Earth gave birth to the Moon

[RecoveringYuppy]

@Nathan, I edited my post while you were posting apparently and included that link. I have a different read on it than you do. It sounded to me like you're talking about the hypothesized second impact which involved a secondary moon that coalesced at a Sun Moon Lagrange point and then crash in to the primary Moon. Theia's origin via a Lagrange point is much less certain.

ETA: Possibly just a disagreement about what is consider part of the hypotheses and what isn't.

 

[nathan]

@Nathan, I edited my post while you were posting apparently and included that link. I have a different read on it than you do. It sounded to me like you're talking about the hypothesized second impact which involved a secondary moon that coalesced at a Sun Moon Lagrange point and then crash in to the primary Moon. Theia's origin via a Lagrange point is much less certain.

ETA: Possibly just a disagreement about what is consider part of the hypotheses and what isn't.

I'd confused Theia-is-impactor with Theia-originates-at-lagrange. I see the latter is a separate hypothesis (necessarily presuming the former). I'm not talking about a subsequent impactor.

 

[RecoveringYuppy]

I'd confused Theia-is-impactor with Theia-originates-at-lagrange. I see the latter is a separate hypothesis (necessarily presuming the former). I'm not talking about a subsequent impactor.

Yeah, I get it. I should have figured out you were talking about Theia's origin.

 

[Vinegar Tom]

Sorry if this is hopelessly wrong - I'm no astrophysicist! - but the accepted theory about the Moon's origin seems to be that another quite large body hit the Earth a very long time ago, and the resulting debris coalesced into the Moon, which therefore has the same ratio of isotopes as the Earth.

But what happened to this other object that collided with our planet? Presumably it would have a different isotope ratio, and the Moon should be partly made from debris from it. So should the Earth - is there one patch of the globe where the isotopes are peculiar because it's actually another planet? And if not, why not?

This theory about the Moon being ejected from the Earth by a spontaneous nuclear explosion, which must have been absolutely gigantic - many orders of magnitude greater than any nuke we can build, since they've tested some pretty big ones without tearing huge lumps off the planet! - sounds a bit far-fetched. Also, it's suspiciously similar to the fate of Superman's home planet Krypton! But if it's feasible, it would rule out the need for the Earth to be hit by something else. Could one of you who understands Lunar geology and so on address these points?

By the way, I dimly remember reading many years ago that there's a place - I think it's in Africa - which is a little bit warmer than it should be because of a natural nuclear reactor deep beneath the ground, and what's more, this thing caused a spontaneous nuclear explosion at some time in the distant past! Presumably this was a fairly small bang, otherwise we'd have a second Moon that used to be Africa. But is this true? I read it so long ago that I can't remember what kind of book it was in, so it may not have been very trustworthy. But it's interesting, so I'd like to know.

 

[BowlOfRed]

Sorry if this is hopelessly wrong - I'm no astrophysicist! - but the accepted theory

I think the theory is actually less-well-accepted today than it was 4 or 5 years ago. Those darn planetary geochemists are screwing everything up.

about the Moon's origin seems to be that another quite large body hit the Earth a very long time ago, and the resulting debris coalesced into the Moon, which therefore has the same ratio of isotopes as the Earth.

But what happened to this other object that collided with our planet? Presumably it would have a different isotope ratio, and the Moon should be partly made from debris from it. So should the Earth - is there one patch of the globe where the isotopes are peculiar because it's actually another planet? And if not, why not?

And this is in fact the problem. At all reasonable spin rates, the moon should be more impactor than earth. You can make everything work out by ramping the spin rate way up (and there have been some new ideas devised that show how such a spin rate could be slowed down to look like it does today).

But changing the spin rate kind of throws a monkey wrench into things. It's now a new degree of freedom. Why this, why that? Although a very high spin rate is possible, there's no direct evidence for it.

This theory about the Moon being ejected from the Earth by a spontaneous nuclear explosion, which must have been absolutely gigantic -

Stupendously gigantic. There's no model I'm aware of that uses that mechanism. In the early solar system, that much energy comes from impactors easily.

