Since its successful landing this February, the Perseverance rover has been prowling Mars in search of signs of life, past or present. Its hunting ground in Jezero Crater is rich in carbonate minerals that are hardy enough to have preserved microscopic fossils for billions of years, meaning that NASA’s new rover might have a shot at finding fossil evidence of alien life.

If Perseverance does find something that looks like it could be a microfossil, scientists on Earth will probably still need to wait for sample return before they can make the final call using more powerful analytical methods available in Earth labs. Determining whether microbe-shaped mineral blobs and filaments are truly microfossils is not always straightforward. Usually, researchers use biogenicity criteria — checklists of characteristics thought to be diagnostic of biological origin — to evaluate potential microfossils. But a recent study published in the journal Gebiology suggests that even with biogenicity criteria, it might not always be possible to distinguish faux fossils from the real deal.

To test the limits of biogenicity criteria, researchers analyzed mineral microfilaments in rocks that formed under Mars-analogue conditions in Earth’s subsurface. They showed that these microfilaments fulfilled many published biogenicity criteria, despite also looking nearly identical to completely inorganic structures formed in microbe-free chemical garden experiments. Given these results, and because the geochemical conditions in which the samples formed could feasibly explain these “dubiofossils,” the researchers argue that we still can’t be sure that they’re microfossils — even though they passed biogenicity criteria. 

Determining the biogenicity of mineral microstructures is hard enough on Earth, where life is a reasonable hypothesis. On Mars, it is the hypothesis of last resort. Every possible abiotic explanation for possible life found on the red planet needs to be eliminated before biology can be considered a reasonable explanation. The search for life beyond Earth will thus rely on our ability to do just that.