It is five years since the NASA rovers named "Spirit" and "Opportunity" have been exploring Mars, sending their close up pictures and especially the microscope images.(Squyre 2004). The images (on the website http://marsrovers.jpl.nasa.gov/gallery) when studied at high magnification (250-400%) are a good source of information and capture one's attention for days on end. Often, when looking at them a second time one discovers things that were missed the first time.
By carefully analyzing the laminated sediments of "Meridiani Planum" of Mars and the strange hematite spherules, known as "blueberries" (Moore 2004), whose images were taken by the rover "Opportunity", the authors suggested that these are organo-sedimentary structures, that is structures produced by micro-organisms and having, though with all the peculiarities of the Martian world, several common features - on a different scale of observation - with the familiar terrestrial stromatolites (Rizzo and Cantasano 2009; #FFF# figures 1, 2, 3).These sediments are characterized by a sequential succession of a pair of sub-millimetre laminae, with a whitish concretion layer (purely skeletal / LA) and a silty fine-grained layer (purely agglutinated / LB), embedding several clusters of microspherules ( calcimicrobes), in linear, serpentine or spheroidal arrays, including the "blueberries" themselves (#FFF# figure 3, 4, 5, 6, 7, 8). Sediment that NASA has already attributed to the presence of water in the past (Squyres 2006; Wacey 2009; Jolliff 2006).
The genetic hypothesis relating to microbial activity is consistent with other scientific findings on Mars (Mackay 1996, Mackay 1998, Schulze-Mazuk 2008), where the presence of methane and formaldehyde was reported (Onstott 2006), where a circadian activity, detected in Vicking experiments, is suspected (Van Dongen 2009); while it seems that the existence of "original" nanobacteria fossils in meteorites of Martian origin has recently been established by NASA(news release).
But what's so peculiar in Mars sediments and where does the microbial hypothesis come from?
The thin lamination, as it is known, is typical of lake sediments or deposition in calm seas. We note, however, that often the laminated structures are gradually replaced by extremely disordered arrangements, with laminar convolutions filled with cavities in which coalescing serpentine forms develop, whose global texture is similar to the typical characteristics of organogenic environments (#FFF# figures 8, 9, 10). It is often observed that the laminae themselves, in the vicinity of blueberries, tend to encircle them, by changing the plane-parallel structure (#FFF# figure 9). These arrays can be observed in the shots taken by both Opportunity and Spirit (Squyre 2006).
With reference to the textural appearance of laminated sediments it can be observed that LA and LB laminae show similar and repeated patterns (#FFF# figure 1a-d), (composed of very small spherules (Ri), with a radial array of stable connections, resulting in a "brick-like" base (BS). BS in LB resembles a wheel with cavities interposed between radial connections, while the concretional film in LA gives BS the appearance of a flower and the laminae show crenellated edges accordingly(#FFF# figure 1e).
In some cases there is a hyaline veil that has similar patterns, showing progressive accretional processes (biomineralization), with transition from transparent hyaline forms to milky-white coloured forms; this accretion is spread randomly on the exposed surfaces; however, in some cases this accretion clearly departs from the basic or complex identified structures (#FFF# figure 4e-f, 5c-e, 5h-i). On the same hyaline film - at times - some scattered granules can be observed, perhaps indicating the beginning of an agglutination process (collagen? #FFF# figure 1b). The size and the structural arrangements of LA, LB and the hyaline films are definitely similar (#FFF# figure 1).
This is, in itself, another anomaly, since from bodies of different composition and in an environment of simple chemico-physical origins one would expect different structures. In addition it is not clear why the accretion phenomena should be more intense around the structures we identified (biogenic origin).
In some cases a skeleton lamina is barely visible, without interposed agglutinated laminae, both because the latter are not very thick and because there is a white film on the surface (with the same aspect of LA), covering the laminated sequence.
At times this film, which in some cases seems to have the density of plastic, is lifted making room to an emerging blueberry , which in such manner carries its remainders upward (#FFF# figure 9a-e). The texture of materials and the processes of "internal" growth (see also #FFF# figure 7s) are inconsistent with the mineral world.
