I’ve long maintained that we’ll find compelling biosignatures on an exoplanet sooner than we’ll find them in our own Solar System. But I’d love to be proven wrong. The recent flurry of news over the interesting findings from the Perseverance rover on Mars is somewhat reminiscent of the Clinton-era enthusiasm for the Martian meteorite ALH8001. Now there are signs, as Alex Tolley explains below, that this new work will prove just as controversial. Biosignatures will likely be suggestive rather than definitive, but Mars is a place we can get to, as our rovers prove. Will Perseverance compel the sample return mission that may be necessary to make the definitive call on life?
by Alex Tolley
Overview of jezero Crater and sample site in article. Credit NASA/MSSS/USGS.
On September 10, 2025, Nature published an article that got wide attention. The authors claimed that they had discovered a possible biosignature on Mars. If confirmed, they would have won the race to find the first extraterrestrial biosignature. Exciting!
One major advantage of detecting a biosignature in our system is that we can access samples and therefore glean far more information than we can using spectroscopic data from an exoplanet. This will also reduce the ambiguity of simpler atmospheric gas analyses that are all we can do with our telescopes at present.
Figure 1. Perseverance’s path through Neretva Vallis and views of the Bright Angel formation. a, Orbital context image with the rover traverse overlain in white. White line and arrows show the direction of the rover traverse from the southern contact between the Margin Unit and Neretva Vallis to the Bright Angel outcrop area and then to the Masonic Temple outcrop area. Labelled orange triangles show the locations of proximity science targets discussed in the text. b, Mastcam-Z 360° image mosaic looking at the contact between the light-toned Bright Angel Formation (foreground) and the topographically higher-standing Margin Unit from within the Neretva Vallis channel. This mosaic was collected on sol 1178 from the location of the Walhalla Glades target before abrasion. Upslope, about 110 m distant, the approximate location of the Beaver Falls workspace (containing the targets Cheyava Falls, Apollo Temple and Steamboat Mountain and the Sapphire Canyon sample) is shown. Downslope, about 50 m distant, the approximate location of the target Grapevine Canyon is also shown. In the distance, at the southern side of Neretva Vallis, the Masonic Temple outcrop area is just visible. Mastcam-Z enhanced colour RGB cylindrical projection mosaic from sol 1178, sequence IDs zcam09219 and zcam09220, acquired at 63-mm focal length. A flyover of this area is available at https://www.youtube.com/watch?v=5FAYABW-c_Q. Scale bars (white), 100 m (a), 50 m (b, top) and 50 cm (b, bottom left). Credit: NASA/JPL-Caltech/ASU/MSSS.
Let’s back up for context. The various rover missions to Mars have proceeded to determine the history of Mars. From the Pathfinder mission starting in 1996, the first mission since the two 1976 Viking landers, the various rovers from Soujourner (1997), Spirit & Opportunity (2004), Curiosity (2012), and now Perseverance (2021), have increased the scope of their travels and instrument capabilities. NASA’s Perseverance rover was designed to characterize environments and look for signs of life in Jezero Crater, a site that was expected to be a likely place for life to have existed during the early, wet phase of a young Mars. The crater was believed to be a lake, fed by water running into it from what is now Neretva Vallis, and signs of a delta where the ancient river fed into the crater lake are clear from the high-resolution orbital images. Perseverance has been taking a scenic tour of the crater, making stops at various points of interest and taking samples. If there were life on Mars, this site would have both flowing water and a lake, with sediments that create a variety of habitats suitable for prokaryotic life, like the contemporary Earth.
Perseverance had taken images and samples of a sedimentary rock formation, which they called Bright Angel. The work involved using the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument to obtain a Raman UV spectrum of rock material from several samples. The authors claimed that they had detected 2 reduced iron minerals, greigite and vivianite, and organic carbon. The claim is that these have been observed in alkaline environments on Earth due to bacteria, and therefore prove to be a biosignature of fossil life. The images showed spots (figure 2) which could possibly be the minerals formed by the metabolism of anaerobic bacteria, reducing sulfur and iron for energy. The organic carbon in the mudstone rock matrix is the fossil remains of the bacteria living in the sediments.
