Lunar craters

• 1. Introduction

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  Lunar craters cover Earth's Moon. On the night of the 30th of November 1609, Galileo turned his newly built telescope toward the Moon and saw something that upended a long-held belief. The Moon was not a perfect sphere. Its surface was punctured by mountains and by bowl-shaped depressions that no one had a name for yet. Those depressions would eventually be called craters, borrowed from the Greek word for a vessel used to mix wine and water. Today the International Astronomical Union recognizes 9,137 of them, and 1,675 have been dated. The questions that follow are not small ones. What made them? How old are they? Who gets to name them, and why does it matter?

• 2. A Long Argument About Origins

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  Robert Hooke put two competing ideas on paper in his 1665 work Micrographia. He proposed that lunar craters were either the result of projectile bombardment from space or the products of volcanic activity beneath the Moon's surface. That debate did not resolve quickly. Volcanic eruption, meteoric impact, and even a theory called the Welteislehre, developed in Germany between the two world wars, which credited glacial motion with carving the craters, all had their defenders across the following centuries.

  Grove Karl Gilbert argued in 1893 that large asteroid impacts were responsible. Ralph Baldwin made the same case in 1949. Around 1960, Gene Shoemaker brought the idea back into serious scientific conversation. David H. Levy described Shoemaker's view plainly: Shoemaker "saw the craters on the Moon as logical impact sites that were formed not gradually, in eons, but explosively, in seconds."

  The Apollo Project settled the matter. Evidence gathered by Apollo missions and by uncrewed spacecraft of the same period proved that meteoric impact, or asteroid impact for the larger craters, was the origin of nearly all lunar craters. That finding extended outward: by implication, most craters on other bodies in the solar system share the same origin.

• 3. What Craters Are Made Of

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  Johann Hieronymus Schröter gave these formations the name "craters" in 1791, extending a term previously applied to volcanic features. The Moon's craters come in a wide range of sizes. The smallest have been microscopic, discovered in rocks returned to Earth from the Moon. The largest crater described as such stretches roughly 290 km across in diameter near the lunar south pole.

  A typical crater carries several recognizable features. The surrounding area is covered with material splashed out during the impact, and it tends to appear lighter in shade than older surrounding terrain because it has been exposed to solar radiation for less time. A raised rim of ejected material sits just at the crater's edge. Inside, the wall slopes downward, and the floor is relatively smooth and flat, accumulating smaller craters of its own as time passes.

  Central peaks appear only in craters larger than roughly 26 km in diameter. These peaks form when the kinetic energy of the impacting object converts to heat, melts lunar material, and sends a splash of it upward. Craters much larger than a couple of hundred kilometers across lose this central peak and are classified instead as basins. Large craters similar in scale to the dark plains called maria, but lacking significant dark lava filling, are sometimes referred to as thalassoids.

• 4. Numbers That Keep Growing

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  At least 1.3 million craters larger than 1 km in diameter exist on the Moon. Of those, 83,000 exceed 5 km across, and 6,972 exceed 20 km. New, small craters form regularly. A survey called NELIOTA, covering 283.5 hours of observation time, found at least 192 craters between 1.5 and 3 m in size created during the observation period alone.

  The biggest crater captured by NASA's lunar impact monitoring program was produced by an impact recorded on the 17th of March 2013. The impact was visible to the naked eye. The object is believed to have been a meteoroid of approximately 40 kg, striking the surface at 90,000 km/h.

  Researchers have also turned to machine learning to find craters that human observers missed. In March 2018, a team at the University of Toronto Scarborough announced the discovery of around 7,000 previously unidentified craters found using a convolutional neural network. A follow-up study in December 2020 identified around 109,000 additional new craters using a deep neural network.

• 5. Classifying What Remains

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  In 1978, Chuck Wood and Leif Andersson of the Lunar and Planetary Lab devised a formal system for sorting lunar impact craters into types. They focused on craters that were relatively unmodified by later impacts, then grouped their results into five categories that together account for roughly 99% of all lunar impact craters.

  The smallest type, called ALC, covers cup-shaped craters with a diameter of about 10 km or less and no central floor; the archetype is Albategnius C. The BIO type is similar but has a small, flat floor and a typical diameter around 15 km, with Biot as its model. SOS craters have a wide, flat interior floor and no central peak, with no terracing on the inner walls; Sosigenes is the archetype. TRI craters are large enough that their inner walls have slumped to the floor; Triesnecker gives the category its model. TYC craters exceed 50 km in diameter and carry terraced inner walls, relatively flat floors, and often large central peak formations, with Tycho as the defining example.

  Beginning in 2009, Nadine G. Barlow of Northern Arizona University began converting the Wood and Andersson database into digital format. Barlow is also constructing a new crater database that covers all impact craters at least 5 km in diameter, based on images from the Clementine spacecraft. The Moon Zoo project, part of the Zooniverse program, asked citizen scientists to map crater sizes and shapes using data from NASA's Lunar Reconnaissance Orbiter; that project has since been retired.

• 6. Names Across the Surface

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  Craters account for 95% of all named features on the Moon. The custom of naming them after deceased scientists and explorers traces back to Giovanni Battista Riccioli, who began the practice in 1651. Since 1919, the International Astronomical Union has regulated the assignment of these names.

  Small craters with particular scientific or exploratory significance, such as those visited by lunar missions, receive human first names: Robert, Jose, Louise, and so on. The crater Apollo, one of the Moon's largest, is named for the Apollo missions. Many smaller craters inside and near it bear the names of deceased American astronauts. Craters inside and near Mare Moscoviense carry the names of deceased Soviet cosmonauts. In 1970, twelve craters were named after twelve living astronauts, six Soviet and six American, a rare exception to the general rule requiring that a person be deceased.

  Most named craters are what are called satellite craters. Their names combine the name of a nearby larger crater with a capital letter, as in Copernicus A, Copernicus B, and Copernicus C. Crater chains have their own naming convention: their Latin names include the word Catena, meaning chain. Catena Davy, for instance, sits near the crater Davy. The Moon's lack of water, atmosphere, and tectonic plates means erosion is minimal, so craters billions of years old still hold their shapes well enough to be studied, categorized, and named.