Sunday, January 7, 2007

Apple #215: Origins of the Earth and the Moon

I just saw two programs on the National Geographic Society channel, and I want to share with you some of the things I learned.

The shows were about the beginning of the solar system and how the Earth was formed, but there was a particularly interesting section about the role that the moon has played in enabling life to begin and to thrive here on Earth.

So I'll tell you the story of the moon and the Earth, as I understand it. I should say up front that this story I'm about to tell is a series of scientific theories as they exist at this point. One story people used to tell about the universe was that the Earth was at the center and everything rotated around it. Now we have a different story to tell, supported by a lot more evidence. But it's possible and even likely that this story I'm about to tell will change in the future, as we learn more about our universe.

Okay, so here it is.

  • So there was this enormous explosion, right? Astronomers see stars blow up all the time, and what happens is all these enormous rocks and boulders and particles go shooting out in all directions. Just a mass of stuff everwhere, a huge cloud of it. And they say at that the beginning of the universe, the same kind of thing happened except on a massive scale.
  • But everything in the universe was spinning. So all the stuff in this huge cloud all had some spin going on. And you know how, when a figure skater does a spin on the ice, she has her arms spread out and she's going kind of slowly, but as she spins, she pulls her arms in and makes her body tighter, and she goes faster and faster the more compact she makes herself? Well, that's what happened to the stuff in the cloud.
  • As they spun, big rocks attracted littler rocks and got still bigger. Like in that video game, Katamari, if you've ever played it, stuff accumulated more stuff. Eventually, the rocks and particles and dust and all that consolidated to form the ancestors of the planets of our solar system.

One drawing of the process by which the solar system formed. Except I'd suggest that it wasn't quite as neat as that, and bear in mind that everything was more or less on fire for a long, long time.
(Drawing from Aerospaceweb)

  • The big, hulking pre-planets started throwing their weight around, moving in their own special, sometimes looping orbits. There were about twice as many planets then as there are now, and their orbits were quite different from the orbits the planets travel today.
  • One of those planets was ultimately going to be the Earth. But back then, Early Earth was enormously hot, covered with molten lava, a steaming, stinking, intolerable inferno. Within that ball of molten stuff, various elements started to consolidate, too. The heavy stuff like iron sunk to the middle of the ball, and the lighter stuff light carbon stayed at the surface.
  • That iron is still there, still molten, and as it swirls around, its motion generates a magnetic field. There's so much iron in the center of the earth, its resultant magnetic field is large enough to encircle the entire planet. And that's what it did way back at the formation of the Earth.
  • Now, remember, there were these other planets loping around the solar system, doing their wacky things, and combining with other, larger planets. Scientists now believe that one of those other planets, smaller than Earth, got sucked toward Earth and collided with it. This made Earth larger, but it also made the moon.
  • When that second planet hit ours, it got pulverized and a bunch of rocks and stuff also flew out from the Earth. Think of throwing a rock into a pile of dirt: a shower of dirt flies up into the air when you do that. The stuff that flew out from the Earth then over the course of about a hundred years, it coalesced and formed what we now know as the moon.

One rendition of a smaller planet colliding into what is now Earth. The resultant spray of rocks and stuff then was pulled together by gravity to form the moon.
(Drawing by David Darling)

  • After its formation, the moon continued to move farther away from Earth (and still is, at 1.5 inches per year) , but because it was smaller, gravity kept it rotating around the Earth even as it slowly cycled ever outwards.
  • That blow from the other planet, by the way, hit Early Earth at an angle, so it was more of a glancing blow rather than a head-on collision. And that impact knocked the Earth off-kilter, to its present tilt. This is crucial because the tilt is what allows our climates to have seasons and vary throughout the year.
But there's more about why the Earth is the way it is today which, surprisingly, has to do with the moon. I'll put that into a separate entry and continue The Story of the Earth and the Moon there.

Some Astronomy Fun
Choose your asteroid and send it hurtling toward a planet of your choice and see the result

National Geographic Society, Naked Science television series, Moon Mysteries and Birth of the Earth
NOVA Transcripts, Origin: Earth is Born and subsequent segments, September 28 and 29, 2004
Ron Cowen, "An early cosmic wallop for life on Earth?" Science News Online, December 2, 2000
Bernard Foing, "Linking the Earth to the Moon," Astrobiology Magazine, August 3, 2006
Jeremy Bailey, Australian Centre for Astrobiology, "The Inner Solar System Cataclysm, the Origin of Life, and the Return to the Moon"
G. Jeffrey Taylor, Origin of the Earth and Moon, Planetary Science Research Discoveries, December 31, 1998

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