Table 2.

Environmental Elements of Microbial Ecosystems on Mars Through Time

Habitable environments	Habitats	Microbial ecotones	Dispersal pathways
Ocean1	Water surface	Shallow ocean margins/land transition	Waves, wind, current, gravity, connectivity networks with estuaries, deltas, streams, and glaciers through subglacial flows
	Water column	Transition zones of temperature, light, pressure, minerals, salts, chemistry, gases	Currents, gravity, fluid fluxes
	Deep sea vents	Temperature, chemistry, gases, minerals	Currents, plumes and fluid fluxes, particulates
	Subseafloor aquifer	Crust/sediment exchange	Fluid circulation, pore pressure and connectivity
Volcanic aquifer1	Pools, springs, spring mounds, geysers, mudpots	Surface water/groundwater interface, chemistry, mineralogy, temperature, biofilms	Current, gravity, infiltration, pressure, plumes and fluid fluxes, wind
	Pore spaces (e.g., catenary, cul-de-sac), grain texture, and mineralogy	Water interface: unsaturated/saturated zone	Recharge, underflow, discharge, current, gravity, pore geometry, fluid pressure
	Deep rock structure, texture, temperature	Rock/confining bed (clays)/clastic sediment interface	Pore geometry, pressure, connectivity, fluid pressure
Stream1	Water column	Surface water/groundwater interface, water chemistry, temperature, nutrient concentration; deltas, areas where water changes speed	Hydrologic connectivity and water intermittency in ephemeral and perennial streams, current, gravity
	Pore spaces (e.g., catenary, cul-de-sac), grain texture, and mineralogy	Water interface: unsaturated/saturated zone	Pore pressure and connectivity, fluid circulation
	Interstitial pore water in sediment and aquifer	Surface water/groundwater interface, chemistry, mineralogy, temperature	Pore connectivity, fluid circulation
	Subglacial	(See ice)	(See ice)
Lake1	Water surface	Shore/river/land transition, surface water/groundwater interface	Wave, wind, current, gravity, connectivity networks with deltas, streams, and glaciers through subglacial flows
	Water column	Temperature, light, pressure, minerals, salts, chemistry, gases	Currents, gravity, evaporation, wind
	Lake bed sediment	Sediment texture, temperature, composition, geochemistry, water chemistry, gases	Currents, gravity, evaporation, desiccation, wind
	Hydrothermal input (impact crater lake, volcanic lake)	(See volcanic aquifer)	(See volcanic aquifer)
	Ice (ice-covered lake)	(See ice)	(See ice)
Ice2–4	Crystals in glacier and snow	Pressure, temperature, gases, composition	Drainage from melt, infiltration, wind
	Spring mound, sediment texture	Temperature, composition, rheology	Stream, current, gravity, infiltration, wind
	Debris-rich basal ice	Solid ice crystal/water-filled veins interface, ice/land transition	Ice movement (in glaciers), meltwater, current, gravity, infiltration, wind
Permafrost5–6	Ice, regolith, sediment, pore spaces, water lenses	Temperature gradient, ice content, multiphase state of water, mineralogy	Melting, thawing, stream, gravity, active layers
Rock7–8	Cracks, pores, vesicles, weathered surfaces, mineral type and optical properties	Mineral, albedo, composition change, face exposure, rock/soil and other transitions (e.g., endolithic, chasmolithic cryptoendolithic)	Pore pressure, fluid circulation, wind, precipitation, thermokarst
Soil/Sediment9	Intergranular spaces, pores, grain texture	Soil/sediment transition, gas composition and content, land/water/ice transitions, geochemical fluxes	Fluid circulation, wind, precipitation, saltation
Salt/Evaporite10	Saline soils, hypolithic and endolithic habitats, hygroscopic minerals	Soil horizons, transitions in salt density, water content gradients, salt type, rock/sediment texture, composition, transparency	Deliquescence solutions, fluid circulations, surface tension in rock matrix, capillary pressure, porosity, permeability, wind
Cavity/Cave1	All habitats above	All ecotones above	All dispersal pathways above

¹See text, references therein, and Table 1; ²Boetius et al., 2014; ³Montross et al., 2013; ⁴Lutz et al., 2017; ⁵Jansson and Taş, 2014; ⁶Steven et al., 2006; ⁷Walker and Pace, 2007; ⁸Blackhurst et al., 2005; ⁹Groffman and Bohlen, 1999; ¹⁰Davila and Schulze-Makuch, 2016.