On the Cover:Viking Orbiter images suggest that rivers may have once flowed on Mars. Erosive features such as scour marks and teardrop-shaped islands, seen in this section of the Kasei Vallis channel, indicate sculpting by liquid water. The impact crater at the bend of the channel is 25 km across. JPL / NASA
4 The Ancient Rivers of Mars: David Pieritakes us on a tour of the signs of ancient rivers flowing across the surface of Mars in Viking Orbiter images.