# Which theory is stronger, that iron came from outside or was formed within earth?

Where is the current body of science learning towards, that iron came from meteor or it was somehow formed on earth?

• Do you mean the element itself (in which case the answers below are sufficient), or deposits of ore or even metallic iron that people could use? – jamesqf Jan 23 '15 at 19:28
• Or the Earth's core? (I assume not, but...) – naught101 Feb 4 '15 at 6:08
• Iron came 'down' from above: bit.ly/1I3vjDs – strangetimes Dec 14 '15 at 10:28

## 5 Answers

The only elements that were formed on Earth are those produced by radioactive decay. There are four natural decay chains that start with transuranic elements and none terminate in iron; neither do the decay chains that are artificial or those that result from cosmic radiation. So all of our iron is from the Earth's formation or meteor impacts since then.

• historically speaking, how far back could this theory be traced? – public static Aug 5 '14 at 21:02
• To 1946 when Fred Hoyle described the nucleosynthesis of iron, meaning that it occurred in stars that predate our solar system. – mistermarko Aug 6 '14 at 2:34
• if this was written say over 1000 years ago would that be unbelievable or you could say it was a good educated guess? – public static Aug 7 '14 at 18:30
• @JackTwain: so? Just because a question is triggered by a theological discussion does not mean that the answer has to be theological. This is a science site. That page has nothing to do with science. – naught101 Feb 19 '15 at 1:42
• Minor disagreement, there are elements on Earth that are formed via fusion and particle bombardment, but granted, these are in small quantities. As there is enough of them produced and put into weapons of mass destruction to kill man many times over, it is not insignificant though. – dlb Sep 30 '16 at 23:12

All the material that eventually formed our solar system is essentially recycled star dust. All iron on Earth was produced by large stars that existed before our Sun formed: the iron was created during nuclear fusion and later released when the parent star(s) exploded, presumably supernova. After our solar nebula had formed and material had been gravitationally compacted, dense elements like iron did not get far from the center of the nebula before "precipitating" out into planets and meteors. Thus the inner solar system has a high abundance of iron. This is in contrast to low-density elements/molecules that were in general able to escape the center of the nebula and make it pretty far before being pulled into a large mass. So we have high iron planets and meteors close to the sun (relatively speaking), and very little iron in the "gas giants" part of the solar system. I would consider this to be material that is "already there" but certainly some iron has been added by meteors.

I believe the element iron (Fe) is formed by stellar nucleosynthesis.

# stellar nucleosynthesis:

it's a process of continuous fusion of the star element to reach heavier and heavier elements. starting from hydrogen (H) till iron (Fe) (the heaviest element the process could reach).

Each 2 atoms of light elements fuse together to form heavier element atom.

For example 2 hydrogen atoms (H --> 1 proton) fuse to form helium (He --> 2 protons).

• Could you explain a little more what stellar nucleosynthesis is in your answer? Thanks! – hichris123 Aug 12 '14 at 23:45

So (naturally occurring) elements on earth can only come to be here in three ways.

Either they are formed here via radioactive decay.
They came via meteor.
They were already here.

As mistermarko stated above iron isn't normally formed via radioactive decay, so we're left with the last two choices.

However if we go back far enough, earth itself was a bunch of meteors and dust that clumped together to form the earth. All matter on earth came from dust, a meteor or a comet. Some of that matter has changed from one element to another, but not iron.

Thus all iron came via meteor.

It isn't a matter of tracing this theory through history, because we know which elements decay into other elements and none of the ones found on earth end in iron.

• I don't think this rules out "they were already here", but I suppose there's a semantic argument around what "already here" means. Can we say, exactly, when the Earth was "formed", and which meteors are part and parcel of the original item, and which came later? – andy holaday Aug 6 '14 at 0:55

Somewhat more from volcanic activity than meteor impacts, but both are important.

The Earth has an iron-nickel ($\ce{Fe}$-$\ce{Ni}$) core that originates from Earth's formation out of the collision of planetesimals which themselves contained iron, rock and ice. The Earth was very hot at that time, and the iron along with some siderophilic elements sunk to the center to form the core. Relatively little of that iron is in the crust. It is mostly sequestered in the core (pages 326, 327).

• The first iron used by humans was meteoric. – Spencer Nov 26 '16 at 16:32