Episode 105: A Fission Origin for the Moon, Part 1
Recap: While I talked briefly about the fission model of lunar origins in Episode 53, it deserves a more in-depth discussion, especially as a setup for the next episode about a fission formation for the planets, from the sun. This episode takes you through a historic context and justifications for the "Big Split," but it ultimately why it has been rejected by the scientific community today.
Puzzler for Episode 105: There was no Puzzler in this episode.
Q&A: There was no Q&A for this episode.
- Logical Fallacies / Critical Thinking Terms addressed in this episode: Argument from Authority
- Relevant Posts on my "Exposing PseudoAstronomy" Blog
Claim: The claim in this episode is that the Moon was formed when it spun off from Earth. Those of you who have been with the podcast for over a year and a half, or who have gone through the archives, may be wondering why I'm talking about this now when I addressed it in Episode 53 on Lunar Formation and Origin. The reason is that there is more to tell, and it has seen a resurgence of support among one particular pseudoscientist with whom I've had a few run-ins.
Historic Idea, Context, and Evidence
To remind everyone of the problem we're trying to solve, Earth's moon is somewhat abnormal so far as moons in the solar system go. It's really big relative to Earth at 1/4 the size; its composition is similar to Earth's, including certain isotopes that we think means they formed either in the same part of the solar system or from the same material; but for its size, the moon lacks a proportional iron core while Earth's is really big; and the Earth-Moon system overall has an abnormally large angular momentum. And so, as scientists, we try to find out how it could have formed that also can explain these weird things.
One idea that was popular for about a century was put forward by George Darwin in 1879. Right there, you have a nice argument from authority for those who like this model, the name "Darwin" being attached to it in some way. In this case, George Darwin was the son of Charles Darwin.
His model, often called the "fission model," or as I called it in Episode 53 the "Big Spin," posits that when Earth was a young, hot, and virile young thang, it was spinning really really fast, and it was molten. Which we still generally think is the case today, just not spinning quite as fast as Darwin did.
Because it was spinning really fast, and it was molten and easier to deform, eventually a piece of it bubbled off, or fissed off, somewhat similar to cellular mitosis. Well, not really ... more like if you had a blob of Mercury and you put it in the middle of a merry-go-round and spun it fast, a piece would bud off.
For the time, this was reasonable, and there wasn't that much evidence against it. Adaptations to the model suggested that a resonance with the sun and solar tides helped pull the moon off of Earth once the bulge or bud had started to form. And, of course, there's the Pacific Ocean! A large chunk of Earth's crust seems to be missing, embodied by the Pacific Ocean basin, and it was proposed by Osmond Fischer, an English geologist and geophysicist who was a contemporary of Darwin.
The fission model was still taught in schools during the Apollo era in the 1960s and early 70s, and it seemed to get support from lunar samples that showed not only was the Moon's crust similar to Earth's in density, but also composition. And there was still that hole that the Pacific Ocean filled.
It also somewhat easily explained why the Moon lacks a big iron core: If it spun off of Earth, one would think it would be made of material nearer to Earth's surface, while the iron-nickel core stays with Earth itself. That doesn't explain why Earth has such a large iron core relative to other terrestrial planets, but we also didn't really know that was the case at that time.
Meanwhile, various calculations can be done to estimate how fast Earth would need to be spinning in order to bud off a moon. When I do the math, equating centrifugal force with gravitational, solving for velocity, and putting in angular velocity equations, I get a result that early Earth would need to spin once on its axis once every 1.5 hours. As opposed to today's 24 hours. Other people with more complicated models get different results but all put the rotation rate somewhere around 2-3-ish hours. Which starts to be a problem.
Reasons Against This
Keep in mind that this is a very, very basic physics calculation. I'm using basic algebra, I'm not sneaking extra terms into the equation, and it's math people have been doing for over a century. But what this means is that Earth would need to be spinning at an untenable rate for this to work out. There is no way that we know of to get Earth to spin so quickly from the basic process of planet formation. Even smacking it with big things, but small enough that they wouldn't destroy the planet.
Not only that, but we also don't know how to slow it down enough once the moon would bud off. You would certainly get a slow-down due to conservation of angular momentum, like an ice skater who's spinning and puts their arms out slows down. But it's not enough, and it's still not enough even with the last 4.5 billion years of spin-down from other processes - mainly tides. You just can't get Earth slowing down that much in that amount of time if you somehow get it to spin that fast in the first place.
