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Microbialites at Gusev Crater, Mars | OMICS International
ISSN: 2332-2519
Journal of Astrobiology & Outreach
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Microbialites at Gusev Crater, Mars

Giorgio Bianciardi1,2*, Vincenzo Rizzo2, Maria Eugenia Farias3 and Nicola Cantasano4
1Department of Medical Biotechnologies, University of Siena, Siena, Italy
2National Research Council-retired, Via Repaci 22, Rende, Cosenza, Italy
3Laboratorio de Investigaciones Microbiológicas de Lagunas Andinas (LIMLA), Planta Piloto de Procesos Industriales Microbiológicos (PROIMI), CCT, CONICET, Tucumán, Argentina
4National Research Council, Institute for Agricultural and Forest Systems in the Mediterranean, Rende Research Unit, Cosenza, Italy
Corresponding Author : Giorgio Bianciardi
Department of Medical Biotechnologies
University of Siena, Siena, Italy
Tel: +39 348 2650891
E-mail: [email protected]
Received: September 28, 2015; Accepted: October 31, 2015; Published: November 03, 2015
Citation: Bianciardi G, Rizzo V, Farias ME, Cantasano N (2015) Microbialites at Gusev Crater, Mars. Astrobiol Outreach 3:143. doi:10.4172/2332-2519.1000143
Copyright: © 2015 Bianciardi G, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Abstract

The Mars Exploration Rover Spirit investigated plains at Gusev crater, where sedimentary rocks are present. The Spirit rover’s Athena morphological investigation shows microstructures organized in intertwined filaments of microspherules: a texture we have also found on samples of terrestrial stromatolites and other microbialites. We performed a quantitative image analysis to compare 45 microbialites samplings with 50 rover’s ones (approximately 25,000/20,000 microstructures). Contours were extracted and morphometric indexes obtained: geometric and algorithmic complexities, entropy, tortuosity, minimum and maximum diameters. Terrestrial and Martian textures resulted multifractals, while terrestrial abiogenic minerals showed a simple fractal structure. Mean values and confidence intervals from the Martian images overlapped perfectly with those from terrestrial samples. The probability of this occurring by chance was less than 1/28, p<0.004. Our work show the presumptive evidence of microbialites in the Martian outcroppings explored by “Spirit”, confirming our previous results concerning the Martian outcroppings explored by Opportunity at Meridiani Planum: unicellular life was widespread on the ancient Mars.

Keywords
Spirit mars exploration rover; Stromatolites; Microbialites; Fractal analysis; Life on mars
Introduction
Rover Spirit touched down the volcanic plains of Gusev Crater on 4 January 2004, for ten years it has runned on the Martian surface. From the landing site to “Pot of Gold” site (on sol 200 about), extensive/ destroyed volcanic outcroppings, often covered by weathering coatings and/or floating on eolic sands, as crater ejecta were evident. The Rover then reached the inner basin of Columbia Hills, the images showed here layered sequences, containing sulfate and other soluble elements, due to ancient large process of aqueous alteration [1-3]. At Home Plate, where are the most evident geologic feature investigated in the Inner Basin, these layered sequences appeared formed by erosion of basaltic rocks and pyroclastic activity, deposited as alluvial fans and/or as eolian sand sheets [4-7]. In effect, ancient volcanic ash, known as “tephra”, and hydrothermal deposits created in a nearby volcanic eruption, initially covered much of the Columbia Hills and the surrounding area [8], and then was covered by deposit of a cold lake [9]. The wind and weathering processes eroded most of that deposit, also carrying away much of the evidence for the ancient lake: an environment where to search for life could be interesting.
Stromatolites/microbialites are a frequently named target of lifedetection missions on Mars [10-13]. Stromatolites/microbialites are an organization of primitive cyanobacteria, analogous to coral reefs that grow/grew in vast colonies. Fossil stromatolites, as well other microbialites, can be identified through their macro-meso-Microcharacteristic structures that result from the growth patterns of their constituent bacteria [14-17].
