There is nothing easy on the search for evidence of life on Mars. Not only do we have to somehow land a rover there, which is extremely difficult. But the rover needs the right instruments, and it has to look in the right place. As of now, Perseverance has ticked these boxes as it continues its mission in Jezero crater.
But there is another problem: there are structures that look like fossils but are not. Many natural chemical processes produce structures that mimic biological ones. How can we tell them apart? How to prepare for these false positives?
Scientists know of dozens of natural processes that can create organic-looking structures. And there are probably many more that have yet to be discovered. Some of the microscopic structures are very attractive and have already fooled scientists.
In a new article, two researchers describe some of the problems with false fossils in finding microscopic true fossils on Mars. The article, published in the Journal of the Geological Society, is titled “False biosignatures on Mars: anticipating ambiguity. “
No article on false indications of 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 – that’s when scientists believe Mars was briefly habitable.
The microscopic structures of the Allan Hills meteorite have a distinct biological appearance. They are only 20 to 100 nanometers long, which is the theoretical size of nanobacteria. However, they are smaller than any known life form, and scientists have since rejected the idea that nanobacteria exist.
“We have been duped by life-mimicking processes in the past.” – Dr Julie Cosmidis, paper co-author, Oxford.
The structures of the meteorite attracted quite a bit of attention, and the matter dragged on for a few years. But eventually, the scientific community moved on, realizing that morphology alone cannot be used to detect primitive life.
The Allan Hills meteorite case has faded, but it is still important and a teaching moment for all of us. The false starts inspired the authors of the new article to take a closer look at the abiotic origins of organic-looking structures.
“We have been deceived by life-mimicking processes in the past,” article co-author Dr Julie Cosmidis said in a statement. Press release. âOn numerous occasions, objects that resembled fossil microbes have been described in ancient rocks on Earth and even in meteorites from Mars, but upon closer examination they were found to have non-biological origins. This article is a uplifting tale in which we call for further research on life imitation processes in the context of Mars in order to avoid falling into the same traps over and over again.
The authors point out that anything that Perseverance finds on Mars that looks like a fossil is likely to have very ambiguous origins. If Perseverance finds something, the “discovery” will spread quickly. But caution is in order and, ideally, scientists will be aware of any fast-disseminating findings about fossilized life on Mars. How can they do that?
Cosmidis and its co-author Dr Sean McMahon, University of Edinburgh, say an interdisciplinary effort is needed to “shed more light on how realistic deposits might form on Mars.”
âAt some point, a Martian rover will almost certainly find something that looks a lot like a fossil, so it is vital to be able to confidently distinguish them from structures and substances created by chemical reactions,â McMahon said. “For every type of fossil, there is at least one non-biological process that creates very similar things, so there is a real need to improve our understanding of their formation.”
Chemical gardens are realistic structures produced only by non-biological chemical processes. The structures are particularly alluring as many of them appear almost identical to real microscopic fossils. In addition, chemical gardens and actual fossils can be found in the same types and ages of rock.
âMany of them are probably fossils – indeed, fossils from a deep biosphere lodged in igneous rocks, with particular relevance to certain life scenarios on Mars – but some are probably not and may result from chemical garden-type processes and / or other types of self-organization that lead to crystals and filamentous aggregates, âthe authors write.
Carbonate-silica biomorphs are another type of abiotic structure that appears to have biological origins. “Although purely inorganic, the curved, sinuous shapes of biomorphs evoke biological objects, such as helical and segmented filamentous microbes, protists, and even plants and animals,” the authors write. They are made from alkaline earth metals and have an amazingly organic appearance. Scientists divide them into three classes: helical filaments, worm-like braids, and flat, leaf-like leaves.
Then there is carbon-sulfur biomorphs. They also appear to be of shocking biological origin and form straight, helical spheres and filaments. The authors of a 2016 paper on carbon-sulfur biomorphs said, âThe morphology and composition of these carbon / sulfur microstructures so closely resemble microbial cellular and extracellular structures that further caution must be applied in interpreting them. Putative microbial biosignatures in the fossil record. “
Human history in some ways is a long story of misunderstanding nature, at least until we came up with the scientific method. So it’s not shocking that we are still grappling with this. In their conclusion, the authors stress how difficult it is to conclude that something is a fossil.
The images clearly show that abiotic processes can produce structures that look very realistic. But what makes detection even more difficult is that these biomorphs mimic biotic structures in ways other than morphology.
âFirst, abiotic processes can mimic not only 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 pieces of evidence for life, âthey explain. Thus, several data sources can suggest biological origins where there is none.
Maybe we’ll get lucky and the Perseverance rover or some other mission will uncover undeniable evidence of ancient Martian life. But it is more likely that we are faced with a series of false positives. “Because life itself is believed to be the product of self-organization in abiotic geochemical reactions, the complexity of natural abiotic products should not be underestimated,” they point out.
Much work has been devoted to studying false biological signatures, and the authors believe more is needed. According to McMahon and Cosmidis, more interdisciplinary work is needed. The key is to create a rich dataset for false positives the same way we created one for biosignatures. They suggest that field experiments in Mars-like locations, as well as lab work, can get us there.
They believe it can be done.
âNonetheless, we are optimistic that the problem of false biological signatures is not unsolvable. The better these phenomena are understood, the more we will be able to distinguish between the true proofs of life and these impostors.