Lunar Maria

Look at the full Moon with your naked eye and you are already mapping its geology. The bright, rugged regions are the ancient highlands — battered anorthosite crust nearly as old as the Moon itself. The dark, smooth patches that form the "Man in the Moon" or the "Rabbit" are the maria (singular: mare, Latin for "sea"). They are the youngest large surfaces on the Moon, and they record the only chapter of lunar history when the Moon behaved like a volcanic world.

What a mare actually is

A lunar mare is a flood-basalt plain sitting inside an impact basin. The story has two distinct acts that are often collapsed into one:

• Act one — excavation. Between roughly 4.0 and 3.8 billion years ago, during the era of heavy bombardment, enormous asteroids and protoplanet fragments slammed into the Moon and gouged out circular basins hundreds to thousands of kilometers across. Mare Imbrium's basin (~1,150 km) was carved by a body perhaps 100 km wide. This impact dug the hole — but the hole was rocky and bright, not dark.
• Act two — flooding. Hundreds of millions of years later, radioactive decay of uranium, thorium, and potassium in the Moon's interior produced partial melting of the mantle. This low-viscosity, iron-rich magma rose through fractures and pooled in the deepest, lowest-lying basins, sheet upon sheet, until the floors were paved with dark basalt.

The crucial insight is that the impacts did not make the lava. The basins were merely the lowest places for later eruptions to collect — like rainwater pooling in old quarries. This is why mare basalt ages (3.0–3.9 Gyr) lag the basin-forming impacts (3.8–4.0 Gyr) by a substantial margin.

Why the maria are dark and smooth

Two independent properties make the maria visually distinct from the highlands.

Dark — because of chemistry. Mare basalt is rich in iron oxide (FeO) and, in many regions, titanium dioxide (TiO₂); the high-titanium basalts returned by Apollo 11 from the Sea of Tranquility contained over 10 wt% TiO₂. These minerals absorb light strongly, giving the maria an albedo (reflectivity) of only about 0.07 — they bounce back roughly 7% of sunlight. The feldspar-rich highlands reflect closer to 12%. To the eye on Earth that modest difference reads as stark dark-versus-light, because the brain stretches the contrast.

Smooth — because of viscosity. Lunar mare lava was extraordinarily runny, far more fluid than typical Earth lava, partly because of its composition and the low lunar gravity. Instead of piling up into steep volcanoes, it flooded outward in thin sheets that could travel hundreds of kilometers, leaving nearly flat plains. Individual flow fronts only tens of meters thick have been mapped from orbit. The result is a surface so level that it became the obvious choice for the first crewed landing: Apollo 11 set down on Mare Tranquillitatis precisely because it was flat.

The near-side / far-side puzzle

The single most striking fact about the maria is their lopsided distribution. They blanket roughly 31% of the near side — the hemisphere permanently facing Earth because the Moon is tidally locked — yet cover only about 1% of the far side. The far side is almost entirely rugged highland. Why?

The favored explanation is crustal thickness, mapped in detail by NASA's GRAIL gravity mission in 2012. The near-side crust averages around 30–40 km thick; the far-side crust is roughly 50–60 km thick, and thicker still in places. Thinner crust is easier for buoyant mantle melt to penetrate, so lava reached the surface readily on the near side and rarely on the far side, where it tended to stall and freeze underground. The near side is also home to the Procellarum KREEP Terrane, a region anomalously enriched in potassium (K), rare-earth elements (REE), and phosphorus (P) — the heat-producing radioactive package that kept the near-side mantle melting for far longer.

Maria versus highlands at a glance

Property	Maria	Highlands
Dominant rock	Basalt (Fe/Ti-rich)	Anorthosite (feldspar-rich)
Albedo	~0.07 (dark)	~0.12 (bright)
Surface age	~3.0–3.9 Gyr	~4.4 Gyr
Crater density	Low (younger)	High (older)
Topography	Flat plains	Rugged, mountainous
Coverage of near side	~31%	~69%
Coverage of far side	~1%	~99%

How we date a lava plain you can see from your backyard

Lunar surface ages rest on two cross-calibrated methods.

• Radiometric dating. The Apollo and Soviet Luna missions returned basalt samples whose crystallization ages can be measured directly using isotopic clocks such as rubidium–strontium and argon–argon. These give absolute ages — Apollo 11's basalts came in around 3.6–3.9 Gyr.
• Crater counting. Impacts accumulate over time, so the density of craters on a surface measures its exposure age. A mare floor pocked by only a few craters is demonstrably younger than the saturated, crater-on-crater highlands. By tying crater densities to the radiometrically dated landing sites, planetary scientists built a calibration curve that lets them date any patch of the Moon from orbital images alone — and then export that clock to Mars, Mercury, and beyond.

The two methods agree, which is why the mare timeline is among the best-constrained chronologies in the solar system. China's Chang'e-5 mission (2020) returned the youngest basalts yet sampled, about 2.0 billion years old, extending the known span of mare volcanism and forcing a rethink of how the small Moon stayed warm so long.

Selected named maria

Mare (Latin)	English	Notable for
Oceanus Procellarum	Ocean of Storms	Largest, ~2,500 km; KREEP-rich
Mare Imbrium	Sea of Rains	Vast basin, ~1,150 km; Apollo 15
Mare Tranquillitatis	Sea of Tranquility	Apollo 11 landing; high-Ti basalt
Mare Serenitatis	Sea of Serenity	Apollo 17 (highland/mare boundary)
Mare Orientale	Eastern Sea	Youngest large basin; bullseye rings

What you find inside a mare

Mare surfaces are not perfectly featureless. They preserve a catalog of volcanic and tectonic structures:

• Wrinkle ridges — long, sinuous compressional ridges formed as the cooling, contracting lava sheets and the load of dense basalt deformed the basin floor.
• Sinuous rilles — winding channels, such as Hadley Rille visited by Apollo 15, carved by flowing lava or collapsed lava tubes.
• Mascons — mass concentrations. The dense basalt fill plus uplifted mantle beneath the big mare basins create positive gravity anomalies strong enough to perturb the orbits of spacecraft, first discovered by Lunar Orbiter tracking in 1968.
• Domes and cones — low shield-like volcanic features marking individual vents.

Why the maria matter

• A clock for the whole inner solar system. The radiometrically anchored mare crater chronology is the yardstick used to date surfaces on Mars and Mercury that we cannot yet sample.
• A window into the lunar interior. Mare basalt is partial-melt of the mantle, so its chemistry is a direct chemical probe of the Moon's deep composition.
• A test of thermal evolution. The duration of mare volcanism — and surprisingly young Chang'e-5 samples — constrains how a body as small as the Moon retained internal heat.
• Landing real estate. Their flatness made the maria the launchpad of crewed exploration, and titanium- and helium-3-bearing mare regolith is a recurring target for future resource use.

Common misconceptions

• The maria are seas of water. No — the name is a 17th-century mistake. They are solid basalt; the Moon has no surface liquid water.
• The impacts melted the rock that became the maria. No — the dark lava erupted hundreds of millions of years after the basins formed, driven by radioactive heating of the mantle.
• The maria are the oldest part of the Moon. The opposite — they are the youngest large surfaces; the bright highlands are far older.
• Maria are evenly spread over the Moon. They are overwhelmingly a near-side phenomenon, a clue to the Moon's asymmetric crust.
• The Moon is still erupting. Major mare volcanism ended about a billion years ago; the Moon today is geologically nearly dead.