Is That a Fossil on Mars? Non-Biological Deposits can Mimic Organic Structures

There is no easy way to find evidence of life on Mars. It is difficult to land a rover there. The rover needs the right instruments to do its job. The Perseverance lander has checked those boxes and is now focused on Jezero Crater.

There are structures that look like fossils but aren't. Natural chemical processes produce structures that are similar to biological ones. How can we tell them apart? How can we prepare for false positives?

There are many natural processes that can create structures. There are likely many more that haven't been discovered. Scientists have been fooled before by some of the tiny structures.

Some of the problems with false fossils are outlined in a new article. The article is about false biosignatures on Mars. The Journal of the Geological Society has published it.

The Allan Hills 84001 meteorite was discovered in 1984 and is the only meteorite that has false indications of life on Mars. Scientists think that the Martian meteorite is four billion years old.

The structures in the meteorite are different from one another. They are only 20 to 100 nanometers in length. They are smaller than any known lifeform, and since then, scientists have discarded the idea that they exist.

We have been fooled by life-mimicking processes in the past.
Dr. Julie Cosmidis is an article co-author.

The structures in the meteorite attracted a lot of attention, and the affair dragged on for a few years. The scientific community realized that they can't detect primitive life alone.

The Allan Hills meteorite has structures that look like living organisms. The image is from NASA and is public domain.

The Allan Hills meteorite affair is still significant and a teaching moment for all of us, even though it has faded. The authors of the new paper were inspired by false starts to take a harder look at abiotic origins.

Cosmidis said in the press release that they have been fooled by life-mimicking processes in the past. Many objects that looked like fossils were described in ancient rocks on Earth and even in meteorites from Mars, but they turned out to be non-biological origins. This article is a cautionary tale in which we call for further research on life-mimicking processes in the context of Mars so that we avoid falling into the same traps over and over again.

The authors point out that if Perseverance finds a fossil on Mars, it's likely to have very ambiguous origins. News of the discovery will spread quickly if Perseverance finds something. It is important for scientists to get in front of any rapidly-spreading conclusions about the existence of life on Mars. How can they do that?

The University of Edinburgh and Cosmidis have collaborated on a paper about how Mars could have deposits that are realistic.

McMahon said that being able to distinguish structures and substances made by chemical reactions is important, as a Mars rover will almost certainly find something that looks a lot like a fossil. There is a need to improve our understanding of how these form because there are at least one non-biological process that creates very similar things.

These images look like chemical gardens but are not. Chemical garden images can be used under a Creative Commons license.

Chemical gardens are structures made from non-biological chemical processes. Many of the structures look almost identical to actual fossils. Both chemical gardens and actual fossils can be found in the same types and ages of rock.

Many of these are fossils from a deep biosphere hosted in igneous rocks, with special relevance to some scenarios for life on Mars, but some are probably not and may result from chemical-garden-like processes.

The carbonate-silica biomorphs look like they have biological origins. The biomorphs evoke biological objects, such as protists and plants and animals. They are formed from Earth metals. They are divided into three classes: helicoidal filaments, worm-like braids, and leaf-like flat sheets.

These images show carbonate-silica biomorphs that are not biological. The image on the left shows structures that look like they are biological. The image on the right shows the blown-up area on the left, which appears to be a branching, flowering organisms. The image is from the Florida State University.

Then there are other types of organisms. They are both straight and helical and appear to be biological. The authors of a 2016 paper on carbon-sulphur biomorphs said that they need to be cautious in interpreting the biosignatures of the fossil record.

There are networks of spheres and filaments that appear biotic in origin. The images were adapted from the book by the same name.
The University of Michigan has a professor named C. Nims.

There are spheres and filaments that appear to be biotic in origin. The images were adapted from the book by the same name.

The University of Michigan has a professor named C. Nims.

Human history is a long tale of misunderstanding nature until we devised the scientific method. We are still struggling with it. The authors point out how difficult it is to conclude that something is a fossil.

The images show that abiotic processes can produce structures that are very life-like. Detection is even more difficult because biomorphs mimic biotic structures in other ways.

A critical attitude is required in all cases and the data from the biosignatures is not necessarily less reliable than other evidence for life. Multiple lines of evidence can show biological origins.

Maybe we will get lucky and the Perseverance rover will find evidence of ancient Martian life. It is more likely that we face false positives. The complexity of abiotic natural products should not be underestimated because they are presumed to be the product of self-organization.

The study of false biosignatures has taken a lot of work and the authors think more is needed. McMahon and Cosmidis say more interdisciplinary work is needed. The key is to build a rich dataset for false positives in the same way that we built one for biosignatures. They suggest that we can do field experiments in Mars-analogue locations.

They think it can be done.

The problem of false biosignatures is not intractable. The better these phenomena are understood, the more we will be able to discriminate between real and fake life.