There’s nothing easy about searching for evidence of life on Mars. Not only do we somehow have to land a rover there, which is extraordinarily difficult. But the rover needs the right instruments, and it has to search in the right location. Right now, the Perseverance lander has checked those boxes as it pursues its mission in Jezero Crater.

But there’s another problem: there are structures that look like fossils but aren’t. Many natural chemical processes produce structures that mimic biological ones. How can we tell them apart? How can we prepare for these false positives?

Scientists know of dozens of natural processes that can create organic-looking structures. And there are likely many more that haven’t been discovered yet. Some of the microscopic structures are very beguiling and have fooled scientists before.

In a new article, a pair of researchers outline some of the problems with false fossils in the search for real microscopic fossils on Mars. The article is titled “False Biosignatures on Mars: Anticipating Ambiguity.” It’s published in the Journal of the Geological Society.

No article about false indications of life on Mars would be complete without mentioning the Allan Hills 84001 meteorite that was discovered in Antarctica in 1984. That Martian meteorite is very old—about four billion years old—which is when scientists think Mars was briefly habitable.

The microscopic structures in the Allan Hills meteorite have a distinct biological appearance. They’re only 20 – 100 nanometers in length, which is the size that theoretical nanobacteria would be. However, they’re smaller than any known lifeform, and since then, scientists have discarded the idea that nanobacteria exist.

“We have been fooled by life-mimicking processes in the past.”

Dr. Julie Cosmidis, article co-author, Oxford.

The structures in the meteorite attracted quite a bit of attention, and the affair dragged on for a few years. But eventually, the scientific community moved on, realizing that morphology alone can’t be used to detect primitive life.

This electron microscope image of the Allan Hills meteorite shows chain-like structures that resemble living organisms. Image Credit: By NASA -  Public Domain,
This electron microscope image of the Allan Hills meteorite shows chain-like structures that resemble living organisms. Image Credit: By NASA – Public Domain,

The Allan Hills meteorite affair has faded, but it’s still significant and is kind of a teaching moment for all of us. False starts like it inspired the authors of the new paper to take a harder look at abiotic origins for organic-appearing structures.

“We have been fooled by life-mimicking processes in the past,” article co-author Dr. Julie Cosmidis said in a press release. “On many occasions, objects that looked like fossil microbes were described in ancient rocks on Earth and even in meteorites from Mars, but after deeper examination, they turned out to have non-biological origins. This article is a cautionary tale in which we call for further research on life-mimicking processes in the context of Mars so that we avoid falling into the same traps over and over again.”

The authors point out that anything Perseverance finds on Mars that looks like a fossil is likely to have very ambiguous origins. If Perseverance does find something, news of the “discovery” will spread rapidly. But caution is required, and ideally, scientists will get in front of any rapidly-spreading conclusions about fossilized life on Mars. How can they do that?

Cosmidis and her co-author Dr. Sean McMahon, from the University of Edinburgh, say that an interdisciplinary effort is needed to “… shed more light on how lifelike deposits could form on Mars.”

“At some stage, a Mars rover will almost certainly find something that looks a lot like a fossil, so being able to confidently distinguish these from structures and substances made by chemical reactions is vital,” McMahon said. “For every type of
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