We've been studying the Moon through telescopes for 400 years, had spacecraft orbit it, and even landed on it, so you might think we'd have its craters pretty well mapped. If you did, you'd be wrong; most craters have yet to achieve official recognition. Now, using a combination of observations from the Chang'e orbiters and artificial intelligence, scientists have spotted 109,956 unrecognized craters, a large advance on the 10,000 registered.
“Impact craters, which can be considered the lunar equivalent of fossils, are the most dominant lunar surface features and record the history of the Solar System,” a team led by Dr Chen Yang of Jilin University, China, wrote in Nature Communications. Consequently, they argue, a more comprehensive knowledge can help us understand the way the Solar System – or at least its inner realms – evolved.
You might expect previously overlooked craters to be small. Instead, the largest are almost 500 kilometers (300 miles) across. Yang and co-authors identified almost 19,000 craters larger than 8 kilometers (5 miles) wide, large enough to house Moon bases the size of small cities.
The reason we have not identified these craters before is that many craters lack the clear outlines we notice when we look to the Moon. Craters partially degraded by subsequent impacts or volcanic activity can be hard to spot.
The Chang’e-1 and Chang’e-2 orbiters provide a record of the lunar landscape at 120 meter (393.7 foot) and 7 meter (22.96 foot) spatial resolutions respectively. Yang and co-authors trained a deep neural network to identify craters by showing it images of some of those confirmed by the human eye. Then they turned it loose on the Chang’e output to see what it could find. Even using a more limited 50 meter (164 foot) resolution from Chang'e-2, it found 100,000 unrecognized craters, although some of these already exist in unverified databases. Anyone skeptical about these craters' validity can pick coordinates from an online database and go look for themselves.
Even at this point, the work is not done – the polar regions were neglected and higher resolution images will no doubt find more.
The ages of larger craters can be estimated based on observable factors such as the number of smaller impacts that have occurred inside them. The authors had the neural network group all craters wider than 8 kilometers (5 miles) into four age ranges, providing a record of the changing rates with which our satellite, and probably the Earth itself, have been hit by large objects over time.
Historically, lunar craters have been named after scientists, particularly astronomers and astronauts. The tradition was started by Giovanni Riccioli in the 17th Century and is now formalized by the International Astronomical Union (IAU). However, it's unlikely the IAU will go looking for 100,000 people to honor – most currently named craters take their name from a larger nearby crater and an additional letter. Nevertheless, many opportunities remain to grant your favorite neglected scientist a piece of near-immortality.