Lunar mare

• 1. Introduction

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  The lunar maria are large, dark plains of basalt on Earth's Moon, and their name means seas even though lava, not water, made them. Together they cover about 16% of the lunar surface, and most of that area lies on the side visible from Earth. The few maria on the far side are much smaller, and they sit mostly inside very large craters.

  These plains look dark to the naked eye because their iron-rich rock reflects less light than the paler highlands around them. That simple contrast has shaped myths, maps and a long scientific argument. How did people read the dark patches before telescopes existed? Why did astronomers keep calling them seas after Galileo described rock? And why do almost all of them gather on the near side, a question scientists have not settled?

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• 2. Faces in the Dark

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  During the Warring States period in China, poems already described the dark marks on the Moon as a rabbit. Other cultures found their own figures there. The best known is the Man in the Moon, an apparent human face. The habit of seeing patterns in the Moon is called lunar pareidolia.

  Neolithic stones in Ireland carry some of the earliest known drawings of the lunar surface. Pre-Columbian art in Mesoamerica also shows the Moon's markings. Ancient Greek philosophers took a more physical view. Thales, Democritus and Anaximenes all considered the Moon earth-like and rocky, with no seas on it.

  The Roman historian Pliny the Elder used the word mascula for the dark areas, meaning spottedness, and he believed the Moon was covered in mud. The biographer Plutarch compared a lunar region called Caspia, which is Mare Crisium in modern terms, with the Caspian Sea on Earth. That comparison may have helped plant the idea of seas on the Moon. Yet Plutarch followed Greek philosophy and explained the spots as differences between higher and lower terrain, without mentioning water.

  The Renaissance painters Jan van Eyck and Leonardo da Vinci are credited with the first realistic drawings of the Moon. Leonardo called its features macchie, which means stains.

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• 3. Kepler's Seas

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  Around the year 1600, the physicist William Gilbert made the first scientific drawing of the Moon, for his book De mundo nostro sublunari philosophia nova. He regretted that no drawings from antiquity survived. Such drawings might have shown whether the Moon's surface had changed over time. Gilbert named features using terms based on seas and islands, but his work was not made public until 1654.

  In 1604 the astronomer Johannes Kepler used the word maria in his work Dioptrice. He paired it with terrae, to describe what looked like seas and land-masses on the Moon. The term survives in the name Mare Crisium. Thomas Harriot, Pierre Gassendi and Michael van Langren appear to have reached the same vocabulary independently.

  Galileo Galilei changed the picture in 1610 with his treatise Sidereus Nuncius. He noticed crater-like features, called the surface rocky, and never mentioned water or mare. After reading Galileo, Kepler changed his mind about the watery nature of the maria. Mapmakers did not follow him, and they kept labelling lunar features as seas, lakes and bays. One of them, Johannes Hevelius, insisted the maria were watery.

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• 4. Mapmakers and Engravers

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  Thomas Harriot was the first person to describe the Moon from telescope observations, in 1609. He produced a detailed telescopic map in 1611, yet it stayed unknown until it was rediscovered and published in 1965.

  Meanwhile, sketches kept improving. Christoph Scheiner made notable advances in 1614, followed by Charles Malapert in 1619, Giuseppe Biancani in 1620 and Christoforo Borri in 1627. Francesco Fontana added his own in 1629 and 1630.

  Pierre Gassendi and his friend Nicolas-Claude Fabri de Peiresc wanted a map with accurate distances between features. They timed moonrises and moonsets at different locations and hired professional artists to draw what they found. Peiresc died before the work was finished. One of the artists, Claude Mellan, produced an engraving in 1637 that was more accurate than anything before it.

  In 1645 Michael van Langren, hired by the Monarchy of Spain, tried to calculate accurate distances and produced a series of detailed lunar maps. Johannes Hevelius followed in 1647 with his Selenographia, sive Lunae descriptio. He hired the engraver Jeremias Falck and printed enough copies for the map to circulate widely. In 1661 Christopher Wren went a step further and built a lunar globe showing the cavities and raised areas of the surface.

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• 5. Naming the Seas

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  Giovanni Battista Riccioli used an improved telescope to make another accurate map of the Moon. He named its features with a mixture of the earlier systems of Van Langren and Hevelius. His nomenclature remains in use today.

  The traditional names include one oceanus, Oceanus Procellarum, along with features called lacus for lake, palus for marsh and sinus for bay. The last three are smaller than maria but share their nature and characteristics.

