Earth gave birth to the Moon

[anglolawyer]

Did the Earth give birth to the Moon in a nuclear event as hypothesised here?

Our Nuclear Moon
It may come as a surprise to many readers, but science cannot fully explain the origin of our own Moon.

Existing models, based on the collision of a young Earth with another, Mars-sized, planet are good at reproducing the angular momentum (orbit, rotation) of the Earth-Moon system, but they can’t explain why Moon rock is so very similar to Earth rock in terms of isotope composition.

We therefore need better models, says Rob de Meijer, Professor Emeritus in Nuclear Geophysics at the University of Groningen. In a paper just published in the journal Chemical Geology (8 May), De Meijer and his colleagues Wim van Westrenen (VU University Amsterdam) and Vladimir Anisichkin (Russian Academy of Sciences) argue that the Earth may have given birth to the Moon after a runaway nuclear explosion deep inside our planet.

‘Collision models predict that 80 per cent of the Moon would originate from the impactor and just 20 per cent from the Earth’, De Meijer explains. But the moon rock that the Apollo missions brought back to Earth tells a different story. It is very similar in composition to the Earth’s mantle.

But could the impactor not be an Earth lookalike? Not according to De Meijer, because planet-formation models rule this out.

For the mentally challenged, in words of one syllable, what is an 'isotope'?

 

[wollery]

Atoms are made up of protons and neutrons. The number of protons decides what element it is. The number of neutrons can vary. Atoms with the same number of protons but different numbers of neutrons are different isotopes of the same element.

For example, carbon always has 12 protons, but can have either 12 or 14 neutrons.

 

[anglolawyer]

Atoms are made up of protons and neutrons. The number of protons decides what element it is. The number of neutrons can vary. Atoms with the same number of protons but different numbers of neutrons are different isotopes of the same element.

Thanks wollery. What difference does it make for an element to have different isotopes? Is this what carbon14 or carbon16 means (if such things exist) and strontium 90 etc? If I had two lumps of carbon with different isotopes could I tell the difference? And do certain isotopes of an element degrade or evolve into other forms of that element?

Congrats on the TLA award btw.

Edit, just saw your edit - substitute 12 for 16 above.

 

[Brian-M]

Thanks wollery. What difference does it make for an element to have different isotopes? Is this what carbon14 or carbon16 means (if such things exist) and strontium 90 etc? If I had two lumps of carbon with different isotopes could I tell the difference? And do certain isotopes of an element degrade or evolve into other forms of that element?

Congrats on the TLA award btw.

Edit, just saw your edit - substitute 12 for 16 above.

From Wikipedia...

For example, the abundant carbon-12 isotope has 6 protons and 6 neutrons, while the very rare radioactive carbon-14 isotope has 6 protons and 8 neutrons.

You can tell the difference because the more neutrons an isotope has, the heavier it is.

Carbon 12 and Carbon 13 are stable, all the other carbon isotopes are unstable (ie, radioactive). Different unstable isotopes have different decay rates.

Carbon 14 has a half-life of 5730 years. This means that if you have a bunch of Carbon 14, after 5730 years half of it would have decayed into something else.

Carbon 16 has a half-life of about 0.748 seconds. This means if you had a bunch of Carbon 16, you lose half of it to radioactive decay every 0.748 seconds.

 

[anglolawyer]

OK, thanks for the isotope primer, which I appreciate. Back to the OP and the reason I asked:

Existing models, based on the collision of a young Earth with another, Mars-sized, planet are good at reproducing the angular momentum (orbit, rotation) of the Earth-Moon system, but they can’t explain why Moon rock is so very similar to Earth rock in terms of isotope composition.

So, this is suggesting the Moon is made of the same stuff as the Earth. Can't it just be that out of the cloud from which the Earth condensed, the moon condensed also? So they are made of the same stuff but one did not come out of the other? Or can the alleged isotope correspondence just be coincidence?

 

[Alareth]

The moon is composed of ejecta from the planetary impact between the proto-Earth and the large impactor.

It only makes sense that they would share the same composition.

 

[Brian-M]

So, this is suggesting the Moon is made of the same stuff as the Earth. Can't it just be that out of the cloud from which the Earth condensed, the moon condensed also?

