As it happened: Nasa rover performs 'flawless' landing on Mars - BBC News
The one-tonne, six-wheeled rover will search for signs of past Martian life.
A picture taken by the Viking 2 lander on Mars Image caption: A picture taken by the Viking 2 lander on Mars

Before the era of robotic spacecraft, scientists and authors alike speculated that Mars might be home to intelligent life with advanced civilisations.

But when the American Mariner 4 spacecraft flew by Mars in 1965, it returned pictures of a dry, cratered, seemingly barren world.

It was becoming clear that any life on the Red Planet must be a godawful small affair, as David Bowie once sang.

Thus, when Nasa launched its two Viking landers to Mars in 1975, the spacecraft carried experiments designed to detect signs of microbes in the Martian soil. The results from one of these tests, known as the labelled release (LR) experiment, remains a source of controversy to this day.

A sample of Martian soil was picked up and placed in a small tube to which a squirt of radioactive carbon was added. Were any Martian micro-organisms present, the hope was that they would recognise the nutrient, "eat" it and give off radioactive gas.

"No-one, including me, was very hopeful this would work on Mars," the experiment's chief scientist Gil Levin said in a 2014 interview with SPIE TV.

"Surprisingly, as soon as the first squirt of radioactive nutrient hit that Martian soil, we got a big bulge of gas coming out. And it came out for the entire seven days, or Martian sols, of the experiment. That was astounding."

Levin and colleagues also ran a "control", to exclude the possibility that chemicals in the Martian soil could react to produce the gas. The results again pointed to microbial production, according to Levin.

But another experiment on Viking had found no native organic matter in the Martian soil.

How could life exist without organic matter? This discrepancy led to the labelled release experiment being judged inconclusive. But a few advocates, such as Dr Levin, continue to believe in the results.