As we sift more and more of the surface of Mars, we’d love to find fossils. But then we may run into a problem that dogs paleontologists on Earth. From the University of Edinburgh:

Rocks on Mars may contain numerous types of non-biological deposits that look similar to the kinds of fossils likely to be found if the planet ever supported life, a study says.

Telling these false fossils apart from what could be evidence of ancient life on the surface of Mars — which was temporarily habitable four billion years ago — is key to the success of current and future missions, researchers say.

University of Edinburgh, “Life on Mars search could be misled by false fossils, study says” at ScienceDaily (November 16, 2021) The paper requires a subscription.

A group of astrobiologists from the Universities of Edinburgh and Oxford have identified dozens of processes that “can produce structures that mimic those of microscopic, simple lifeforms that may once have existed on Mars.”

Dr Sean McMahon, Chancellor’s Fellow in Astrobiology at the University of Edinburgh’s School of Physic and Astronomy, said: “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. For every type of fossil out there, there is at least one non-biological process that creates very similar things, so there is a real need to improve our understanding of how these form.”

University of Edinburgh, “Life on Mars search could be misled by false fossils, study says” at ScienceDaily (November 16, 2021) The paper is open access.

The paper goes into considerable detail; for example, for Mars:

The difficulty of correctly determining the biogenicity of life-like geological substances and structures has continued to plague palaeontology, and the new field of astrobiology, often despite laudable efforts to practice the ‘care, patience, and critical attitude’ recommended by Cloud. For example, McKay et al. (1996) reported multiple lines of evidence in the martian meteorite ALH84001 that were suggestive of past life on Mars, making news bulletins around the world. These included carbonate globules and magnetite crystals resembling bacteriogenic precipitates, polyaromatic hydrocarbon compounds and worm-like microstructures interpreted as morphological fossils. The origins of these features are still unclear today, but abiotic explanations have been offered for all of them, and the overall case for life in ALH84001 no longer seems compelling (Martel et al. 2012). This example warns us that it is not enough to have ‘multiple lines of evidence’ for biogenicity (an oft-repeated mantra) if each line of evidence is ambiguous.

False biosignatures on Mars: anticipating ambiguity, Sean McMahon and Julie Cosmidis, Journal of the Geological Society, 17 November 2021, https://doi.org/10.1144/jgs2021-050

and for Earth:

Other ongoing debates concern the oldest fossil and geochemical evidence of life on Earth. Schopf (1993) presented fossil ‘cyanobacteria’ from the 3.5-Gyr-old Apex Chert of Western Australia, which were later reinterpreted as abiotic carbon organized around spherulitic quartz growth and/or worm-like delaminated clay booklets (Brasier et al. 2002, 2005; Wacey et al. 2016a). Recent work highlights the possibility that organic matter in such cherts might be biogenic even if it has been abiotically redistributed (Duda et al. 2018), while Schopf et al. (2018) have claimed that morphology-specific carbon isotope values confirm the biogenicity of the Apex microstructures and reveal them to include methane-cycling archaea; this controversy continues (e.g. Alleon and Summons 2019). Nutman et al. (2016) described triangular features exposed on 3.7-Gyr-old metasedimentary rocks from the Isua supracrustal belt in Greenland and interpreted them as fossil stromatolites; these have been reinterpreted by some researchers as abiotic deformation features on the basis of 3D imaging and geochemical analysis (Allwood et al. 2018; Zawaski et al. 2020), although Nutman and co-workers stand by their original view (Nutman et al. 2021). Dodd et al. (2017) described hematite tubules in a c. 4-Gyr-old hydrothermal chert from the Nuvvuagittuq Greenstone Belt in Canada as the mineralized sheaths of ancient Fe-oxidizing bacteria; these have been queried as possible chemical gardens (McMahon 2019), but further analyses are needed.

False biosignatures on Mars: anticipating ambiguity, Sean McMahon and Julie Cosmidis, Journal of the Geological Society, 17 November 2021, https://doi.org/10.1144/jgs2021-050

The trouble is, simple life forms leave only evidence, not messages so it can sometimes be hard to be sure.

On Earth, we can also be fooled about human tools or footprints. For example, from New Scientist we learn that sometimes, stones smashed by horses or monkeys can be mistaken for fragments of ancient tools.

Or it can go the other way: Just recently, it was discovered that footprints thought to be made by a bear were more probably made by humans 3.7 million years ago:

As bears walk, they take very wide steps, wobbling back and forth,” says senior author Jeremy DeSilva, an associate professor of anthropology at Dartmouth. “They are unable to walk with a gait similar to that of the Site A footprints, as their hip musculature and knee shape does not permit that kind of motion and balance.” Bear heels taper and their toes and feet are fan-like, while early human feet are squared off and have a prominent big toe, according to the researchers. Curiously, though, the Site A footprints record a hominin crossing one leg over the other as it walked — a gait called “cross-stepping.”

“Although humans don’t typically cross-step, this motion can occur when one is trying to reestablish their balance,” says McNutt. “The Site A footprints may have been the result of a hominin walking across an area that was an unlevel surface.”

Dartmouth College, “Footprints from site a at Laetoli, Tanzania, are from early humans, not bears” at ScienceDaily (December 1, 2021) The paper is open access.

Other researchers urge caution about the footprints — which reinforces the difficulty of certainty.

It won’t be surprising if we hear heated controversies about whether new finds on Mars are really fossils. But it’s a step in the right direction if astrobiologists have something concrete to argue about.