Table 2.
Comparison of Sedimentology, Environmental Conditions, Habitability, and Potential Biosignature Preservation in the 3.5–3.33 Billion-Year-Old Kitty's Gap and Josefsdal Chert Formations and the Sediments in Gale Crater, Yellowknife Bay Formation
| Early Earth: Kitty's Gap, Josefsdal Cherts (3.5–3.33 Ga) | Mars, Yellowknife Bay Formation, Gale Crater (∼3.6 Ga) | |
|---|---|---|
| Depositional environment | Mostly subaqueous, shore face to tidal | Lacustrine to fluvial; eolian |
| Sediment composition | Basaltic (ultramafic, mafic) some more evolved sediments [rhyolitic; primary hydrothermal/evaporitic silica; evaporite minerals (gypsum, aragonite, high-Mg calcite, halite)] | Basaltic sediments (pyroxene, olivine, plagioclase); some more evolved volcanics; amorphous phases |
| Secondary minerals, alteration | Volcanics altered to phyllosilicates and anatase; hydrothermal secondary minerals—silica, siderite, pyrite | Volcanic clasts altered to phyllosilicates (Fe/Mg smectites), magnetite, secondary minerals—Ca, Mg, Fe sulfates … |
| Cement | Silica | Phyllosilicates, amorphous phases? |
| Habitable environment | pH acidic–neutral | pH neutral (possibly partly acidic and partly alkaline) |
| high salinity ∼6% | dilute brines | |
| Anaerobic with micro amounts of abiotic O2 | Anaerobic (with hypothesized micro amounts of abiotic O2) | |
| Moderate-high temperatures >50°C? | Probably low temperatures | |
| Energy source | Organics of abiotic origin (hydrothermal or extraterrestrial broken down and hydrolyzed in water) and biogenic origin; redox reactions at the surfaces of reactive minerals, e.g., olivine, hydrothermal sulfide; H2 (serpentinization) from hydrothermal vents; photons | Organics of abiotic origin (hydrothermal or extraterrestrial broken down and hydrolyzed in water) and possibly biogenic origin, if there was life; redox reactions at the surfaces of reactive minerals, e.g., olivine, hydrothermal sulfide; H2 (serpentinization) from hydrothermal vents if they were active (no sign yet, except possibly CH4); photons |
| Organics | Biological, extraterrestrial (see above), hydrothermal | Certainly extraterrestrial (see above), possibly biological, if life present, possibly hydrothermal, if vents present |
| Life | Life already well established by 3.5–3.33 Ga | Not yet detected in Gale Crater (conditions for the emergence of life here not present), but the environment is potentially habitable if viable cells could have been transported there |
| Potential life-forms | Anaerobic chemolithotrophs, chemoorganotrophs, phototrophs | Anaerobic chemolithotrophs, chemoorganotrophs; little possibility of phototrophs (lack of evolutionary possibilities) |
| Potential distribution | Ubiquitous, on surfaces of volcanic detrital particles; in hydrothermal fluids (including silica gel); on sediment bedding-plane surfaces (phototrophs) including hydrothermal silica gel | Primarily on surfaces of volcanic detrital particles; possibly in hydrothermal fluids if they existed; possibly on sediment bedding-plane surfaces if phototrophs existed |
| Preservation | Rapid silicification | Entombment by cementing phyllosilicates and/or amorphous phases? |
| Biosignatures | Morphological; organic; metabolic | Morphological only if cementation was rapid or if phototrophs developed (MISS, cf. Noffke, 2015); organic (degraded remnants of organisms); possibly metabolic fractionation of carbon |