It is not easy to look for evidence of life on Mars. We don’t just have to somehow land a rover there, which is extremely difficult. But the rover needs the right tools and it has to look in the right place. At the moment, the Perseverance lander has ticked these boxes while pursuing its mission in the Jezero crater.
But there is another problem: there are structures that look like fossils but are not. Many natural chemical processes create structures that mimic biological ones. How can we tell them apart? How can we prepare for these false positives?
Scientists are familiar with dozens of natural processes that can create organic-looking structures. And there are probably many more that have not yet been discovered. Some of the microscopic structures are very beguiling and have fooled scientists in the past.
In a new article, two researchers outline some of the problems with fake fossils in finding real microscopic fossils on Mars. The article is entitled “False Biosignatures on Mars: Anticipating Ambiguity”. It is published in the Journal of the Geological Society.
No article on false references to life on Mars would be complete without mentioning the Allan Hills 84001 meteorite, discovered in Antarctica in 1984. This Martian meteorite is very old – around four billion years old – and scientists believe that Mars was briefly habitable.
The microscopic structures of the Allan Hills meteorite have a distinct biological appearance. They are only 20-100 nanometers long, which corresponds to the size of the theoretical nanobacteria. However, they are smaller than any known life form, and since then scientists have discarded the idea that nanobacteria exist.
The structures in the meteorite caused quite a stir, and the affair dragged on for several years. But eventually the scientific community moved on and realized that morphology alone cannot be used to discover primitive life.
The Allan Hills meteorite affair has faded, but it is still significant and a kind of teaching moment for all of us. False starts, as if they had inspired the authors of the new paper to examine the abiotic origins of structures that appear organic.
“We have been deceived by life-mimicking trials in the past,” said co-author of the article, Dr. Julie Cosmidis, in a press release. âOn many occasions, objects that looked like fossil microbes have been described in ancient rocks on Earth and even in meteorites from Mars, but upon closer inspection they were found to be non-biological in origin. This article is a cautionary story in which we call for further research into life-mimicking processes in the context of Mars so that we don’t fall 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 persistence finds something, the news of the “discovery” will spread quickly. But caution is advised, and ideally, scientists will draw conclusions about petrified life on Mars before any rapidly spreading conclusions. How can you do that?
Cosmidis and her co-author Dr. Sean McMahon of the University of Edinburgh said an interdisciplinary effort is needed to “… shed more light on how lifelike debris might form on Mars”.
âAt some point, a Mars rover will almost certainly find something that looks very much like a fossil. It is therefore important to clearly distinguish these from structures and substances that arise from chemical reactions, âsaid McMahon. âFor every type of fossil there is at least one non-biological process that produces very similar things.
Chemical gardens are lifelike structures that are produced purely by non-biological chemical processes. The structures are particularly beguiling because many of them are almost identical to real microscopic fossils. Also, both chemical gardens and actual fossils can be found in the same rock types and ages.
âMany of these are likely fossils – actually fossils from a deep biosphere housed in igneous rocks, with particular relevance to some life scenarios on Mars – but some are likely not and may be from chemical garden-like processes and / or others this results in types of self-organization that lead to filamentous crystals and aggregates, âthe authors write.
Carbonate-silica biomorphs are another type of abiotic structure that appear to be of biological origin. “Although purely inorganic, the curved and sinuous shapes of the biomorphs are reminiscent of biological objects such as spiral and segmented filamentous microbes, protists, and even plants and animals,” the authors write. They are made of alkaline earth metals and look amazingly organic. Scientists divide them into three classes: helicoidal filaments, worm-like braids, and leaf-like flat leaves.
Then there are carbon-sulfur biomorphs. They also appear shockingly biological in origin, forming spheres and filaments both straight and spiral. The authors of a 2016 paper on carbon-sulfur biomorphs said: “The morphology and composition of these carbon / sulfur microstructures are so similar to microbial cellular and extracellular structures that new caution is required when interpreting putative microbial biosignatures in the fossil record. â
Human history is in some ways a long history of misunderstanding nature, at least until we have developed the scientific method. So it’s not shocking that we are still struggling with this. In their conclusion, the authors point out how difficult it is to infer a fossil.
The images make it clear that abiotic processes can produce structures that appear very lifelike. But what makes it even more difficult to prove is that these biomorphs mimic biotic structures in ways other than just morphology.
âFirstly, abiotic processes can not only imitate morphological biosignatures, but also chemical / molecular, mineralogical, isotopic and textural biosignatures; a critical attitude is required in all cases, and morphological data is not necessarily less reliable than other possible evidence of life, âthey explain. So multiple lines of evidence can point to biological origins where there are none.
Maybe we’re lucky and the Perseverance rover or some other mission will uncover indisputable evidence of ancient Martian life. But it is more likely that we will face a number of false positives. “Since life itself is presumably the product of the self-organization of abiotic geochemical reactions, the complexity of abiotic natural products should not be underestimated,” they emphasize.
Much work has been put into the study of false biosignatures, and the authors believe that more is needed. According to McMahon and Cosmidis, more interdisciplinary work is needed. The key is to build a large dataset for false positives the same way we built one for biosignatures. They suggest that field experiments at Mars-analogous locations as well as laboratory work can lead us there.
They are confident that it can be done.
âNevertheless, we are optimistic that the problem of incorrect biosignatures is not unsolvable. The better these phenomena are understood, the more sensitively we will be able to distinguish between true signs of life and these deceivers. “