Mineralized biosignatures in ALH-77005 Shergottite - Clues to Martian Life?


 The ALH-77005 Martian meteorite was found in Allan Hills on Antarctica during the Japanese National Institute of Polar Research (1977-1978) mission. One thin section sample was studied by optical microscopy for microtexture and by FTIR-ATR microscopy for interpretation of biogenic minerals and embedded organic materials. The geochemical data (biogenic elements, δ13C) of ALH-77005 meteorite from literature implementing recent results were compared to terrestrial geological samples. The ALH-77005 has poikilitic textures with coarse pyroxenes and brown olivines, and with recrystallized melt pocket. The coarse-grained minerals do not contain any alteration along the grain boundaries. Melt pocket and vicinity of opaque minerals contain biogenic signatures as filamentous, coccoidal forms of iron-oxidizing bacteria. The biosignatures were determined by 1) coccoidal, filamentous forms, 2) presence of embedded organic material, 3) presence of biogenic minerals, like ferrihydrite, goethite, and hematite. The other signatures for biogenicity of this meteorite are strong negative δ13C, enrichment of Fe, Mn, P, Zn in shock melt support scenario. This study proposes presence of microbial mediation on Mars.


Introduction
The ALH-77005 Martian meteorite was found in the Allan Hills, in South Victoria Land on Antarctica in 1977-1978(Yanai, 1979. Nyquist et al. (2001) indicated lherzolitic texture of ALH-77005. Moreover, Ikeda (1994) suggested that the origin of this meteorite and its shergot-tite formation with shallow magma reservoir (such as lava lake) is on Mars. The aim of this study is to investigate potential mineralized biosignatures in the spinifex textured melt pockets. The petrographical, petrological composition and shock metamorphic features were described by Ikeda (1994); Nagy et al. (2012); Gyollai et al. (2013).

Methods
The FTIR-ATR measurements and optical microscopy were performed at HAS Research Centre for Astronomy and Earth Sciences, IGGR. The mineral assemblages and textures were characterized by a NICON Eclipse LV600POL optical microscope with magnification 40x, 100x, 200x, 400x and 1000x. We used Bruker Vertex70 with Hyperion 2000 FTIR-ATR microscope with a 20x ATR objective and MCT-A detector, with spatial resolution of 2 µm, for the determination and distribution of micro-mineralogy and organic compounds. During the infrared analysis, the minerals in the thin section were contacted by the tip of the germanium (Ge) crystal (selected 1 N pressure) of 100 µm in diameter with spatial resolution of 2 µm. All measurements were performed for 32 scans at 4 cm −1 resolution in the 600-4000 cm −1 range. Bruker Optics' Opus 5.5. software was used for manipulation of the resultant spectra (e.g. baseline correction, atmospheric compensation etc.). To avoid the interpretation of environmental conditions of the measurements in the sample, the spectra of dichloromethane, glass rode were used as background. The measurements were done on 35 µm thick thin section (Figures 3-4). A total 77 of spectra were acquired.

Petrography
The ALH-77005 consists of pyroxene, olivine and feldspar. The ALH-77005 has coarse-granular texture with locally microgranular and poikilitic texture regions and melt pockets with recrystallized needle-like crystallites in glassy matrix.
In the environment of melt pockets resorption rim can be observed with toast-like texture in some cases and infiltration of dark melt. The melt pockets are darker (darkbrown-black in plane polarized lights) than its environment (well-crystallized coarse crystals).
In the vicinity and inside of melt pockets several textures can be observed. The needle-like crystals are feldspar and pyroxene in melt pockets. The lengths of needles are between 10-75 µm, and their width falls into 1-5 µm range.
Near to melt pockets, isotropic lath-shaped plagioclase, maskelynite occur. But, according to presence of weak feldspar band of Raman and FTIR spectra, this alteration is transient, the shock pressure did not exceed 30 GPa. In the olivines, parallel to the fractures, kink-band system can be observed. The poikilitic fractured pyroxene grain contains olivine with thick one-set kink bands.

Mineralized putative biosignatures in ALH-77005 meteorite
The recrystallized shock melt with spinifex texture contains putative microbially mediated features as wellresembling to mineralized microbially produced texture (MMPT) -in the form of pearl necklace-like textures with vermiform inner signatures, which are embedded in needle-like crystals (olivine, pyroxene, feldspar) in the rapidly cooled shock melt ( Figure 5). The coarse minerals do not contain microbially mediated texture. It occurs only near to opaque minerals and in shock melt pocket. The MMPT consists of micrometer-sized microbial filamentous elements and clusters in their boundary region. The MMPT is very extensive, reaches 70-80 vol.% of the shock melt pocket and is intimately woven in the full crosssection of the melted part. All of recrystallized melt pocket sections showed signs of Fe mobilization and oxidation (brown haloes around mineral grains, brown filaments, Figure 5).

