The 1976 Viking mission's negative microbial life test results further dampened the prospects of even the tiniest forms of Martian life.
But the recent discovery of methane plumes in Mars's atmosphere, a tantalising clue pointing to potential subsurface life, has raised the hopes of some experts.
Photo: MRO's view of Victoria Crater on Mars(NASA/JPL-Caltech/Univ. of Arizona/Cornell/Ohio State Univ.)
Take a look at some of the twists and turns in our search for life on the Red Planet.
The Viking orbiters see evidence of past water on Mars.
Viking orbiters mapped Mars in unprecedented detail and returned strong evidence of past flowing water. This finding suggested that the Red Planet may have once supported life.
American probes look for microbes in Martian soil.
The first spacecraft to land on Mars were the American Viking 1 and 2 probes in 1976. Their mission included testing the planet's soil for life. However, despite promising early test results, life was not found.
A series of missions finds astonishing features on the Red Planet.
The unmanned Mariner missions gradually revealed Mars's extreme landforms. Perhaps most impressive was Olympus Mons, a volcano three times the size of Mount Everest.
Brian Cox explains that methane on Mars is a tantalising indication of potential life.
Professor Brian Cox explains how methane plumes recently discovered in Mars's thin atmosphere are a tantalising indication of potential subsurface microscopic life.
As the Sun ages, Mars may reawaken and support life.
As the Sun gets brighter towards the end of its life, it is predicted that water ice on Mars may melt, transforming the Red Planet into a place that could support human life.
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