Question about J White's moon rock composition

[Luke Pemberton]

I['m?] always somewhat irritated when I see the way Hollywood depicts an asteroid belt.  They have asteroids bouncing of each other and spacecraft dodging them, while in real life asteroids are hundreds of kilometers apart.

Yeah, but when you're a kid it leads to classic scenes:

I have to agree though, I wonder how many such misconceptions are created by Hollywood and the actual harm it does to education. It's sometimes very difficult to shift ideas in kids. Maybe films should come with scientific warnings, much in the same way that tobacco packets have warnings about impending death. Should films have warnings stating that 'if you believe the content without question you will be stupid.'

[ka9q]

The impact theory explains a little more than the geology, it is also a good candidate to explain the relatively high angular momentum of the Earth-Moon system.

Do we have any idea of the length of the earth's day before the impact?

Given the similar size and mass of Earth and Venus, and the latter's extremely long (and backward) day and lack of a moon, I wonder if they were even more alike before the impact.

[Bob B.]

Do we have any idea of the length of the earth's day before the impact?

I don't know what it was before the impact, but I read somewhere that the rotation period immediately after the Moon's formation was about 6 hours.

[ka9q]

Yes, that I can believe. Since its formation the moon itself has lost angular momentum as it tidally locked with the earth and then lengthened its day (and our month) as it moved away. But it gained considerably more angular momentum in its orbit, decreasing the earth's own angular momentum and significantly lengthening our day.

But can we even tell how long was an earth day before the Theia impact? Maybe it was even as long as Venus?

Here's how we might estimate the pre-impact day length. We know Theia was about the size of Mars, so we know its mass. We know it co-formed in the earth's orbit, most likely at L4 or L5 until it got too massive to be stable. From that we can determine the probable impact velocity.

Somebody has probably determined the angle at which it must have struck the earth to produce the moon we now have, so from that we could determine how Theia's angular momentum was divided into escaped debris, the earth-moon system's orbit around the sun, the moon's orbit around the earth, and the rotation of the earth and moon around their own axes. And from all that we could work backward to what the earth's angular momentum would have been prior to the impact.

[Bob B.]

I wonder how many such misconceptions are created by Hollywood

Nebulae is another thing that they routinely misrepresent.

[Luke Pemberton]

Nebulae is another thing that they routinely misrepresent.

This really grates you doesn't it? You're making me laugh. You and I are the original grumpy old men. I really dilsike laser weapons and their representation. In the Empire Strikes Back, I estimate that Darth Vader had about 30 nanoseconds to fend off Han Solo's blaster. That's some reaction time, even for a Sith Lord.

ETA: Correction to seconds conversion. I can't convert to nanoseconds anymore, what is wrong with me?

[Luke Pemberton]

Do we have any idea of the length of the earth's day before the impact?

I've seen similar figures to Bob, around 6 days.

[Dalhousie]

Do we have any idea of the length of the earth's day before the impact?

I've seen similar figures to Bob, around 6 days.

I would be sceptical of such figures, they are model-based and therefore assumption controlled.  Unlike modelling of giant impacts, they don't lead to testable predictions.

[Luke Pemberton]

I would be sceptical of such figures, they are model-based and therefore assumption controlled.  Unlike modelling of giant impacts, they don't lead to testable predictions.

Agreed, I would not take that number to be definitive. Having previously worked in model based analysis for 15 years and written various Monte Carlo simulations myself, I understand the confidence with such models. I think we all understand that they are models, but they do make for interesting scientific discussion? After all, someone must have confidence with their credence to write them in the first place and then publish the results so they become part of the annals of science.

Someone must have done the sums to explain how the impact event explains the high angular momentum of the Earth-Moon system. It has to be derived from some point.

[frenat]

Nebulae is another thing that they routinely misrepresent.

This really grates you doesn't it? You're making me laugh. You and I are the original grumpy old men. I really dilsike laser weapons and their representation. In the Empire Strikes Back, I estimate that Darth Vader had about 30 nanoseconds to fend off Han Solo's blaster. That's some reaction time, even for a Sith Lord.