Exciting, no? Possible proof that life once existed on Mars. The authors submitted a paper to Nature with the title, “Detection of a Potential Biosignature by the Perseverance Rover on Mars“. The title was clearly meant to catch the scientific and popular attention. At last, NASA’s “Follow the Water” strategy and exploration with their last rover equipped to detect biosignatures had found evidence of fossil life on Mars. It might also be a welcome boost for NASA’s science missions, currently under funding pressure from Congress.
Then the peer review started, and the story seemed less strong. Just as 30 years ago, when the announcement from the White House by the US president, Bill Clinton claimed that a Martian meteorite retrieved from the Antarctic, ALH8001, was evidence of life on Mars proved very controversial. Notably, slices of that meteorite viewed under an electron microscope showed images of what might have been some forms of bacteria. These images were seen around the world and were much discussed. The consensus was that the evidence was not unambiguous, with even the apparent “fossil bacteria” being explained as natural mineral structures.
Well, the new paper created one of the longest peer review documents I have ever read. Every claimed measurement and analysis was questioned, including the interpretation. The result was that the paper was published as the much drier “Redox-driven mineral and organic associations in Jezero Crater, Mars”. There are just 3 uses of the term biosignatures, each prefaced with the term “potential”, and the null hypothesis of abiotic origin emphasized as well. One of the three peer reviewers even wanted Nature to reject the paper, based on what might be another ALH8001 fiasco. A demand, too far.
What were the important potential biosignature findings?
Organisms extract energy from molecules via electron transfer. This often results in the compounds becoming more reduced. For example, sulfur-reducing bacteria convert sulfates (SO4) to sulfide (S). Iron may be reduced from its ferric (Fe3+) state to its ferrous (Fe2+) state. Two minerals that are often found reduced as a result of bacterial energy extraction are greigite Fe2+Fe3+2S4] and vivianite [Fe2+3(PO4)2·8H2O]. On Earth, these are regarded as biosignatures. In addition, unidentified carbon compounds were associated with these 2 minerals. The minerals were noticed as spots on the outcrop and identified with the Planetary Instrument for X-ray Lithochemistry (PIXL), which can identify elements via X-ray spectroscopy. The SHERLOC instrument identified the presence of carbon in association with these minerals.
Figure 2. An image of the rock named “Cheyava Falls” in the “Bright Angel formation” in Jezero crater, Mars, collected by the WATSON camera onboard the Mars 2020 Perseverance rover. The image shows a rust-colored, organic matter in the sedimentary mudstone sandwiched between bright white layers of another composition. The small dark blue/green to black colored nodules and ring-shaped reaction fronts that have dark rims and bleached interiors are proposed to be potential biosignatures. Credit: NASA/JPL-Caltech/MSSS.
To determine whether the carbon associated with the greigite and vivianite was organic or inorganic, the material was subjected to ultraviolet rays. Organic carbon bonds, especially carbon-carbon bonds, will respond to specific wavelengths by vibrating, like sound frequencies can resonate and break wine glasses. Raman spectroscopy is the technique used to detect the resonant vibrations of types of carbon bonds, particularly specific arrangements of the atoms and their bonds that are common in organic carbon. The spectroscopic data indicated that the carbon material was organic, and therefore possibly from decayed organisms. This would tie together the findings of the carbon and the 2 minerals as a composite biosignature. However, the reviewers also questioned the interpretation of the Raman spectrum.. The sp2 carbon bonds (120 degrees) seen in aromatic 6-carbon rings, in graphene, graphite, and commonly in biotic compounds, should show both a G-band (around 1600 cm-1) and a D-band (around 2700 cm-1), yet the spectrum only clearly showed the G-band. Did this imply that the organic carbon may not have been found? The reviewers also questioned why the biological explanation was favored over an abiotic one. No one questioned the greigite and vivianite findings, other than that they are not exclusively associated with anaerobic bacterial metabolism.
