The moon formed sometime last night. Or I mean, the night before. Or possibly right now. It’s of course hard to say. I was in New York City this weekend premiering a film, which is a lousy excuse for potentially failing to see a rock the size of Mars hurtling towards the still-young Earth and then — insert comic-book words here — WHAM! BLAM! ZAP! BLASH! Debris blasting into space, circling the molten, shuddering earth, coalescing quickly to make the moon.
That’s more or less the idea. Not long after the Earth initially condensed, an object astronomers call Theia slammed into it. When exactly? A paper last April in Nature, by Seth Jacobson et al, pinned the date at 95 million years after the initial formation of the Earth, give or take 32 million years. So with my current scale, where 1 day of my time is equivalent to 33.6 million years, this lands the moon formation somewhere in Jan 3 - 5.
In other words, maybe I can imagine this is happening right now. Theia is exploding itself onto the Earth. Rocks from the two bodies are intermingling as everything blends and blasts together in a most violent planetary kiss. Energy melts the rocks. The Earth grows. Ejected debris encircle the hot planet. In less than a hundred years - or one-third of a second on my time-scale! - the Moon forms. The molten, liquid Earth adjusts. Heavy iron and nickel descend to the core. Lighter elements float to the surface. Theia + Earth = new Earth + Moon. Au revoir, Theia!
This is all very formative, and there is much to discuss. Maybe tomorrow, as Cambridge's snowfall melts to slush, I’ll sort through the science of this early violence. For now, I’m closing my eyes trying to picture this collision without the aid of any computer-generated graphics.
If you lob a snowball in the air, and then quickly throw another snowball at that first snowball, do you make a moon? My father knew a guy in Germany who could throw an apple so hard at a tree, the apple would stick to the tree, as if it became part of the bark. In France today, somewhere, two old men are playing pétanque, lobbing silver spheres in the gravel. When one sphere strikes another, the ping rings out across the hills, echoing back onto itself a thousand thousand times as the bemused sun peers down and glints.