For Mars to have rivers, it must once have had streams, rain, clouds and an atmosphere, a planet just like the Earth today-if these were rivers. They had to be sure.

The answers would come in 1998 with the launch of Mars Global Surveyor. It was equipped with the latest high-resolution electronic cameras. Sections of the valleys were revealed in fantastic detail. Some valleys had been eroded and filled in with sand. It was impossible to say how they'd been formed. They could have been carved by water, or soil, or carbon dioxide. Then, after they'd searched through thousands of images they found this- a winding valley, two kilometers wide, and at a bend in the canyon, a tiny channel, the unmistakable trace of an ancient river.

This is the best evidence we have that climate in the past was different from what it is today.

Only a river would leave a meandering trace like this. Only a river would leave the traces of silt they saw in the bend, and only water could carve the winding channel feeding into it. But they soon found more than just a few rivers. They discovered evidence that water had once existed on Mars in colossal quantities.

The evidence came from another probe that reached Mars in 1997. It landed in an enormous channel and sent back images of huge boulders scattered around the Martian surface. Scientists were puzzled. What could this strange pattern signify? One man looking on knew exactly what those scattered stones meant-Jim Rice.

Mars Global Surveyor:U. S. mission to Mars launched in 1996. Main instruments include a camera, laser altimeter, thermal emission spectrometer, and magnetometer. During the primary mission, the spacecraft circled the planet once every 118 minutes at an average altitude of 378 kilometers (235 miles). The extended mission phase began February 1, 2001. The spacecraft may function eventually as a communications satellite to relay data back to Earth from surface landers launched as part of future Mars missions

meander:to follow a winding and turning course
meandering:winding