Table 1.

Mars Early Evolution—Earth-Like, But… (Almost) All Things Being Unequal

	Time (Ga)	Events/Models and implications
Atmosphere	4–5/4.2	Sun T-Tauri stage. No thick primary CO2 atmosphere1.
	4.3	Catastrophic outgassing of volatiles2–4.
	4.2–3.8	Secondary CO2 atmosphere from volcanic activity5 and heavy bombardment6.
	4.2–3.8	Decrease in solar activity and soft X-ray flux1.
	4.1–Present	Loss of atmosphere from sputtering by solar wind and radiation7–8.
Magnetosphere	>4.1	Loss of magnetosphere9. Lack of magnetization in Hellas. Termination of magnetic field before the end of the valley network formation10.
		Differences in magnetic properties between both hemispheres due to the formation of the global dichotomy (see below), or an original single-sphere dynamo11 from nonuniform temperature across the core-mantle boundary12.
Global dichotomy	4.3?	Its formation affects geography, climate, hydrology, geology13, and dispersal pathways for life. Externally driven models: A megaimpact or multiple impacts14–18 or a southern polar giant impact by a bolide 0.1–1.0 lunar mass19. The resulting magma ocean solidifies to form the thicker crust of the southern hemisphere20.
		Internally driven models: Removal of the basal lowland crust by mantle convection21–23 or plate tectonics24.
		The global dichotomy forms during accretion or right after25, making it the most ancient geological feature of Mars26–27.
Impact basins	4.1–3.9	Most large impact basins form during the Noachian (Hellas, Argyre, Isidis, Utopia, other)27–29.
Structure Volcanism	3.8–3.5	Valles Marineris develops and is subsequently modified by erosion and sedimentation into the Amazonian30–31. Volcanic activity into recent geological past32.
	3.8	The rise of Tharsis generates rifting in Valles Marineris and Noctis Labyrinthus.
	3.4–3.1	Elysium develops33–34, with activity into the Amazonian35.
Hydrology	4.3–3.8	Hydrological provinces and flow directions structurally defined early with the formation of the global dichotomy and the heavy bombardment36.
	4.0–3.7	Peak of valley networks 27,37–39 and lake formation40–42. Localized activity in the Amazonian. Networks are not fully integrated with the landscape, suggesting only short favorable conditions for their development39–47.
	3.7>	Outflow channels. Peak formation in the Hesperian with residual activity in the Amazonian27.
	4.1> (?)	Ocean in the first billion years of history from subpermafrost aquifer modeling48 and from proposed geomorphic evidence49–51. Disputed model52.
	3.7>	Oceanus Borealis (Hesperian ocean)53–56. Cons: Difficulty to reconcile the volumes required to form the outflow channels with known volatile sources and sinks27, lack of fine-grained sediments57, evaporites58, carbonates59–60, no evidence for Noachian glaciation61, and already thin Noachian atmosphere62–63. Pros: Lack of carbonates explained by destruction by precipitated H2SO464, a globally acidic atmosphere65, or the short lifetime of ocean episodes54.
		Other potential evidence: The global distribution of Hesperian deltas66, possibly tsunami deposits67–69, and shorelines49–51,70–73. The existence of shorelines has been debated since originally proposed53,74. The water global equivalent layer (GEL) has been used as a pro or con argument depending on models: D/H enrichment factors argue for a large Pre-Noachian water loss (GEL ≥137 m)75. Ion-microprobe analysis implies a Pre-Noachian water loss greater than during the rest of the martian history (GEL >41–99 m vs. GEL >10–53 m) and an undetected subsurface water/ice reservoir (∼100–1000 m GEL) that exceeds the current observable inventory of ∼20–30 m GEL)76–77. A recent global reevaluation of eroded volumes of valley networks results in an even larger GEL of ∼5 km78. Other potential evidence includes polygonal ground in the lowlands79 and subdued impact craters, whose anomalous depth/diameter ratio could be associated with a Late Hesperian ocean episode.53,80–83
Climate and environmental change	4.1>	Episodic periods of warmer and thicker atmosphere after the loss of the magnetosphere contributed by orbital forcing84–85 and volcanism. Sulfur86 and sulfates58,87–88 suggest the release of greenhouse gases89–92. Other warming mechanisms (≤10 million years) include carbonate-silicate cycles93. Impact cratering provided transient environmental/atmospheric changes94.
		Excursions of warmer periods on a cold early Mars are supported by the presence of phyllosilicates95–99 and increasingly more arid conditions from the Hesperian on, with an abundance of sulfates and evaporites58. Significant variations in obliquity, eccentricity, and precession84 have also led to a redistribution of water in the polar ice deposits to lower latitudes to create ice ages regularly during martian history100.

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