The Moon’s familiar face, visible even without a telescope, displays prominent dark and light markings. These features offer a narrative of the Moon’s deep past and geological evolution, helping to unlock the mysteries of our closest celestial neighbor.
What Are the Lunar Maria?
The lunar maria, pronounced “mah-ree-uh,” are the large, dark, and relatively smooth plains that contrast sharply with the Moon’s brighter, heavily cratered highlands. Early astronomers mistakenly named them “maria” (Latin for “seas”), believing they were bodies of water. These features are instead vast, solidified plains of volcanic rock, covering approximately 16% to 17% of the Moon’s surface, predominantly on the Earth-facing side.
The Fiery Origins of Lunar Maria
The formation of lunar maria began with massive impact events billions of years ago, creating enormous, deep basins on the Moon’s surface. Following this intense bombardment, significant volcanic activity commenced within the Moon’s interior.
Molten lava, more fluid than Earth’s, rose through cracks in the lunar crust, flooding these impact basins. Extensive volcanic eruptions occurred primarily between 3.9 and 3.5 billion years ago, filling the low-lying areas and solidifying into smooth plains. While most mare volcanism concluded by 3.2 billion years ago, some activity persisted in certain regions until 2.03 billion years ago.
Unique Characteristics of Lunar Maria
Lunar maria are primarily composed of basaltic rock, a dense, dark volcanic material similar to Earth’s basalts. Their iron and magnesium-rich composition gives them a characteristic dark appearance, as these minerals absorb more sunlight than the lighter rocks of the lunar highlands. Maria basalts also tend to have a fine-grained texture.
The volcanic flows smoothed out pre-existing topography within the basins, creating relatively flat surfaces. Maria also have a significantly lower density of impact craters compared to the highlands. This indicates mare surfaces are geologically younger, having formed after the most intense period of early solar system bombardment. Unlike terrestrial basalts, lunar basalts are notably deficient in water and other volatile elements.
Maria’s Story: Clues to the Moon’s Past
The lunar maria serve as important records of the Moon’s geological evolution, offering insights into its internal structure and thermal history. Studying their composition and distribution helps scientists understand the timing and extent of ancient volcanic activity. Rocks collected by Apollo missions provide direct evidence of the Moon’s volcanic past.
Maria are unevenly distributed, with the vast majority on the Moon’s near side. This asymmetry is linked to differences in crustal thickness, where the near side’s thinner crust allowed magma to reach the surface. The distribution of heat-producing elements within the Moon’s interior may also have contributed. These features show a Moon that was once geologically active, despite its current static appearance.