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Trajectories of Martian Habitability

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  • Review Article

    Trajectories of Martian Habitability

    Charles S. Cockell

    Abstract

    Beginning from two plausible starting pointsan uninhabited or inhabited Marsthis paper discusses thepossible trajectories of martian habitability over time. On an uninhabited Mars, the trajectories follow pathsdetermined by the abundance of uninhabitable environments and uninhabited habitats. On an inhabited Mars,the addition of a third environment type, inhabited habitats, results in other trajectories, including ones wherethe planet remains inhabited today or others where planetary-scale life extinction occurs. By identifyingdifferent trajectories of habitability, corresponding hypotheses can be described that allow for the varioustrajectories to be disentangled and ultimately a determination of which trajectory Mars has taken and thechanging relative abundance of its constituent environments. Key Words: MarsHabitabilityLiquid waterPlanetary science. Astrobiology 14, xxxxxx.

    Introduction

    Assessing the habitability of Mars has been an ob-jective of scientists for a long time, but it has recentlybecome a sustained focus in light of data being returned from theplanet and growing knowledge about life in extreme environ-ments (e.g., McKay and Davis, 1991; Boston et al., 1992; Ja-kosky and Shock, 1998; Cabrol et al., 1999; Weiss et al., 2000a;Fairen et al., 2005, 2010; Schulze-Makuch et al., 2005; Dartnellet al., 2007; Nisbet et al., 2007; Davila et al., 2008, 2010; Toscaet al., 2008a; Javaux and Dehant, 2010; Stoker et al., 2010;Cockell et al., 2012; Ulrich et al., 2012; Cousins et al., 2013;Grotzinger et al., 2014; Michalski et al., 2013; Westall et al.,2013). This endeavor is important for elucidating the history ofMars and providing a comparative data point for understandingthe geological and biological history of Earth (Taylor, 2011). Byinvestigating the habitability trajectories possible for terrestrial-type planets, we gain information with which to assess thehabitability of diverse rocky extrasolar planets (Fridlund, 2008).

    Assessing the habitability of martian environments de-pends on the use of an operational definition of habitable. Inthis paper, it is taken to be an environment that has thenecessary conditions for at least one known organism to beactive, where active means metabolically active as mainte-nance, growth, or reproduction. Habitability is necessarilydefined by reference to specific organisms. An anaerobiclocation is not habitable to an iron-oxidizing microorganismthat requires oxygen as the terminal electron acceptor; but,for example, it might be habitable to certain anaerobic iron-reducing microorganisms if ferric compounds and accessi-ble 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 andchemical conditions. The more energy sources and essentialnutrients an environment contains, the greater the potentialdiversity of life that the environment is likely to support.

    Habitability is therefore a conservative term, circumscribedby the limits of our knowledge of the range of possible life-forms, their environmental and metabolic capabilities, and theirenergy requirements (e.g., Hoehler, 2007). A habitable envi-ronment could exist for a very short period of time as a tran-sient environment. The suitability of environments to hosthabitable conditions over geological timescales is a morecomplex calculus, and it is determined by the interplay offactors such as plate tectonics, the hydrological cycle, andplanetary temperature regimens (Nisbet et al., 2007).

    A common departure point for investigations on the bio-logical potential of Mars is to assume that Mars was habitableand inhabited and then to assess the surface and subsurface ofthe planet against known metabolic capabilities of microor-ganisms on Earth (see discussions in Boston et al., 1992;Nisbet et al., 2007; Javaux and Dehant, 2010).

    At the current time, there is no unequivocal evidence forlife on past or present Mars. Therefore, any assessment as tohow the biological potential of that planet has changed overtime must start with the formation of early Mars, when itwas uninhabitable, and then consider the diversity of tra-jectories that result on a planet that became inhabited orremained uninhabited.

    One way to achieve such a systematic characterization oftrajectories is to bin environments into three types (Cockellet al., 2012; Fig. 1). On a Mars that was always uninhabited,

    UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK.

    ASTROBIOLOGYVolume 14, Number 2, 2014 Mary Ann Liebert, Inc.DOI: 10.1089/ast.2013.1106

    1

  • there are two potential types of environment: uninhabitableenvironments and uninhabited habitats. An uninhabitableenvironment is defined here to be an environment in whichno known organism can be active. An uninhabited habitatis defined here to be an environment capable of supportingthe activity of at least one known organism but containingno such organism.

