Martian Biosignatures and the Problem of “False Positives”

Credit: NASA.

One stated goal of the current generation of Mars rovers is to search for possible ancient biosignatures on Mars. However, the question of what a genuine biosignature would look like is still open and subject to ambiguity. A recent paper by Sean McMahon of the University of Edinburgh and Julie Cosmidis of Oxford University underlines this issue of potentially “false positive” biosignatures, making the case that the search for such traces of early life on Earth has historically been fraught with the misidentification of abiotic processes as biotic.

The authors describe a classic example of such ambiguity: the announcement in 1996 that fossil structures of microbial origin had been discovered in the martian meteorite ALH84001. While the origin of these features is still unclear, subsequent research has cast doubt on the argument that these structures represent fossil life. Such seemingly biological but actually inorganic structures are common in terrestrial geology and can be deceiving, even with much better data-collecting methods and geological context than would be possible for NASA’s Perseverance or ESA’s Rosalind Franklin rovers on Mars. Thus, the authors argue that there is a need to study and effectively catalog such instances from the terrestrial literature so that we have better context to understand any potential discoveries from these missions.

In addition to “pseudofossils” that appear morphologically similar to actual fossils, organic materials found in rocks could have complex origins not necessarily related to the presence of life. Simple carbon-bearing organic matter may have undergone a number of processes that generate much more complex carbon-bearing molecules even without the intervention of life. Understanding the analytical capabilities of the current generation of rovers, and what data could and could not tell us about the biogenic source of such material, is of paramount importance to correct interpretation of any discovery of a potential biosignature. READ MORE