By the way, I dimly remember reading many years ago that there's a place - I think it's in Africa - which is a little bit warmer than it should be because of a natural nuclear reactor deep beneath the ground, and what's more, this thing caused a spontaneous nuclear explosion at some time in the distant past!

It's not that it's warmer, it's that the isotopes are off from natural decay. It's consistent with a nuclear reaction (not an explosion) creating fission decay products. This isn't a mechanism for the moon at all.

 

[Hellbound]

And this is in fact the problem. At all reasonable spin rates, the moon should be more impactor than earth. You can make everything work out by ramping the spin rate way up (and there have been some new ideas devised that show how such a spin rate could be slowed down to look like it does today).

But changing the spin rate kind of throws a monkey wrench into things. It's now a new degree of freedom. Why this, why that? Although a very high spin rate is possible, there's no direct evidence for it.

Just out of curiosity, does the spin rate of the impactor play a part at all? I can see the earth spin rate play a part in "mixing" the impactor material and spreading out the damage, just wondering if the same might hold true for the impactor.

Also, what if it were multiple impactors rather than a single large one? Don't know if that would have the same effect or just make things worse, though. But how would smaller high-speed impactors over a (geologically) short period of time (like ten thousand years or so) affect the models?

Or, if no one feels like typing, where can I find some good information on how these variables would affect the models?

 

[Metullus]

Just out of curiosity, does the spin rate of the impactor play a part at all? I can see the earth spin rate play a part in "mixing" the impactor material and spreading out the damage, just wondering if the same might hold true for the impactor.

Also, what if it were multiple impactors rather than a single large one? Don't know if that would have the same effect or just make things worse, though. But how would smaller high-speed impactors over a (geologically) short period of time (like ten thousand years or so) affect the models?

Or, if no one feels like typing, where can I find some good information on how these variables would affect the models?

A TARDIS would come in handy right about now...

 

[BowlOfRed]

Just out of curiosity, does the spin rate of the impactor play a part at all? I can see the earth spin rate play a part in "mixing" the impactor material and spreading out the damage, just wondering if the same might hold true for the impactor.

From my understanding (which is limited), not much. It's not so much mixing the material as it is extra energy to get material from the earth (rather than the impactor) to leave the planet.

Also, we only have certain samples from the moon. There is some chance that the upper crust of the moon is significantly different from lower layers which changes what is needed from a model.

Also, what if it were multiple impactors rather than a single large one? Don't know if that would have the same effect or just make things worse, though. But how would smaller high-speed impactors over a (geologically) short period of time (like ten thousand years or so) affect the models?

Or, if no one feels like typing, where can I find some good information on how these variables would affect the models?

No idea on the mathematical models. There were a couple of papers in Science back in October on these ideas.
http://www.sciencemag.org/content/338/6110/1040.summary?sid=72b365ff-1888-4c35-862d-32b547fd9130
http://www.sciencemag.org/content/338/6110/1052.abstract
http://www.sciencemag.org/content/338/6110/1047.abstract

Also, there was a Science Friday talk around then with some discussion. I don't know if there is a transcript, but I thought this one was good about bringing up some of the ideas behind the papers. http://www.sciencefriday.com/segment/10/19/2012/scientists-in-the-dark-over-birth-of-the-moon.html

 

[Hellbound]

Thanks, BowlofRed!

Digesting the articles now (I have a bit of time at work as I'm letting test code run, waiting for results, so it's a nice time filler for those odd 5 minutes here and there )

ETA: Well, I was going to read them, but have no login. Darnit!

Seems I can get ot your final link, though

 

[BowlOfRed]

Yup. I don't have a subscription. You can try your library. I'm sure some folks here have access as well.

 

[Hellbound]

Yup. I don't have a subscription. You can try your library. I'm sure some folks here have access as well.

I'll check the library this weekend. Been a while since I've been by there anyway

 

[Puppycow]

This is funny:

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.

