International Journal of Astrobiology 8 (4): 267–280 (2009) Printed in the United Kingdom 267 doi:10.1017/S1473550409990152 f Cambridge University Press 2009 Possible organosedimentary structures on Mars Vincenzo Rizzo1 and Nicola Cantasano2 1Earth Science Department, Florence University, Via L. Repaci snc, 87036 Rende (Cs.), Italy 2National Council of Research, I.S.A.Fo.M. U.O.S., Via Cavour 4/6, 87036 Rende (Cs.), Italy e-mail: [email protected] Abstract: This study, using the Microscopic Imager (MI) of NASA Rover Exploration Mission’s (REM) ‘Opportunity’, aims to explain the origin of laminated sediments lying at Meridiani Planum of Mars, and of the strange spherules, known as blueberries, about which several hypotheses have been formulated. To this purpose, images of the sedimentary textures of layers and fragments captured by REM have been analysed; sediments that NASA has already established as ‘pertinent to water presence’. Our study shows that such laminated sediments and the spherules they contain could be organosedimentary structures, probably produced by microorganisms. The laminated structures are characterized by a sequence of a thin pair of layers, which have the features of skeletal/agglutinated laminae and whose basic constituents are made by a partition of septa and vacuoles radially arranged around a central one. The growth of these supposed organosedimentary masses is based on the ‘built flexibility’ of such a basal element; it may be a coalescing microfossil formed by progressive film accretion (calcimicrobe), in a variety of geometrical gross forms, such as a repeated couplet sequence of laminae or domal mass and large composite polycentric spherule, both in elevation. The acquired structural and textural data seem to be consistent with the existence of life on Mars and could explain an origin of sediments at Meridiani Planum similar to that of terrestrial stromatolites. The Martian deposits, probably produced by cyanobacterial activity, and the embedded blueberries could represent a recurrent and multiform product of colonies with sheath forms, resembling in shape those of the fossil genus Archaeosphaeroides (stromatolites of Fig Tree, South Africa). Received 11 June 2009, accepted 24 June 2009, first published online 28 August 2009 Key words: blueberries, calcimicrobes, cyanobacteria, life, Mars, stromatolites. Introduction for several reasons: the identification of Polycyclic Aromatic Hydrocarbon (IPA) on the meteorite was regarded as en- The possibility of life on Mars, even in a fossilized state, vironmental contamination; the micro-rings on its surface opens up new horizons for the human colonization of the seemed so small as to be completely unlike any form of ter- Solar System. The presence of sedimentary layers, probably restrial bacteria; the signs of loadstone incorporated in the produced by the metabolic activity of microorganisms living meteorite’s carbonatic matter were artificially reproduced in under extreme environmental conditions, indicates that pri- laboratory and, therefore, they were considered as inadequate mordial forms of life can arise from very little, and could then evidence (McKay et al. 1998). be more widespread than we can imagine. It is even more NASA’s latest Rover Exploration Mission (REM, em- amazing to make a comparison with ancient terrestrial forms ploying rovers ‘Opportunity’ and ‘Spirit’), has captured, of life, and to consider the relevant cultural implications this from 2004 until now, the Mars landscape of Meridiani relationship implies. Planum and Gusev Crater, recording macroscopic (PanCam) The hypothesis of the presence of life on Mars was advanced and microscopic (MI) imagery. The pictures of the Mars in 1984, when American researchers found a small meteorite, outcrops brings forth once again an old problem, further weighing 8 kg and 20 cm in diameter, on the Allan Hills in emphasized by the presence on the Martian ground of several the Transantarctic Range. Initially the fragment, named blue-coloured spherules, which Steve Squyres, team leader of ALH84001, was assigned as a ‘diogenite’: a kind of igneous NASA’s mission, nicknamed ‘blueberries’ (Fig. 1). Squyres meteorite, a sliver of the Vesta asteroid. Finally, in 1993, David himself, in NASA’s press conference of March 2, 2004, ad- McKay, a NASA biochemist, declared that the meteorite was, vanced two hypotheses: the Aeolian–sedimentary origin of in fact, a Martian fragment which dated back 4.5 billion years, the subcrops of Meridiani Planum and the concretionary and he proposed its biological origin (McKay et al. 1996). nature of the spherules (Squyres et al. 2004). Some years later, McKay’s hypothesis was refuted by other Later, other scientists (McCollom & Hynek 2006) sug- NASA research teams and in 2001 was definitively put aside gested the hypothesis of a volcanic origin of the sediments 268 V. Rizzo and N. Cantasano between Mars and a meteorite, involving deposition from a ground-hugging turbulent flow of rock