A while back, I saw a show on the Science Channel about the moon which claimed that the moon formed when the young earth collided with the planet Orpheus. The earth became the earth and Orpheus became the moon. Is this some fringe theory or is this the generally accepted theory of how the moon came into being? I hadn’t heard of this before and I haven’t heard about it since.
Thanks,
Rob
Orpheus? Some call it Theia.
The theory that a Mars-sized object hit the earth and threw off a molten chunk on the crust that became the moon (it probably rates theory status by now) is accepted by most astronomers. I haven’t heard it called Orpheus, but somebody somewhere might be doing so.
The notion isn’t very new either. There was a major conference at the University of Arizona on this in 1998, and it became a book, Origin of the Earth and Moon ed. by Robin M. Canup & Kevin Righter.
A good popular science book on the subject is The Big Splat, or How Our Moon Came to Be, by Dana Mackenzie.
This theory explains the composition of the moon and its orbital status better than any of the accreting bodies hypotheses, which is why it became the norm so quickly. It’s bolstered by the evidence of the moon rocks the astronauts brought back.
Definitely. If you take Astronomy 101 at just about any university, that’s the only theory of the formation of the moon that you’re likely to learn in class. If the professor has a special interest in moon formation or history of astronomy, you might hear about some other ones, but only as theories that have been rejected.
I haven’t heard it called Orpheus or Theia, either- just a “Mars-sized object”.
You’re wrong in one detail, though- Orpheus wouldn’t have become the moon. It would have collided with Earth and been destroyed. The material that made up Orpheus, together with some material splattered off Earth, would have made up a ring of debris around Earth, and the moon would have eventually formed from material in that ring. We’re pretty sure there are moons in the solar system that are captured objects, but those formed without a collision between the planet and the object, and also tend to be much smaller relative to their planet than the moon is relative to Earth.
I just love that one of the names for the theory is “The Big Whack”. 
I’m sorry if anything I said implied that. I meant specifically that the material that became the moon is thought to have formed from the earth’s crust - and possibly also the object’s crust, because the moon lacks most of the heavy metals that formed the earth’s core, which is a major problem with the earlier accretion theories.
I remember catching that show in progress and never getting to see the whole thing. It was all very interesting but IIRC, there was some discussion to the effect that the Earth (I think they called it Earth 1) may have been around long enough that life may have appeared and possibly even complex life.
The savage degree of The Big Whack was so sever that the entire crust of E1 would have been returned to the unstable lava-covered surface of a time following it’s beginning. The fact that Earth may have had life form even preceding the Great Dying that killed like 90% of all life is mind boggling to me. Wiped out without so much as a fossil to show for it.
The recent discussions that keep pushing back the first origins of life to even earlier than recently thought gives even more credence to this concept.
Anyone know the title of this program?
You didn’t; I’m pretty sure she meant the OP:
If There Was No Moon.
Perhaps it was “If There Were No Moon”
And now…The Moon.
This theory is, as others have noted, now generally accepted. Interesting to note, however, is that it was rejected by the scientific community at first and the guy responsible for the theory, Bill Hartmann, spent a fair amount of his professional career being mocked for having such a stupid idea. 30 years later he’s considered to be a pillar of the scientific community. Such is science.
Now, the mass of Earth and the Moon would have to be very close to the mass of E1 and Theia/Orpheus/Mars-sized-object, ja? I’d be inclined to think that the Sun had already showed up at this point. So how exactly do two planet sized objects collide with one another while under the influence of a star like the Sun? As I understand it the Earths orbit is a fairly predictable dance that doesnt change much. With an orbit changing so slowly wouldn’t two planets get close to each other VERY slowly, over millions of years, and prehaps “clip” each other before they collide? The Earth orbits Sun at what… 24 kilometers a second? Was this Mars-sized object hurtling through space without an orbit? Or more like a comet? I just don’t get it.
He’s in good company, then. Both the Big Bang and plate tectonics went through decades of being ridiculed and now look at them – fundamental theories of their respective sciences.
The Wikipedia article on Theia (see Squink’s link above) suggests that it spent the first years[sup]1[/sup] of its life in one of Earth’s Lagrange points, either L4 or L5. But those points are only stable for objects that weigh a small fraction (I think about 1/25) the mass of the smaller fo the other two bodies. Mars-sized means it weighed about a tenth the mass of hte Earth, so its orbit would have become unstable once it grew too massive. After that it would only be a matter of time until it crashed into another planet or a close encounter with one gave it enough energy to escape the Solar System.
[sup]1[/sup] Well, 34 million years. Only an eyeblink in the history of the Solar System, Theia, we hardly knew ya!!