Geological Chronicle
Mare Imbrium, or the Sea of Showers, is an expansive basaltic lava plain spanning 1,145 kilometers across the northwestern quadrant of the lunar near side. Formed approximately 3.85 billion years ago during the Early Imbrian epoch, the basin was excavated when a protoplanetary body roughly 250 kilometers in diameter struck the young Moon. The collision represents the defining event of the lunar stratigraphic timescale, giving its name to the Imbrian Period and dispersing impact melt and debris across both lunar hemispheres.
The excavation generated three concentric rings of uplifted crust. The outermost ring, measuring 1,300 kilometers in diameter, forms the towering arcs of Montes Apenninus, Montes Caucasus, and Montes Carpatus, which rise more than 5 kilometers above the mare floor. High-velocity ejecta ploughed outward across 800 kilometers of terrain, gouging deep radial furrows known as the Imbrium Sculpture, first recognized by geologist Grove Karl Gilbert in 1893. Seismic energy traveling directly through the Moon's interior converged at the exact antipodal point on the far side, shattering the crust at Van de Graaff crater.
Centuries after the impact, extensive volcanic eruptions flooded the depression with iron-rich, low-viscosity basalt lava. These flows accumulated in multiple layers between 2 and 5 kilometers thick, burying most of the inner mountain rings and leaving only isolated peaks such as Mons Pico and Mons La Hire projecting above the dark plains. The dense basalt fill, combined with an uplifted mantle plug beneath the crust, created the largest mass concentration (mascon) on the Moon, detected through spacecraft orbit perturbations in 1968.
Mare Imbrium has served as a primary destination for surface lunar exploration. In November 1970, the Soviet Luna 17 lander deployed Lunokhod 1, the first robotic wheeled rover on an extraterrestrial body, which traversed 10.5 kilometers across the western plains. In July 1971, Apollo 15 touched down at Hadley-Apennine along the southeastern rim, where astronauts collected the 4.1-billion-year-old Genesis Rock and drove the first crewed Lunar Roving Vehicle. In December 2013, China's Chang'e 3 lander and Yutu rover investigated northern Imbrium, using ground-penetrating radar to detect at least nine distinct subsurface basalt layers.