This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Forthcoming exploration of Mars aims at identifying fossil biosignatures within ancient clay-rich formations. The subsurface of Mars has mostly acted as a giant freezer for the last 4 Gyr, thereby preserving potential remains of martian life. Yet, volcanism and impactors have periodically triggered the circulation of hydrothermal fluids, inevitably causing alteration of potentially fossilised biogenic organic materials. It thus appears crucial to quantify the impact of hydrothermal processes on organic biogeochemical signals in the presence of clay minerals. Here, we submitted RNA to hydrothermal conditions in the presence of Mg-smectites. Results show heterogeneous organo-mineral residues, with sub-micrometric phosphates, carbonates and amorphous silica particles together with Mg-smectites with interlayer spaces saturated by N-rich organic compounds. Although the chemical structure of RNA did not withstand hydrothermal conditions, clay minerals efficiently trapped organic carbon, confirming the relevance of drilling for organic carbon in ancient martian sediments. In addition, the degradation of RNA in the presence of Mg-smectites led to the precipitation of a quite uncommon mineral assemblage that could be seen as a biosignature per se. Martian targets exhibiting this mineral assemblage will thus constitute high priority and highly relevant candidates for sample return.

No abstract

Noachian surfaces on Mars exhibit vertical assemblages of weathering horizons termed as weathering profiles; this indicates that surface water caused alteration of the rocks that required a different, warmer climate than today. Evidence of this early Martian climate with CO2 vapor as the main component causing greenhouse warming has been challenged by the lack of carbonate in these profiles. Here we report the analysis of Compact Reconnaissance Imaging Spectrometer for Mars L‐detector data leading to the detections of carbonates using a spectral signature exclusively attributed to them. The carbonates are collocated with hydroxylated minerals in weathering profiles over the Martian surface. The origin of CO2 for the formation of carbonates could be the atmosphere. The widespread distribution of weathering profiles with carbonates over the surface of the planet suggest global interactions between fluids containing carbonate/bicarbonate ions with the surface of Mars in the presence of atmospheric water until around 3.7 billion years ago.

Carbonaceous meteorites are fragments of asteroids rich in organic material. In the forming solar nebula, parent bodies may have accreted organic materials resulting from the evolution of icy grains observed in dense molecular clouds. The major issues of this scenario are the secondary processes having occurred on asteroids, which may have modified the accreted matter. Here, we explore the evolution of organic analogs of protostellar/protoplanetary disk material once accreted and submitted to aqueous alteration at 150 °C. The evolution of molecular compounds during up to 100 days is monitored by high resolution mass spectrometry. We report significant evolution of the molecular families, with the decreases of H/C and N/C ratios. We find that the post-aqueous products share compositional similarities with the soluble organic matter of the Murchison meteorite. These results give a comprehensive scenario of the possible link between carbonaceous meteorites and ices of dense molecular clouds.

Widespread occurrence of Fe,Mg‐phyllosilicates has been observed on Noachian Martian terrains. Therefore, the study of Fe,Mg‐phyllosilicate formation, in order to characterize early Martian environmental conditions, is of particular interest to the Martian community. Previous studies have shown that the investigation of Fe,Mg‐smectite formation alone helps to describe early Mars environmental conditions, but there are still large uncertainties in terms of pH range, oxic/anoxic conditions, etc. Interestingly, carbonates and/or zeolites have also been observed on Noachian surfaces in association with the Fe,Mg‐phyllosilicates. Consequently, the present study focuses on the dioctahedral/trioctahedral phyllosilicate/carbonate/zeolite formation as a function of various CO2 contents (100% N2, 10% CO2/90% N2, and 100% CO2), from a combined approach including closed system laboratory experiments for 3 weeks at 120°C and geochemical simulations. The experimental results show that as the CO2 content decreases, the amount of dioctahedral clay minerals decreases in favor of trioctahedral minerals. Carbonates and dioctahedral clay minerals are formed during the experiments with CO2. When Ca‐zeolites are formed, no carbonates and dioctahedral minerals are observed. Geochemical simulation aided in establishing pH as a key parameter in determining mineral formation patterns. Indeed, under acidic conditions dioctahedral clay minerals and carbonate minerals are formed, while trioctahedral clay minerals are formed in basic conditions with a neutral pH value of 5.98 at 120°C. Zeolites are favored from pH ≳ 7.2. The results obtained shed new light on the importance of dioctahedral clay minerals versus zeolites and carbonates versus zeolites competitions to better define the aqueous alteration processes throughout early Mars history.

