Lunar MariaEdit
Lunar maria are among the most striking features of the Moon’s surface. These expansive, dark plains spread across large basins on the nearside of the Moon and are formed by ancient volcanic activity that flooded impact basins with basaltic lava. The term mare (plural maria) comes from the Latin for “sea,” a holdover from early observers who mistook these dark regions for lunar seas. In reality, they are solidified lava flows and crustal plains that record a long chapter of the Moon’s volcanic and magmatic evolution. Maria cover roughly a sixth of the Moon’s surface and are concentrated on the side facing Earth, where the crust is thinner in places, allowing magma to reach the surface more readily. The distinction between the maria and the highland terrain is both geological and visual: the maria are smoother and darker, while the highlands are heavily cratered and lighter in color.
Researchers use a combination of remote sensing, sample analysis, and crater counting to build a picture of maria that integrates basalt chemistry, volcanic processes, and planetary differentiation. The study of maria intersects multiple fields, including planetary science, geology, and astrogeology, and it relies on data from orbital missions such as Lunar Reconnaissance Orbiter, as well as historical samples brought back by the Apollo program.
Geology and composition
Lunar maria are primarily basaltic plains formed by volcanic eruptions that filled large impact basins with lava. The basalts that make up the maria are generally low in potassium, rare earth elements, and phosphorus in some regions, while others exhibit higher titanium contents. This variation leads to a classification of mare basalts into distinct geochemical groups, including low-Ti and high-Ti basalts. The high-Ti basalts are especially prominent in parts of the eastern nearside maria, where the lava sourced from deeper mantle regions contributed to a different surface composition than the more typical low-Ti basalts found elsewhere.
The lava flows that created the maria cooled to form relatively smooth, glass-rich surfaces in many places, though subsequent impacts and local tectonic activity have added complexity. The typical density, mineralogy, and texture of mare basalts indicate an origin from partial melting of the lunar mantle, followed by fractional crystallization and crystal-poor, rapidly erupted lavas that spread across the basins. The mare soils (regolith) result from billions of years of micrometeoroid bombardment and space weathering, overlaying the solid basalt.
For broader context, see basalt and Lunar geology; the mare regions also relate to the Procellarum KREEP Terrane, a geochemical province that has played a role in late-stage volcanic activity.
Formation and distribution
The maria formed as the Moon’s crust was thickening early in its history and then experienced episodic thinning in regions where magma could reach the surface. Large impact basins created deep cavities that later served as repositories for magma. When portions of the mantle partially melted, magma began to pool and eventually erupt onto the surface, flooding basins and creating extensive lava plains.
On the near side, the distribution of maria is far more extensive than on the far side. This asymmetry is attributed to differences in crustal thickness between the near and far hemispheres, with the nearside crust being thinner in some regions and more conducive to magma ascent. The major maria—such as Mare Imbrium, Mare Serenitatis, Mare Tranquillitatis, Mare Nubium, and Oceanus Procellarum—define a network of large basaltic plains that dominate the nearside geography. In contrast, the far side is cratered and highland-dominated, with far fewer large basaltic plains.
The timing of mare volcanism spans a broad interval. Radiometric dating of returned samples and crater counting indicate that significant volcanic activity occurred from roughly 3.0 to 1.0 billion years ago, with initial flooding beginning earlier in some basins and continuing into later epochs. The precise ages vary by basin and basalt type, reflecting differences in mantle melting, magma supply, and crustal dynamics over time.
Notable maria and features
- Oceanus Procellarum (Ocean of Storms) — the largest mare region on the Moon, located on the western nearside. Its geochemical signature is distinctive, and it hosts a variety of volcanic features and rilles. See Oceanus Procellarum.
- Mare Imbrium (Sea of Rains) — one of the largest and most prominent maria, formed in a massive impact basin that was later flooded. See Mare Imbrium.
- Mare Serenitatis (Sea of Serenity) — a prominent, relatively smooth basaltic plain with high scientific interest for its well-preserved lava flows. See Mare Serenitatis.
- Mare Tranquillitatis (Sea of Tranquility) — the site of the Apollo 11 landing; features diverse basalts and a long volcanic history. See Mare Tranquillitatis.
- Mare Nubium (Sea of Clouds) — another extensive basaltic plain with classic mare textures. See Mare Nubium.
- Mare Crisium (Sea of Crises) — a circular mare basin near the eastern limb of the nearside; its basaltic plains provide insights into crustal structure. See Mare Crisium.
- Mare Fecunditatis (Sea of Fertility), Mare Nectaris (Sea of Nectar), and Mare Vaporum (Sea of Vapors) — smaller but scientifically important lunar basins with characteristic basalt compositions. See Mare Fecunditatis, Mare Nectaris, Mare Vaporum.
- Other notable features include Rimae/>rille networks and volcanic plains that reveal the tectonic and magmatic evolution of the lunar surface.
Exploration and dating
Knowledge of the maria rests on multiple lines of evidence. High-resolution imagery and topography from orbiters reveal the extent and morphology of lava flows, while spectrometers map surface composition and identify basaltic rocks. The deepest insights come from direct analysis of samples collected during the Apollo program, whose petrology and radiometric ages constrain the timing of volcanic activity. Crater counting, calibrated against radiometric ages, provides estimates of surface ages across different maria and adjacent highlands. More recent data from missions such as Lunar Reconnaissance Orbiter and other lunar orbiters refine the distribution, thickness, and emplacement history of mare basalts and illuminate variations in crustal thickness and mantle composition.
Remote sensing has shown differences in basalt chemistry among maria, reinforcing the view that mantle melting, magma supply, and crustal interactions varied from one basin to another. The role of the Procellarum region and the nearby KREEP (potassium, rare earth elements, phosphorus) signature is a topic of ongoing study, linking late-stage volcanism to the Moon’s global magmatic evolution.