Our Earth Formation Theories Could Be All Wrong, New Experiment Suggests

When scientists came up with currently accepted theories about how the Earth formed soon after the sun itself came into being about 4.5 billion years ago, one of the most important factors in their calculations was what is thought to be the composition of our planet’s core and mantle. But a new experiment, whose results were shared Sunday at a conference on geochemistry, casts doubt on those theories, suggesting they could be wrong and need revision.

We don’t fully understand the interior of the Earth, but it was thought to contain almost no zinc in its core. Researchers from Institut de Physique du Globe de Paris (IPGP)conducted an experiment to simulate the “core-mantle differentiation at the time of the Earth's formation,” which they did by melting mixtures of metals rich in iron and compounds of silicate that contained zinc (Zn) and sulfur (S). This was done at temperatures of about 4,100 Kelvin (about 6,900 degrees Fahrenheit; the sun’s surface is about 6,000 Kelvin) and under pressures of up to 80 gigapascals (which is about one-and-a-half times the pressure needed to synthesize diamonds).

“We found that under conditions similar to those estimated when the Earth formed, Zn has a tendency to be distributed between the core and mantle differently than we had thought, i.e. there will be a significant amount of it bound up in the Earth’s core. Based on previous models, if we can place more Zn in the core, then by association you place more S in the core as well, much more in fact than most current observations suggest,” Brandon Mahan of IPGP explained in a statement Sunday.

The researchers put their results about the distribution of Zn and S into computer models based on current theories of Earth’s formation, but none of them models came close to showing the same sulfur-to-zinc ratio of the present-day mantle.

It is thought that soon after the formation of the sun, the dust around the star coalesced into larger particles, which in turn gravitated toward each other to become larger rocks and eventually into terrestrial planets like Earth. However, the material for each planet came from a relatively narrow space, meaning Earth was largely formed by the coming together of members from a very small number of meteorite subclasses.

The main subclass of these non-metallic stony meteorites (called chondrites as a category) that is thought to have made up Earth is called enstatite chondrites. The discovery of another subclass, called CI chondrites, in a few places around the world, led scientists to think Earth formed essentially those two types of stony meteorites.

“However, this new work indicates that the Earth needs to have formed from a more S-poor source; in terms of the geochemistry, the best candidate for this material is the metal rich CH chondrites,” Mahan said in the statement. “CH chondrites were first classified in 1985, and only a few dozen examples have been identified. They are rich in metallic iron and poor in easily vaporized elements, which indicates formation at very high temperatures, but they also contain a few percent of water-bearing minerals, which paradoxically indicates low temperatures,” he added.

Referring to the amount of sulfur in the Earth’s crust, as capped at 2 percent by “most leading estimates,” Mahan said using known meteorites as a source for Earth’s formation doesn’t concur with currently accepted values, thereby precluding “any of the solar system materials that have previously been proposed” as the source material for Earth.

“But if the building blocks of the Earth were something like the CH chondrites, this could give us an Earth pretty similar to the one we see today,” he said.

The results of the experiment were presented at the ongoing Goldschmidt2017 conference in Paris.