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How did lunar maria form?
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Moon
(Left) Near side of Earth's Moon, photographed by the Galileo spacecraft on its way to Jupiter. (Right) Far side of the Moon with some of the near side visible (upper right), photographed by the Apollo 16 spacecraft.
Lunar maria (singular: mare) are vast, dark, basaltic plains that formed through a two-stage process on the Moon:
- Basin Formation: Beginning approximately 3.9 billion years ago, the Moon experienced a period of intense bombardment by asteroid-sized bodies. These massive impacts created giant basins across the lunar surface.1
- Volcanic Flooding: Following these impacts, a long sequence of volcanic events occurred between 3.9 and 3 billion years ago. Extremely fluid magma from the lunar interior rose to the surface, flooding the low-lying impact basins and congealing into basalt.12
While giant impact basins are distributed across both the near and far sides of the Moon, most far-side basins remain unfilled. This is likely due to the Moon's crustal asymmetry; the far-side crust is roughly twice as thick as the near-side crust, making it less likely to have been fully ruptured by impacts to allow for volcanic flooding. Most maria are associated with mascons, which are regions of dense lava that cause gravitational anomalies.1