Experimental clues for detecting biosignatures on Mars

Abstract

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.

Constraining the preservation of organic compounds in Mars analog nontronites after exposure to acid and alkaline fluids

“…Interestingly, we identified a decrease of the NH 3 + component (at 401.6 eV) and an increase of the NH 2 contribution (at 400.2 eV) in both samples, being more noteworthy in the gly-acNon pellet. Previous works have shown that Al-smectites and Mg-smectites favored the accumulation of N-rich organic residues after exposing ANR under hydrothermal conditions 28,29 . In our experiments, the fact that the fragmentation of the glycine molecule (mainly driven by decarboxylation) and the increment of the NH 2 / NH 3 ratio are more noteworthy in the acid-treated sample, might also indicate that glycine photodegradation produces an enrichment of amino-rich moieties over the samples, as previously suggested by Tzvetkov et al 44 .…”

“…These results are consistent with the work of Vinogradoff et al 30 that showed that the starting compositional nature of phyllosilicates (Al vs Fe-rich) strongly determines the chemical evolution of organic matter in asteroids. On the other way around, recent investigations have also revealed the influence of post-depositional hydrothermal alteration in the preservation of organic compounds in smectites and on the final mineral assemblages 28,29 . All these works highlight the importance of studying in detail the nature of the host minerals and their chemical evolution as the key to improve the searching for traces of life beyond Earth.…”

“…The importance of clay minerals as targets for biosignature preservation has been widely recognized 7,[11][12][13][14][26][27][28][29][30] but, to the best of our knowledge, there is no previous works analyzing the particular effect of the type of activation, alkaline or acidic, on the efficiency of clays as preservers of organic compounds under Mars-like surface conditions. The exposure to different diagenetic fluids can induce changes in the smectite structure (e.g., surface area and layer charge) that might affect chemical interactions with organic molecules and compromise its preservation capacity on the long-term.…”

Biogenic Metal Nanoparticles: A New Approach to Detect Life on Mars?

“…Such microbial biomass, regardless of their physiological characteristics, might still be preserved in Martian sedimentary rocks or fluids [46,53], or any biological molecules might be trapped in mineralized cells [47]. It is known that the subsurface of Mars has mostly acted as a giant freezer, potentially preserving any remains of Martian life [54]. Under these assumptions, we propose a method for preliminary screening of life that presents several advantages: it is simple, relatively fast, cheap, without the need of advanced, specialized, and dedicated equipment and personnel, and easy to perform and implement either on Earth or, with some adaptations, in situ.…”

Inherited geochemical diversity of 3.4 Ga organic films from the Buck Reef Chert, South Africa

“…Although the general perception in paleobiology has long been that thermal maturation processes lead to a converging composition of organic materials from different origins 7 , several studies have demonstrated that organic molecules may undergo only partial degradation during their geological history [8][9][10][11][12][13][14][15] . In parallel, laboratory experiments have evidenced that burial-induced thermal degradation processes may not completely obliterate organic geochemical signals [16][17][18][19][20][21][22][23][24][25][26] and have suggested that initial organic molecular heterogeneities can withstand diagenesis and be recognized in the fossil record 27 .…”