*5.1.1 Sedimentological context and possible biogenic structures*

Curiosity landed on a flat plain (the Bradbury landing site) to the North of Mount Sharp in August 2012. Subsequently, the rover travelled to reach the extensive strata of a lacustrine sedimentary sequence at the base of Mount Sharp (around Sol 750), detecting along this track a heterogeneous assemblage of sedimentary rocks, representing a fluvial-deltaic-lacustrine environment (the Yellowknife Bay

formation). The basal Sheepbed and Gillepsie members of the Yellowknife formation are characterized by silts and mudstones showing mutual unconformity; such sediments were deposited in an intertidal region of a shallow lacustrine environment, representing the latest stage of transport and deposition of fan sediments inside Gale Crater lake [1].

In the first part of the Curiosity survey, interesting chemical and structural data emerged. Hence, with reference to the Sheepbed formation, at John Klein and Cumberland sites, SAM detected organic chemicals referable to molecules of chlorobenzene as well as the occurrence of all the chemical components of life [49]. Very interesting structures and morphologies were also observed, in addition to the many already cited, possible microbialites on the Sheepbed mudstone and structures recalling terrestrial microscopic induced by sedimentary structures, known as MISS, locally present as "erosional remnants," "pocket," "mat chips, "roll ups," "desiccation cracks," and "gas domes" [14]. Other possible microbially induced structures were also observed at this site, e,g., burst bubbles, filaments, mini-atolls, oncoids, domes and many other atypical sedimentary structures known as microbialitic [17, 39, 50], and some of which resemble stromatolites of the "Conophyton" type [18]. However, the morphology together with the chemical data mutually support the presence of ancient life, even if analyzed individually, they suggest alternative abiotic interpretations. For this reason, they will be individually considered, according to the criteria established by Astrobiology Field Laboratory (AFL) at http://mepag.jpl.nasa.gov/reports/index.html, as "possible biosignatures".

All previous described putative organosedimentary macrostructures were not observed when Curiosity moved away from the intertidal zone and entered (at Pahrump Hills location) a clay sedimentary sequence, the Murray formation at the base of the Mount Sharp. This was an area well investigated by the rover between Sols 750 and 930, and where deposits of mudstone and siltstone outcrops reveal a very thin, sub-millimetric lamination, proving the occurrence of a hydrodynamically stable environment. Particularly worthy of attention are the very thin laminated outcroppings which contain a few widely distributed aggregated harder structures [51], some up to several cm in size (**Figures 4** and **5**). Such dendritic, nodular and laminate concretions, comparable to the host rocks, show notable Mg-enhancement and a strong depletion of other major elements [52].

Chemical results indicate that the Gale Crater sediments were strongly influenced by early, subaqueous diagenetic reactions that produced, and sometimes filled a variety of pore types [53]. Since some sulfur was also detected in a dendritic feature, Mg occurrence was likely to be associated with a MgSO4 phase: a mineral present as precipitated cement within sediment pores [2] and an indicator of a local very acid environment, confined to the hardest structures and, intriguingly, in a target (Mojave) where organic molecules, such as kerogen and one of its polyaromatic fragments (thiophene), were found [54]. Considering the lack of contact with host sediment, these harder structures appear not to be transported/deposited elements but diagenetic structures, formed after sediment deposition. In fact, the two step heating experiments made by Martin to date K-Ar ages of primary (at 930°C) and secondary (at 500°C) mineralogical components at Mojave 2 target gave respectively 4.07 +/− 0.03 Ga (associated to detrital plagioclase of lake sediments) and 2.12 +/− 0.36 Ga (associated to sulphates and other secondary components of the diagenetic structures including the "rice grain"). Such data also suggest a post 3 Ga aqueous phase occurred in Gale Crater, at a time after surface fluvial activity on Mars was thought to have largely ceased [7].

Hence, at the landing site of Gale Crater, conditions for the evolution of Martian life beginning with chemotrophic and anaerobic bacteria and which survive in fossilized traces has been hypothesized [11]. Moreover, chemical and textural data

**109**

**Figure 4.**

*distribution.*

*Concretions found on the Murray Formation, Mars (C-F) in comparison to some Permian lacustrine terrestrial stromatolites (frames A,B; from: https://www.paleodirect.com/strx005-permian-lacustrinestromatolite-colony-fossil). Both Martian samples and terrestrial stromatolites counterparts show harder structures consisting of lumps/nodules, that developed in close proximity (B, D) or became randomly aligned forming a branched (F) or overlapping structure (C), starting from single point (F and C) or from scattered "germination points" (D-F). One can also observe similarities on their surface pattern (B-E yellow circles) and* 

*Life on Mars: Clues, Evidence or Proof? DOI: http://dx.doi.org/10.5772/intechopen.95531*

#### **Figure 4.**

*Concretions found on the Murray Formation, Mars (C-F) in comparison to some Permian lacustrine terrestrial stromatolites (frames A,B; from: https://www.paleodirect.com/strx005-permian-lacustrinestromatolite-colony-fossil). Both Martian samples and terrestrial stromatolites counterparts show harder structures consisting of lumps/nodules, that developed in close proximity (B, D) or became randomly aligned forming a branched (F) or overlapping structure (C), starting from single point (F and C) or from scattered "germination points" (D-F). One can also observe similarities on their surface pattern (B-E yellow circles) and distribution.*

#### **Figure 5.**

*The two upper images (taken at the same outcropping, at sols 810 and 890), show irregularly hardened lamination. This case (irregularly developed in layers) is conceptually similar to those shown on Figure 2 and shows local transition to nodules (A, B). For this reason these hardened structures may suggest a syngenetic or early diagenetic origin, due to different spatial activity of colonies, rather than the effect of erosion. Note the morphological similarity to terrestrial microbialites (frames A-A' to an encrusting thrombolites; frames B-B′ to a cerebrotic surface of stromatolites).*

testify to the presence of almost neutral lake water and to considerable diagenetic variations in pH [52]. The latter may suggest in very locally confined areas, that the acid micro-environment could also be attributed to microbial activity. Hence, the presence of elongated and curved filaments inside these structures and on their surface both in terrestrial stromatolites and Martian samples is particularly noteworthy (**Figures 6** and **7**).
