Amazonian

The Amazonian is the youngest and longest-duration geologic system and chronostratigraphic period of the planet Mars, spanning roughly two-thirds of Martian history from the end of the Hesperian period at ~3.0 billion years ago (Ga) (ranging from 2.9 to 3.3 Ga depending on the lunar-to-Martian cratering chronology function) to the present day (0 Ga). Defined by Kenneth L. Tanaka (1986) from the smooth, sparsely cratered volcanic and sedimentary plains of Amazonis Planitia (centered at 24°N, 196°E in the northern lowlands west of the Tharsis Montes), the Amazonian marks the third and final epoch of the Martian environmental triad—following the phyllosilicate-forming, valley-network Noachian (~4.1–3.7 Ga) and the sulfate-depositing, outflow-channel Hesperian (~3.7–3.0 Ga).

In Jean-Pierre Bibring and colleagues' (2006) OMEGA mineralogical stratigraphy, the Amazonian corresponds to the "Siderikian" (anhydrous ferric oxide) era: a cold, hyperarid, tenuous-atmosphere regime in which global liquid-water weathering had largely ceased and slow, dry oxidative weathering by atmospheric photochemistry coated the Martian surface in nanophase ferric oxides (hematite, goethite, and ferrihydrite), giving Mars its modern rust-red color.

Planetary geologists subdivide the Amazonian into three epochs—Early Amazonian, Middle Amazonian, and Late Amazonian—calibrated by cumulative impact crater densities N(D) per 10⁶ km² mapped across reference volcanic plains by Michael H. Carr, Ronald Greeley, Kenneth Tanaka, and Gerhard Neukum / William K. Hartmann (Tanaka et al., 2014):

Although Mars's global internal mantle heat flow declined steadily across the Amazonian as its stagnant-lid lithosphere thickened to >150–300 km, decompressional mantle plumes remained active beneath two giant volcanic provinces—Tharsis and Elysium—producing the largest shield volcanoes and some of the youngest flood-lava plains in the Solar System:

1. Olympus Mons and the Tharsis Montes: Rising 21.9 kilometers above the Martian datum across a 600-km basal edifice encircled by an 8-km basal scarp, Olympus Mons erupted low-viscosity basaltic lava flows repeatedly throughout the Amazonian. High-resolution crater counts on small fans and caldera floors using Mars Reconnaissance Orbiter (MRO) CTX and HiRISE imagery by Gerhard Neukum and colleagues (2004) yielded model surface ages of 115 to 200 Ma inside the six nested summit caldera collapses and as young as 2 to 25 Ma on western flank lava flows and rootless cones.
2. Cerberus Fossae, Athabasca Valles, and Marte Vallis (Very Late Amazonian): Southeast of Elysium Mons, extensional volcanotectonic fissures (Cerberus Fossae) erupted both low-viscosity flood basalts—which covered >1,000,000 km² of western Elysium Planitia and Marte Vallis within the past 10 to 100 million years, exhibiting pristine platy-ridged pahoehoe-like surface textures—and catastrophic bursts of pressurized groundwater down Athabasca Valles and Grjótá Valles at ~2–20 Ma (Burr et al., 2002; Vaucher et al., 2009). Even more recently, NASA's InSight lander seismometer (SEIS, 2019–2022) detected a cluster of low-frequency tectonic and magmatic marsquakes originating directly beneath Cerberus Fossae, beneath a young dark pyroclastic mantle dated to just ~53,000 to 210,000 years ago (Horvath et al., 2021; Stähler et al., 2022)—proving that Martian magmatism has persisted into the geological present.
3. Resolution of the Martian Meteorite (Shergottite) Age Paradox: Over 80% of the >300 known Martian meteorites recovered on Earth are shergottites (basaltic and lherzolitic igneous rocks), whose Sm–Nd, Rb–Sr, Lu–Hf, and U–Pb crystallization ages cluster between 150 and 600 Ma (Late Amazonian), alongside the ~1.3 Ga (Middle Amazonian) nakhlites and chassignites (Nyquist et al., 2001). Because coherent, unfractured young Late Amazonian lava sheets transmit impact spallation shock waves far more efficiently than pulverized ancient Noachian regolith—and because young rayed craters such as Mojave Crater (58 km diameter, ~3–5 Ma) and Tooting Crater excavated young Tharsis/Elysium volcanic terrains—Martian meteorite deliveries to Earth selectively sample Amazonian volcanism.

Unlike Earth—whose axial tilt (obliquity ε ≈ 23.4° ± 1.3°) is stabilized against secular planetary resonances by the large gravitational torque of the Moon—Mars lacks a large satellite. Groundbreaking numerical integrations by Jacques Laskar and colleagues (2002, 2004) demonstrated that Mars's obliquity is chaotic over timescales >10 million years, wandering between 0° and >60° (with a long-term mean near ~38° during the Late Amazonian) and oscillating by 15° to 35° on quasi-periodic 120,000-year and 1.2-million-year cycles.

