In the US Viking project two identical space probes were dispatched to Mars, where an orbiter mapped the surface and a lander returned pictures and other data from the surface. In addition the lander sampled and analysed the soil. The probes were launched in August and September 1975, and the intention was that the first, Viking 1, should set down its lander on Mars on July 4, 1976, to celebrate the bicentenary of the US. But on arrival in orbit, pictures returned from orbit of the landing site on the plain of Chryse showed the terrain to be too rough. An alternative landing site had to be chosen, which delayed the landing until July 20. Viking 2's selected landing site in the Cydonia region also proved unsuitable, and an alternative was chosen on the plain of Utopia, where the lander set down on 3 September. Both probes were outstandingly successful. Viking 1 was amazingly long-lived, the orbiter functioning for over four years, the lander for over six years. Viking 2 had a much shorter life, the orbiter lasting for less than two years, the lander for less than four. The Viking orbiters mapped virtually the whole of the Martian surface and, using filters, were able to send back data from which coloured pictures were obtained. The lander pictures revealed remarkably similar terrain at both landing sites - loose, rust-red soil, with scattered small rocks. Analysis of the soil by the on-board laboratory revealed no sign of organic matter, which might have indicated the presence - past or present - of living organisms. But strange results were obtained, revealing peculiar chemistry at work on Mars. The soil minerals included superoxides of iron not known on Earth. It is they that impart the rusty colour to the landscape of Mars: well is it called the Red Planet.
