Pre-Noachian

The Pre-Noachian is the earliest informal geological eon of the planet Mars, spanning approximately 400 million years from initial planetary accretion and core differentiation at ~4.53 billion years ago (Ga) to the formation of the Hellas multiring impact basin at ~4.10 Ga (which stratigraphically defines the base of the formal Noachian System, followed by the Hesperian and Amazonian periods). Formally synthesized in Martian chronostratigraphy by Herbert Frey (2003–2006), Kenneth L. Tanaka, and Michael H. Carr and James W. Head (2010), the Pre-Noachian is the Martian temporal and geodynamic counterpart of Earth's Hadean Eon (4.54–4.03 Ga) and the Lunar Pre-Nectarian Period (4.51–3.92 Ga).

Although virtually all surface geomorphological landforms constructed during the Pre-Noachian were obliterated, buried, or thermally reset by subsequent giant impact basins and Early Noachian resurfacing, the Pre-Noachian established the permanent first-order geophysical architecture of Mars and its subsequent potential for planetary habitability and astrobiology. During these first 400 million years, Mars accreted rapidly as a stranded planetary embryo, crystallized a global silicate magma ocean within ~20 Myr, formed its planet-scale hemispheric crustal dichotomy (the 5-kilometer elevation step separating the northern Borealis lowlands from the southern highlands), generated a vigorous liquid-iron core magnetic dynamo that magnetized the ancient southern crust in Terra Sirenum and Terra Cimmeria, and outgassed a primordial steam–CO₂–H₂ atmosphere that underwent extreme ultraviolet (XUV)-driven hydrodynamic escape before condensing the earliest water on Mars.

Hf–W Isotopic Chronometry: Mars as a Stranded Planetary Embryo

High-precision radiometric measurements of the short-lived hafnium–tungsten (¹⁸²Hf → ¹⁸²W, half-life t_1/2 = 8.9 Myr) and samarium–neodymium (¹⁴⁶Sm → ¹⁴²Nd, t_1/2 = 68–103 Myr) decay systems in Martian meteorites (the SNC clan—Shergottites, Nakhlites, Chassignites—together with the ancient orthopyroxenite ALH 84001 and the regolith breccia Northwest Africa [NWA] 7034 / 7533, "Black Beauty") demonstrate that Mars accreted far more rapidly than Earth:

• Because lithophile hafnium (Hf) remains in the silicate mantle while siderophile tungsten (W) partitions into the metallic iron–nickel–sulfur core during core–mantle differentiation, the elevated radiogenic tungsten anomaly (ε¹⁸²W ≈ +0.3 to +3.0) preserved in Martian mantle reservoirs proves that Mars reached half of its final mass within 1.8 ± 0.8 Myr after the condensation of Calcium–Aluminum-rich Inclusions (CAIs at 4,567.3 Ma) and completed >90% of its accretion and core segregation within 2 to 4 Myr (Dauphas & Pourmand, 2011).
• Unlike Earth—which grew over 30 to 100 Myr through stochastic collisions of Mars-sized embryos culminating in the Moon-forming giant impact—Mars escaped runaway late-stage embryo mergers and survived as a primordial oligarchic planetary embryo, accounting for its small mass (M_Mars = 6.417 × 10²³ kg = 0.107 M_⊕) and low surface gravity (g = 3.721 m s⁻²).

Core Differentiation and Primordial Crustal Extraction (~4.54–4.47 Ga)

Energetic release from accretionary impacts, core segregation of an iron–nickel–sulfur–oxygen liquid core (R_core ≈ 1,830 ± 40 km as measured by NASA's InSight lander seismic receiver functions), and decay of short-lived radionuclides (²⁶Al and ⁶⁰Fe) melted the outer silicate mantle of newborn Mars into a global magma ocean extending to depths of 1,000 to 1,500 km.

