Rediscovering Mars

For Mars researchers who depend on complicated, expensive interplanetary probes for their investigations, data seem to come either in trickles or in floods. From 1977 to 1996, only two probes were launched toward Mars—and both were unsuccessful. The delay was frustrating to researchers, because previous missions had revealed Mars to be a very strange planet. Indeed, at times, parts of Mars almost seemed to be from two different planets. Although most terrain in the south was rough, heavily cratered, and therefore ancient, most of the Northern Hemisphere had been resurfaced to a nearly billiard ball smoothness. This hemispheric dichotomy was accentuated by an average altitude difference of several kilometers between north and south. Superimposed on this background were some of the Solar System's highest volcanoes and some of its deepest craters.