Chondritic hypothesis: New discovery shakes beliefs of Earth to the core

Mar 29, 2012
Discovery shakes beliefs of Earth to the core
Photo by Flickr/FlyingSinger

(PhysOrg.com) -- For a century, scientists have assumed that the Earth has same chemical make-up as the sun. But this belief has been challenged by scientists at The Australian National University.

Professors Ian Campbell and Hugh O’Neill from the Research School of Earth Sciences at ANU said their research shakes up our understanding of the Earth’s chemistry – right to the core.

“For decades it has been assumed that the Earth had the same as the Sun, as long the most volatile elements like hydrogen are excluded. 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,” Professor O’Neill said.

As it is easier to measure the chemical make-up of chondritic meteorites, planetary geologists have long used these to more precisely determine the sun’s composition – and therefore the composition of the Earth. From this, scientists have concluded that the Earth has a ‘chondritic’ composition.

Professor Campbell said this thesis has been challenged again and again.

“Recent discoveries have shown that the ratio of two of the rare earth elements in Earth’s volcanic rocks is higher than in chondritic meteorites. Many scientists have explained this by arguing that there must be a hidden reservoir of these elements near the centre of the Earth to balance this ratio out. This reservoir would also be enriched in the heat producing elements uranium, thorium and potassium,” he said.

Professor Campbell spent twenty years researching mantle plumes – columns of hot rock that rise from the boundary of the Earth’s core and are the mechanism that removes heat from the Earth’s centre.

“The problem with the idea of a hidden reservoir is that although these elements could be hidden we would be able to detect the heat they produce,” he said.

“However, 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 .”

Professor O’Neill has developed an explanation as to why the Earth’s composition may differ from chondrites.

“The Earth is thought to have formed by collision of planetary bodies of increasing size. In our research we suggest that by the time these planetary bodies had reached a moderate size they developed an outer shell that contained a significant amount of heat-producing elements,” he said.

“During the final stages of the Earth’s formation this outer shell was lost by a process called ‘collisional erosion’. This produced an Earth that has fewer heat producing than chondritic meteorites, which explains why the doesn’t have the same chemical composition as chondritic meteorites.”

The research has been published in Nature.

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User comments : 9

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Graeme
5 / 5 (1) Mar 29, 2012
rare earth elements are very strongly bound to oxygen and I do not think the compounds will be heavy enough to sink in molten iron. So they should not be dissolved in the core or sunk to the inner core.
Callippo
4 / 5 (1) Mar 29, 2012
In our research we suggest that by the time these planetary bodies had reached a moderate size they developed an outer shell that contained a significant amount of heat-producing elements
Vesta planetoid is rather small and it already contains enough of heat-producing elements. http://science.na...aplanet/
DarkHorse66
3 / 5 (2) Mar 29, 2012
For anyone doesn't yet know what a chondritic meteorite is (I hadn't heard of them either):

http://en.mimi.hu...ite.html
http://www.lexic....ondritic
Nice photos here:
http://www.arizon...ication/
Cheers, DH66.
SleepTech
5 / 5 (3) Mar 29, 2012
Clearly Earth was once from another solar system or a nomad planet. On it's way in, knocked Uranus off it's axis. Clearly.
Callippo
1 / 5 (1) Mar 29, 2012
Clearly Earth was once from another solar system or a nomad planet. On it's way in, knocked Uranus off it's axis. Clearly.
IMO the properties of Earth are good average of Mars and Venus planets. I don't see a lot of evidence for this hypothesis, despite it's presented in form of joke or "pointless verbiage".
SleepTech
5 / 5 (5) Mar 29, 2012
IMO the properties of Earth are good average of Mars and Venus planets. I don't see a lot of evidence for this hypothesis, despite it's presented in form of joke or "pointless verbiage".


I'll take your opinion as "pointless verbiage" until you can present your own evidence. Someone can't take light humor.
julianpenrod
1 / 5 (1) Mar 29, 2012
As opposed to stony, meatal or stony metal meteorites, which all seem to show signs of melting and recrystallizing, chondritic ones previously were though to be essentially the remains of comets after all the volatiles boiled away.
In fact, also traditionally, the earth was not considered to be similar to the sun in the predominances of elements heavier than helium. Thermodynamic equilibtium effects, early on, were described as causing most light volatile elements and compounds to escape the inner solar system and accumulate in the gas giants. That theory seems to have been discarded, for whatever reason, but this brings it back.
Interestingly, the very effect that created chondrites was also the reason used to explain the earth's composition. "Science" accepted it for one but not the other.
julianpenrod
2.3 / 5 (3) Mar 29, 2012
In an earlier comment, I used the word "assume" and was jumped on by TheGhostofOtto1923. Yet, here, "science" is described as making "conclusions" about the composition of the earth based on chondritic meteroites simply because they were the easiest to assay. TheGhostofOtto1923 doesn't seem to be interested in questioning this illegitimate methodology, simply because "scientists" did it and, in the opinion of such as The GhostofOtto1923, anything "scientists" say must be defended, no matter how non credible. It should be mentioned, too, though, that this demonstrates a trait of "science", namely, pretend universal wisdom by always having an "answer", even if it comes from an illegitimate methodology.
CardacianNeverid
5 / 5 (1) Apr 02, 2012
In an earlier comment, I used the word "assume" and was jumped on by TheGhostofOtto1923. Yet, here, "science"...[snip] -julianTard

And you will be "jumped on" every time you put the word science in quotation marks.

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