Looking for biosignatures in a pristine Mars analogue environment on Earth
Authors: Palma Vera , Jiménez-Morillo Nicasio , Sauro Francesco , Massironi Matteo , De la Rosa José M. , González-Pérez José A. , Onac Bogdan , Tiago Igor , Caldeira Ana Teresa , Miller Ana Z. 
Published: February 2023

The Selvagens Islands (Madeira, Portugal), located in the North Atlantic, are a small archipelago of volcanic origin formed by two main islands, which emerged in the Oligocene (25-29 Ma). These oceanic islands are recognized as a unique example of marine and terrestrial biodiversity, characterized by many endemic species. Still unblemished by civilization, this isolated and undisturbed ecosystem makes the volcanic caves from Selvagens a promising model system for investigating biosignatures preserved in the rock record valuable for astrobiology.&#160;The Inferno Cave, one of the three main terrestrial caves of the Selvagens Islands, has copious amounts of fine white powder crusts scattered throughout the cave.&#160;From the mineralogical point of view, the whitish powdery deposits are mainly composed of gypsum and minor amounts of clay minerals.&#160;Thermal analyses (TG/DTG-DsC) revealed the presence of labile and stable organic matter (OM), with contrasting relative abundances among the gypsum samples.&#160;The stable isotope composition of carbon was determined by elemental analysis coupled to isotope ratio mass spectrometry (EA/IRMS) to decipher the origin of the organic fraction preserved in the gypsum deposits. Two well-differentiated organic matter pools were distinguished: one comprising &#120575;13C values > &#8211;19 &#8240; related to microbial activity (i.e., microbial degradation of fresh organic matter), and another with &#120575;13C < &#8211;20 &#8240;), which may suggest the preservation of recalcitrant biomarkers from aboveground vegetation. U-series results from the gypsum deposits were used to produce isochron ages needed to generate an age-depth model for the 1-m thick gypsum deposit.A detailed study of the organic fraction preserved in the gypsum deposits was conducted by pyrolysis gas chromatography/ mass spectrometry (Py-GC/MS) and by GC-MS after extraction of total lipids. Preliminary results show the presence of&#160;n-alkanes of low molecular weight (<C21). In addition, long-chain&#160;n-alkanes (C>21) were also observed, which indicates a direct contribution of plant biomass from the overlying surface. Interestingly, the Py-GC/MS analysis also shows relatively large contents of mid-chain branched alkanes associated with a direct biological source focused on microorganisms since they are known to biosynthesize such alkanes.&#160;A predominance of chemolithotrophic microbial communities was found based on the 16S rRNA gene analysis, which is consistent with the biological origin of the mid-chain branched alkanes.&#160;The mineralogical, biogeochemical, and microbiological characterization of the gypsum samples from the pristine Selvagens Islands is thus a reliable way to infer the biogenicity of cave mineral deposits, recognize biosignatures, and determine paleoenvironmental changes from a natural environment, free from anthropogenic influence. Considering the analogies with Mars, the Selvagens Islands are also suitable for space research, including astronaut training and rover testing for future planetary missions.Acknowledgements:&#160;This work received support from the Portuguese Foundation for Science and Technology (FCT) under the MICROCENO project (PTDC/CTA-AMB/0608/2020). The financial support from the Spanish Ministry of Science and Innovation (MCIN) under the research project TUBOLAN PID2019-108672RJ-I00 funded by MCIN/AEI/ 10.13039/501100011033 is also acknowledged. A.Z.M. was supported by the CEECIND/01147/2017 contract from FCT, and the Ram&#243;n y Cajal contract (RYC2019-026885-I) from the MCIN.&#160;