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dtilque
2006-Jul-30, 07:59 AM
Current theory says that a planet with about one tenth the mass of the Earth (i.e. Mars-sized) smacked the Earth upside the head and the resulting debris coalesced and formed the Moon. Several questions about this:

1. Has anyone named this other planet? I have a vague memory that someone has, but don't remember what name they've given it.

2. Do we have an idea of when the Big Splat occured? I think I remember that it was about 300 million years after the Earth formed, but perhaps someone has pinned it down a bit more exactly.

3. What was this other planet doing before the Big Splat happened? I imagine it must have been somewhere in the inner solar system, but that's a long time to be in what eventually turned out to be an unstable orbit. I have two hypotheses of my own about where it was: A) in what's now the asteroid belt and B) co-orbital with the Earth or Mars.

For A, obviously Jupiter must have perturbed the orbit to send it in an other-planet crossing orbit and it eventually hit one of them. For B, I'm thinking of a Janus-Epimetheus arrangement, which perturbations by the other planets eventually destabilized.

Any other ideas?

astromark
2006-Jul-30, 09:41 AM
We can run computer simulations of what the planetary disk was like in the early period of this solar systems life. Some calculations can be made of the approximate mass but, none of this 'science' is absolute. What set of the chain of events that bought about the collision that's debris resulted in the Moon. We can not with absolute certainty know.
I notice that there is now available a simple PC program that lets you build a solar system any way you like and you can run it and watch as the spiraling planets dance and eject them selves away when a second star gets inside the inner solar system. From memory i think 'solar' got me to it. It was interesting for the first ten hours. What I did notice was that almost any fiddling with this system bought about a catastrophic conclusion.

t@nn
2006-Jul-30, 12:16 PM
The object has been named Theia. One theory states that it formed at one of the Earth's Lagrangian points (L4 or L5) and later became unstable.

dtilque
2006-Jul-31, 08:10 AM
Thanks for the name, t@nn. That got me the Wikipedia article which says that the impact occured some 34 million years after the formation of the Earth. So my memory was wrong about the timing of it.

jlhredshift
2006-Jul-31, 12:41 PM
I have often wondered if there is any connection with the Moon forming event and Venus's slow retrograde rotation, possibly being turned upside down relative to solar system norm. There is no way to know for sure of course other than to say that the formation period had to have been chaotically catastrophic. (please forgive the imprecise description of Venus's condition by me).

dtilque
2006-Aug-01, 07:02 AM
Lots of people blithely assume that Venus's retrograde rotation must have been the result of a collision with another object, however that isn't necessarily so. Several years back, there was a report of computer simulations that showed that it could have been the result of solar tides in Venus's dense atmosphere. These tides exerted a braking motion on the planet's direct rotation, and apparently it overshot the tidal-locked position and went retrograde. I'm not sure if the tides will correct this and brake the retrograde motion or not. The report didn't say.


As far as Theia goes, can we tell the difference between it originating in a Lagragian point or a Janus-Epimethues-type co-orbit?

Ozzy
2006-Aug-01, 11:29 AM
As the mineral composition of planets gets less metallic and more gaseous as you move away from the sun, can the mineral compostion of the moon tell us from whereabouts in our solar system Theia Big Splat originated. Is the moon made from fragments of Theia, solidified Earth magma or both?