Earth’s Core is a Recycling Product

The planets of the solar system, including the Earth, formed about four and a half billion years ago from a swirling disk of gas and dust that was left over from the newly formed Sun. However, we do not understand, why the Earth ended up being different from other Earth-like or «terrestrial» planets and how the earliest features, like the metallic core, developed. Research at ETH Zurich by Professor Alex Halliday, to be published in this week’s edition of Nature, claims to have found some answers.

It has generally been assumed that the Earth’s metallic core, which generates the magnetic field, formed by segregating dense metallic iron from the accumulated mixture of metal and silicate that was the primitive Earth. New modelling provides evidence that the core was most likely partly built by direct mixing of earlier formed planetary cores during extremely energetic collisions. Surprisingly, these proto-planets appear not to have had chemical compositions like the present Earth or even its closest neighbour the Moon. Instead, some of them were more like Mars. Indeed, Mars may be a very good analogue of what the Earth was like in its earliest stages of development.

«Embryoplanets» formed Earth

The Earth-like planets, including also Mercury, Venus and Mars, are thought to have been built up gradually, initially by sticking together of dust and rocky debris. When these objects reached the size of a kilometre or so gravity would have started to exert a major influence and a process called runaway growth would have consumed all of the debris in the vicinity. The bigger the planet the stronger its gravity and so the more it will attract other objects. However, this only builds objects that are about 1% of the mass of the Earth. Nearly all of the material in the inner solar system would then have been in the form of numerous roughly 1,000 km diameter planetary embryos. To get objects to be as big as the Earth requires that these embryos repeatedly collide by chance and gradually fuse into a much smaller number of discrete planets as we have today. These collisions would have been incredibly energetic and would have melted the colliding objects and even vaporized some of the rock and metal. It is thought that the Earth’s Moon formed from the debris produced in such a collision.

Date of Earth formation is hard to predict

Because this collision process is somewhat random it is also hard to predict. However, the various models that have been proposed differ with respect to the amount of time over which it is expected that the Earth would have taken to form. Some theories have predicted that the Earth would have formed in much less than one million years. Other, more widely accepted theories predict that it took ten to a hundred million years. Still other schools of thought have proposed something between these extremes. Extinct radioactive isotopes have proved particularly powerful in defining just how quickly planet formation occurred – allowing these theories to be tested. The isotopes of tungsten and lead are especially useful because they have been affected by the decay of radioactive hafnium and uranium respectively. The biggest change in hafnium to tungsten and uranium to lead ratio takes place during the core formation that accompanies planetary growth. This allows the isotopic compositions of tungsten and lead to be used to determine a rate of planetary growth. The data indicate that the Earth formed over tens of millions of years and that the Moon formed late, consistent with the theories of more protracted formation.

Earth’s Core formed from Earlier Planets

However, Halliday shows that the story is not so simple. The two isotopic clocks used, hafnium-tungsten and uranium-lead, actually give distinctly different timescales for planet formation when calculated in the same manner. Halliday shows that there is only one likely explanation for this. Some portion of the Earth’s core formed as a result of the coagulation of earlier cores from the colliding planets. This is different from the general view of core formation – that the iron metal from each colliding planet first mixed with the rocky outer parts of the Earth and then simply settled to the centre of the Earth because of its higher density. Furthermore, it means that the time-scales of formation of the Earth and Moon have been under estimated. Recently it was estimated that the Moon-forming Giant Impact took place at about 30 million years after the start of the solar system. An age for the Moon of closer to 50 million years now appears more likely.

Earth and Mars are relatives

The isotopic compositions of tungsten and other elements in the Moon can be used to deduce what the chemical composition of its impacting parent planet was like. It turns out it was probably much more like Mars – a relatively volatile-rich and oxidized planet. The Earth and the proto-planets that made the Earth most probably lost volatiles during growth. Given this history a big question that remains is how Earth acquired its water. This is, in fact, one of the most puzzling remaining problems about Earth-like planets.