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Fresh fracture surfaces of the martian meteorite ALH84001 contain abundant polycyclic aromatic hydrocarbons (PAHs). These fresh fracture surfaces also display carbonate globules. Contamination studies suggest that the PAHs are indigenous to the meteorite. High-resolution scanning and transmission electron microscopy study of surface textures and internal structures of selected carbonate globules show that the globules contain fine-grained, secondary phases of single-domain magnetite and Fe-sulfides. The carbonate globules are similar in texture and size to some terrestrial bacterially induced carbonate precipitates. Although inorganic formation is possible, formation of the globules by biogenic processes could explain many of the observed features, including the PAHs. The PAHs, the carbonate globules, and their associated secondary mineral phases and textures could thus be fossil remains of a past martian biota.

Assessing the habitability of martian environments depends on the use of an operational definition of habitable. In this paper, it is taken to be an environment that has the necessary conditions for at least one known organism to be active, where active means metabolically active as maintenance, growth, or reproduction. Habitability is necessarily defined by reference to specific organisms. An anaerobic location is not habitable to an iron-oxidizing microorganism that requires oxygen as the terminal electron acceptor; but, for example, it might be habitable to certain anaerobic iron-reducing microorganisms if ferric compounds and accessible organics are present, provided that all other requirements for the organisms exist, such as supplies of the elements C, H, N, O, P, S, liquid water, and appropriate physical and chemical conditions. The more energy sources and essential nutrients an environment contains, the greater the potential diversity of life that the environment is likely to support.

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A categorization of the possible trajectories of martian habitability through time can therefore be addressed by separating out trajectories based on the existence of combinations of these three environments and then considering, within each class of trajectory, the relative abundance and characteristics of these environments at different scales (Fig. 2).

Trajectories of martian habitability. Different trajectories of the habitability of Mars through time, beginning with the branch point of an uninhabited and inhabited Mars. Experimental investigations of Mars will allow for the determination of which trajectory applied to Mars and the relative abundance of the different environments. Trajectory 4 is the trajectory that Earth has taken.

The presence of deformation bands (Okubo et al.,2009) is suggested to show that water flow in the past subsurface of Mars might have been influenced, and channeled, by these features. Discoloration along the boundaries of the bands is interpreted to show aqueous alteration of primary minerals (Okubo et al.,2009). Variations in discoloration along the bands are taken to suggest heterogeneity in past martian subsurface water flow and spatial differences in subsurface water geochemistry.

Hydrogen atoms are available from water throughout the martian depth profile, which could be split radiolytically in the subsurface (Lin et al.,2005). Hydrogen could also be generated in chemical reactions. The presence of serpentine in impact crater uplifts (Ehlmann et al.,2010, 2011; Quantin et al.,2012) suggests the possibility of hydrogen production through serpentinization reactions, particularly when water flow through ultramafic rocks was more extensive in the Noachian.

Nitrogen gas is present in the modern atmosphere at 2.7%. Fixed nitrogen compounds have been reported in martian meteorites (Wright et al.,1992; Grady et al.,1997) and confirmed on the surface of Mars (Ming et al.,2014). They have been predicted to include nitrate and ammonium based on terrestrial analogues (Mancinelli and Banin, 2003). To be used in biological systems, nitrogen must be in a fixed form. One potential pathway is biological fixation, which was shown to be possible at a pN2 of 5 mbar, but not below 1 mbar, suggesting that this pathway was plausible in a denser early martian atmosphere but unlikely today (Klingler et al.,1989). Nitrogen fixation on Mars could occur by abiotic processes, including impact events, lightening, and volcanic activity (Segura and Navarro-Gonzlez, 2005; Summers and Khare, 2007; Manning et al.,2009), or by processes analogous to reduction by hydrogen in deep subsurface systems on Earth (Brandes et al.,1998). The concentrations reached and the depths achieved by nitrogen fixed in such processes throughout martian history are unknown. Boxe et al. (2012) used a one-dimensional model to show that fixed nitrogen species, some produced photochemically, for example, , NO, HNO3, could be generated on the surface of Mars and then transported into near-surface environments in thin water films. Similarly, NO and other abiotically fixed species have been suggested as nitrogen sources and biological electron sinks on early Earth (Ducluzeau et al.,2008). This transient photochemically produced nitrogen cycle on Mars could provide a source of fixed nitrogen species today, but the depth of its penetration would be low because of lack of surface liquid water. Without a continuous flow of fixed nitrogen into the deep subsurface of Mars, particularly following the cessation of widespread surface hydrological activity on Mars in the Noachian, nitrogen might be, and might have been, one of the limiting factors for life. Despite the detection of fixed nitrogen in meteorites and directly on the surface of Mars, determining the distribution and form of fixed nitrogen in the martian crust, past and present, remains one of the most important challenges in constraining martian habitability (Mancinelli and Banin, 2003).

