Lunar Mare

A lunar mare (plural lunar maria, from the Latin for "seas") is a vast, dark, topographically low plain of solidified basalt lava on the Moon formed by ancient effusive flood volcanism that filled giant multi-ring impact basins and adjoining structural depressions across the inner Solar System. Covering approximately 16% to 17% of the total lunar surface (~6.3 × 10⁶ km²), the lunar maria display a stark hemispherical asymmetry: they cover roughly 31% of the Earth-facing nearside hemisphere (producing the familiar dark features of the "Man in the Moon" visible to the naked eye) yet occupy less than 1% to 2% of the lunar farside. Although early seventeenth-century astronomers initially mistook these smooth, dark expanses for liquid oceans, petrological and radiometric analyses of samples returned by NASA's Apollo missions, Soviet Luna robotic landers, and China's Chang'e-5 and Chang'e-6 missions established that the maria consist of iron- and titanium-rich basalts erupted between 4.2 and 2.0 billion years ago.

When Galileo Galilei published the first telescopic drawings of the Moon in Sidereus Nuncius (1610), he distinguished two fundamental lunar terrains: the bright, rugged, heavily cratered highlands (which he termed terrae, or lands) and the smooth, dark lowland plains. Johannes Kepler (1609) and Michael Florent van Langren (1645) introduced hydrographic nomenclature for these dark plains, which was standardized in 1651 by the Italian Jesuit astronomers Giovanni Battista Riccioli and Francesco Maria Grimaldi in their lunar map Almagestum Novum.

Riccioli named the largest continuous western expanse Oceanus Procellarum ("Ocean of Storms," spanning over 2,500 km north-to-south across 4 × 10⁶ km²), designated circular basin-filling plains as maria (such as Mare Imbrium, Mare Serenitatis, Mare Tranquillitatis, Mare Crisium, Mare Fecunditatis, Mare Humorum, and Mare Nectaris), and applied smaller water-related Latin terms to subsidiary features: lacus ("lake," e.g., Lacus Mortis), palus ("marsh," e.g., Palus Putredinis), and sinus ("bay," e.g., Sinus Iridum and Sinus Medii).

The chemical composition and origin of lunar mare basalts in planetary science are rooted directly in the crystallization of the global lunar magma ocean following the Moon's formation via the Giant-Impact Hypothesis (~4.51–4.42 Ga). As the initially molten outer several hundred kilometers of the Moon cooled:

1. Dense magnesium-rich olivine and orthopyroxene cumulates crystallized first (during the first ~75%–80% of solidification) and sank to form the deep lunar mantle at depths of 150 to 500 km.
2. Once plagioclase feldspar (anorthite, CaAl₂Si₂O₈) reached saturation, its low density (~2.73 g/cm³) caused it to float upward through the denser iron-rich residual magma (~3.1 g/cm³), building the 35-to-60-km-thick bright feldspathic highland crust (composed of >85%–90% plagioclase with 25–30 wt% Al₂O₃).
3. In the final 5% of magma ocean crystallization, dense iron-titanium oxide minerals—principally ilmenite (FeTiO₃)—crystallized alongside clinopyroxene below the crust, accompanied by an incompatible-element-enriched residual liquid termed urKREEP (enriched in potassium [K], rare-earth elements [REE], phosphorus [P], and the heat-producing radioactive isotopes ²³⁵U, ²³⁸U, ²³²Th, and ⁴⁰K).

Because the late-stage ilmenite-rich cumulate layer (~3.7–3.8 g/cm³) was denser than the underlying magnesium-rich olivine mantle (~3.3 g/cm³), gravitational Rayleigh–Taylor overturn caused dense ilmenite and KREEP-bearing pods to sink deep into the lunar mantle. Over the subsequent hundreds of millions of years, radiogenic decay of U, Th, and K heated these hybridized olivine–orthopyroxene–ilmenite cumulate reservoirs at depths of 150 to 400 km, triggering partial melting (~5% to 15% melt fraction).

When giant impacts during the Late Heavy Bombardment (~4.1–3.8 Ga) excavated deep nearside multi-ring basins and fractured the underlying lithosphere, these dense basaltic melts ascended along deep lithospheric dikes and erupted onto the basin floors. While cryptomare basalts and high-aluminum clasts began erupting as early as 4.2 to 3.9 Ga, the main pulse of Imbrian mare flood volcanism occurred between 3.85 and 3.10 Ga (hundreds of millions of years after the host basins formed, proving that impact heating itself did not generate the mare lavas). Eruptions persisted into the Eratosthenian period inside Oceanus Procellarum, where China's Chang'e-5 mission returned pristine basalts from the Mons Rümker region in December 2020 yielding a lead–lead crystallization age of 2.030 ± 0.004 Ga—demonstrating that lunar mantle volcanism remained active for more than 2.5 billion years.

Lunar mare basalts are classified geochemically by their titanium dioxide (TiO₂) content into three major suites (Neal & Taylor, 1992): High-Ti basalts (>6 to 14 wt% TiO₂, sampled at Apollo 11 and Apollo 17), Low-Ti basalts (1.5 to 6 wt% TiO₂, sampled at Apollo 12, Apollo 15, and Chang'e-5), and Very-Low-Ti (VLT) basalts (<1.5 wt% TiO₂, sampled at Apollo 17 and Luna 24).

