Where did the moon come from? It seems silly to ask, given that it has shone down upon us, brightening our nights with its cool white light since humans – since life itself – ever existed. But it’s an anomaly. Venus has no moon. Neither does Mercury. Mars has two tiny chunks of rock orbiting it…captured asteroids, most likely. But nothing that affects it like our moon does us. The moon is a quarter the size of Earth. Only Pluto – now relegated to dwarf planet status – has a moon such a considerable fraction of its own size (and Pluto is smaller than our moon.) All the other planets with moons have a host of much smaller bodies in their hold…fascinating worlds in their own right, of course, but still nothing like the unique relationship we have with our pale celestial partner. How did this come to be?
This video from National Geographic illustrates the current theory of the moon’s origin very nicely. First proposed by Dr. William K. Hartmann, astronomer and one of my favorite space artists, together with Dr. Donald Davis, this theory proposes that the moon was formed in a catastrophic collision of our primordial Earth and a Mars-sized planet over 4.5 billion years ago, when the Earth was still a sphere of molten rock. For a period of time after, the early Earth had a ring of ejected material from the collision, which then coalesced, cooled and eventually became the moon as we know it today. It’s a radical theory, sure, but it seems to fit with the results we are familiar with seeing in the sky every night. There’s a host of reasons…the iron-poor composition of the moon compared to the Earth, the matching isotopes in the lunar material, the speed and trajectories of the orbits…read more about them on the PSI site link above. But if this theory is right (and there’s really no standing reason why it isn’t) then when you look up at the shining crescent of the moon, you’re looking at a world born of our own planet billions of years ago, a reminder of an incredibly violent event that ultimately turned out very well for us.
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View more of Dr. Hartmann’s astronomical artwork on his site here. For more information and artwork like it, check out his books The Grand Tour and The History of Earth, which he wrote and illustrated with fellow artist Ron Miller. They’re two of my favorite reads on the subject of our planet and the solar system.