Earth may not have formed quite like once thought (Image: NASA/Suomi NPP)

Shaking Up Theories Of Earth’s Formation

Article Updated: 23 Dec , 2015

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Researchers from The Australian National University are suggesting that Earth didn’t form as previously thought, shaking up some long-standing hypotheses of our planet’s origins right down to the core — literally.

Ian Campbell and Hugh O’Neill, both professors at ANU’s Research School for Earth Sciences, have challenged the concept that Earth formed from the same material as the Sun — and thus has a “chondritic” composition — an idea that has been assumed accurate by planetary scientists for quite some time.

 

Chondrite meteorites are composed of spherical chondrules, which formed in the solar nebula before the asteroids. (NASA)

Chondrites are meteorites that were formed from the solar nebula that surrounded the Sun over 4.6 billion years ago. They are valuable to scientists because of their direct relationship with the early Solar System and the primordial material they contain.

“For decades it has been assumed that the Earth had the same composition as the Sun, as long the most volatile elements like hydrogen are excluded,” O’Neill said. “This theory is based on the idea that everything in the solar system in general has the same composition. Since the Sun comprises 99 per cent of the solar system, this composition is essentially that of the Sun.”

Instead, they propose that our planet was formed through the collision of larger planet-sized bodies, bodies that had already grown massive enough themselves to develop an outer shell.

This scenario is supported by over 20 years of research by Campbell on columns of hot rock that rise from Earth’s core, called mantle plumes. Campbell discovered no evidence for “hidden reservoirs” of heat-producing elements such as uranium and thorium that had been assumed to exist, had Earth actually formed from chondritic material.

“Mantle plumes simply don’t release enough heat for these reservoirs to exist. As a consequence the Earth simply does not have the same composition as chondrites or the Sun,” Campbell said.

The outer shell of early Earth, containing heat-producing elements obtained from the impacting smaller planets, would have been eroded away by all the collisions.

“This produced an Earth that has fewer heat producing elements than chondritic meteorites, which explains why the Earth doesn’t have the same chemical composition,” O’Neill said.

The team’s paper has been published in the journal Nature. Read the press release from The Australian National University here.

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gopher65
Member
gopher65
March 30, 2012 7:04 PM

Why would already formed planets (or planetoids or whatever) have a different composition than the sun? I don’t understand why it would matter to Earth’s elemental composition which way it formed.

Jason Major
Guest
March 30, 2012 7:22 PM

Elements over time would differentiate into layers within larger planet-sized bodies that had molten interiors, and when combining they would separate out differently than if the full-sized object (i.e., Earth) had gradually amassed from the initial cloud (like chondritic meteorites.) Heat-producing elements would thereby have been kept away from the interior of the Earth.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
March 30, 2012 9:13 PM

Besides what we are talking about here, isotope ratios besides D/H (which are affected by how much hydrogen is lost to space, hydrogen (H) is preferentially lost over deuterium (D) ), FWIW there is also a gradient of material over a forming protoplanetary disk. For example, volatile ices freezing out farther away. Not exactly different composition compared to the sun, but between bodies.

(Samples shows that the disk material mixed a lot though, so for example comets IIRC have grains that looks to have melted close to our sun.)

Peter
Member
Peter
April 1, 2012 12:40 AM

Torb, please, you sound like you understand something here but you make it difficult to follow you. One, please use complete sentences. The last two in the first paragraph are not sentences at all. Two, less anagrams please. Three, verbs agreeing would help make things clear. Unless you are just writing these for your own pleasure, give us the pleasure of learning from those who know.

squidgeny
Member
squidgeny
April 2, 2012 12:28 PM

Those are acronyms, not anagrams. I happen to agree with that point, but remember English isn’t Torbjorn’s first language

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
March 30, 2012 8:56 PM
This is an interesting hypothesis of course, predicting the observed 142Nd/144Nd and Sm/Nd ratios or Earth as well as the general differentiation observed in Vesta and other large asteroids. Consequences would be both a delay of Earth core formation (from ~ 30 My to ~ 40 My @ ~ 85 % of mass in an equilibrium model) and a delay of crust formation (due to the explicit ablation!) as well as Earth-Moon impactor. For the general context as well as the time scales of Earth formation, see Chronometry of Meteorites and the Formation of the Earth and Moon, Kleine et al, Elements, 2011. They refer to this hypothesis by a 2008 O’Neill and Palme publication, btw. Notable in… Read more »
Jason Major
Guest
March 30, 2012 9:10 PM

There is this recent paper too (sorry, paywall) http://www.sciencemag.org/content/335/6075/1477.abstract showing evidence against a chondritic Earth origin. It mentions assumed reservoirs as well.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
March 30, 2012 9:47 PM

Thanks! The non-chondritic origin gains momentum seems like, and the enstatites always sat awkward. (Atypical, rare, IIRC originates closer to the Sun, et cetera.)

Rishard Ismail
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Rishard Ismail
April 2, 2012 7:38 PM

huh

Charv
Guest
Charv
March 30, 2012 11:08 PM

In light of recent articles postulating an over-abundance of rogue planets in our galaxy – and the compositional disparity between our own, would it be wildly speculative to consider the possibility one or more of our solar system’s planets were captured by the sun and not “home grown”?

If that were the case I’d imagine it could throw quite a large wrench into our understanding of astrogeology.

Just a thought.

Ivan3man_At_Large
Member
Ivan3man_At_Large
March 31, 2012 7:05 PM

According to this National Geographic article, “any captured planets would have extremely large, distant, and elliptical orbits.

“These rogue planets might gently sink into orbit around an object at distances perhaps ten thousand times greater than Earth is from our sun.”

Jeffrey Boerst
Member
Jeffrey Boerst
March 31, 2012 6:35 AM

Let’s keep this simple..: It was formed in 7 days by a big guy with a beard and a mean streak… >.<

John Campbell
Guest
April 2, 2012 4:02 AM

I know creationist’s can’t normally be bothered by facts but really! 7 days you say? sorry but that is incorrect on several levels the most humorous being that according to every bible that I ever heard of he did it in six and rested on the 7th. If your going waste everyone’s time by posting fairy tales on a website devoted to science don’t you think you should at least get the story right?

Tony
Guest
April 2, 2012 10:22 AM

It was only sarcasm John, sarcasm!

John Campbell
Guest
April 4, 2012 1:38 AM

Sorry about that Jeff even if it weren’t sarcasm my response was now that I think about it unnecessarily harsh.

d.w. kobza
Guest
d.w. kobza
March 31, 2012 2:15 PM

so the bible was right after all!

Ron 1958
Member
Ron 1958
March 31, 2012 8:08 PM

If there is not enough uranium and thorium to produce mantle plumes, then how has the earth managed to keep a molten outer core for the past 4.6 billion years? Why hasn’t that heat already dissipated?

John Campbell
Guest
April 2, 2012 4:10 AM

If you have ever had the misfortune of biting into a hot pocket pizza roll after taking it from the microwave and letting it cool for what you thought was a safe amount of time only to scald the inside of your mouth when you tried to eat it …well you get the Idea. The earths surface area to volume ratio makes for a terribly inefficient heat sink and though it’s quite thin the rocks of the crust are fairly poor thermal conductors basically most of the heat generated by it’s formation has yet to radiate into space.

hillsider62
Guest
hillsider62
April 1, 2012 4:35 AM

Now, I got confused. Somebody theorize that a planet was cast out from the solar system; then, these guys are thinking that the Earth was a rogue planet captured by the sun. Well, definitely, other thinkers may propose that the moon was from other planet or we have two moons but lost one.

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