The Coevolution of Life and Environment on Mars: An Ecosystem Perspective on the Robotic Exploration of Biosignatures

Abstract

Earth's biological and environmental evolution are intertwined and inseparable. This coevolution has become a fundamental concept in astrobiology and is key to the search for life beyond our planet. In the case of Mars, whether a coevolution took place is unknown, but analyzing the factors at play shows the uniqueness of each planetary experiment regardless of similarities. Early Earth and early Mars shared traits. However, biological processes on Mars, if any, would have had to proceed within the distinctive context of an irreversible atmospheric collapse, greater climate variability, and specific planetary characteristics. In that, Mars is an important test bed for comparing the effects of a unique set of spatiotemporal changes on an Earth-like, yet different, planet. Many questions remain unanswered about Mars' early environment. Nevertheless, existing data sets provide a foundation for an intellectual framework where notional coevolution models can be explored. In this framework, the focus is shifted from planetary-scale habitability to the prospect of habitats, microbial ecotones, pathways to biological dispersal, biomass repositories, and their meaning for exploration. Critically, as we search for biosignatures, this focus demonstrates the importance of starting to think of early Mars as a biosphere and vigorously integrating an ecosystem approach to landing site selection and exploration. Key Words: Astrobiology-Biosignatures-Coevolution of Earth and life-Mars. Astrobiology 18, 1-27.

FIG. 1.

Framework for a coevolution on Mars—uncertainties about the origin(s) of life on Earth and the early martian environment, and the use of Earth's chronology for early life in the context of Mars, limit any notional coevolution model. Here, the variables are the plausible environments of origin(s) for life on Mars in view of terrestrial theories and what is known of the evolution of habitability on Mars. The fix parameter is life as we know it, with the chronology provided by discoveries made on Earth to date. We assume a timing for the earliest evidence of life consistent with Earth at 4.28 Ga (references in the text) and focus on early life colonization and dispersal pathways from these environments of origin.

FIG. 2.

Polyextreme environmental evolution of Mars. Mars became polyextreme very early. Its capability to preserve subaerial habitats, ecotones, and microbial dispersal pathways during the transitional period between the Noachian and Amazonian would have depended on fluctuating interactions between multiple environmental extremes and their relative dominance at any given time, which in part depended on time and obliquity. (A) Evolution of the abundance and diversity of microbial habitats, ecotones, and dispersal pathways over time as a function of increase in polyextremes (number and intensity). The impact is most severe for those at the surface (solid black line); it would have been less severe for those in association with the subsurface (dashed black line) and least severe for those of the deep underground (dotted line), where survivability would have depended primarily on the evolution of geothermal energy and deep water circulation. The straight lines show temporal trends, while evolution would have varied with obliquities. (B) Interactions between extreme environmental parameters. White arrows = promote; black arrows = prevent; left-right black arrows = counter; left-right gray arrow = combine; dotted thin arrows show loop cycles.

FIG. 3.

Primary dispersal pathways and biomass repositories—the red arrows represent potential primary biological dispersal pathways; blue arrows are hypothetical shallow dispersal paths along costal marine currents; boxes indicate primary deep biomass repositories. The suggested surface areas are representative only. They include the following: (1) The south polar–Argyre–Chryse trough drainage system (Clifford and Parker, 2001), where Argyre collects polar basalt meltwaters. Surface discharges occurred through the Chryse trough (Parker et al., ; Fairén et al., 2016) into west Valles Marineris and an early northern ocean. Surface flow is questioned by Hiesinger and Head (2002), who favored subsurface drainage of a lake. Evidence supports an early habitable environment (e.g., Moore and Wilhelms, ; Hiesinger and Head, ; Fairén et al., ; Williams et al., 2017). (2) Hellas presents similar habitability potential as Argyre (e.g., Gulick, ; Schulze-Makuch et al., , and references herein; Wilson et al., 2010). Hellas is a closed basin and the deepest point on Mars at −7152 m. Its seasonal atmospheric pressure ∼89% higher than the surface (Grassi et al., 2007) allows transient surface liquid water episodes and glacial processes (e.g., Haberle et al., 2001) and the release of deep materials to the surface. (3) Sirrenum and Memnonia Fossae are structural troughs connected to Tharsis. They mark the origin of large lakes and channels, including in the Mangala and Ma'adim Vallis regions. The latter is additionally linked to volcanic/hydrothermal systems from impact cratering (e.g., Gusev Crater) and volcanic activity (e.g., Apollinaris Patera). The exploration of Gusev confirmed a Noachian habitable environment, with geomorphic and mineralogical evidence presented as possible bioconstruct analogues (Ruff and Farmer, 2016). (4–5) Sustained volcanic and hydrothermal activity with cyclic accumulation of volatiles in equatorial aquifers makes the Tharsis/Valles Marineris and Elysium regions high-priority areas for deep biomass repositories. (6) Arabia Terra is an outstanding candidate repository. Topographically, it has been a surface, subsurface, and deep underground collection area over the entire history of Mars. Late Amazonian volcanism (Broz et al., 2017) shows modern magmatic processes and a potential for hydrothermal circulation that drains from Valles Marineris toward Arabia. The region is characterized by higher epithermal neutron count (e.g., Feldman et al., 2002) and methane plumes (e.g., Mumma et al., ; Oehler and Etiope, 2017). (7) The northern plains repository could theoretically be composed of biomass released from the highland through catastrophic releases of equatorial aquifers and from oceanic habitats. Other biomass repositories and deep dispersal pathways might include the equatorial belt at depth, where the highlands/lowlands flow circulation concentrated underground over billions of years; the planet's overall deep interior (≥500 m)—with water at depth, combined lithostatic pressure and geothermal gradient have maintained conditions to develop possible deep habitats over time since accretion. Microbial organisms migrating from subseafloor and/or volcanic aquifers could have colonized deep aquifers, caves (e.g., from chemical dissolution, ancient magma chambers, lava tubes, and underground rivers), cavities, mineral surfaces, and pore spaces (see Table 2). Mars analog studies have abundantly demonstrated the suitability of these environments for a broad range of microbial communities. Credit: The basemap was prepared by Daniel Macháček.