Rocks on the surface of the earth have a different composition than the earth itself, because most of the heavier elements tended to sink into the core. But the moon isn't big enough for this effect to be as great, so I'd guess that moon rocks should have higher proportions of the heavier elements, but don't.

 

[nathan]

So, this is suggesting the Moon is made of the same stuff as the Earth. Can't it just be that out of the cloud from which the Earth condensed, the moon condensed also? So they are made of the same stuff but one did not come out of the other?

Exactly. The Theia hypothesis has the impactor forming at a Lagrange point. It's therefore going to be coalescing from the same stuff as Earth. To me (not an expert), it is unsurprising that the Earth and Moon are made of similar stuff.

 

[WhatRoughBeast]

Much worse, I rthink, is the idea that there was a "runaway nuclear reaction".

I say worse because this would result in massively skewed ielementtal abundances and sotope ratios, showing a whole lot of daughter species. Think of the Oklo http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor event but on a planetary scale.

 

[anglolawyer]

Exactly. The Theia hypothesis has the impactor forming at a Lagrange point. It's therefore going to be coalescing from the same stuff as Earth. To me (not an expert), it is unsurprising that the Earth and Moon are made of similar stuff.

I have to look up Lagrange point now

But what about this idea that planets can spontaneously explode? That doesn't sound right. Is there a mechanism for planetary explosion? Are we about to explode again?

 

[catsmate]

Thanks wollery. What difference does it make for an element to have different isotopes? Is this what carbon14 or carbon16 means (if such things exist) and strontium 90 etc? If I had two lumps of carbon with different isotopes could I tell the difference? And do certain isotopes of an element degrade or evolve into other forms of that element?

Chemically isotopes behave the same (there are exceptions where the difference in mass is significant, mostly hydrogen and light elements) and in most chemistry the difference between isotopes is ignored.
Generally the radioactive isotopes can't change into non-radioactive forms of the same element as there isn't a suitable mechanism.
For example:
²³⁵U --> ²³¹Th + α
here a Uranium nucleus loses a chunk of its constituent particles and becomes a Thorium nucleus. Fairly quickly the Thorium nucleus will also change, with one of its neutrons turning into a proton (releasing a beta particle in the process), becoming Protactinium.

¹⁴C --> ¹⁴N + β−
Here a carbon-14 nucleus undergoes beta decay, turning into a nitrogen-14 nucleus


You can consider the nucleus as a constantly flexing, writhing, mass of neutrons and protons; sometimes it flexes in such a way that part of it escapes (usually an alpha particle, a cluster of two neutrons and two protons) or the whole thing breaks apart (fission).
There are two forces at work in the nucleus and the balance between them determines the stability of the configuration. The electromagnetic force of repulsion between positively charges protons and the strong nuclear force which causes attraction between all the particles. The strong force is more powerful but shorter ranged and falls off more quickly than the electromagnetic force, hence for heavy elements (beyond lead) no configuration is stable.

I have to look up Lagrange point now

The stable points in a system with gravitational interaction between multiple bodies. The wiki page is pretty good.

But what about this idea that planets can spontaneously explode? That doesn't sound right. Is there a mechanism for planetary explosion? Are we about to explode again?

Planets are held together (mostly) by gravity, they don't explode without an enormous energy input. The gravitational binding energy of the Earth is ~2.25x10³¹ Joules or around 5.4 million million million megatonnes.

Or the energy produced by mixing 250 billion tonnes of anti-matter and a similar amount of matter with complete efficiency (and ignoring the loss of ~50% of the energy as neutrinos)

 

[wollery]

Oops, brainfart on the number of protons/neutrons in Carbon!

My excuse is that I was sitting in a meeting and posting from my phone.

 

[nathan]

I have to look up Lagrange point now

In this case it's one of the points 60degrees in advance of Earth or following. Those are stable. If you do the math for large Sun and Earth, but negligible mass there, it's a gravity well. As the mass at that point increases it becomes more unstable.

The Trojan asteroids are trapped in the same points WRT Jupiter.

But what about this idea that planets can spontaneously explode? That doesn't sound right. Is there a mechanism for planetary explosion? Are we about to explode again?