Phosphate has been reported in martian meteorites (Boctor et al.,1998) and on the surface of Mars in a number of missions. For example, Mssbauer, MiniTES, and APXS spectra from the Mars Exploration Rovers are interpreted to suggest apatite concentrations (wt %) at between 0.1% and 2.4% (McGlynn et al.,2012). Rocks with P2O5 abundances (Wishstone Class) of over 5% were observed in Gusev Crater by the Mars Exploration Rovers in which the primary phosphate-bearing mineral may be merrillite (Usui et al.,2008). Phosphorus was also observed in alkaline basalts studied in Gale Crater at

The presence of reduced sulfur species such as sulfides, which have been found in martian meteorites (Scott, 1999) and on the surface of Mars (Leshin et al.,2013; Ming et al.,2014), suggests the possibility of sulfur species oxidation (Grotzinger et al.,2014). However, anaerobic conditions prevent chemolithotrophic sulfur species oxidation with oxygen as the terminal electron acceptor. Sulfur can be oxidized with the use of ferric iron as the electron acceptor (Pronk et al.,1992). This reaction occurs in acidic conditions, and elemental sulfur has been tentatively identified on Mars (Morris et al.,2007). Liquid water on the surface in the Noachian would have allowed for more favorable conditions for sulfur anoxygenic photosynthesis. This would require the co-location of reduced sulfur species, light, and other requirements for habitability at microbial scales.

Other chemolithotrophic redox couples could include methanogenesis and acetogenesis, both with the use of CO2 from the atmosphere or from dissolved carbonates and H2 from serpentinization reactions as observed in the subsurface of Earth (Kotelnikova and Pedersen, 1998; Moser et al.,2005; Harris et al.,2007). Methane itself can be oxidized by microorganisms as a source of energy and can be produced abiotically (Berndt et al.,1996). Serpentinized ultramafic rocks are known to host thriving microbial communities in the subsurface of Earth and in surface discharge (Blank et al.,2009; Okland et al.,2012; Szponar et al.,2013) and could provide analogies to potential water-rock-microbial interactions for the martian subsurface.

Although biogenic methanogenesis cannot be ruled out in the present day, to date a surface detection of methane that would be consistent with such a hypothesis remains elusive (Webster et al.,2013). The presence of CO in the atmosphere, which can be used as an electron donor in anaerobic carboxydotrophy, has also been suggested to indicate the lack of a significant biological sink (Weiss et al.,2000a). Although martian sources and sinks of CH4 and CO are not fully understood, the data do not at the current time provide any evidence for a present-day active chemolithotrophic biosphere on Mars.

All trajectories of martian habitability begin with the formation of Mars. From early planetesimals (Debaille et al.,2007), an uninhabitable planet formed. As water condensed and the environment cooled, the planet was at a branch point in its long-term trajectory of biological conditions. In one set of trajectories, the planet is defined by its condition as uninhabited (neither an origin of life occurs nor does life transfer to the planet from Earth in meteoritic matter). In the second set of trajectories, the planet is defined by the establishment of life, an event that changes the use of habitable conditions and through feedback effects would itself change the habitability of environments (Nisbet et al.,2007).

Mars might have always lacked a fundamental requirement for life at sufficient concentrations. Although fixed nitrogen has been detected in martian meteorites (Wright et al.,1992; Grady et al.,1997) and on the surface of Mars (Grotzinger et al.,2014; Ming et al.,2014), if these species had not been produced at sufficient concentrations over large scales or co-localized at micron scales with other requirements for life, then this element could have been limiting to life (Mancinelli and Banin, 2003).

Trajectory 7 is a scenario in which uninhabitable environments during much of martian early history in the Noachian ultimately transitioned to uninhabited habitats on an uninhabited Mars at some later stage during the Hesperian/Amazonian. This trajectory, which supposes a planetary-scale improvement in the conditions for life, is difficult to reconcile with the known hydrological and geological evidence, since Noachian Mars had more abundant liquid water than Hesperian and Amazonian Mars (Lasue et al.,2013). be457b7860