Discussion
The brown signature of melts and olivine could be caused by magnetotactic bacteria (Hoffmann et al., 2008), which has 0.4-0.7 wt.% MnO (Ikeda, 1994) as bioessential element which support our scenario. The low Ca pyroxenes also contain 0.4-0.6 wt.% MnO (Ikeda, 1994). The dark melt has 19-26 wt.% FeO and 0.4-0.7 wt.% MnO (Edmunson et al., 2002), which are bioessential elements of iron oxidizing bacteria. The P 2 O 5 content in melt pockets varies between 3-5 wt.% (by melt of apatite and whithlockite) (Calvin & Rutherford, 2005), which is a essential chemical component for microbial activity and could also be consumed as nutrient for iron oxidizing bacteria. The mentioning of signature for Martian life was mentioned by Mikouchi et al. (1997), who found PAHs and magnetotactic nanobacteria structured microfossils with biominerals troilite pyrite and carbonate in ALH-84001 meteorite. Our recent results fit well with their interpretation, similarly to Mikouchi et al. (1997), these features were found in melt pocket and near pyrite -troilite grains of ALH-77005 meteorite: organic material, morphology, biogenic minerals were found. The troilite grains contain 0.04 wt.% Zn based on Ikeda (1994), which could solve as nutrient for iron oxidizing bacteria.
Noble gas compositions, cosmic-ray exposure age (2.9 ±0.7 my), and K-Ar age (1.33 ± 0.13 by) are obtained by Miura et al. (1995) for ALH-77005 shergottite. However, they mentioned, that this meteorite is heavily contaminated by elementally fractionated gases -which were proposed by microbial alteration of terrestrial origin. But comparing the textures to the secondary terrestrial alteration should follow the fractures and grain boundaries, but in our sample, the microbial alteration was observed mostly in melt pocket, the fractures of coarse silicates remained clear. Alternatively, Takenouchi et al. (2018) suggest formation of brown olivine by heterogeneous shock p-T conditions, where Fe-rich nanoparticle formed along crystal defects of shocked olivine due to shear stress during impact event: the high pressure phases (ringwoodite, wadsleyte) backtransformed to olivine during the decompression. However, this interpretation does not fit with the complex filamentous microbial microtexture. Wright et al. (1992) measured negative C isotope values for SNC meteorites, where δ 13 C of ALH-77005 is −36% (for further meteorites data are the following: Zagami δ 13 C: −34 % , Nakhla δ 13 C: −31 % , Shergotty δ 13 C: −31 % , -Lafayette δ 13 C: −29 % ). The carbonate fraction of EETA 79001 reach −25 % (Wright et al., 1988). The negative δ 13 C imply organic carbon contribution supporting biogenicity (Polgári et al., 2012a).
Comparing our results to biogenicity aspects, our data confirm: Dense and invasive microbially mediated processes in the ALH-77005 meteorite, supported by microtexture, micromineralogy, embedded organic compounds, and enrichment of bioessential elements, which effected most of the mass of the melted pocket of the sample. Also, strongly negative δ 13 C values as "vital effect" support this scenario. These fit well on 5 hierarchical levels (isotope, element, molecule, mineral and texture) with complex terrestrial biogenicity features, and also results on other meteorites (Polgári et al., 2017a,b, 2018a, 2019. In the case of these meteorites, the complex dense biosignatures affected most of the mass (up to 80%) of the samples, which raised that if the biosignatures are terrestrial contamination, it is of no effect to interpret the so called transformation products happened on the parent body, as it is a general custom based on literature. This arises a contradiction: how could it be that all these classical transformations have been occurred to happen on the parent bodies, while the microbial processes happened separately, in terrestrial conditions, although the MMTP had interwoven all the textural and mineralogical assemblage (Polgári et al., 2019).
Comparing recent results and interpretation with other meteorites, it can be raised, that on these similarities the microbially mediated biosignatures can be proposed microbial mediation by FeOB on Mars.

Conclusion
The microbial alteration occur only in recrystallized shock melt pocket and near to opaque minerals. Microbial mediation along rims and fractures of coarse grains are not present, which dismiss the terrestrial alteration origin. The microbial mediation by FeOB is supported by 1) morphology (coccoidal, filamentous form of FeOB; 2) presence of embedded organic material (aliphatic CH, PAHs); 3) enrichment of bioessential elements (Fe, Mn, P, Zn); 4) negative δ 13 C values (−36% ). The other Martian meteorites have similarly strong negative δ 13 C values (−29 -−36 % ) and alteration by Fe-rich clays, iron-oxi-hydroxides.
Comparing recent results and interpretation with other meteorites, it can be concluded, that on these similarities the microbially mediated biosignatures can be proposed microbial mediation by FeOB on Mars.

Highlights
-Putative filamentous iron oxidizing bacteria were found in shock melt pocket of shergottite. -Biosignatures were determined by optical microscopy and by FTIR-ATR. -Bioessential elements and carbon isotope data from references support biogenicity. -Comparison of similar results of Mező-Madaras, Mócs, Knyahinya, support biogenicity on Mars.