Abstract EANA2026-52

Lipids are structural components of cell membranes, mainly represented by fatty acids and related compounds such as alkanols and alkanes, and are among the most compelling biomolecules for detecting life on other planetary bodies due to their ubiquity in living systems on Earth and their chemical recalcitrance. Yet, they can be degraded under the radiolytic effects associated with Galactic Cosmic Rays (GCRs), which generate secondary ionizing radiation, including gamma (γ) rays [1]. A recent study suggested that the C10–C12 n-alkanes detected in Martian mudstones exposed for ~80 Myr could derive from more complex organics, such as fatty acids, after fragmenting and/or losing their carboxyl group under GCR-driven radiolysis [2]. To elucidate the origin of these molecules and distinguish biogenic from abiotic sources, we need to understand radiation-driven taphonomic processes that govern the alteration and complete destruction of lipid biomarkers under Martian conditions.

To address this, we irradiated seven synthetic linear compounds of plausible biogenic origin (n-alkanes, n-alkanols, and n-alkanoic acids) at different doses of γ-rays (50, 500, and 1300 kGy), corresponding to ~0.6, 6, and 20 Myr of cumulative exposure on the Martian surface, based on RAD instrument data [3] and the GEANT4 model [4]. A range of short- to long-chain organics was analyzed for the three lipid families to assess their radiolytic response as a function of chain length: two n-alkanes with 17 (heptadecane) and 30 (triacontane) carbons; two n-alkanols with 14 (tetradecan-1-ol) and 22 (docosan-1-ol) carbons; and three n-alkanoic acids with 12 (dodecanoic acid), 22 (docosanoic acid), and 28 (octacosanoic acid) carbons. Each compound was analyzed in triplicate before (control, 0 kGy) and after the three irradiation doses. In addition, a mixture of the two alkane analytes (heptadecane and triacontane) was irradiated to investigate potential cross-linking reactions between the compounds under simulated Martian irradiation conditions. All controls and irradiated analytes were analyzed using gas chromatography–mass spectrometry (GC–MS) to identify molecular changes and potential radiolytic by-products, and gas chromatography–isotope ratio mass spectrometry (GC–IRMS) to assess compound-specific carbon isotopic shifts.

The results show that short-chain molecules, regardless of their chemical family, are more susceptible to degradation and to the formation of radiolytic by-products under γ-radiation, whereas longer-chain molecules survive even at the highest dose. n-Alkanols and n-alkanoic acids generate new compounds through loss of their –OH and –COOH functional groups, respectively. In contrast, a mixture of heptadecane and triacontane does not yield new compounds under irradiation. Preliminary compound-specific isotopic analysis suggests a slight shift toward less negative δ¹³C values as a consequence of the higher lability of the chemical bonds bearing the lighter isotopes. Although preliminary, these results are relevant and could have implications in the identification of biological signatures, because an apparent ¹³C-enrichment during irradiation could imply an attenuation of the characteristic ¹³C‑depleted signature of life, thereby hampering their distinction from purely abiotic signatures. 

Funding: This work was funded by the Spanish Ministry of Science and Innovation/State Agency of Research MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe” through the projects CNS2024-154541 (L.S.G. & L.S.), PID2021-126746NBI00 (L.S.G., P.L.F., & V.P.), and PID2022-140180B-C21 (D.C.).

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