As analyzed by Linda T. Elkins-Tanton et al. (2003–2005) and Bouvier et al. (2018), this Martian magma ocean solidified from the bottom upward within 15 to 25 Myr:

1. Cumulate Mantle Overturn: Early crystallization precipitated magnesium-rich olivine and orthopyroxene near the base of the magma ocean, followed by denser iron-rich clinopyroxene and garnet-majorite cumulates near the top. This gravitationally unstable density stratification triggered a rapid solid-state Rayleigh-Taylor cumulate overturn that transported dense, heat-producing incompatible elements (U, Th, K) and iron-rich phases affecting mantle serpentinization and oxidation state while leaving behind a stably stratified, chemically heterogeneous mantle that prevented Earth-style global plate tectonics.
2. 4.48–4.43 Ga Zircons in NWA 7034/7533 ("Black Beauty"): Uranium–lead (U–Pb) and Lu–Hf isotopic dating of detrital zircon and baddeleyite grains inside the Martian regolith breccia NWA 7533/7034 (Humayun et al., 2013; Bouvier et al., 2018) yields crystallization ages of 4,476 to 4,428 Ma with unradiogenic chondritic hafnium isotopes, proving that Mars extracted a primordial 40- to 60-kilometer-thick evolved basaltic-to-andesitic primary crust within <90 Myr of Solar System formation.
3. 4.091 Ga Crystallization of ALH 84001: The coarse-grained orthopyroxenite meteorite Allan Hills 84001 crystallized as a deep plutonic cumulate in the Pre-Noachian southern highlands at 4,091 ± 30 Ma (Lapen et al., 2010), preserving carbonate rosettes precipitated from low-temperature aqueous fluids at ~3.9–4.0 Ga alongside paleomagnetic remanence from the Pre-Noachian dynamo.

Formation of the Martian Hemispheric Crustal Dichotomy (Borealis Impact)

The most prominent global structural feature of Mars is its hemispheric crustal dichotomy: the northern third of the planet (Vastitas Borealis) sits at an average elevation 4 to 6 kilometers lower and possesses a much thinner basaltic crust (average thickness ~32 km) than the heavily cratered southern highlands (average crustal thickness ~58 km), separated by a sinuous escarpment inclined ~25° to the equator.

While early endogenic models attributed the dichotomy to a degree-1 mantle convection plume or cumulate overturn, three-dimensional gravity and topographic inversions stripping away the younger Noachian–Hesperian Tharsis volcanic load by Jeffrey C. Andrews-Hanna, Maria T. Zuber, and W. Bruce Banerdt (2008) accompanied by hydrocode impact simulations by Margarita Marinova et al. (2008) and Francis Nimmo et al. (2008) demonstrated that the northern lowlands outline a single, continuous elliptical megabasin—the Borealis Basin—measuring 8,500 km by 10,600 km across (covering ~40% of the planet's surface, four times the diameter of the Hellas basin). This megabasin was excavated at ~4.45–4.50 Ga by the oblique collision (at 30°–60° incidence angle and 6–10 km s⁻¹ velocity) of a Pluto-sized planetary body (~1,600–2,700 km diameter, impact energy ~10²⁹–10³⁰ J), stripping the primordial crust from the northern hemisphere while depositing a thick circum-basin ejecta ring across the southern highlands.

Buried Pre-Noachian Impact Basins and Quasi-Circular Depressions (QCDs)

Using Mars Orbiter Laser Altimeter (MOLA) digital elevation models and Bouguer gravity anomalies, Herbert Frey (2003–2008) identified more than 20 giant buried impact basins exceeding 1,000 km in diameter—termed Quasi-Circular Depressions (QCDs)—that formed after the Borealis dichotomy but prior to the 4.10 Ga Hellas basin, including Utopia Planitia (~3,300 km diameter), Ares (~3,300 km), Chryse (~1,700 km), Acidalia (~2,000 km), and Solis (~1,400 km). The cumulative crater density N(1,000) of these buried Pre-Noachian megabasins is three to five times higher than that of the visible Early Noachian highland surface, confirming that intense impact bombardment and widespread thermal and volcanic resurfacing occurred between 4.4 and 4.1 Ga.

Although present-day Mars lacks an active global dipolar magnetic field, the Magnetometer and Electron Reflectometer (MAG/ER) aboard Mars Global Surveyor (Acuña et al., 1999; Connerney et al., 1999–2005) and MAVEN discovered intense crustal remanent magnetic anomalies reaching 1,500 nT at 100 km altitude (corresponding to crustal magnetizations of 10–30 A m⁻¹, an order of magnitude stronger than terrestrial oceanic crust) concentrated across the ancient Pre-Noachian/Early Noachian southern highlands of Terra Sirenum and Terra Cimmeria (150° E–240° E, 20° S–80° S).

