Shaking Up Theories Of Earth’s Formation

by Jason Major on March 30, 2012

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

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.

About 

A graphic designer in Rhode Island, Jason writes about space exploration on his blog Lights In The Dark, Discovery News, and, of course, here on Universe Today. Ad astra!

gopher652003 March 30, 2012 at 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 March 30, 2012 at 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.

Torbjörn Larsson March 30, 2012 at 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.)

Peristroika April 1, 2012 at 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 April 2, 2012 at 12:28 PM

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

Torbjörn Larsson March 30, 2012 at 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 the Kleine et al paper is their own disequilibrium model of core formation. It contrasts to the traditional equilibrium model mentioned above, where the aggregating impactors are able to melt, by their fragmentation, and equilibrate their non-siderophile (non-”iron loving”) elements with the growing mantle. That material exchange predicts nicely the lowered siderophile content of the mantle compared to chondrites.

But Kleine et al show that as little as 40 % of equilibration suffice for predicting the same abundances. And that opens up the time window for a smaller scale (and simpler) aggregation rate model and a late Earth-Moon impactor.

I am not sure I understand the O’Neill et al model. That radioactive elements such as radioactive lanthanides and actinides are lithophile (“rock-loving”) and would be ejected off Earth by impact erosion I can dig. But what are these previously assumed “hidden reservoirs” in the molten rock mantle? How much difference in composition between mantle and crust are we talking about?

Above all, they would have to constrain by recent neutrino measurements of Earth radioactivity: “observations indicate that heat from radioactive decay contributes about half of Earth’s total heat flux.”

Jason Major March 30, 2012 at 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.

Torbjörn Larsson March 30, 2012 at 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 April 2, 2012 at 7:38 PM

huh

Charv March 30, 2012 at 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 March 31, 2012 at 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.”

Jeff Boerst March 31, 2012 at 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 April 2, 2012 at 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 April 2, 2012 at 10:22 AM

It was only sarcasm John, sarcasm!

John Campbell April 4, 2012 at 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 March 31, 2012 at 2:15 PM

so the bible was right after all!

joseph yates March 31, 2012 at 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 April 2, 2012 at 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 April 1, 2012 at 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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