dougettinger wrote:Then I should assume that any sizable ejecta that is flung beyond escape velocity after a major impact is from the impacted body.
Yes, except for a case like the formation of the Moon, where you have two huge bodies of similar size. The collision dynamics in that case are very different. I'm assuming we're talking about "typical" collisions, where an asteroid a few km in diameter or smaller strikes a rocky planet. In that case, the impactor is vaporized, along with a large volume of the material around the impact site. Some of this vaporized material may be ejected to space, but most will probably remain in the atmosphere and rain back down over time. There will be a region a little outside the impact point where planetary material may avoid vaporization but still get enough energy transferred to it to escape the planet. That is how we have Martian meteorites on Earth.
Can we assume that 3.9 billion year ago the Earth was molten and much softer under its developing crust when the impact occurred?
Not molten, but certainly hotter, and with very different crustal composition. But I don't think that matters in the slightest. The energy released from two planets colliding is so huge that the result will be the same whether those planets have molted cores or are frozen bodies.
I question the miracle of how the Moon's final resting place is in an orbit with the same ecliptical plane.
Why is that a miracle? The collision was between two bodies that likely both lay near the ecliptic. And there are very strong tidal forces that will seek to flatten the lunar orbit. Keep in mind that dynamically, the Moon isn't even in orbit around the Earth, it is actually in orbit around the Sun.
And why was only one moon formed ? Should there not exist some larger collisional debris that did not form into a spherical shape?
The more bodies you have, the more unstable the system is- especially if the bodies are in similar orbits. As collisional material coalesced, you'd expect the largest body to act as a sort of vacuum cleaner, sweeping up or ejecting other material. If the amount of ejected material was much less, that might be different. But what formed was more of a binary planet than a planet-moon system.
Why does the moon have a comparatively large iron core?
Because both bodies involved in the collision were differentiated planets with iron cores. The iron that was ejected settled to the core of the Moon as that body differentiated and cooled.
Why does not the Moon crustal materials match those of the Earth's mantle and crust?
In most important respects, it does. Of course, it has undergone some modification through the Moon's own early volcanism, and the Earth's crust is continually being modified.
And where does such a large impactor come from in the early solar system?
Most models that attempt to describe the formation of the Solar System have more planets than we see today. Gravitational interactions can result in rapid and substantial shifting of orbits (that this can happen is a fact; whether it actually happened in our own system is a matter of uncertainty).
How does Jupiter actually perturb the long standing orbits of asteroids?
All realistic orbits in a system with more than two bodies are unstable. An asteroid is orbiting the Sun, but it is also subjected to a varying force from Jupiter and the other planets. Resonances are particularly effective at altering asteroid orbits, and occur when perturbing forces peak in the same place during each orbit. That's why the asteroid belt looks like it does, with big gaps where there is no material (and Saturn's rings, as well).
Martian Moon Phobos from Mars Express