ETA: Correction to seconds conversion. I can't convert to nanoseconds anymore, what is wrong with me?

Star Wars blasters are not lasers.  They are supposed to be a particle beam weapon, though I couldn't find any info on the speed of the blaster "bolt".
http://starwars.wikia.com/wiki/Blaster

[Luke Pemberton]

Star Wars blasters are not lasers.  They are supposed to be a particle beam weapon, though I couldn't find any info on the speed of the blaster "bolt".

I never knew that. I feel much more content now, and shall correct others that lobby the same criticism at the incorrect portrayal of blaster weapons.

[ka9q]

Someone must have done the sums to explain how the impact event explains the high angular momentum of the Earth-Moon system. It has to be derived from some point.

When they talk about the "high angular momentum of the earth-moon system", I think they're excluding the angular momentum in the earth-moon system's orbital revolution around the sun. They're considering only the rotation of the earth, the rotation of the moon, and the revolution of the moon around the earth.

What actually had to be conserved through the Earth/Theia collision was the total angular momentum of Earth and Theia, including their orbital angular momenta around the sun (which would have greatly exceeded the angular momenta of their own rotations).

This "pool" of angular momentum was redistributed by the collision. Whatever angular momentum didn't go into the moon's orbit (which probably dominates the local angular momentum of the earth-moon system) would have gone into the angular momentum of the earth-moon system around the sun and to any escaped debris.

If the collision was a glancing one, I would expect more of this momentum to go into the rotation of the earth and the moon's orbit (the part considered "high" by those studying lunar origins). If the collision were more direct, then I'd expect it to go mostly into the earth-moon system's angular momentum around the sun. It would all have to be conserved, so any change in the "local" angular momentum of the earth-moon system would have to be compensated for by a change in the length of our year.

So that's where it was "derived from".

[Luke Pemberton]

If the collision was a glancing one, I would expect more of this momentum to go into the rotation of the earth and the moon's orbit (the part considered "high" by those studying lunar origins). If the collision were more direct, then I'd expect it to go mostly into the earth-moon system's angular momentum around the sun. It would all have to be conserved, so any change in the "local" angular momentum of the earth-moon system would have to be compensated for by a change in the length of our year.

That's my understanding. In fact any more than a glancing blow, and the Earth may not have survived to have a year, so I was referring to the angular momentum of the Earth and Moon (does my inclusion of system imply the Sun too, as I need to remove the ambiguity in that case?)

I'm sure that this was discussed at length in the BBC's 'The Planets.'

[ka9q]

That's my understanding. In fact any more than a glancing blow, and the Earth may not have survived to have a year, so I was referring to the angular momentum of the Earth and Moon (does my inclusion of system imply the Sun too, as I need to remove the ambiguity in that case?)

Obviously the angular momentum of the solar system as a whole has to be conserved, but I'm assuming tidal effects from the sun are negligible. It only rotates once every 28 days so it's almost a perfect sphere, and we're also a considerable distance from it. So it's probably a good approximation to say that the total momenta of Earth and Theia were conserved through the collision and evolution of the moon.

[Jason Thompson]

Star Wars blasters are not lasers.  They are supposed to be a particle beam weapon, though I couldn't find any info on the speed of the blaster "bolt".
http://starwars.wikia.com/wiki/Blaster

In addition to which, Vader was reacting as much to Solo's drawing, aiming and firing of the weapon as he was to the speed of the bolt, and he had the advantage that he probably expected Solo to pull a gun on him the moment the door opened.

Furthermore, I know the prequel trilogy is seen as blasphemous to many, but remember that Qui-gon Jinn described the Jedi's (and by extension the Sith's) reflexes as partly being so rapid because they can sense things before they happen, which would give Vader even more of an advantage even if the gun had been firing a laser bolt: provided his hand is up and in place before the bolt leaves the gun it will be blocked.