Rizzo observed by a qualitative visual inspection that images of Martian outcroppings (selected frames from images obtained by Opportunity and Spirit rovers) presented features resembling terrestrial stromatolites/microbialites [18-20]. Recently, we performed a fractal morphometric quantitative analysis of the Martian outcroppings explored by the Rover Opportunity revealing evidences of the presence of microbialites at Meridiani Planum [21]. In order to search for possible signature of microbialites in other Martian regions, we have performed a morphometric analysis of the Martian outcroppings shot by the ATHENA facility aboard of the Rover Spirit, comparing fractal morphometric indexes of the microtextures evidenced in the Martian outcropping with the ones of terrestrial microbialites.
Materials and Methods
All the Martian images elaborated in this work are referred to float or weathered rocks, somewhere ejected by meteoric impact.
Samples
This study undertakes a systematic analysis of black and white Microscopic Images (MI) obtained by Athena (Figure 1), a camera mounted on the NASA Mars Exploration Rovers (MER) “Spirit”, selecting all the images that presented microspherules and filaments of microspherules (V.R.). The field of view of Athena is 1024*1024 pixels in size and its optics provide a square frame of 32 mm of-field sampling at the working distance of about 63 mm from the front of the lens barrel to the object plane, consequently having a resolution of about 30 micrometers [22]. In particular, 20 selected MI images obtained by the rover Spirit have been chosen and 50 samplings obtained (Figures 1 and 2), approximately corresponding to the presence of 20,000 microspherules/intertwined filaments of microspherules.
These cuttings having dimensions of about ¼ of the original images (8-10 mm) were subjected to a slight contrast increase (Microsoft Publisher software) for an optimal vision of the microstructures. The studied cuttings were related to polished surfaces by MER’s Rock Abrasion Tool (RAT) or to exposed surfaces of the Martian outcroppings.
As regards terrestrial stromatolites, unambiguously determined as biogenic, 20 quoted images and 45 samplings were obtained (Figure 3) from the WEB (from which one sample of terrestrial stromatolite photographed by Athena and one from cultured cyanobacteria, see Figure 2 or photographed at the Regional Museum of Natural Sciences, Turin, approximately corresponding to 25,000 microspherules/ intertwined filaments of microspherules (Figure 2). They have been acquired and magnified in order to obtain the same dimensional scale (X30, ± 10%), resolution and acutance of the Athena imagery (V. R.). They were analyzed using the same procedures of the Martian images.
Image analysis
The contours present in the terrestrial and Martian images (biogenic stromatolites and selected images shot by Opportunity) were automatically extracted from the images and converted to single pixel outlines by a canny-edge filter (Digital Image Magnifier software by Strikos Nikolaos: http://www.softoxi.com/digital-image-magnifier.html), (fixed sigma and low threshold values, equals to 0.9 and 12, respectively) (Figure 4).
The obtained textures were characterized by analyzing their geometrical complexity, Entropy (Information Dimension), algorithmic complexity (L-Z, randomness) and tortuosity (Dmin). Minimum and maximum diameters of the microspherules were also measured.
Geometrical Complexity, D0
To evaluate the geometrical complexity of the patterns, the local fractal dimension was measured using the box-counting algorithm. Resulting our texture multifractals, as identified by the two straight lines on the log-log plot (Figure 5), the algorithm was applied for the two regions: 200-10 pixels=2 mm - 0.1 mm and 10-5 pixels=0.1 mm-0.05 mm. Briefly, each image was covered by a net of L square boxes and the number of boxes containing any part of the outline Nb(L) was counted. The slope of the log-log plot of Nb(L) vs. 1/L represented the fractal dimension of the distribution (Figure 5) [23]. The existence of log-log straight lines (p<0.001) justified the use of the fractal analysis, applied here as a tool to obtain the morphometric indexes. The method was validated by measuring computer-generated Euclidean and fractal shapes of known fractal dimensions (Circumference=-0.7%; Square=+0.4%; Triadic Koch island=-0.9%; Sierpinski’s Triangle=-1.5%).
