went almost without saying that where water flows, there could be life. The data were returned to Earth in 1972, just as NASA was preparing the Viking missions. The discoveries were timely because the main emphasis of those missions was to search for life. Once again, however, the outcome was disappointing: Viking did not even find organic molecules suggestive of the presence of life on the planet's surface--much less life itself.

After the Viking program, the pace of Mars exploration slowed. The focus shifted from the direct and rapid detection of life to acquiring a better understanding of the planet. That still meant looking for water, or at least for where it might have been. In the meantime, public attention drifted elsewhere, until two events renewed wider interest in Mars.

The first event was the announcement in 1996 that a meteorite from Mars contained evidence--possibly fossilized bacteria--suggestive of ancient life. The second event was the extraordinary success of NASA's Pathfinder rover in 1997. The martian meteorite that caused such a fuss in 1996 is generally no longer considered to contain any fossils, and nonbiological explanations of the observed mineral formations now seem more appropriate. Yet the search for water--and life--on Mars has hardly been abandoned. The new convergence of spacecraft is proof enough of that, all of them trying to help answer essentially the same questions that fired the imaginations of Herschel and Lowell: Has liquid water ever been abundant on the martian surface? And if so, has it enabled the planet to support life?

The geology of Mars is a spectacle to behold. The planet's southern hemisphere bears the scarring of heavy bombardment by meteorites: the craters, much like the ones that pockmark the highlands of our Moon, clearly date to the era, sometime before 3.8 billion years ago, when all the bodies of the inner solar system were subject to heavy meteorite bombardment [see "Moonstruck," by G. Jeffrey Taylor, September 2003].

The northern martian hemisphere, however, is sparsely cratered, indicating that the old cratered surface there has been buried by younger materials. What are these materials? They could be volcanic, but Timothy J. Parker and his coworkers at NASA's Jet Propulsion Laboratory in Pasadena, California, have speculated that they are sediments in what were once ocean basins. Their elevations are some three miles lower than those of the cratered southern uplands. Perhaps, then, the old, cratered surface is partly buried by marine sediments. But what exactly caused the northern depression is unknown.