Birth of the Moon: a Runaway Nuclear Reaction?

An anonymous reader writes "How the Moon arose has long stumped scientists. Now Dutch geophysicists argue that it was created not by a massive collision 4.5 billion years ago, but by a runaway nuclear reaction deep inside the young Earth."

Comments

Nuclear Reactor

[heavygravity]

Can't get to the article, but - if you haven't heard of this before, it's pretty cool: the Oklo Natural Fission Reactor in Gabon. And while you're at it, you can read about how this natural reactor has scientists rethinking how constant the fine structure constant really is.

Re:Impactors all the way

[Canazza]

We may never *know* for certain. We can have hypothesis after hypothesis, and although the giant impact fits the data nicely, and is unlikely to be wrong, the only way we'll really challenge that is by having other ideas. What really throws this theory out for me however (And I admit, I can't view the page, it's been /.ed) was that most of Earth's fissle material is in the crust, not the core. So any 'deep explosion' would have to have been in the crust or mantle, not the center.

Re:Impactors all the way

[mcgrew]

My problem with it is simple that the impactor idea seems to fit all the data so well I think it's unlikley to be wrong.

Further on they say that the impactor theory doesn't exactly fit the data. I'd blockquote, but I'm stuck on page three, I think we slashdotted it.

They give several reasons; one is that the object would have had to hit at a precise angle to become the moon and not completely vaporize the earth. Another is that the object would have had to have been formed very near the earth; they calculate from the moon rocks it would have had to be between Venus' and Mars' orbits.

Re:Teach the hypothetical controversy!

[MikeBabcock]

Christian-minded skepticism would sound a lot less idiotic (no offense to those of you who can't stand that), and something like:

Why do we think this might have happened? Because it might be possible. Do we have any proof of it? None whatsoever. Does it seem likely or probable? Not enough data. Could the moon have been spontaneously created by an infinitely powerful being instead? Sure.

From TFA

[mcgrew]

I know you're joking, but

In a major breakthrough reported in the U.S. journal Science in 2005, Earth scientists Maud Boyet and Richard Carlson of the Carnegie Institution in Washington DC, concluded that both a partition between the Earth's mantle and core, and another within the mantle, formed within 30 million years of the planet being born.

This internal partition isolated the lower mantle, the D''-layer, from the rest of the mantle. Boyet and Carlson arrived at their conclusion by investigating the rare earth elements samarium (Sm) and neodymium (Nd). Samarium-146 is a radioactive element that decays relatively speedily, with a half-life of 103 million years, to neodymium-142.

At present hardly any samarium-146 is left on Earth. Theoretically, terrestrial rock should contain just as much neodymium as the primordial material from which the Earth was formed - samples of which sometimes still reach the Earth in meteorites.

But the researchers discovered something odd. Rock from the Earth's mantle contains more neodymium than these meteorites. The only conceivable explanation is that samarium was distributed unevenly throughout the planet, because the overall concentration should be equal to that in meteorites.

But where can this neodymium-poor rock be? Not in the Earth's core, because neither samarium nor neodymium can bond chemically to iron. That only leaves the D''-layer. This chunky boundary layer between core and mantle must be low in neodymium.

Boyet and Carlson discovered that the Moon has a peculiarity too: rocks that are just as rich in neodymium as the Earth's mantle. This makes the impact hypothesis very improbable indeed, according to van Westrenen.

"Considering that at this giant impact 4.5 billion years ago the Earth's core and Theia's core fused, it is most improbable that isolated layers deep within the planet survived the impact. Yet this is what the data from Carlson and Boyet suggest."

Carlson was candid about this over the telephone: "Our data show a strong similarity between terrestrial and lunar rock, but there is no good explanation for that at all."

How the impact with Theia took place, and how the D''-layer survived this impact while the Earth's core fused with the core of the impactor, is beyond Carlson's comprehension as well.

How Big the Earth, How Thin the Crust

[Alien54]

Just how much crust there is is often misunderstood.

Example: imagine a model of the earth where 1 mm = 1 mile. (or you can use 1mm = 1 km, if you like)

The earth is 7926.28 miles (12756.1 km) in diameter.

