In his new work, published in the International Journal of Astrobiology, Vincenzo Rizzo from the National Research Center in the Italian city of Cosenza asks a provocative question. Why do many scientists do not want to use geological methods to identify biological processes on Mars, while on Earth these methods are widely used?
He points to one case that took place in Germany in 1908. Then a scientist named Ernst Kalkowsky suggested that multilayer hummocks, columns and sheet-like sedimentary rocks, called stromatolites, are biological in nature. Contemporaries did not believe him. But it was later proved that Kalkovsky was right when it became known for what reason stromatolites are formed. And they arise due to the fact that biofilms consisting of cyanobacteria and other microorganisms capture sedimentary deposits. Stromatolites are the oldest evidence of life on Earth since they are at least 3.5 billion years old and still exist in some remote regions, such as Shark Bay in Australia.
In his work, Rizzo follows in the footsteps of Kalkovsky, analyzing images taken on Mars by the rovers Spirit, Optitude, and Curiosity. These images indicate the presence of biological macrostructures such as stromatolites. He believes that unless a different, non-biological explanation is found, removed by the Mars rovers should be considered Martian stromatolites. Rizzo demonstrates a large number of structures that are strikingly similar to terrestrial stromatolites.
In principle, I am very doubtful of evidence that is based only on external resemblance or morphology, since the human brain tends to see or fill with familiar images, even where they do not exist. But Rizzo in his analysis is not limited to appearance. He made me realize that if (and this is big if) stromatolites really existed at an early stage in the life of Mars, then they looked exactly like the samples that he found in the images taken by all-terrain vehicles.
Here comes to mind the famous saying of Carl Sagan (Carl Sagan):
“Unusual statements require unusual evidence.”
For each structure that appears as a result of biological processes, there is a certain geochemical or physical process that imitates it. Some of these processes may not be known to us, because they occur on other planets such as Mars. On the other hand, we now know more about Mars than in the past. We know that at the early stage of the history of this planet, there were lakes, and possibly oceans, including in the Gale Crater, where he made his photographs of Curiosity. Organic matter was found in that place, and we know that pieces of rock can get from Earth to Mars (and vice versa), and also that microbes can survive such a journey. What, then, is a more “unusual” statement? What on Mars during the early usable period were such life forms that were similar to terrestrial ones, and which produced similar biogenic structures? Or that Mars has always been and remains a dead planet?
One way or another, the bar for applications for the discovery of life on Mars (both in the past and in the present) must be set very high. Even on Earth, it is sometimes difficult to decide whether a particular structure is the result of biological processes. Now we have more opportunities to explore Mars, but we still cannot do what we do on Earth all the time: go out into the street with a magnifying glass or other device, and study this or that mysterious feature.
If in the future we conduct an isotopic analysis of the structures discovered by Rizzo, which are similar to stromatolites, we will be able to obtain new evidence or refutation of their biological origin (life prefers lighter isotopes that we can use for verification). But I have suspicions that this hypothesis will face the same fate as Kalkovsky’s assumption. The final verdict will be issued much later, most likely when Mars will begin to investigate people who arrived there. And I hope they prove him right.