Olympus Mons, Mars' extinct volcano, 1976.
© National Aeronautics & Space Administration / Science & Society
This false colour image, taken by the Viking 1 Orbiter, exaggerates the differences in the lava flows that have accumulated over time. Two Viking spacecraft were launched towards Mars in 1975, each carrying a lander spacecraft and an orbiter. Both succesfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms - none were found.