Abstract_EANA2026-95

Abstract EANA2026-95

Elusive traces of microbial life on Mars : a case study from the 3.45 Ga Kitty’s Gap Chert, Pilbara, Australia

Westall, F.(1), Purvis, G.(2), Sano N.(3), Sheriff, J.(4), Clodoré, L.(1), Foucher, F.(5), Milojevic, T.(6)

1. CNRS-Centre de Biophysique Moléculaire, Orléans, France 2. School of Natural and Environmental Sciences, Newcastle University, UK 3. Ionoptika Ltd., Chandler’s Ford, Eastleigh, UK 4. Department of Engineering, University of Newcastle, UK 5. CNRS- Conditions Extrêmes et Matériaux: Haute Température et Irradiation, Orléans, France 6. Institut für Elektronenmikroskopie und Nanoanalytik, University of Graz, Austria

The 2028 ExoMars mission aims at detecting traces of martian life, while both the Curiosity and Perseverance rovers in Gale and Jezero craters, respectively, are also searching for martian life (and may have found tentative organic signs of it, Freissinet et al., 2025). The most optimistic scenario is that organisms similar to phototrophs developed and left both macroscopic and chemical traces detectable at the landing sites (Hickman-Lewis et al., 2023). A more likely scenario is that martian life may not have developed phototrophic capabilities, at least not necessarily in the areas of the landing sites. However, a more likely scenario is that martian life, if it existed at the landing sites, remained in a primitive, chemotrophic stage. The remains of such organisms are not readily observable and their geo-organic signatures are very subtle.

The 3.45 Ga Kitty’s Gap Chert hosts the oldest known cellular remains, remains that are both morphologically preserved, as well as organically preserved. They occur in volcanoclastic sediments formed on a coastal mud flat, similar to the shallow water scenarios for the Mars rover landing sites. However, the cells are very small, < 1 µm, the colonies are very small, generally < 20 µm, and the organic content in the fossilised matter very low (Westall et al. 2025). While the colonies of small coccoidal silicified microbial cells from the Kitty’s Gap Chert were well documented, and the carbon isotope signature of the semi-bulk sediments suggested a biological origin, the previously published results were either ignored or considered to be artefacts of contamination. Westall then had to await the development of a new instrument with increased precision and sensitivity (Cluster SIMS, Ionoptika, UK) that was able to document, in situ, the co-location between aromatic and aliphatic molecular fragments containing C, H, N, and O and exhibiting even carbon number compositions, and the cellular structures. Together with the morphological biosignatures previously documented, this suggests indeed a biological origin. Moreover, molecular fragments also containing Si indicate that the coccoids had been silicified at the same time as the matrix sediments and therefore not a later contamination (during sample preparation, for example). The intimate association of the microfossil colonies with the surfaces of the volcanoclastic particles and dust strongly suggest a chemolithotrophic metabolism, particularly considering evidence of tunnelling in the surfaces of some of the clasts.

This 25-year long study shows that detection of martian life will be arduous and verification of potential biogenicity will not be straight forward.

Freissinet, C., et al., 2025, PNAS, 122(13), e2420580122.

Hickman-Lewis, K., et al., 2023, Geology, 51(1), 33-38.

Westall, F. et al., 2025. Nat. Astron. 9, 1615