TRABAJOS Y APORTES ANTERIORES A EINSTEIN

Mars has been the subject of much scrutiny in the study of the possible existence of life elsewhere in the universe, and Martian meteorites represent the only samples of the Martian crust directly available for study. Here, a methodology was developed for the evaluation of astrobiologically interesting samples to help guide future exploration work. This was achieved using a variety of techniques widely used in the assessment of

geological materials in terrestrial laboratories (including optical microscopy, scanning electron microscopy, in situ micro X-Ray diffraction, electron probe microanalysis, and Raman spectroscopy).

This consisted of the mineralogical and geochemical characterization of a suite of Martian meteorites from an astrobiology perspective. Once a thorough reconnaissance study was carried out to characterize the mineral species present in these Martian meteorites, a more in depth investigation of their environmental characteristics and potential habitability could be done. The results from the study of mineral substrates present in the rocks and the alteration conditions that they were exposed to allowed for an evaluation of the potentially habitable environments they represent.

Los Angeles is a coarse-grained basaltic shergottite displaying a subophitic texture. It is more differentiated than most other Martian rocks, found to be composed of 40% large

breakdown material is present throughout the meteorite, as a symplectic hedenbergite- fayalite-silica assemblage resulting from the metastable breakdown of an iron-rich pyroxene phase. The Zagami meteorite is a basaltic shergottite. It is fine-grained and less differentiated when contrasted with the Los Angeles meteorite, with an ophitic texture and slight pyroxene foliation. Quenched glassy mesostasis was found in both shergottites, and minor phosphates (merrillite), sulfides (pyrrhotite), and oxides (titanomagnetite with some ilmenite) are present as accessory phases. Nakhla is a clinopyroxenite (nakhlite) displaying a subcumulate texture. The majority of the rock is made up of augite crystals. Olivine, associated mesostasis (glassy and feldspathic), and minor oxides (primarily titanomagentite), sulfides (pyrrhotite), and aqueous secondary minerals are also present in Nakhla.

Micro X-Ray diffraction was shown to be an effective technique for the in situ

identification of mineral species. It requires minimal sample preparation and does no damage to the sample. It allowed for identification of all major mineral species in these three meteorites. Targeted micro X-Ray diffraction analysis of alteration material in the Nakhla meteorite provided a detailed mineral assemblage present within alteration veins. This material is composed primarily of iron oxides (including goethite and hematite) and smectites (including saponite and nontronite), with some serpentine and the carbonates ankerite and siderite. Targeted in situ Raman spectroscopy of Nakhla alteration material was also able to provide information on the mineral assemblage present in altered veins. These spectra allowed for the identification of iron oxides, smectites, and carbonates, confirming the assemblage obtained from µXRD.

EPMA geochemical analyses provided in situ compositional information on the mineral phases present in the meteorites with minimal damage to the samples. The major

ferromagnesian phase in all three of these meteorites is pyroxene. Augite and pigeonite make up equal proportions of Los Angeles and Zagami, while augite is the pyroxene present in Nakhla. Los Angeles and Zagami have much more variation in their pyroxene composition than Nakhla, showing considerable zoning with iron-rich pyroxene rims. Pyroxferroite in Los Angeles was found to be enriched in iron when compared to the two stable pyroxene phases. Los Angeles is the most iron rich meteorite studied here; the iron content of primary ferromagnesian (pyroxene) phases in Los Angeles (33-39 wt%) were found to be significantly higher than that of Zagami (22-28 wt%), with Nakhla pyroxenes (16 wt%) being iron-poor compared to the two shergottites.

Nakhla contains fayalitic olivine that is significantly enriched in iron content (52 wt%) compared to similar volcanic olivines on Earth, with an average composition of Fa65.5. Maskelynite in Los Angeles was found to be slightly more anorthitic than that of Zagami, with both shergottites containing minimal potassium in their feldspathic phases.

Pyrrhotite in the shergottites was found to be very iron-rich, with an average composition of Fe0.935S. Mesostasis in LA and Zagami is glassy and silicon-rich, while mesostasis in Nakhla is composed of coarse anorthitic feldspar laths and silica-rich glassy material with iron enrichment in some areas.

The alteration mineral assemblage identified here using mineralogical techniques (micro X-Ray diffraction and Raman spectroscopy) differs from previously reported assemblages

reevaluation of the environment of alteration. The assemblage reported here suggests a relatively high Fe/Mg fluid ratio, temperatures of slightly below or approaching 150oC, fluid pCO2 conditions of >50-100 mbar, and circumneutral fluids pH transitioning from reducing to oxidizing. The presence of siderite and ankerite in the alteration products suggests hydrothermal reworking of carbonates by aqueous fluids.

Endolithic microbes in terrestrial environments require an energy source for metabolism to take place (e.g., Benzerara et al., 2009; Colwell et al., 1997; Liu et al., 2012; Jorgensen et al., 2007; Mortimer et al., 1997; Stetter et al., 1990). Serpentine identified here in Nakhla suggests methane may be a potential energy source. Reduction and oxidation of iron is the major metabolic pathway used by endolithic microorganisms in terrestrial settings (e.g., Thorseth et al., 2003). Los Angeles, Zagami and Nakhla contain large amounts of iron in their primary phases, in significant excess to similar terrestrial rocks (Bridges et al., 2006). Iron-rich sulfides, oxides, and ferromagnesian phases are

intergrown with fine-grained textures and extensive mineral interfaces throughout these rocks. These three meteorites also contain patches of mafic glassy material, a large potential source for energy for microbes (e.g., Thorseth et al., 2001; Furnes et al., 2004; Banerjee et al., 2009; Izawa et al., 2010). Olivine in Nakhla is very iron-rich and is heavily fractured with evidence of extensive aqueous activity within fractures.

The limiting factor in most lithospheric microbial habitats appears to be the aqueous flux and what (if any) sources of energy and nutrients this provides. Los Angeles and Zagami show no evidence of interaction with any fluids, while Nakhla displays evidence of interaction with aqueous fluids in the Martian subsurface. These fluids were found to be

similar to those supporting terrestrial subsurface endolithic communities based on fluid chemistry, pH, pressure, and temperature (e.g., Engelen et al., 2008; Peckman et al., 2008; Walton, 2008; Thorseth et al., 2003). This suggests that the subsurface horizons represented by Los Angeles and Zagami would be attractive microbial substrates if aqueous environmental conditions were present, and that Nakhla was a potential substrate for microbial colonization (though the extent of the aqueous hydrosphere at depth is poorly understood).

The results reported here show that iron-rich basaltic rocks (such as the subsurface Martian rocks represented by the Los Angeles, Zagami, and Nakhla meteorites) are attractive astrobiological targets where they occur in settings with evidence of aqueous activity at depth. Rocks in the Tharsis region that have interacted with fluids (such as those in hydrothermal environments) are high priority targets in the search for endolithic basaltic habitats on Mars. This study developed a methodology for investigating Martian rocks with consideration of their potential to act as microbial substrates in the Martian subsurface. This work will help to understand how best to guide future exploration missions on the Martian surface. Studies such as this are critical in preparation for Martian sample return during future exploration to allow for effective sample selection during these missions.