At this scale, you can make out significant mountain ranges, etc. The Atmosphere would be 4 or 5 inches deep. The crust is an inch or 2 thick.

And the Earth itself is more than 8 yards across. That inch or two of crust is sitting on a chewy molten insides. (check volcano flows, etc.)

The Earth is really a molten droplet spinning in space with the thinnest external layer where life has happened to accumulate, like the layer of tarnish on a coin.

Re:Wouldn't there be an empty space?

[WhiplashII]

The sun's gravity accelerates things near the Earth by 0.006 m/sec^2. Two objects close enough to be meaningfully gravitationally linked while orbiting the sun in the Earth's orbit will have a maximum differential acceleration of maybe a thousandth of that. So to get to 1000m/s takes 5 years.

So this essentially posits that an explosion had enough force to blown the planet apart, and send the pieces into space, but not to escape velocity (11.2 km/s) but instead to a velocity just short of that (11.19 km/s or so), so that the moon goes flying away for 2.5 years but 2.5 years later comes back and settles into a nice, circular orbit.

That would be hard to accomplish on purpose - saying an accident did it is beyond belief.

Robert Heinlein, "Blowups Happen"

[dpbsmith]

It's really not the same at all, but the article did call this story immediately to mind.

"Blowups Happen" is a classic 1940s SF story about a future in which society is total dependent on nuclear power plants. The engineering theory behind them shows that they are intrinsically safe and cannot blow up like a bomb. Then someone discovers that there is a false assumption in the equations and that, in fact they can blow up like bombs.

Meanwhile, an expert in the theory of lunar formation has concluded the lunar craters cannot have been formed by meteor impact, because of the "rays." There had to have been enough energy to "crack an entire planet." The only possible explanation, he says, is that the Moon was once an inhabited planet with an atmosphere and that "Here at Tycho was located their main power plant, and here at Copernicus and Kepler, on islands of the middle of the great oceans, were secondary power stations."

In other words, not only can they blow up like bombs, but that is what reduced the Moon to its present airless, lifeless, cratered and cracked state.

As I say, that's a completely different theory from the one being discussed. Nevertheless, I would bet a nickel that at least one of the authors of that article had read "Blowups Happen."

Re:Wouldn't there be an empty space?

[jc42]

A Spherical estimation is good enough for most purposes.

True, at least by the old engineer's rule of thumb that only three places are significant for practical purposes.

But there are some situations where it's not good enough. One is if you're dealing with the orbits of satellites. To the satellites, the Earth is decidedly lumpy, enough so to affect orbits on a time scale of weeks or months.

A fun case I ran across some years ago was a geography trivia question: Name the three "highest points on Earth", and for each, give the definition of "highest point" that it satisfies.

The only answer that most people know is Mount Everest, which is the point that's the highest above the local "geoid" (which is the extension of "sea level" to handle areas far from the closest open ocean).

Some people know another answer: Mauna Kea, which is the point that's the highest above the mean level of the surrounding land. Everest rises some 3,000 m above the surrounding land, the Tibetan Plateau, Mauna Kea rises from the bottom of the central Pacific Ocean, and it's a much taller pile of rock than Everest. Its peak is more than 10 km above its base.

Hardly anyone can even guess the third answer. It turns out to be Mount Chimborazo, which is on the equator in Ecuador, and is the point that's farthest from the Earth's center. It's a good-size volcano that rises some 2,500 m above the surrounding land, but its peak is estimated at 6,384.4 km above the Earth's center, several km higher than the peaks of Everest or Mauna Kea.

All of these "highest point" claims are mentioned in the wikipedia articles about them (which is where I checked the numbers). And you could probably find them reasonably quickly by googling for that phrase, though I haven't tried it. I also wonder if there are other definitions of "highest point" that have different answers.

(And Chimborazo is one of the answers to another trivia question that's fun in "global warming" discussions: What are the two places where there are glaciers on the equator? So far, nobody I've asked this one has got either answer right, though some people get close to the other answer. Both places' glaciers are retreating rapidly, and are predicted to disappear in a few decades.)