The surface of Mars is less inhabitable than previously thought, according to researchers at the University of Edinburgh.

Key points: Toxic cocktail formed when chemical compounds the soil were exposed to UV light

Toxic cocktail formed when chemical compounds the soil were exposed to UV light Life on Mars still possible, but the search will have to go deeper into Mars

Life on Mars still possible, but the search will have to go deeper into Mars Less risk now of human contamination of Mars through hitchhiking bacteria

Their findings mean more work needs to be done to explore how to grow food on Mars, before human colonisation is viable.

The study found a toxic cocktail formed when chemical compounds in the soil, called perchlorates, were exposed to UV light.

The perchlorates were known to exist in Martian soil, but in experiments researchers on Earth exposed the compounds to short-wave ultra-violet radiation, similar to that found on Mars.

A bacteria, which is a common contaminant found on spacecraft, was destroyed within minutes.

The research suggests that two other parts of the Martian soil, iron oxides and hydrogen peroxide, work with the perchlorates to further increase the toxicity.

Simon George, professor of organic geochemistry at Macquarie University, said the paper published in the journal Scientific Reports confirmed what scientists expected.

"There's no ozone layer [on Mars] so UV rays penetrate down to the surface," he said.

NASA plans to send humans to the Red Planet in the 2030s. ( NASA )

"And they're very toxic, they give you a really bad sunburn if you're on the surface.

"But what happens is this UV radiation interacts with perchlorates and produces side products, probably chlorite and others, which are the things that are really toxic to life."

Life on Mars 'still possible'

Despite the finding, Professor George said there was still a possibility of finding life on Mars — but they will have to search deeper.

"We need to be able to be drilling maybe 20 centimetres, maybe metres, down below the Martian regolith [the mixture of dust, soil and broken rocks] and be looking for life in the rocks and the sediments below the surface, out of the way of this UV radiation," he said.

The good news is that there is less risk of human contamination of Mars, through hitchhiking bacteria.

"We are worried that spacecraft coming to Mars might be bringing Earth-based microbes to Mars and that they might survive there," he said.

"And in fact the level of toxicity of this perchlorate means that is perhaps less likely."

Professor George says there is still a possibility of finding life on Mars. ( NASA )

The Martian's potato farm appears unlikely

NASA plans to send humans to the Red Planet in the 2030s.

It wants to find out if Mars was ever home to microbial life, and whether it could be a safe home for humans.

Works of fiction have long imagined what it would be like, such as the 2015 film The Martian.

When stranded on Mars, Matt Damon's character builds a greenhouse, fertilising the soil with his own faeces — and soon grows potatoes.

But the results of this study make that scenario appear farfetched.

David Willson, from NASA's Planetary Science and Astrobiology division in California, has been looking at the possibility of treating Martian with enzymes to put the perchlorates to good use and release oxygen.

"You need to flush out the perchlorates and then you can grow vegetables," he said.

"But you would have to add nutrients into the soil as well.

"So you'd have to do more than just pick up the soil, you'd have to work the soil."