  The names of maria follow a pattern. Some refer to sea features, such as Mare Humorum, Mare Imbrium, Mare Insularum, Mare Nubium, Mare Spumans, Mare Undarum, Mare Vaporum and Mare Frigoris. Others refer to sea attributes, such as Mare Australe, Mare Orientale, Mare Cognitum and Mare Marginis. The final group of names refers to states of mind, including Mare Crisium, Mare Ingenii, Mare Serenitatis and Mare Tranquillitatis.

  Mare Humboldtianum and Mare Smythii were named before the states-of-mind scheme was accepted, so they fit none of these groups. The most unusual case is Mare Moscoviense, discovered by Luna 3 and proposed by the Soviet Union. The International Astronomical Union accepted the name on the justification that Moscow is a state of mind.

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• 6. Lava Not Water

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  In 1665 Robert Hooke published Micrographia, in which he described the Moon's surface as the product of geological activity. He linked both craters and maria to volcanism on the Moon. Centuries of later study have supported the lava explanation.

  By Earth's classification schemes, every mare basalt is tholeiitic. Scientists group them into three series by major element chemistry. These are high-titanium basalts, low-titanium basalts and very-low-titanium basalts, abbreviated VLT. The Apollo samples suggested the groups were distinct. Global remote sensing data from the Clementine mission later showed a continuum of titanium concentrations, with the high-titanium end the least abundant.

  Titanium dioxide can reach up to 15 weight percent in mare basalts, while most terrestrial basalts hold much less than 4 weight percent. A special group, the KREEP basalts, is unusually rich in potassium, rare-earth elements and phosphorus.

  The largest difference from Earth's basalts is water. Lunar basalts have a near-total absence of water in any form. They lack hydrogen-bearing minerals such as amphiboles and phyllosilicates, which are common in terrestrial basalts because of alteration or metamorphism. The substance early observers expected to find is the one these rocks almost entirely lack.

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• 7. Ages and Buried Lava

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  The Schiller-Schickard region has been extensively covered by ejecta from the Orientale basin impact. It is an example of cryptomaria, which are lunar volcanic deposits hidden under debris thrown out by crater and basin impacts. Cryptomaria make up about 18% of the total mare basalt area. They can often be spotted by dark halo impact craters, which punch through the cover and bring up dark material.

  Direct radiometric dating gives mare basalt ages from about 3.16 to 4.2 billion years. Crater counting gives a youngest age of about 1.2 billion years. Updated measurements of samples from the Chang'e-5 mission show that some lunar basalts could be as young as 2.03 billion years old. Even so, most mare basalts appear to have erupted between about 3 and 3.5 billion years ago.

  The few eruptions on the far side are old. The youngest flows lie in Oceanus Procellarum on the near side. Many basalts erupted into low-lying impact basins or flowed into them. Yet Oceanus Procellarum, the largest expanse of volcanic units, does not match any known impact basin.

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• 8. The Near-Side Puzzle

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  Initial mare volcanism generally seems to have begun within 100 million years of a basin forming. Because many basalts fill low basins, it was once assumed that the impact itself triggered the eruption. The timing and length of volcanism in several basins cast doubt on that idea. The authors who studied it felt 100 million years was too long a gap for a link. Still, the oldest and deepest basalts in each basin are likely buried and out of reach, which biases the sampling. Current data may not rule an impact trigger out.

  Another suggestion is that Earth's gravity lets eruptions happen on the near side but not the far side. In a frame rotating with the Moon, the centrifugal acceleration exactly balances Earth's gravitational pull, so no net force points toward Earth. Earth's tides do deform the Moon into an elongated ellipsoid. Its high points lie at both the Earth-facing and the opposite point, much as Earth has two high tides per day.

  Crust thickness is a third candidate. Mare basaltic magma is denser than the upper crustal anorthositic material, so eruptions might favor low, thin-crusted places. But the far-side South Pole-Aitken basin holds the Moon's lowest elevations and is only sparingly filled with basalt. Its crust is predicted to be much thinner than beneath Oceanus Procellarum. Crust thickness may modulate how much lava reaches the surface, but it cannot be the only control.

  A fourth idea ties the basalts to the Moon's synchronous rotation around Earth. Tidal despinning arises from the moments of inertia of the body, and the mare basalts barely contribute to those. Despinning is also predicted to have finished within thousands of years, while most basalts erupted about a billion years later.

  Data from the Lunar Prospector mission point somewhere else. A large share of the Moon's heat-producing elements, in the form of KREEP, sits within Oceanus Procellarum and the Imbrium basin. This unique geochemical province is now called the Procellarum KREEP Terrane. Its extra heat is almost certainly tied to the length and intensity of volcanism there. How KREEP came to be concentrated in that one region remains unresolved among scientists.

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