The moon was formed when it was washed out of the right eye of the god of the land while he was bathing. Tsukuyomi-no-Mikoto, the moon god of Japanese folklore, then lived forever in the heavens after climbing a giant celestial ladder from his father’s bathroom.
What I love is how the whimsy of folklore is equalled, if not surpassed, by modern explanations for the origin of the moon.
For Natsuki Hosono of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) in Yokohama, the origin of the moon is the most interesting topic in all of planetary science. And that’s not just because his name contains the kanji for “tsuki,” the word for “moon.”
Hosono’s study, which was published in April in the journal Nature Geoscience, helps explain why we have a very large moon when the other rocky planets nearby don’t (Mars has two moons but they are very small). It also sheds light on something that has puzzled geologists for 50 years, since Neil Armstrong and Buzz Aldrin brought back samples of rocks gathered from the lunar surface.
The Apollo rocks showed that the moon is made of the same stuff as the Earth. What that means, scientists have surmised, is that the moon was born of the early Earth.
The story Hosono’s team has pieced together goes like this. Once upon a time there were five “inner” planets between the sun and Jupiter: Mercury, Venus, Earth, Theia and Mars. I say Earth, but back then, and we’re talking about 4.5 billion years ago, the third planet from the sun was a different size to the one we live on. So it’s better to say proto-Earth or early Earth. Theia was a planet about the size of Mars, which is smaller than Earth.
Theia’s orbit wasn’t settled. This was only 50 million years or since the solar system itself had formed, and everything was a bit chaotic. In an accident that set off an extraordinary series of events, Theia crashed into the proto-Earth. There was an explosion of unimaginable scale and energy.
George Darwin — son of Charles — was the first to suggest this as the origin of the moon in 1898, and it’s since become known as the giant-impact hypothesis. To mix our folkloric examples, Theia is the name of the ancient Greek Titan who gave birth to the moon goddess Selene. (Mixing of legends, by the way, is acceptable astronomical practice: Selene is the name of a moon orbiter launched by Japan in 2007 that is also known as Kaguya — the moon princess from the famous Japanese fairy tale “The Tale of the Bamboo Cutter.”)
Until recently, all the models for the formation of our moon ended up showing that it should be formed by rocks from Theia and not Earth. Hosono’s new model seems to have solved the problem by thinking of Theia colliding into an Earth covered by an ocean of magma. After the titanic impact, a monstrous plume of molten rock was smashed into space, and there it gradually cooled into a solid mass — our moon. According to JAMSTEC’s simulations, the rest of Theia eventually fell back and sank into the Earth.
There are many missions being planned to the moon by countries such as the U.S., Japan, India, China and Israel, plus several private space companies, including Blue Origin, owned by Amazon founder Jeff Bezos. Hosono is positive about the developments.
“I think the simplest next step is to carry out high precision measurements of as many lunar fragments as possible,” he says.
What about human settlements on the moon?
“If there is a village on the moon, we might do on-site measurements,” he says. “That sounds interesting.”
In a way, Hosono’s story of the formation the moon is analogous to the story from Japanese folklore, which described it as being formed from a teardrop washed from his father’s eye. In Hosono’s explanation, the moon was formed in the distant past, from a giant teardrop of molten rock, smashed from its parent planet.
Rowan Hooper is managing editor of New Scientist magazine. He tweets at @rowhoop and his book, “Superhuman: Life at the Extremes of Our Capacity,” is out now, published by Simon & Schuster.