Compared to terrestrial mid-ocean ridge or Hawaiian basalt (which contains ~48–51 wt% SiO₂ and 9–12 wt% FeO), lunar mare basalts are depleted in silica (SiO₂ ≈ 38–47 wt%) and enriched in divalent ferrous iron (FeO ≈ 18–23 wt%), with virtually zero ferric iron (Fe³⁺) due to the Moon's low oxygen fugacity and anhydrous state. Because FeO and TiO₂ depolymerize silicate melt networks, molten lunar mare lava at eruption temperatures of 1,300–1,400 °C possessed an ultra-low dynamic viscosity of 0.1 to 1.0 Pa·s—roughly 10 to 100 times more fluid than terrestrial Hawaiian basalt.

This high fluidity produced a distinctive suite of lunar volcanic landforms without building tall shield volcanoes like Olympus Mons on Mars:

• Flat Flood Sheets and Sinuous Rilles: Individual mare lava flows spread across distances of 300 to 1,000 km with thicknesses of only 5 to 30 m. Thermal and mechanical erosion by turbulent, superheated lava incised winding channels known as sinuous rilles (such as Hadley Rille, explored by Apollo 15 astronauts), as well as roofed underground lunar lava tubes detected via skylight collapse pits in Mare Tranquillitatis and Marius Hills.
• Pyroclastic Dark Mantle Deposits: Where ascending mantle magma carried trace volatile gases (principally CO alongside S, F, Cl, and trace H₂O), fire-fountain eruptions sprayed droplets of molten lava through the lunar vacuum, quenching in flight into microscopic orange and green volcanic glass beads (such as the 3.64-Ga Apollo 17 orange soil).
• Wrinkle Ridges, Concentric Graben, and Mascons: Because solidified iron-rich mare basalt (~3.3–3.4 g/cm³) is denser than the surrounding porous anorthositic crust (~2.55 g/cm³), 2-to-5-km-thick mare fills atop uplifted mantle plugs created positive free-air gravity anomalies termed mascons (mass concentrations). Downward lithospheric flexure under the mascon load compressed the interior of each mare basin into concentric wrinkle ridges (dorsa) while stretching the outer basin margins into circumferential extensional graben.

The concentration of lunar maria on the Earth-facing nearside (~31% surface coverage vs. ~1% on the farside) is explained by two coupled geophysical factors:

1. The Procellarum KREEP Terrane (PKT): Orbital gamma-ray mapping by Lunar Prospector revealed that more than 60% of the Moon's near-surface radioactive thorium (5 to 12 ppm Th), uranium, and potassium is concentrated within a single oval province on the western and central nearside encompassing Oceanus Procellarum and Mare Imbrium, designated the Procellarum KREEP Terrane by Jolliff et al. (2000). The radiogenic heat output of this KREEP-enriched mantle maintained elevated nearside mantle temperatures for over two billion years.
2. Crustal Thickness and Hydrostatic Magma Buoyancy: GRAIL gravity inversions show that the nearside crust averages 20 to 35 km in thickness, whereas the farside feldspathic highlands average 50 to 60 km in thickness. Across the thick farside highlands, dense iron-rich ascending dikes stalled within the mid-crust as intrusive plutons, whereas across the thin nearside crust and deep nearside basins, pressurized dikes breached the surface to flood the lunar maria.

Why do the lunar maria look so dark from Earth?

Lunar mare basalts have a low visual normal albedo of only 0.07 to 0.10 (reflecting just 7% to 10% of incident sunlight, comparable to fresh asphalt) compared to 0.12 to 0.18 for the bright anorthositic highlands. This darkness results from high concentrations of light-absorbing iron-bearing pyroxene, olivine, and opaque black ilmenite (FeTiO₃), reinforced by micrometeorite and solar-wind space weathering that coats soil grains in submicroscopic nanophase metallic iron (npFe⁰).

Did the asteroid impacts that created the basins cause the volcanic lava to erupt right away?

No. Radiometric dating of Apollo samples proved that the major mare flood basalts erupted 100 million to over 1.5 billion years after the host impact basins formed. For example, the Imbrium impact basin formed around 3.91–3.92 Ga, whereas the lavas filling Mare Imbrium erupted across multiple phases from 3.5 Ga down to 2.0 Ga. The basins acted as pre-existing topographic bowls with thinned, fractured crust where later radiogenically heated mantle magmas could reach the surface.

Why did China's Chang'e-5 2.0-billion-year-old basalt samples surprise planetary scientists?

Prior to Chang'e-5, all Apollo and Luna mare basalt samples dated between 3.9 and 3.1 Ga, and geophysicists assumed that a 2.0-Ga eruption in northern Oceanus Procellarum would require either high concentrations of radioactive KREEP elements or high mantle water content to lower the melting point of the cooling lunar mantle. However, isotopic and volatile analyses of the Chang'e-5 samples in 2021 proved that their mantle source was both dry (1 to 5 ppm H₂O) and depleted in KREEP, showing instead that ilmenite-rich clinopyroxene cumulates with lower solidus temperatures enabled late-stage lunar volcanism.

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