Figure 3 – Raman spectrum with interpolated curves to highlight the G-band in the 4 samples taken at the location.
So what to make of this? Clearly, the authors backed down on their more positive interpretation of their findings as a biosignature.
What analyses would we want to do on Earth?
Assuming the samples from Perseverance are eventually retrieved and returned to Earth, what further analysis would we want to do to increase our belief that a biosignature was discovered?
A key analysis would be to analyze the carbon deposits. The Raman UV spectra indicate that the carbon is organic, which is almost a given. You may recall that the private MorningStar mission to Venus will do a similar analysis but use a laser-induced fluorescence that detects aromatic rings [1]. Neither of these techniques can distinguish between abiotic and biogenic carbon. The carbon may even be in the form of common polycyclic aromatic hydrocarbons (PAH), a form that is ubiquitous and is easily formed, especially with heat.
One useful approach to distinguish the source of the carbon is to measure the isotopic ratios of the 2 stable carbon isotopes, carbon-12 and carbon-13. Living organisms favor the lighter carbon-12, and therefore, the C13/C12 ratio is reduced when the carbon is from living organisms. This must be compared to known abiotic carbon to confirm its source. This analysis requires a mass spectrometer, which was not included with the Perseverance instrument pack.
The second approach is to analyze the carbon compounds. Gas chromatography followed by infrared spectroscopy is used to characterize the compounds. Life restricts the variety of compounds compared to random reactions, and can be compared to expectations based on Assembly Theory [2], although exposure to UV and particle radiation for billions of years may make the composition of the carbon more random.
Lastly, if the carbon were once protein or nucleotide macromolecules, any chirality might distinguish its source as biotic.
Isotopic analysis can also be made on the sulfur compounds in the greigite. As with carbon, life will preferentially use lighter isotopes. Bacteria reduce sulfate to sulfide for energy, and the iron sulfide mineral, greigite, is a waste product of this metabolism. Of the 2 stable sulfur isotopes, sulfur-32 and sulfur-34, if the S34/S32 ratio is reduced, then this hints that the greigite was formed biotically.
Lastly, opening up the samples and inspecting them with an electron microscope, there may be physical signs of bacteria. However, any physical features will need to be identified unambiguously to avoid the ALH8001 controversy.
Unfortunately for these proposed analyses, the Mars Sample Return (MSR) mission has been cut with the much-reduced NASA budget. When, or whether, we get these samples for analyses on Earth is currently unknown.
My view on the findings
If the findings and the interpretation of their compositions is correct, then this would probably be the most convincing, but still not unambiguous biosignature to date. If the samples are returned to Earth and the findings are extended with other analyses, then we probably would have detected fossil life on Mars. In my opinion, that would validate the idea that Martian life existed, and further exploration is warranted. We would then want to know if that life was similar or different from terrestrial life to shed light on abiogenesis or panspermia between Earth and Mars. As the formation of the Moon would have been very destructive, if life emerged on Mars and was spread to Earth, this might provide more time for living cells to evolve compared to the conditions on Earth. It would also stimulate the search for subsurface life on Mars, where interior heat and water between rock grains would support such a niche habitat as it does on Earth.
It seems a pity that without an MSR, we may have the evidence for Martian fossil life, packaged for analysis, but kept frustratingly remote and unavailable, mere millions of kilometers distant.
The paper is Hurowitz, J.A., Tice, M.M., Allwood, A.C. et al. “Redox-driven mineral and organic associations in Jezero Crater, Mars.” Nature 645, 332–340 (2025). https://doi.org/10.1038/s41586-025-09413-0
Other readings
Tolley A, (2022) Venus Life Finder: Scooping Big Science web: https://www.centauri-dreams.org/2022/06/03/venus-life-finder-scooping-big-science/
Walker, S. I., Mathis, C., Marshall, S., & Cronin, L. (2024). “Experimentally measured assembly indices are required to determine the threshold for life.” Journal of the Royal Society Interface, 21(220). https://doi.org/10.1098/rsif.2024.0367