No. The energy to explode an Earth-sized planet is stupidly large.

http://www.blastr.com/2011/09/astronomer-explains-why-w.php

http://en.wikipedia.org/wiki/Orders_of_magnitude_(energy)

ETA: I've just done the math for TNT explosion. It appears one'd need 3x10²²m³ of tnt, which is approximately 38 times Earth's volume. (I'm neglecting TNT compressibility due to it's own gravity well)

 

[nathan]

Planets are held together (mostly) by gravity, they don't explode without an enormous energy input. The gravitational binding energy of the Earth is ~2.25x10³¹ Joules or around 5.4 million million million megatonnes.

Phil Plait's article and the wikipedia page I just quoted claim it's 2x10³² -- ten times bigger.

 

[anglolawyer]

Is the moon at a Lagrange point between Earth and Sun? I am surely not getting this, despite reading the wiki link. I understand the Earth and Sun both orbit a common centre of gravity, probably located somewhere inside the Sun but when you throw a third object into the mix, does that also orbit that common CofG but at a one of five intermediate points/orbits?

 

[anglolawyer]

Oops, brainfart on the number of protons/neutrons in Carbon!

My excuse is that I was sitting in a meeting and posting from my phone.

Yes, I picked it up on the wike. Carbon 14 is 6 protons and 8 neutrons. Fear not. I got that far . If anyone has the patience to attempt to explain how the heck we are able to know this with complete precision I would appreciate it.

 

[RecoveringYuppy]

Exactly. The Theia hypothesis has the impactor forming at a Lagrange point. It's therefore going to be coalescing from the same stuff as Earth. To me (not an expert), it is unsurprising that the Earth and Moon are made of similar stuff.

The impactor forming at a Lagrange point? How would we know and Lagrange point between what two objects?

ETA: http://en.wikipedia.org/wiki/Giant_impact_hypothesis This hypothesis has a secondary collision between two proto moons happening due to unstable Lagrange points. The origin of Theia object doesn't appear to be known or assumed.

Is the moon at a Lagrange point between Earth and Sun?

No, the Moon is in nearly circular orbit about the Earth. The nearest Earth Sun Lagrange point is about a million miles away (see SOHO mission).

 

[nathan]

Is the moon at a Lagrange point between Earth and Sun?

No. the moon is not at a lagrange point. The stable lagrange points in Earth's orbit are where Earth was 2 months ago and where it will be in 2 months time (1/6th of the way round the orbit).

The Earth-moon system is pretty much like a double-planet. The barycenter (the common center of rotation) is about 1000kmmiles below the surface of the Earth. The size and speed of the moon's orbit around Earth are such that it is never convex (or retrograde) to the sun. An observer at the sun will always see the Moon travelling in the same direction, and the chord joining any two points on its path never crosses the path at an intermediate point (Hopefully I've phrased that correctly).

I am surely not getting this, despite reading the wiki link. I understand the Earth and Sun both orbit a common centre of gravity, probably located somewhere inside the Sun but when you throw a third object into the mix, does that also orbit that common CofG but at a one of five intermediate points/orbits?

Once you throw more objects in, you run into what's known as 'the three body problem' . There is no analytical solution for 3 or more objects obeying Newton's laws of gravitation. One ends up approximating. Lagrange's approximation is that the 3rd mass is negligible. Treating the Earth-moon system as a single mass in such a system presumes that tidal forces can be ignored. That's true if one's sufficiently far away. That's true when determining the orbit around the sun, and also (I suspect) of the two Lagrange points we're talking about.

The other 3 Lagrange points are unstable, btw. Meaning that a slight perturbation will cause the object to drift further away. The 2 points being discussed here are stable -- a slight perturbation will return to equilibrium.

ETA:Units

 

[catsmate]

Phil Plait's article and the wikipedia page I just quoted claim it's 2x10³² -- ten times bigger.

Let me check my calculations........

 

[nathan]

The impactor forming at a Lagrange point? How would we know and Lagrange point between what two objects?

Yes, that's the Theia hypothesis. An object coalesced at a Lagrange point of the Earth-Sun system. As it grew it got progressively more unstable -- advancing and retarding along Earth's orbit. Eventually colliding with Earth.

https://en.wikipedia.org/wiki/Giant_impact_hypothesis I can't find an animation showing Theia's position in sun-centered rotating frame of reference.