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., 1993; 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, 2001; Hiesinger and Head, 2002; Fairén et al., 2016; Williams et al., 2017). (2) Hellas presents similar habitability potential as Argyre (e.g., Gulick, 1998; Schulze-Makuch et al., 2007, 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., 2009; 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.