And, not only that, but dynamic models that tried to model the moon budding off Earth showed that the budding moon would cause an instability in Earth's spin. This would lead to the spin slowing down before the budding could be completed, and the moon would NOT separate from planet.
If you're not sure how to visualize that, think of a large ball of pizza dough. Nice and elastic. You start to spin it really fast, and arrange it so that a piece starts to bud off to one side. As you're spinning it, and at least when I did this experiment over a clean work surface so I could still use the dough, the entire mass just would no longer spin nicely. It wobbled and wiggled and, effectively, it went unstable. As with all analogies this one isn't perfect, it might give you a bit of a better idea on how to visualize the problem.
Another issue is that if this happened, the Moon should orbit along Earth's equator. Instead, the moon orbits about 5.1° away from the equator.
Yet another issue is that models show that, in this fission scenario, the new moon should be around 20% as massive as Earth. While it's about 25% Earth's diameter, because mass goes as the cube of diameter if you have the same density but the moon is less dense, the moon is really only about 1/80th the mass of Earth. There's no good way to explain that rather large discrepancy.
Oh, and it doesn't explain the abnormally large angular momentum of the Earth-Moon system.
Also damning was two other lines of evidence that showed the Moon could not possibly have sprouted from Earth's Pacific Ocean: Radiometric dating and plate tectonics.
In that order, the first problem is that several decades ago, we could date both the ocean floor and the Moon, especially once samples were brought back with the Apollo astronauts. The ocean floor of the Pacific was no older than about 200 million years, million with an "m." Practically all of the lunar samples were instead several billion years old, billion with a "b." The oldest rock, called the Genesis Rock, was brought back by Apollo 15 astronauts and contains minerals that are 4.46 billion years old. Over 20 times as old as the oldest Pacific Ocean crust.
The second major problem against the Pacific Ocean being a moon-formed scar is plate tectonics and continental drift. Darwin's idea was proposed in 1879. While many scientists before him and who were contemporary with him had proposed continental drift, it wasn't until the late 1800s and early 1900s that the geological community really started to advocate the idea in force. Alfred Wegener was the first to present the idea that the continents formed a single landmass that broke apart, and he presented this at the German Geological Society on January 6, 1912. Lots of evidence - not only the shape of the continents but where fossils appeared on continents separated by large oceans - was presented over the years.
But, it was very controversial for several decades, with various persons advancing gigantic transient landbridges to account for the fossils appearing on multiple continents.
This controversy persisted throughout the mid-20th century, which is why the fission model was probably still being taught in schools as viable during the Apollo era. Now-a-days, there is overwhelming evidence in favor of continental drift and plate tectonics, from not only the shapes of the landmasses and the fossils, but magnetic striping, earthquake epicenters, and that we can actually measure continents moving, not only with GPS, but also with astrometry - the very precise measurements of star positions as seen from various locations on Earth.
And so, not only are the ages of the Pacific Ocean and the moon wildly different, but the Pacific Ocean did not exist in its present state 10s to 100s of millions of years ago.
And of course, all those other reasons I've spent the last 5-10 minutes talking about as to why this model does not work.
More Recent "Support"
Pretty much all scientists today no longer consider the fission model in any way a viable model for the moon's formation. It simply doesn't work. It doesn't explain all the observations, it itself doesn't work dynamically, and there isn't even any decent evidence for it beyond outdated and disproven ideas for Earth's structure.
You would think that it would be a dead idea at this point, relegated to the history books like geocentrism. But, as you learned in Episode 78, there are modern geocentrists. And, just as there are modern geocentrists, there are modern people who think the Moon budded off Earth.
One of these people was Tom Van Flandern, last spoke of during the two-parter on whether the asteroid belt was a planet, perhaps best known for his ideas about exploding planets. While he is now deceased, his Meta Research website remains online so anyone can read it, and I've linked in the shownotes to the relevant page.
Ongoing champions of Van Flandern's failed models reside in Richard Hoagland and his one-time co-author, Mike Bara. In his book, The Choice, Mike talks about how the moon formed through the fission process. Since I do not own the book, I'm not going to comment further on it.
BUT, in the next episode, we're going to take this argument further and apply it to planets, because Mike Bara - stemming from Tom Van Flandern, is a large advocate of the fission model for planetary formation, whereby planets are spun off from the sun, in pairs.
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10:21pm on Wednesday, April 2nd, 2014
"People on the street wearing bread-boards." Of course you meant "sandwich boards." A bread-board is what you cut bread on -- probably in your kitchen.