Mars [2.36] [2.37] (Figure 2.2).
2.8 Rio Tinto: A Geochemical and Mineralogical Terrestrial Analog of Mars
In 2000, Christensen et al. [2.24] [2.25] reported the occurrence of extensive outcrops of hematite in the region of Terra Meridiani. This mineral is abundant in Rio Tinto as the result of the maturation of ferric acidic minerals. This enigmatic enrichment led to the selection of Meridiani as the landing site for the Opportunity rover in 2004. During the exploration of this area, Opportunity detected hematite occurring in spheroidal concretions that were named blueberries. However, the most intriguing and revealing finding was the abundance of sulfates [2.93] [2.101] [2.80] where the main mineral phase was jarosite, as detected by Mössbauer spectroscopy on board the Opportunity [2.63]. Later on, it was observed that the acidic sulfates were very extensive in different Mars regions, supporting the idea that Mars had been exposed to a low-pH episode lasting hundreds of millions of years [2.79] [2.33] [2.29], whose origin has been related to the weathering of metallic sulfides [2.34] [2.112]. Very recently, the formation of iron sulfides associated with hydrothermal materials that are similar to the hydrothermal rocks of the Rio Tinto basement was reported [2.78]. This suggests that the biogeochemical cycles of the putative biosphere that could have emerged on Mars might have been dominated by S and Fe metabolism, as is observed in Rio Tinto.
In this context, Rio Tinto becomes an excellent terrestrial reference in two aspects. On one hand, it provides direct information about the physicochemical and biological processes that are involved in the formation of the iron-rich acidic minerals such as iron oxides and sulfates [2.57] [2.58] [2.37]. On the other hand, the evolution of the acidic system over the last 30 million years is recorded in the form of alteration materials and iron-rich terraces that show the diagenetic pathways followed by the iron-rich minerals over time [2.35] [2.37] [2.41]. The association of modern and ancient deposits at Rio Tinto can provide key information to understanding the origin and maturation of the iron-rich deposits that are found in vast areas of Mars like Aram Chaos, Meridiani Planum, Valles Marineris, Mawrth Vallis and Syrtis Major [2.63] [2.79] [2.33] [2.29] [2.71]. From an astrobiological perspective, Rio Tinto shows us how biological activity is produced and recorded under extreme acidic conditions over millions of years. The modern environment and its ancient acidic materials of surface and subsurface areas show a wide diversity of biological traces, including microstructures, minerals, stable isotope signatures and molecular compounds, suggesting that these biosignatures survived the diagenesis and maturation processes over 30 million of years of geological evolution [2.43]. As discussed before, the micron-sized crystals of Fe-bearing carbonates likely produced by Acidiphilium sp. are found in the acidic sediments going from modern to the oldest ferruginous materials of over 30 million years [2.42]. Although carbonate minerals are highly unstable under the acidic conditions of Rio Tinto, mineral precipitation is favored by metabolic activity in micron-size cellular scale sites that allow net preservation in the oldest acidic materials of the basin. Furthermore, the occurrence of microbial-derived minerals is also observed associated with microbial structures in the deep subsurface of the Rio Tinto crust [2.43]. Over the course of the last 30 million years, iron and sulfur biogeochemical cycling in the basement has been recorded as carbonate-bearing microstructures that formed through metabolic processing of hydrothermal minerals by using sulfur as electron acceptors or donors (e.g., barite and pyrite), depending on the redox conditions. In the Rio Tinto subsurface, some of the reactions that followed the microbes to obtain energy mimic or are the reverse of the biochemical processes that would have appeared in a S and Fe world prebiotic scenario for an emergent metabolism [2.43] [2.104]. Therefore, there is a close connection between a substrate that promotes the geochemical cycles of S and Fe and the microbial metabolism that couples them to obtain energy in the most favorable ways. If life arose on Mars, the dominance of S and Fe geochemical processes rooted in an early metabolism very likely would have produced the same metabolic minerals in the interior of the red planet [2.43].
It is often claimed that acidic environments are inhospitable for the preservation of molecular traces of life since the highly oxidizing and aggressive pH would rapidly destroy the biological and organic compounds. The microscopic study of modern and ancient Rio Tinto materials [2.39] has shown that the high mineralization rates of the acidic solutions encourages the preservation of biological structures in great detail (Figure 2.6a, b1, b2). Paradoxically, in the molecular analysis of samples, the preservation of organic compounds was not considered under such low pH and high Eh conditions. Later on, the sample analysis of the different terrace and gossan levels showed that fragments of biomolecules and complex organic compounds are found in all deposits from the most modern sediments to the oldest materials of the acidic system. Using peptide extraction techniques on ancient acidic materials from the Rio Tinto terraces, Colín-García et al. [2.22] showed preservation of protein fragments during the last 2.1 million years. Such peptidic sequences provided qualitative information about the organisms in the past of the acidic system, which coincide with some microorganisms living in the modern system. Very recently, the sample analysis of surface and subsurface using TOF-SIMS detected diverse organic compounds that are associated with microbial structures (Figure 2.6a, c1 to c3). The preservation of molecular traces of life in acidic conditions is even more extraordinary as these rocks have been exposed to intense diagenesis that has changed the composition of the mineral matrix from sulfates to iron oxides under varying pH and Eh conditions. The occurrence of biomolecules and organic compounds of biological origin in the Rio Tinto rocks demonstrates the preservation of molecular traces of life in materials formed under extreme acidic conditions and despite exposure to drastic diagenesis.
The preservation of different biosignatures in the ancient acidic materials of Rio Tinto strongly supports that, if life emerged on Mars, traces of its activity in acidic deposits are just as likely to remain as they are in materials that formed under mildly neutral conditions in the red planet [2.28]. Given that acidic environments were abundant in the Late Noachian to Hesperian ages (more than 3.5 billion years ago), the next astrobiological missions should increase the chances of finding traces of life on the red planet by seeking them in acidic deposits.
Figure 2.6 Preservation of different traces of life in the oldest Rio Tinto terrace (a) that has been dated as 7 to 2.1 million years old [2.32, 2.37]. b1 and b2 show preservation of plants and micron-sized bacterial filaments, respectively. The analysis of the terrace samples by TOF-SIMS shows the detection of filament structures outlined by the distribution of the negative fragments of different lipids biomarkers like Myristic (c1), Pentadecanoic (c2), and Palmitic acids (c3). (Image credit: the authors).
2.9 Conclusions
In the last fifty years, important advances in microbiology have challenged the view generated by the Viking Missions, which were very pessimistic about the existence of life on Mars. Thanks to research on extremophiles, we now know that life is extremely robust and able to adapt to extraordinarily diverse conditions, which has increased the probability of finding life on other planetary bodies. Of the different extremophiles, acidophiles are of special interest because they are the only ones that create the extreme conditions of the environment in which they thrive. The characterization of the Rio Tinto basin, an extreme acidic environment 92 km long, has answered some basic questions, like the origin of the extreme acidic conditions of the ecosystem, the identification and