    Uninhabited habitats are rare on Earth (Cockell, 2011). Anatural example would be a newly formed lava flow tran-siently uncolonized by metabolically active cells. An arti-ficial example would be a Petri dish containing uninoculatednutrient agar. Indeed, all microbiological media are recipesfor making uninhabited habitats. Cockell et al. (2012) pre-dicted that, if habitable conditions exist on Mars, uninhab-ited habitats could exist, and they could be detected byshowing that all the requirements for habitability are met ina candidate environment but that there are no organics as-sociated with life. The report of a habitable environment inGale Crater, Mars (Grotzinger et al., 2014), but no definitivedetection of biologically associated organics (Ming et al.,2014), would be consistent with an uninhabited habitat, al-though destruction of organics over billions of years couldconfound determining whether the environment ever con-tained life.

    On an inhabited Mars, there is an additional environmenttype to consider: inhabited habitats. An inhabited habitat isdefined here to be an environment containing life whoseactivity can be supported by that environment.

    A categorization of the possible trajectories of martianhabitability through time can therefore be addressed by sepa-

    rating out trajectories based on the existence of combinationsof these three environments and then considering, within eachclass of trajectory, the relative abundance and characteristics ofthese environments at different scales (Fig. 2).

    In this synthesis, six major potential trajectories of hab-itability on Mars are identified. Experimental requirementsand their associated hypotheses are discussed that can beused to determine which of these trajectories Mars hastaken.

    Requirements for Microbial Activity on Mars,Past and Present

    To identify trajectories for habitability, three require-ments for habitability (liquid water, nutrients, and energy)and four factors that can influence the distribution of hab-itable conditions on Marsradiation, pH, the presence ofbrines, and porosityare taken into consideration. This al-lows for (1) the identification of trends, particularly withrespect to changing liquid water availability over time,which allows for the selection of the habitability trajectoriesdiscussed in the following section, and (2) the identificationof physical and chemical factors that might have influencedthe changing abundance of uninhabited and inhabited hab-itats and uninhabitable environments on Mars globally andlocally over time.

    Liquid water through time

    Liquid water is the essential solvent for life, and thehistory of its abundance on Mars drives the habitability

    FIG. 1. Environments on Mars. Types of environments and habitats that may exist and have existed on Mars in twodifferent scenarios of Mars: one in which it has always been uninhabited and one in which it is and/or was inhabited. Thearrows and corresponding text show how locations, even at microscopic scales, could transition between these states asenvironmental conditions alter (adapted from Cockell et al., 2012).

    2 COCKELL

  • trajectories suggested in this synthesis. Without liquid wa-ter, the presence of other requirements for habitability isirrelevant.

    Past liquid water

    The presence of ancient liquid water on Mars is supportedby the observations of phyllosilicates associated with an-cient crustal units exposed by craters and erosion (Pouletet al., 2005; Bibring et al., 2006; Ehlmann et al., 2013). Forexample, the Nili Fossae region of Mars (Fig. 3) exhibits anextraordinary range of minerals and is composed of distinctunits. Information on this region has been obtained by theMars Reconnaissance Orbiter (MRO), Compact Re-connaissance Spectrometer for Mars (CRISM), and the HighResolution Science Experiment (HiRISE) instruments. Oneunit in the Nili Fossae is a brecciated Fe/Mg-smectite-bearing unit that contains meter- to kilometer-sized blocksof altered and unaltered rock and is inferred to represent theancient crust of Mars, which was torn up in subsequentimpact events (Ehlmann et al., 2009). Fe/Mg-smectite claysare the most abundant clays on Mars, followed by chlorites.A second, olivine-rich unit with evidence for aqueous al-teration may be impact melt or early stage lava from theSyrtis Major (Wray and Ehlmann, 2011). The presence ofprehnite suggests low-temperature (200350C) alteration,and the wide variety of hydration products, including kao-linite, chlorite, mica, opal, zeolites, sulfates, serpentine,smectites, and carbonates, suggests complex water-rock in-teractions (Ehlmann et al., 2009, 2010, 2011; Wray andEhlmann, 2011).

    Asteroid and comet impact craters, which can be regardedas n