How is a completely wacky idea like that a "better model" than a plausible model that may have some unanswered questions about minor aspects but basically explains the thing fairly well?

 

[shadron]

This is funny:

How is a completely wacky idea like that a "better model" than a plausible model that may have some unanswered questions about minor aspects but basically explains the thing fairly well?

Mainly because it is their idea. However, it is in the essence of science to allow for this sort of thing. Now all they have to do is create a theory which satisfies astronomy, geology and nuclear physics simultaneously, and start making some verifiable predictions.

 

[shadron]

By the way, I dimly remember reading many years ago that there's a place - I think it's in Africa - which is a little bit warmer than it should be because of a natural nuclear reactor deep beneath the ground, and what's more, this thing caused a spontaneous nuclear explosion at some time in the distant past! Presumably this was a fairly small bang, otherwise we'd have a second Moon that used to be Africa. But is this true? I read it so long ago that I can't remember what kind of book it was in, so it may not have been very trustworthy. But it's interesting, so I'd like to know.

You probably are remembering a reading about the natural uranium reactors at Oklo^WP. These formed by the slluvial concentration of uranium ores, 2 billion years ago, back when the percentage of naturally occurring U235 was high enough to approach being reactor fuel. The reactors ran on hourly-to-daily cycles of boiling away and then recapturing moderating water, and ran for several hundred thousand years, before depleting their U-235 resources. That have long since shut down, and have lately become ores for France's uranium mining concerns.

 

[Reality Check]

If anyone is interested, there is a pre-print of the paper available: Forming the Moon from terrestrial silicate-rich material
The actual expulsion of the Moon from the Earth by a nuclear explosion is an interesting idea. The explosion is supposed to have generated ~2.5 * 10^30 J.
The authors show that "The calculations above show that without additional concentration factors, U, Th and Pu concentrations in the CMB are insufficient to reach criticality" and then mention a couple of extra concentration factors: pressure waves from an impact event and the development of compositional heterogeneities.

I can see a few problems.
The authors do not talk about the concentrations of the fissionable isotopes which are much lower then the natural element.
Uranium-235: "Uranium-235 is an isotope of uranium making up about 0.72% of natural uranium".
Plutonium is very rare and its fissionable isotopes are basically a tiny percentage in uranium deposits or artificial.

There is no evidence of a molten surface caused by this large release of energy (this is also a problem for the Giant impact hypothesis).

I would expect there to be detectable daughter products (as a previous poster noted).

 

[Sockor]

235U makes up 0,72% of natural uranium today. It was more abundant in the past: off the top of my head about 3-4% when Aklo reactor was running some 1,8-2 billion years ago. There would have been even more of it earlier, perhaps even some plutonium.
But to have enough fissiles for a big bada boom? I certainly doubt that.

 

[Reality Check]

235U makes up 0,72% of natural uranium today. It was more abundant in the past: off the top of my head about 3-4% when Aklo reactor was running some 1,8-2 billion years ago.

Whoops - you are right, about 45.5% when the Earth formed (85 times more abundant).

 

[Hellbound]

Reality Check:

Regarding the evidence of a molten surface, my understanding was that the impact theory (and I would assume, by extension, this one) occurred before the earth fully cooled, while the surface (or much of it) was still molten (or mostly molten). Thus, there wouldn't really be any indication of a molten surface.

Likewise, the moon would have been around long enough for any surface features to be largely lost by meteorite impacts over time (or at least lost without an extensive survey of the surface).

Am I incorrect in that understanding? I haven't studied any of this in-depth, just an interested layman

 

[Sockor]

I made some calcultations with an axe and wood: it would take only 6 650 000 kg of 235U nuclei to fission to have an atomic explosion yield the total amount of energy cited (2,5*10^30 J) under same conditions as a human-made bomb. That is the mass of the fissioning nuclei put together, not the critical mass or the whole amount of uranium present at any particular place. Also some of that energy will not do work to put stuff to orbit to achieve the cited figure. It was just a fun exercise in scale.

 

[Alferd_Packer]

Are we about to explode again?

That's no moon.