Entropy (Information Dimension, D1)
To evaluate the information (entropy) present in the patterns, information dimension, D1, a robust estimate from a finite amount of data that gives the probability of finding a point in the image, was calculated. The set was covered with boxes of linear size, d, from 200 to 10 pixels and from 10 to 5 pixels as above, keeping track of the mass, mi (the amount of pixels) in each box, and calculated the information entropy I (d) from the summation of the number of points in the i-th box divided by the total number of points in the set multiplied for its logarithm [24]. The slope of the log-log plot of Information entropy vs. 1/box side length represented the information dimension of the distribution. The method was validated by measuring computer generated Euclidean and fractal shapes of known information dimensions. The existence of log-log straight lines (p<0.001) justified the use of the fractal analysis, applied here as a tool to obtain the morphometric indexes.
Algorithmic complexity (“randomness”, L-Z)
To determine the algorithmic complexity (“randomness”) of the patterns, relative Lempel-Ziv, L-Z, values were calculated according to the Kaspar and Schuster algorithm [25] using the Chaos Data Analyzer version 2.1 software package (CDA; Pro, Academic Software Library, North Carolina State University, USA). Briefly, patterns of the original image were transformed into 16.732 points containing one dimensional vector, where each datum point was converted into a single binary digit according to whether the design is touched (=1) or not (=0). Relative L-Z values are close to 0 for a deterministic equation, close to 1 for totally destructured random phenomena.
Tortuosity (Dmin)
Tortuosity, or the fractal dimension of the minimum path, Dmin, was computed for each cluster present in the image from the power law Ic=rDmin , where Dmin is the exponent that governs the dependence of the minimum path length between two points (Ic) on the Pythagorean distance r between them in a fractal random material. To obtain Dmin, the maximum diameter and the half perimeter of the microstructures present in the textures were measured using an automated procedure (Image Pro Plus software, Media Cybernetics, USA). For each image 100-500 microstructures were measured. The slope of the log-log plot (maximum diameter vs. perimeter) represented Dmin. The existence of a log-log straight line (p<0.001) justified the use of the fractal analysis in order to obtain the morphometric index. The method was validated with the original one by Hermann and Stanley [26] with a maximum shift of ± 3%.
Geometrical Complexity and Information Dimension were calculated using the Benoit 1.3 software, (TruSoft Int'l Inc: http:// trusoft-international.com/benoit.html). Algorithmic complexity and Dmin were calculated using a software written by us. All this four methods are routinely performed by one of us in biomedical works [26-31]. Minimum and maximum diameters of the microspherules (Earth and Mars) were automatically measured by Image Pro Plus software (Media Cybernetics, USA).
Statistical analysis
Mean intra-and inter-observer coefficients varied <2.0% and <3%, respectively. Comparisons between the groups were analyzed by the Mann-Whitney U test and chi-square test; t-test was applied in order to verify the linearity of the log-log plots.
Results
In the images photographed by Spirit as well from the images obtained from terrestrial microbialites a continuum pattern of microspherules and intertwined filaments of microspherules, dimensions of about 0.1 mm–0.3 mm, are present. Figure 6 show the same microstructures observed at high magnification after Scanning Electron Microscopy.
On Earth, these reports are prevalently referred to the microbialitic environment, where the microspherule, somewhere aggregate in linear arrays, is known as peloids [32-34].
The morphometric analysis reveals that both textures, from microbialites (Earth) and from selected MI images (Mars), present a multifractal aspect, as revealed by the two straight lines in the log-log plot (Figure 5). Two textures are present: the texture as a whole (200-10 pixels, corresponding to 2 mm-0.1 mm) and the microstructure inside the microspherules and the intertwined filaments of microspherules (10-5 pixels, corresponding to 0.1 mm-0.05 mm).
