Follow the Water

The search for life on Mars has always been closely associated with the search for evidence of water. As you can read in the LIFE section, it was the supposed evidence of "canals" which made late 19^th century astronomers and writers of fiction think there might be a dying Martian civilization, trying to hang as its planet dried out.

When Mariner 9 went into orbit around Mars, there was certainly plenty of dust. The planet’s surface--by then known to be moon-like with lots of craters but no canals--was shrouded in thick clouds of dust. As they cleared and settled, however, the first thing scientists saw were the tops of giant volcanoes. They realized this meant Mars was once geologically active, belching out gases and producing a much thicker atmosphere. Could there once have been water?

When the Viking orbiters flew over Mars, beginning in 1976, they returned images that seemed to support that idea. There were winding channels that looked like the oxbows and meanders of rivers here on Earth. There were tear-drop shaped islands which might have been carved by running water. What remained a mystery was whether Mars once had an atmosphere thick enough to allow water to stay liquid on its surface for a long time. Some researchers argued instead that periodic asteroid or comet impacts must have suddenly melted sub-surface ice which would gush out over the surface. Others thought volcanic activity triggered short-lived floods which might have carved the river-like shapes.