In the oldest and driest desert in the world, the dried-up remains of an ancient river delta are like a little slice of Mars here on Earth.

Water flowed through the clay-rich mud at the Red Stone site in Chile’s Atacama Desert around 100 million years ago, but the environment was still dry and harsh — and today, the once-flowing water has dried up and the mud has hardened and been compressed into stone. It’s geologically very similar to Jezero Crater, the Perseverance rover’s home base on Mars, complete with iron-rich hematite deposits and calcite, both revealing the presence of ancient water. And like Jezero Crater, it’s both a compelling and challenging place to look for biosignatures — the chemicals left behind by microbes and their activities.

And a recent experiment with samples from Red Stone suggests that the search for Martian life may face a problem. If traces of ancient life are buried in Martian rocks and regolith, the instruments on the rovers now trundling around the surface of Mars — and even the next generation of rovers like the ESA’s Rosalind Franklin — may not be sensitive enough to detect them.

Needle in a hay stack

Astrobiologist Armando Azua-Bustos (of the Centro de Astrobiología in Spain) and his colleagues used versions of some of the Curiosity and Perseverance rovers’ instruments, as well as one that Rosalind Franklin will carry to Mars in the next few years, to look for biosignatures — chemical traces of life — in samples of rock and dirt from Red Stone. The instruments they tested included replicas of the Perseverance Rover’s SuperCam instrument and the Curiosity Rover’s ChemCam instrument, which both blast rocks with a laser and then measures the chemical makeup of the vaporized debris; and Rosalind Franklin Rover’s Mars Organic Molecule Analyzer (MOMA), which will measure the chemical contents of samples by sorting individual molecules by their mass.

They didn’t find much. The SuperCam and ChemCam replicas aced the job of measuring what the rocks and soil were made of, but finding organic molecules that might point to life, not so much. MOMA detected organic molecules in just a few samples, and that only with some extra processing.

But when Azua-Bustos and his colleagues analyzed samples from Red Stone using commercial-grade laboratory instruments, which are much more sensitive than the ones on board current and upcoming rovers, it turned out that Red Stone had been teeming with life 160 to 100 million years ago — and still hosts some hardy microbial communities today.

An instrument called a gas chromatography-mass spectrometer (GC-MS), which also measures the chemical makeup of a sample by sorting its molecules by their mass, found traces of chemicals that might be signs (or at least building blocks) of life, including lipids and an amino acid called proline. Other laboratory tools extracted fragments of DNA from the samples, revealing the presence — either now or in the past — of diverse communities of bacteria.

The GC-MS instrument Azua-Bustos and his colleagues used was about 10 times more sensitive than the version on board the Curiosity rover’s Sample Analysis at Mars instrument, and it had decent luck with the Red Stone samples. But the rocks at Red Stone weren’t exactly packed chock full of organic compounds; the chemical traces showed up in very small amounts, right at the bare edges of the lab instrument’s sensory abilities.

“Similarly low levels of organics will be hard, if not impossible, to detect in Martian rocks, depending on the instrument and technique used,” write Azua-Bustos and his colleagues. According to them, Curiosity’s SAM instrument should be able to detect organic compounds, but only if they’re at least as abundant as they are at Red Stone, where life thrived much more recently than on Mars, and where its remains haven’t been bombarded by cosmic rays.

All told, the experiments suggest that rovers like Perseverance and even Franklin, as advanced and remarkable as they are, may miss traces of past life on Mars if they’re relying on their onboard instruments. Our best chance at finding evidence of Martian life is to bring samples back to Earth.

Digging (Literally) Into the Details

The instruments aboard the current generation of Mars rovers are cutting-edge, but they’re less sensitive than what you’ll find in a well-equipped university lab. Everything about a rover like Perseverance or Curiosity is designed not only to withstand years of driving around on Mars, but to fit on board the rocket that will launch it from Earth. So it’s almost impossible to equip a rover with instruments that can compete with the ones we can install in laboratories here on Earth, where weight and size aren’t limiting factors.

That’s part of why decades of Mars rovers have struggled in the search for solid evidence of life on Mars. Perseverance has found traces of ring-shaped compounds that form the basic structure of most of the molecules we think of as “the building blocks of life.” But we don’t have enough information yet to be sure whether those compounds are remnants of ancient life or just stray molecules dropped onto the Martian surface by meteors and comets.

The other challenge is that any evidence of life on Mars is probably billions of years old, and it’s likely just chemical traces buried in soil and rock, which means they’ll be faint and sparse at best — and buried deep, if they have any chance of remaining intact. If cosmic rays haven’t totally degraded all traces of past life on Mars, we’ll still need incredibly sensitive instruments to find them. That’s why NASA plans to eventually bring some pieces of Mars home for a closer look.

The future of the past

“Our results stress the importance of returning samples to Earth for conclusively addressing whether life ever existed on Mars,” write Azua-Bustos and his colleagues.

But the Red Stone experiments also demonstrate why analogs — environments here on Earth that share important features with environments on Mars — are so important in the search for Martian life. Places like Red Stone give scientists and engineers a chance to find out exactly what kind of performance they can expect from instruments on the robots they send to explore Mars.

“If the biosignatures can’t be detected in Earth samples, where both current and ancient life is clearly documented, we should not expect these instruments to be capable of detecting evidence of life from Mars’ early history,” writes astrobiologist Carol Stoker (of NASA’s Ames Research Center) in a paper commenting on the study.

Perseverance spent the end of 2022 and the early weeks of 2023 building a cache of samples for a later mission to pick up and carry back to Earth; 10 sealed titanium tubes of Mars rock and regolith are now waiting for a planned sample retrieval lander to arrive in 2033 (or later). Meanwhile, Perseverance and Curiosity will keep drilling small holes in the ground, and zapping rocks with lasers. Sometime after 2028, the ESA’s Rosalind Franklin Rover and its MOMA instrument will join them on Mars.

Meanwhile, we may want to manage our expectations even when it comes to the Mars Sample Return.

“Any biological activity in these samples presumably took place billions of years ago, and only a few small samples can be brought back to Earth. It remains to be seen if unambiguous signatures of life can be found in those limited samples,” writes Stoker. “We must be cautious about interpreting absence of strong evidence of life as evidence of its absence.”

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