The Moon’s surface is marked by light and dark areas that have fascinated observers for centuries. These dark, relatively smooth plains were once mistakenly identified as bodies of water by early astronomers, leading to their Latin designation as maria (seas). Among the most prominent of these features is the vast, dark expanse known as the Sea of Showers, a feature so immense it dominates the appearance of the Moon’s northern hemisphere. This immense basin was created by a catastrophic event billions of years ago.
Identifying the Sea of Showers
The name “Sea of Showers” is the direct English translation of the Latin term Mare Imbrium. Like all lunar maria, this area is a large, low-lying plain with a darker color and smoother texture compared to the surrounding rugged highlands. Its dark appearance is caused by the basin being filled with low-viscosity basaltic lava, an igneous rock rich in iron and magnesium.
Early observers believed these dark regions were actual oceans, which is how the Latin name Mare originated. The specific name Imbrium translates to “of the rains” or “of the showers,” reflecting this historic interpretation. This vast, smooth plain is second only to Oceanus Procellarum in overall size among the maria.
Pinpointing the Location on the Moon
The Sea of Showers is prominently located on the near side of the Moon, the face that is always turned toward Earth. Its position makes it easily observable with binoculars or a small telescope, particularly when sunlight strikes it at a low angle near the first quarter phase. This angle allows the surrounding mountain ranges to cast long, defining shadows.
The center of this immense feature is situated in the Moon’s northern hemisphere, centered near 32.8° North latitude and 15.6° West longitude. It is positioned northwest of the center of the visible lunar disk and is bordered by several other notable lunar seas and craters.
Geological History and Formation of the Imbrium Basin
The Sea of Showers is the visible part of the much larger Imbrium impact basin. This colossal basin was created approximately 3.9 billion years ago during the Imbrian period, a time of intense bombardment on the Moon. The impactor, estimated to have been a proto-planet up to 250 kilometers in diameter, struck the Moon with tremendous force.
The impact event excavated a multi-ring structure and scattered a blanket of ejecta across vast portions of the lunar near side. This widespread ejecta layer serves as a key marker for dating other geological events on the Moon. The mountainous rings surrounding the basin, such as the Montes Apenninus and Montes Carpatus, are the uplifted remnants of the original rim, rising more than five kilometers above the mare surface.
The dark, smooth plain formed hundreds of millions of years after the impact. Volcanic activity erupted low-viscosity basaltic lavas, which flowed across the basin floor and solidified between about 3.3 and 3.5 billion years ago. This process covered the original impact floor, creating the flat, dark surface of the mare. The Imbrium basin also possesses the Moon’s largest mass concentration, or mascon, a region of high gravity caused by the dense, subsurface lava flows and material that rebounded after the initial impact.
Significance and Associated Exploration
The Imbrium Basin holds significant value for scientists because it serves as a geological time marker. The basin’s rim features several well-known landmarks, including the Montes Alpes and the prominent crater Archimedes, which is partially flooded by the mare basalts. The sinuous rille known as Hadley Rille, a vast channel 1.5 kilometers wide and 300 meters deep, is a volcanic feature located near the southeastern edge of the basin.
The region has been a frequent target for robotic and crewed missions. The Soviet Luna 17 mission in 1970 successfully deployed the Lunokhod 1 rover directly onto the mare surface. Later, the Apollo 15 mission in 1971 landed near Hadley Rille at the base of the Apennine Mountains. The astronauts collected 77 kilograms of lunar material, including samples of mare basalts and older highland rocks from the basin’s upthrown ramparts. China’s Chang’e-3 mission in 2013 also landed in the northern part of the mare, deploying the Yutu rover to analyze the composition of the younger, high-titanium basalts.