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The scientific instruments we have sent to Mars may not be able to detect signs of life there. Tests of these instruments on samples from the Atacama desert in Chile have shown that they may not be sensitive enough to spot biological material, even if it does exist on Mars.

at the Spanish Astrobiology Centre in Madrid and his colleagues took samples in a region of the desert called Red Stone, where the dust is red because it is full of hematite, the same mineral that gives Mars its rusty colour. “This is probably the most Mars-like place on Earth,” says Azua-Bustos. “Being there is almost like being on Mars, except for the colour of the sky.”

When they used state-of-the-art scientific instruments – the sort only available in laboratories on Earth – to analyse the makeup of their samples, they found up to 1 microgram of DNA per gram of soil. This included DNA from 19 species of bacteria and two fungi.

However, nearly half of the microbial DNA didn’t match anything in the genetic databases we have, leading the researchers to refer to those microbes as “the dark microbiome”. This could mean that they are organisms that haven’t previously been discovered, or that they are relics from organisms that lived in the area hundreds of millions of years ago. “We know the microorganisms are there, we have the sequences, but we cannot tell you what they are,” says Azua-Bustos.

When the researchers tested their samples using instruments comparable to the ones on current Mars rovers and those planned for the near future, those instruments could barely detect any microbial material, dark or otherwise. “If you were an alien coming to Earth and you happened to land in the Atacama desert with instruments like the ones we have on Mars, you might say Earth is uninhabited,” says Azua-Bustos. “If those instruments are not able to detect the things that we know are on this site, how are they going to see anything on Mars where we don’t even know what we’re going to find?”

If there was ever life on Mars, this means that our spacecraft probably wouldn’t be able to find convincing evidence of it. “We must be cautious about interpreting absence of strong evidence of life as evidence of its absence,” wrote at NASA Ames Research Center in California in a comment piece accompanying the paper. To be sure about any detection of signs of life on Mars, we’ll have to bring samples back to Earth to analyse them – a task which NASA plans to undertake in the late 2020s.

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