fragments, salts, sulphides, brines and ice produced by the impact (Knauth et al. 2005). McKay’s theory, consistent with a biological origin of the carbonate globules covering the Martian meteorite ALH 84001, resurfaced in 2005. Spanish researchers at the Madrid Astrobiology Center, and later American groups, put for- ward the proposal that the strange spherules on Meridiani Planum could be related to a community of chemolitho- autotrophic bacteria still living on Earth in acid environments enriched by iron minerals (Parro et al. 2005; Jepsen et al. 2007). Recently, other authors have found further evidence of life on Mars by detection of methane and formaldehyde (Onstott et al. 2006; Schulze-Makuch et al. 2008). The bio- logical theory was also supported by Italian researchers (Caiola & Billi 2007); they supposed that some terrestrial organisms, from the cyanobacteria group and similar to the genus Chroococcidiopsis, could be adapted to Martian con- ditions. Finally, in July 2008, NASA admitted that the geo- logical conformation of the Martian surface encountered by the space probes ‘Opportunity’ and ‘Spirit’ was consistent with the presence of water in remote ages. NASA declined to give an opinion about the biological origin of the blueberries, but stated that the search for life on Mars would be the main goal of the next missions in 2009. This study aims to describe the structures of layers and blueberries in the area of Meridiani Planum in order to understand their debated origin. Materials and methods The study undertakes a systematic analysis of MI shot by Athena, a camera mounted on the NASA Mars Exploration Rover (MER) ‘Opportunity’, and of panoramic photos shot by a wide-angle camera (PanCam) mounted on the same rover. The field of view of both instruments is 1024r1024 Fig. 1. Panoramic picture (PanCam) of spherules known as pixels in size. The images in this study, captured by MI, are ‘blueberries’. monochromatic because there is a single broad-band filter mounted on the MI. The MI optics provide ¡3 mm depth- of-field at 30 mm/pixel sampling. The working distance is and spherules, but this interpretation was superseded by 63 mm from the front of the lens barrel to the object plane. NASA’s concretional theory (Squyres et al. 2006) after stu- In this condition the field of view is about 3 cm in size and dies of the chemical composition of the spherules indicated the texture of bodies smaller than 100 mm cannot be visu- the presence of haematite, a mineral whose basic constituents alized. There are several images of regards polished sur- grow in aquatic environments. Moreover, the substratum was faces, about 7 cm in diameter, prepared by MER’s circular made up of silicate and sulphate minerals, while the rocks in Rock Abrasion Tool (RAT); this tool is able to grind about which the spherules were embedded appeared highly lami- 5 mm of sediments near the midpoint, tapering toward its nated, so that their presence inside the layers was considered border. as the concretional product coming from diagenetic pro- As regards to the methodological approach; first, various cesses. forms of microfossils have been individualized and classified, Other authors (Catling 2004; Chan et al. 2004; Coleman and the most-often occurring kinds identified. The structure et al. 2005), following up Squyres’ concretional theory, made of the laminated sediments and the nature of the spherical a comparison between the Martian blueberries and similar bodies embedded amongst the layers of the surface sediments forms present on Earth such as pisolits, oolits and other have been successively analyzed, focusing both on polished similar formations. Also advanced was the attractive theory surfaces and on exposed ones; structural and morphological that these spherules could have been raised from an impact relationships were used to assume their origin. In this context, Organosedimentary structures on Mars 269 fragments and eroded bodies were useful in understanding and BS type (some of them indicated by white arrows in Fig. internal structure. 4), and in some cases by regular or irregular internal cell partitions (Fig. 4(c)). The variety of shapes and external Results structures (each one different) could be seen as the result of different constructions built from further smaller constituents The study of a selected set of amplified MER MI images (including the cell partition, whose dimensions are about shows the presence of objects which have traits of prevalently 0.1 mm–0.3 mm). The BS structure is observed on the spher- microfossil-like artefacts resembling early life stages (Fig. 2). ule surface, has a dimension of about 1 mm, and comprises a They include spatially structured chambers and septa some- sheet of a radial array of 5–10 cells around a central cell, this what spirally organized (Fig. 2(m), (n), (o), (p)), bodies last occupied by an Ri element. As seen in Fig. 2(l) the BS showing a ‘from interior’ growth Fig. 2(a), difficult to explain structures are coalescing, and as displayed in Fig.