Noachian

Noachian is the earliest formal geological period and chronostratigraphic system of the planet Mars, spanning approximately 4.1 to 3.7 billion years ago (Ga) (preceded by the Pre-Noachian epoch from 4.5 to ~4.1 Ga and succeeded by the Hesperian and Amazonian periods). Formally defined in 1978–1986 by the United States Geological Survey planetary geologists Donald U. Wise, Kenneth L. Tanaka, and David H. Scott using Viking orbiter imagery, the system is named after its type region, Noachis Terra ("Land of Noah"), an expanse of heavily cratered ancient highlands west of the Hellas impact basin in the Martian southern hemisphere. In planetary science and astrobiology, the Noachian Period represents the prime habitable window of Martian history: a time when a thicker early atmosphere, magnetic dynamo remnants, and volcanic outgassing supported sustained or episodic surface liquid water, carving thousands of branching dendritic valley networks, filling open-basin crater lakes (such as Jezero Crater and Gale Crater), and weathering basaltic crust into neutral-to-alkaline phyllosilicate (smectite clay) and carbonate minerals prior to the Hesperian transition to acidic sulfate brines.

Because radiometric ages from returned Martian bedrock samples are not yet completed across the global stratigraphic column (pending planetary protection-certified Mars Sample Return), planetary geologists subdivide the Noachian System into Early, Middle, and Late Noachian epochs using the cumulative spatial density of impact craters N(D)—defined as the number of impact craters with diameter ≥ D (for D = 1, 2, 5, and 16 km) per 10⁶ km² of surface area—calibrated against the Lunar Apollo radiometric cratering chronology (Hartmann and Neukum chronology systems):

Three planetary-scale geophysical events shaped the lithosphere and volatile budget of Noachian Mars:

1. The Hellas, Isidis, and Argyre Impact Basins: The base of the Early Noachian Epoch is stratigraphically anchored by the formation of the Hellas Planitia multiring basin (~4.1–3.98 Ga), a 2,300-kilometer-wide impact structure excavated to a depth of 9 kilometers below the surrounding highland rim—the deepest topographic depression on Mars. Ejecta blankets from Hellas, Isidis (~3.96 Ga), and Argyre (~3.92 Ga) buried older Pre-Noachian terrains under kilometers of impact breccia (megaregolith) with high fracture porosity (20–30%) capable of storing deep crustal aquifers.
2. Shutdown of the Martian Core Dynamo (~4.1–4.0 Ga): Magnetometer mapping by Mars Global Surveyor discovered intense crustal remanent magnetic anomalies (up to 1,500 nT at 100 km altitude—ten times stronger than terrestrial crustal anomalies) preserved across the ancient Noachian southern highlands (Terra Sirenum and Terra Cimmeria). Crucially, the interior floors of the Hellas, Argyre, and Isidis basins are completely demagnetized, proving that Mars possessed a vigorous liquid-iron core convective dynamo prior to ~4.1 Ga that shut down before or during the Hellas–Argyre impact sequence, leaving the upper atmosphere exposed to solar-wind ion pick-up and photochemical escape.
3. Emplacement of the Tharsis Magmatic Province: Over the course of the Noachian Period, plume-driven volcanism emplaced the Tharsis bulge—a 5,000-kilometer-wide igneous plateau containing ~3 × 10⁸ km³ of basaltic magma (~10²² kg, or ~1.5% of the mass of Mars). As demonstrated by Roger J. Phillips and collaborators (2001), the lithospheric load of Tharsis induced true polar wander, fractured the Western Hemisphere into radial grabens, established the global hydraulic head that directed Noachian valley networks toward Chryse and Margaritifer Terra, and degassed an estimated 1.5 bar of CO₂ and a 120-meter global equivalent layer (GEL) of H₂O into the Noachian atmosphere.

Whereas Hesperian outflow channels (such as Kasei Valles and Ares Vallis) were carved rapidly over weeks by subsurface aquifer bursts without tributaries, Late Noachian valley networks (such as Warrego Valles, Parana Valles, and Nanedi Vallis) display classic Strahler stream-order dendritic branching (orders 4 to 7), drainage divides, and meandering inner channels fed by distributed precipitation (rain or snowmelt) and surface runoff.