These dramatic obliquity swings act as the pacemaker of Amazonian climate and cryospheric redistribution (Head et al., 2003; Forget et al., 2006):

• High-Obliquity Glacial Epochs (ε > 35°–45°, Last Occurring ~0.4–2.1 Ma and 5–20 Ma): When Mars's axial tilt exceeds ~35°, summer solar insolation at the geographic poles increases more than twofold, sublimating massive volumes of water ice (H₂O) out of the North and South Polar Layered Deposits (PLD) and raising atmospheric water vapor pressure by 10- to 100-fold. Atmospheric general circulation models show that this moisture is transported equatorward and precipitates as snow and ice across the western flanks of the Tharsis Montes and Olympus Mons (where massive tropic-zone drop moraines cover >100,000 km²; Forget et al., 2006) and across the 30°–55° N/S mid-latitude belts, forming thick debris-covered piedmont glaciers, lobate debris aprons (LDAs), lineated valley fills (LVFs), and concentric crater fills (verified by MRO SHARAD radar sounding to consist of >80%–90% pure water ice up to 500–800 m thick beneath a thin 1–10 m lag of rocky sublimated till; Holt et al., 2008).
• Low- to Moderate-Obliquity Interglacials (ε ≈ 15°–25°, Including Present-Day ε = 25.19°): Beginning roughly 400,000 years ago, Mars entered a low-obliquity regime. Mid-latitude snow accumulation ceased; water ice became thermodynamically unstable in the shallow regolith equatorward of ~45°–50° latitude and sublimated back to the cold polar caps, leaving behind a desiccated, ice-cemented polygonally patterned latitude-dependent mantle (LDM) while building the young rhythmically bedded North Polar Layered Deposits (Planum Boreum, ~3 km thick, <4–5 Ma) and South Polar Layered Deposits (Planum Australe, ~10–100 Ma) capped by seasonal and perennial CO₂ dry-ice frost.

With liquid water unstable at the surface over most of Mars (where mean surface barometric pressure is ~6.1 hPa—right at the triple point of H₂O, 6.116 hPa at 273.16 K—and mean temperature is ~210 K / −63 °C), aeolian (wind) erosion and transport have served as the dominant daily geomorphic agents throughout the Amazonian:

• Over 3 billion years of saltating basaltic sand and thermal-fatigue fracturing carved streamlined yardangs into weak volcaniclastic deposits (such as the Medusae Fossae Formation and the upper Mount Sharp / Aeolis Mons sequence in Gale Crater), assembled the circumpolar sand sea of Olympia Undae, and generated seasonal regional and planet-encircling dust storms.
• Alcove-Channel-Apron Gullies and Recurring Slope Lineae (RSL): On steep pole-facing crater walls at 30°–60° latitude, thousands of young gullies incise Late Amazonian dunes and mantle deposits. Multi-year HiRISE monitoring demonstrates that present-day gully activity and dark seasonal slope streaks are driven primarily by the spring sublimation of seasonal translucent CO₂ dry-ice slabs (the Kieffer gas-jetting and dry granular flow mechanism; Dundas et al., 2019), possibly supplemented during higher-obliquity excursions (~0.5–2 Ma) by transient melting of snowpacks or deliquescent calcium- and magnesium-perchlorate (ClO₄⁻) brines.

From an astrobiology perspective, the Amazonian surface has remained hostile to unshielded terrestrial life due to extreme desiccation (water activity a_w ≪ 0.6), unattenuated solar ultraviolet (200–300 nm) and galactic cosmic-ray irradiation (~76 mGy yr⁻¹ measured by Curiosity RAD), and oxidizing soil perchlorates (~0.5–1 wt% ClO₄⁻ discovered by the Phoenix lander). However, two Amazonian habitability refugia remain active targets for life-detection missions:

1. The Deep Subsurface Geothermal Hydrosphere (3–10+ km Depth): Where radiogenic and magmatic heat raises crustal temperatures above the freezing point of saline groundwater, potentially sustaining chemolithoautotrophic subsurface ecosystems analogous to terrestrial deep-crustal hydrogenotrophic methanogens and sulfate reducers (Michalski et al., 2013).
2. Transient Magmatic-Hydrothermal Ice-Melt Systems and Buried Mid-Latitude Ice: Where young Amazonian dike intrusions (such as at Cerberus Fossae) or impact craters melted thick cryospheric ice sheets to create short-lived subsurface lakes, while buried mid-latitude glacier ice preserves an archive of ancient organic molecules shielded from cosmic-ray radiolysis.

1. Tanaka, K. L. (1986). "The Stratigraphy of Mars." Journal of Geophysical Research: Solid Earth, 91(B13), E139–E158. https://doi.org/10.1029/JB091iB13p0E139
2. Head, J. W., Mustard, J. F., Kreslavsky, M. A., Milliken, R. E., & Marchant, D. R. (2003). "Recent ice ages on Mars." Nature, 426(6968), 797–802. https://doi.org/10.1038/nature02114
3. Laskar, J., Correia, A. C. M., Gastineau, M., Joutel, F., Levrard, B., & Robutel, P. (2004). "Long term evolution and chaotic diffusion of the insolation quantities of Mars." Icarus, 170(2), 343–364. https://doi.org/10.1016/j.icarus.2004.04.005
4. Neukum, G., Jaumann, R., Hoffmann, H., Hauber, E., Head, J. W., Basilevsky, A. T., et al. (2004). "Recent and episodic volcanic and glacial activity on Mars revealed by the High Resolution Stereo Camera." Nature, 432(7020), 971–979. https://doi.org/10.1038/nature03231
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