Oceanus Procellarum

• 1. Introduction

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  Oceanus Procellarum is the only plain on the Moon that astronomers have named an ocean. Its Latin name translates roughly as the Ocean of Storms. Every other dark lowland on the lunar face is a mere sea. This one earned the grander word through sheer size, and it sits on the western edge of the side of the Moon that faces Earth. Yet size is not the strangest thing about it. Scientists still argue about what carved the region in the first place. Some of its rock turned out to be far younger than anyone expected from a world this small. Robotic probes and Apollo astronauts have both touched down on it. And on one tight schedule in the 1960s, it came close to hosting the first crewed landing.

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• 2. A Plain of Frozen Lava

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  More than 2500 km separate the northern and southern ends of Oceanus Procellarum. It covers roughly 4000000 km2, which makes it the largest plain on the Moon, and it takes up 10.5% of the entire lunar surface. Its floor is a nearly flat blanket of basalt. Ancient volcanic eruptions poured lava across the region, and the flood hardened into a thick layer of solid magma. Some of that basalt has been estimated at about one billion years old. The edges are crowded with smaller dark patches. Sinus Roris lies to the north, while Mare Nubium and Mare Humorum lie to the south. In the northeast, the Carpathian Mountains separate the ocean from Mare Imbrium. Two craters stand out against the dark ground. On the northwest edge sits Aristarchus, 32 km wide, and it is the brightest feature on the near side. Copernicus lies within the eastern edge, with bright rays of pale material sprawling over the darker lava. One question hangs over all of this geology. A true lunar sea normally fills a single, well-defined impact basin, and this ocean may not.

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• 3. What Made the Ocean

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  Researchers have proposed several explanations, and none has settled the matter. The leading idea is a giant impact on the near side very early in lunar history, while the Moon's magma ocean still existed or had just ended. The basin from such a collision would measure more than 3,000 kilometers across. That would rank it among the three largest craters in the Solar System. The same blow would have thrown 5-30 km of crustal material onto the far side, building its highlands. Later impacts and volcanism would then have erased the rim and central peak. Support comes from the chemistry. Incompatible elements, grouped under the name KREEP, are concentrated around the region, along with low-calcium pyroxene. A rival idea comes from the GRAIL mission, which mapped gravity gradients across the Moon. It found square formations, resembling rift valleys, hidden beneath the lava plains. A round crater would not leave that pattern. The team read it as a sign that internal heating and cooling shaped the basin, not an impact. A third proposal is stranger. Debris from the giant impact may have formed a second moon, about 1,200 km in diameter. A few tens of millions of years later it collided gently with the Moon and piled up on one side as the far side highlands.

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• 4. A Surprisingly Young Fire

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  Features on the ocean's surface looked too fresh to be ancient. They had eroded very little, and that hinted at volcanic activity within the last 2 billion years. In 2020, the Chang'e-5 sample return mission put a number on it. The rock it brought back measured 1963 ± 57 million years old. That is over a billion years younger than any lunar sample returned before. The result was a puzzle, because the Moon is much smaller than Earth. Its interior heat should have drained away around three billion years ago, so something extra must have kept the lava flowing. The obvious suspects were heat-producing elements such as potassium, thorium and uranium. Earlier studies expected this region to be rich in them. The returned samples told a different story. Their concentration of suspected radioactive elements fell well short of what long heating would require. The source of that late warmth remains an open question.

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• 5. Landings on the Ocean

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  Four robotic probes reached the surface here in the 1960s. Luna 9 came down southwest of Galilaei crater in 1966. Luna 13 followed later that year, southeast of Seleucus crater. Surveyor 1 landed north of Flamsteed crater in 1966, and Surveyor 3 arrived in 1967. In November 1969, Apollo 12 astronauts Charles Pete Conrad and Alan Bean set the Lunar Module Intrepid down nearly 165 meters from Surveyor 3. Their landing site became known as Statio Cognitum, Latin for "to be known from experience". Apollo planners nearly sent Apollo 11 somewhere else entirely. They worried about sunlight angles at the landing site, so backup targets shifted steadily westward, following the terminator. A two-day delay would have redirected Apollo 11 from Mare Tranquillitatis to Sinus Medii. Another two days would have pointed it at ALS5, a site in Oceanus Procellarum. In December 2020, the Chinese probe Chang'e 5 landed at Statio Tianchuan on Mons Rümker and collected 1.73 kg of lunar rock. The ocean has also reached music. A song named after it appears on Sirens, a 2004 album by the Greek metal band Astarte. Another appears on The Study of Losses, a 2025 album by Beirut.

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