Carbon-rich organic molecules, which serve as the building blocks of life, may be present on Mars after all, say scientists - challenging a widely-held notion of the Red Planet as barren.

When Nasa's two Viking landers picked up and examined samples of Martian soil in 1976, scientists found no evidence for carbon-rich molecules or biology.

But after the Phoenix Mars Lander discovered the chlorine-containing chemical perchlorate in the planet's "arctic" region in 2008, scientists decided to re-visit the issue.

They travelled to the Atacama Desert in Chile, where conditions are believed to be similar to those on Mars.

After mixing the soil with perchlorate and heating it, they found that the gases produced were carbon dioxide and traces of chloromethane and dichloromethane - just like the gases released by the chemical reactions after the Viking landers heated the Martian soil more than three decades ago