Fresh insight into the origins of Planet Earth

Jun 03, 2010
Fresh insight into the origins of Planet Earth
Around 100 million years in the making: the Moon and the Earth

For the first time, an international team of researchers has incorporated extensive geochemical data on the formation of Earth into a model - with surprising results: more models can be used for the process of Earth’s accretion than previously assumed.

Earth was formed during the creation of our Solar System when Moon and Mars-sized protoplanets collided, leaving the Earth to gradually “grow”; just how long it took for the Earth to reach its eventual size and what the accretion of the planet was like, however, is much disputed among the experts. “The latest models reveal that an accretion period of around 100 million years is the most consistent with the formation of the Moon and the Earth”, says Bernard Bourdon, a professor from the Institute of and Petrology at ETH Zurich, Switzerland. However, there are also models that clearly suggest the Earth reached 70% of its size in just 10 million years.

The models for a rapid accretion of the Earth have to proceed from the basic premise that the colliding protoplanets blended fully with one other when the Earth was formed and achieved a chemical equilibrium between the elements in the Earth’s metallic core and the mantle, explains Bourdon. This would have occurred when the iron-loving elements sank to the Earth’s center and the “silicate-loving” elements remained in the Earth’s mantle.

The scientist harbored doubts that the Earth could have evolved in only 10 million years; he teamed up with John Rudge, a guest scientist from Cambridge, and Thorsten Kleine, at the time one of Bourdon’s colleagues and currently a professor at the University of Münster, in an attempt to test the theory by fully considering all the known parameters in a model for the first time: on the one hand, the hafnium-tungsten and uranium-lead radioactive clocks; on the other hand, the distribution of the between the core and the mantle. This distribution depends on the pressure and temperature conditions during the core formation, which probably varied during the accretion. The conditions of core formation can thus be extrapolated from the element distribution in the Earth’s interior.

In their study published in Nature Geoscience, Bourdon and his team now demonstrate that there are several models that are compatible with the chemical observations. The notable thing is that there is no need for a full equilibrium between metals and silicates: “Up to now, it was always assumed that you could only explain the distribution of the elements through equilibrium; we show, however, that the distribution is just as easy to explain in disequilibrium”, says Thorsten Kleine.

The observations are also compatible with a state of equilibrium of only about 40 percent; this means the cores of the colliding protoplanets could have reached the Earth’s core directly without a major equilibration with the Earth’s mantle. For Bourdon, the crucial thing is that the results show that the notion of a full equilibrium might be wrong since a disequilibrium would require more time for Earth’s full accretion, thus being more consistent with the time when the Moon was formed. “If we assume that the Earth’s mantle preserved signatures of the protoplanets, the end of Earth’s accretion and the Moon’s age coincide”, says Kleine.

The age difference had always puzzled the scientists; after all, the termination of the Earth’s accretion should actually coincide with the Moon’s age as it ended due to the impact of a Mars-sized protoplanet that formed the Moon.The “radioactive clocks” are supposed to have been partially reset by this catastrophic collision. According to the new study, a large part of the probably formed rapidly; however, it took at least 100 million years in all to reach its completion. The rapid accretion at the beginning and a slow completion are consistent with the time of the Moon’s formation, says Bourdon.

Explore further: Lava from Hawaii volcano picks up speed

More information: Rudge JF, Kleine T & Bourdon B: Broad bounds on Earth’s accretion and core formation constrained by geochemical models: Nature Geoscience (2010), 3, 439 - 443 Published online: 23 May 2010 doi:10.1038/ngeo872

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Caliban
not rated yet Jun 03, 2010
Ok, so it makes sense that that accretion would slow over time, as the protoplanetary material would have been depleted while the process was ongoing- still continues, in fact, although at a nearly infinitesimal pace.

Why the assumption of equilibrium between silicate and ferrous minerals, I know not. Since the interior temperature of the earth is supposed to be high enough for melting, then it quickly becomes a matter of the specific gravity of the minerals themselves, at various pressures/temperatures.

The greater the component of Iron in the mineral, the higher the specific gravity- therefore centerward displacement. The higher the concentration of Silica, the lower the specific gravity, and displacement "upwards".

It would be nice to get a fix on the radioisotope ratios, and get a clearer picture of total accretion time.

Finally, a question: would it be possible to perform some orbital forensics and locate the "Mars-sized protoplanet" responsible for the Moon?
PinkElephant
not rated yet Jun 04, 2010
@Caliban,
would it be possible to perform some orbital forensics and locate the "Mars-sized protoplanet" responsible for the Moon?
That's a trick question. The proto-planet that hit proto-Earth would have been destroyed in the process. Some of it may survive in the form of near-Earth asteroids. Some of it survives in the sum total of our Moon. And most of it merged with the proto-Earth, so you're sitting on it and probably a significant fraction of your body is composed of it...
mysticshakra
1 / 5 (2) Jun 04, 2010
Nonsense. Planetary accretion is a silly theory that simply does not work, ignores missing planets, does not explain planet pairing, etc. See Tom Van Flandern's work.

The moon is full of artificiality caused by outside tinkering. Likely is was put there by someone purposefully.
yyz
not rated yet Jun 04, 2010
Do you mean the same Tom Van Flandern who "authored a book, Dark Matter, Missing Planets and New Comets: Paradoxes Resolved, Origins Illuminated, in which rejected and offered replacements for the fundamental theories of modern physics (especially special relativity, general relativity, and quantum mechanics), and challenged prevailing notions regarding dark matter, the big bang, and solar system formation, and advocated the theory that the asteroid belt consists of the remains of an exploded planet.(per wiki)"?

And why would I want to peruse his work again? Entertainment?

mysticshakra
1 / 5 (1) Jun 04, 2010
Prevailing notions of physics are wrong. They are a dead end. To pretend that after a few hundred years of looking we have it all nearly figured out is monumentally ignorant and shows limitless arrogance. This type of attitude is why we have made so little progress since Maxwell.

Still no explanation for gravity, still clinging to dark matter, black holes and all the rest.

Try reading the book instead of wikipedia. Consensus reality is always shown to be wrong, get ahead of the curve. If prevailing theories were adequate we wouldn't have spent the last 70 years treading water.