Connected to these valley networks are more than 210 open-basin paleolakes—impact craters equipped with both an incised inlet valley (often depositing a stepped Gilbert-type or bird's-foot fluvial fan-delta) and a breached outlet canyon proving that water rose to the spillover elevation of the crater rim:

• Jezero Crater (Perseverance Mars 2020 Landing Site, 18.38°N, 77.58°E): A 45-km-diameter Noachian crater on the western margin of Isidis Planitia where the western inlet channel (Neretva Vallis) built a prominent clay- and carbonate-bearing fan-delta into a 250-meter-deep lake before breaching the eastern rim via Pliva Vallis.
• Gale Crater (Curiosity MSL Landing Site) & Eberswalde Crater: Preserving hundreds of meters of finely laminated lacustrine mudstones (Sheepbed and Murray formations) recording millions of years of neutral-pH lake sedimentation across the Late Noachian–Early Hesperian transition.

Reconciling this abundant geomorphological evidence of liquid water with stellar astrophysics poses the Noachian Faint Young Sun Paradox: at 3.8–4.0 Ga, standard solar evolution models dictate that the young Sun's bolometric luminosity was only ~75% of its present value, which at Mars's semi-major axis (1.524 AU) yields an effective blackbody temperature of ~197 K (−76 °C). Because a pure 1-to-2-bar CO₂–H₂O atmosphere undergoes Rayleigh scattering and CO₂ ice cloud condensation before reaching 273 K, climate modelers debate two end-member Noachian regimes:

1. Warm and Semi-Arid Greenhouse (CO₂ + H₂ / CH₄ Collision-Induced Absorption): Volcanic outgassing and serpentinization of ultramafic crust released 1% to 5% molecular hydrogen (H₂) and methane (CH₄) into a 1–2 bar CO₂ atmosphere; CO₂–H₂ and CO₂–CH₄ collision-induced absorption (CIA) closed the infrared spectral window at 250–500 cm⁻¹, maintaining above-freezing mean surface temperatures for 10⁶–10⁷ years (Wordsworth et al., 2017).
2. Cold and Icy Highlands with Episodic Melt ("Late Noachian Icy Highlands"): Under a 0.5–1 bar atmosphere, snow and ice accumulated preferentially at high elevations (> +1 km) on the southern highlands (adiabatic cooling trap), and episodic volcanic SO₂/H₂S greenhouse pulses, orbital obliquity cycles, or impact thermal plumes triggered centuries-long summer snowmelt episodes that incised the valley networks.

Near-infrared hyperspectral mapping by Jean-Pierre Bibring (Mars Express OMEGA) and Bethany L. Ehlmann (Mars Reconnaissance Orbiter CRISM) established a three-era mineralogical evolution of Mars in which the Noachian corresponds to the Phyllosian (Phyllosilicate) Era:

• Neutral-to-Alkaline Clay Weathering (pH 6.5–8.5): Across more than 4,000 Noachian outcrops, primary olivine and pyroxene basalts were aqueously altered at high water-to-rock ratios into Fe/Mg-smectite clays (nontronite and saponite), chlorite, serpentine, and illite/muscovite, frequently capped by leached Al-rich kaolinite/montmorillonite horizons characteristic of top-down pedogenic (soil) weathering profiles.
• Nili Fossae and Jezero Marginal Carbonates: In the Noachian Nili Fossae region and along the inner rim of Jezero Crater, olivine-rich units weather directly into magnesium carbonate (magnesite/hydromagnesite, MgCO₃), demonstrating that Noachian surface waters were not strongly acidic (since low pH dissolves carbonates and blocks smectite precipitation in favor of jarosite).
• Biosignature Preservation Potential: Because expandable 2:1 smectite clays possess enormous internal interlayer surface areas (~600–800 m² g⁻¹) and negatively charged aluminosilicate sheets that bind and shield polar organic molecules against ionizing radiation and oxidation, Noachian phyllosilicate mudstones and lacustrine carbonates represent the highest-priority astrobiological targets in the inner Solar System for preserving prebiotic chemistry or ancient Martian microfossils.

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