The similarity between the Martian structures / textures and those of stromatolites and, more generally, of terrestrial organo-sedimentary sediments, covers several aspects: aspects that develop at different scales, from the minimum scale of observation up to the macroscopic scale of outcrops (#FFF# figure 2, 3). Generally, random morphological similarity between different bodies is a sporadic fact and a coincidence; it is hard to imagine such a coincidence repeated at different scales, which is more inherent in the internal structure of bodies rather than in their simple morphology
But what is most interesting is that almost all sedimentary bodies observed at sub-millimetre scale, while having different resulting forms are made up of coalescing Ri microspherules , whose different spatial organization (in lines, planes or clusters; according to regular or disordered geometries ) gives rise to more complex structures.
The size of the Ri microspherules are within the limit of resolution, equal to 0.04 mm up to 0.3 mm, while the basic structures of laminated bodies BS and Ri blueberries themselves are about 0.1 mm (#FFF# figure 1).
Therefore it is possible to observe, within the size range of Ri and up to a few millimetres, some rather bizarre products of coalescence, such as (#FFF# figure 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12):
- • arrays of equidimensional or regularly sized spherules (#FFF# figure 4);
- • rolling-up spiralled spherules (#FFF# figure 5);
- • BS imbricate structures (#FFF# figure 1);
- • arrays of serpentine and / or tubular spherules (#FFF# figure 9), sometimes overlapping (figure 1, 4, 5, 10);
- • sometimes arranged side by side forming regular / irregular dislocations either in closed shape or in bundles (#FFF# figure 2);
- • sudden transitions from straight to curved arrays (#FFF# figure 2);
- • gradual transitions from planar laminae to convoluted laminae/ disordered / thrombolitic / dendritic, clusters with cavities (presumably primary #FFF# figures 2, 8 e 9);
- • clusters of microspherules, making up lumps, sheaths, and larger spherules, including ooids (oolites / oncolites) and the blueberries themselves (#FFF# figure 6 e 7).
The much discussed "blueberries", in particular, are not homogeneous and have varying shapes and structures (#FFF# figure 7).They show both radial polycentric structures of "interconnected Ri" and poly-spherical structures (microspherules emerging from the surface, side-by-side, intersected or even protruding from the " mother " spherule) and also rolled-up laminated structures (sometimes with variables convolutions within the spherules themselves)
The polycentric and rolled-up laminated structures of "blueberries" can be seen as the result of a different Ri dislocation, where the external appearance and internal structure depends on the predominance of their dimensional asymmetry and spatial interconnection links, and, in fact, some reveal intermediate forms, with combined polyspheric and spiralling characteristics. (#FFF# figure 7).
All these structures are quite atypical, and in many respects resemble the structures of carbonate sediments of microbial origin (Riding 2000; Riding 2002) and it is difficult to imagine how they could be the result of a simple physico-chemical process. So that the "trivial microspherules" become "interesting subjects", due to the nature and complexity of their products, some of which play a "key role" for interpretative purposes. The diversity and variety of forms suggest that they are produced by "builders" smaller than themselves, with coalescence, orientation and accretion features and with repetitive building patterns (in the plane and in the space) at different scales. Moreover, the most basic structures closely resemble those of the bacterial world: the so-called calcimicrobes, which are nothing but coalescing microbial colonies , isorientated and capable of facilitating / triggering concretional processes from the surrounding environment, mostly of spherical or discoid shape(#FFF# figure 3). Recent studies have shown that ooids, whose aggregation forms the well-known terrestrial oolitic concretions, can be obtained in the laboratory and are the result of bacterial activity (Brehemen 2004). Concretions very similar to oolites also exist on Mars, where the internal structure shows arrays consistent with the structures described above (#FFF# figure 6), lacking however the characteristic concentric layers with repeated colours that characterize the terrestrial deposits, due to the recurring climatic changes and, for this reason as well, it is likely that they may be of biogenic origin.
Subject to the assumption on stromatolites , briefly stated in the #FFF# figure 12, the hypothesis that some other selected shapes (#FFF# figures 13, 14 e 15) may represent specific species and other forms of life in fossil state and in part still alive (#FFF# figure 13, image X1) cannot be ruled out. Apart from the limitations to a clear understanding and characterization due to the scantiness of findings, their existence strengthens the hypothesis that, on Mars, other forms of life exist, or at least have existed.
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Geologia, Università di Firenze, Consiglio Nazionale delle Ricerche