Three lines of evidence constrain the operation and termination of this Pre-Noachian core magnetic dynamo:

1. Surface Paleointensity in ALH 84001: Quantum-diamond microscope and SQUID paleomagnetic measurements of the 4.09 Ga orthopyroxenite meteorite ALH 84001 (Weiss et al., 2002; Mittelholz et al., 2020) record a stable thermoremanent magnetization acquired in an ancient Martian surface magnetic field of ~20 to 60 μT—comparable in strength to Earth's modern geomagnetic field (30–60 μT).
2. East–West Magnetic Lineations: In Terra Sirenum and Terra Cimmeria, crustal magnetic anomalies form alternating positive and negative east–west-trending quasi-linear bands 150–200 km wide and up to 2,000 km long, recording either early lithospheric dike intrusion and crustal spreading or hydrothermal serpentinization producing magnetite (Fe₃O₄) and pyrrhotite (Fe_1−xS) during polarity reversals of the Pre-Noachian dynamo.
3. Thermal Demagnetization of Giant Impact Basins (~4.1–4.0 Ga): The interior floors of the Utopia, Hellas, Argyre, and Isidis giant impact basins—as well as the northern Borealis lowlands (except for isolated weak anomalies analyzed by InSight and MAVEN)—lack strong crustal magnetic anomalies. Because impact shock heating and melt-sheet cooling across a 2,000-km basin takes 10⁵ to 10⁶ years, the absence of thermoremanent magnetization inside Hellas, Utopia, and Argyre proves that the Martian core dynamo shut down near the Pre-Noachian / Early Noachian boundary at ~4.1 to 4.0 Ga (though a brief late reactivation near ~3.7–3.8 Ga has been debated from weak magnetization over Lucus Planum and InSight Homestead Hollow).

During magma ocean solidification at ~4.50 Ga, degassing of dissolved volatiles from the Martian interior released a dense primordial steam and carbon dioxide atmosphere containing an estimated 50 to 300 bars of H₂O and 10 to 50 bars of CO₂, CO, H₂, and CH₄. Once impact energy fluxes declined enough for the radiative skin temperature to drop below the critical point of water (647 K), this steam atmosphere condensed within ~1–10 Myr into a hot primordial hydrosphere (creating the earliest water on Mars).

However, during the first 100 to 400 Myr of the Pre-Noachian, the young pre-main-sequence and zero-age main-sequence Sun emitted an extreme-ultraviolet (EUV/XUV, λ < 91.2 nm) flux 20 to 100 times more intense than the modern solar EUV output. Combined with Mars's low escape velocity (v_esc = 5.03 km s⁻¹, meaning thermal Jeans and hydrodynamic escape require only one-quarter the energy per unit mass needed on Earth), solar XUV irradiation photodissociated upper-atmospheric H₂O and drove supersonic hydrodynamic blowoff of hydrogen into space, dragging along heavy noble gas isotopes (mass-fractionating primordial ³⁶Ar/³⁸Ar and non-radiogenic xenon ^124–136Xe) and stripping the bulk of the primordial atmosphere prior to 4.1 Ga.

For astrobiology, the Pre-Noachian represents a critical precursor epoch: although sterilizing basin-scale impacts (such as Borealis, Utopia, and Ares, which vaporized surface oceans and heated the upper tens of meters of crust above 400 K) periodically interrupted surface habitability, the deep fractured megaregolith (at 2 to 8 km depth) maintained warm hydrothermal circulation cells fueled by impact melt sheets and radiogenic heat, hosting widespread subsurface serpentinization (producing H₂ and CH₄) and Fe/Mg-smectite phyllosilicate alteration that seeded the Noachian biosphere window.

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2. Andrews-Hanna, J. C., Zuber, M. T., & Banerdt, W. B. (2008). "The Borealis basin and the origin of the martian crustal dichotomy." Nature, 453(7199), 1212–1215. https://doi.org/10.1038/nature07011
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4. Dauphas, N., & Pourmand, A. (2011). "Hf–W–Th evidence for rapid growth of Mars and its status as a planetary embryo." Nature, 473(7348), 489–492. https://doi.org/10.1038/nature10077
5. Bouvier, L. C., Costa, M. M., Connelly, J. N., et al. (2018). "Evidence for extremely rapid magma ocean crystallization and crust formation on Mars." Nature, 558(7711), 586–589. https://doi.org/10.1038/s41586-018-0222-z