Abiogenic pseudostromatolites, as well other abiogenic minerals, reveal always a simple fractal (and not multifractal) textures (Figure 7) [20,21].
The morphometric analysis also shows that the Martian and terrestrial textures were extremely similar to each other; the average values and confidence intervals of the eight independent morphometric parameters of the terrestrial and Martian images perfectly overlapped with each other (Table 1). In these eight separate comparisons, the probability of this occurring by chance is 1/28 (p<0.004).
Discussion
The search of life on Mars is the main motivation behind the different research programs on the Martian surface. Our knowledge on Mars has remarkably increased after the last NASA missions, especially those called Mars Explorer Rover (MER, Opportunity, Spirit and Curiosity rovers) held, from 2004 on the Mars landscape at Meridiani Planum and Gale Crater, respectively. Indeed, the photo reportages realized by the Martian rovers confirmed the presence of water deposits on the surface of Mars [35].
The search on Mars of extraterrestrial microorganisms and, in particular, of cyanobacteria, the main building materials of terrestrial microbialites/stromatolites [36] has been underlined many times. A century of research on stromatolites has revealed diverse fabric and many structures together with a contentious history and various definitions.
Besides, most of microbialites/stromatolites are carbonate in composition, but siliceous, phosphatic, iron, manganese and sulphate examples also occur: in recent view, stromatolites may be defined as microbialites, macroscopically layered authigenic microbial sediments, namely organosedimentary rocks [17]. In this frame, the outcroppings on Mars surface at Meridiani Planum (Opportunity Rover) or at Gusev Crater (Spirit Rover) represent an interesting field for microstructure/ textures investigations [4,37].
In this work we analyzed enlarged photographic images, at the same magnifications, resolution and acutance, of stromatolites and of the Martian images obtained with the ATHENA system of the Spirit Martian Rover. At our range of magnification and resolution is possible to study microstructures in the 1-0.05 mm size range: a field characterized in the terrestrial microbialites/stromatolites by clotted fabric, tipically formed by widespread micritic clots or lumps, agglutinated and set in an irregular sponge-like network structure or in intertwined turf-like communities [15-17], giving origin to the aspects of microspherules and intertwined filaments of microspherules (see, Figure 6, to look these microstructures at high magnification).
All these structures and aspects are very interesting for astrobiological purposes because they are difficult to explain as abiogenic artifacts. In fact, while the sediment structure follows universal and repetitive laws (revealing in the present work a simple fractal structure, Figure 7, right), the biological structure, within the same environment, at the same meso-microscopic scales, appears to vary depending on the metabolic activity of the living organisms [38-43] (revealing in the present work a multifractal structure, Figure 5, left).
In this paper, a tangle of microspherule and intertwined filaments of microspherules has been evidenced on the Martian sediments photographed by the Spirit rover: a textural pattern that is also present in living microbialites as well in recent and fossil stromatolites [18-20]. Both the textures, from microbialite images (Earth) or from analyzed MI images (Mars), are multifractals, as revealed by the two straight lines present in the log-log plots (Figure 5). At a quantitative analysis, our objective morphometric approach reveals indexes of geometric and algorithmic complexity, entropy, and tortuosity, minimum and maximum diameters of the microtextures extremely similar among them: average values and confidence intervals of the eight independent fractal parameters between Earth (stromatolites and other microbialites) and Mars samples perfectly overlap. The probability of this occurring by chance is less than 1/28 (p<0.004).
We may note that at Spirit location we obtained the same results we have had analyzing sediments photographed by Opportunity Rover at Meridiani Planum [21], a region very far away from Gusev crater. On February 2015, Nora Nofke [44], in a visual inspection of images shot by Curiosity Rover (landed in a further region of Mars, at Gale crater) noted signs of the presence of stromatolites in the Martian landscapes. Evidences of microbialites in the Martian outcroppings are multiplying: unicellular life was widespread on the ancient Mars.
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