Expanding our capabilities to unambiguously identify ancient traces of life in ancient rocks requires laboratory experiments to better constrain the evolution of biomolecules during advanced fossilization processes. Here, we submitted RNA to hydrothermal conditions in the presence of a gel of Al-smectite stoichiometry at 200 °C for 20 days. NMR and STXM-XANES investigations revealed that the organic fraction of the residues is no longer RNA, nor the quite homogeneous aromatic-rich residue obtained in the absence of clays, but rather consists of particles of various chemical composition including amide-rich compounds. Rather than the pure clays obtained in the absence of RNA, electron microscopy (SEM and TEM) and diffraction (XRD) data showed that the mineralogy of the experimental residues includes amorphous silica and aluminosilicates mixed together with nanoscales phosphates and clay minerals. In addition to the influence of clay minerals on the degradation of organic compounds, these results evidence the influence of the presence of organic compounds on the nature of the mineral assemblage, highlighting the importance of fine-scale mineralogical investigations when discussing the nature/origin of organo-mineral microstructures found in ancient rocks.

Radioactive caesium pollution resulting from Fukushima Dai-ichi and Chernobyl nuclear plant accidents involves strong interactions between Cs+ and clays, especially vermiculite-type minerals. In acidic soil environments, such as in Fukushima area, vermiculite is subjected to weathering processes, resulting in aluminization. The crystal structure of aluminized clays and its implications for Cs + mobility in soils remain poorly understood due to the mixture of these minerals with other clays and organic matter. We performed acidic weathering of a vermiculite to mimic the aluminization process in soils. Combination of structure analysis and Cs + extractability measurements show that the increase of aluminization is accompanied by an increase in Cs + mobility. Crystal structure model for aluminized vermiculite is based on the interstratification of unaltered vermiculite layers and aluminized layers within the same particle. Cs + in vermiculite layers is poorly mobile, while the extractability of Cs + is greatly enhanced in aluminized layers. The overall reactivity of the weathered clay (cation exchange capacity, Cs + mobility) is then governed by the relative abundance of the two types of layers. The proposed layer model for aluminized vermiculite with two coexisting populations of caesium is of prime importance for predicting the fate of caesium in contaminated soil environments.Existing studies devoted to the fate of radioactive caesium in polluted soil environments due to nuclear plant accidents systematically suggest that strong interactions occur between this pollutant and clay minerals, especially vermiculite-type minerals [1][2][3][4][5][6][7][8][9][10] . The layered structure of vermiculite involves an octahedral sheet sandwiched between two tetrahedral sheets. Isomorphic substitutions by less charged cations in tetrahedral sheets induce a permanent negative charge. This charge is compensated by the presence of exchangeable interlayer cations, which produces the high cation exchange capacity (CEC) values of vermiculites and their strong reactivity in cation adsorption/desorption processes.In the Fukushima area, Cs + is preferentially adsorbed in vermiculite-type minerals derived from the weathering of micaceous minerals 11,12 . However, in acidic soil environments, such as those encountered in the Fukushima area, aluminium is released into solution by the partial dissolution of vermiculite or micaceous minerals and is then commonly adsorbed again and partially hydroxylated in the interlayer space of the mineral 13 . This aluminization process in turn strongly impacts the reactive properties of the vermiculite minerals by leading to a decrease in the CEC [14][15][16] . The effects of acidic weathering and the associated aluminization of vermiculite in soils on the interactions with Cs + remain poorly understood. Indeed, existing studies on this topic have either evoked an increase in Cs + selectivity 1,9,17,18 or a decrease in the amount of poorly mobile Cs 6,[19][20][21][22] . Interpretations have been syst...