Earth was “born” from a giant melting pot of molten rock

Around 4.6 billion years ago, our solar system was a very different place. Instead of planets, all that was floating around the sun was clouds of gas and dust.

Gravity eventually drew these clouds together into clumps, which grew into pebbles, boulders, then planet-sized rocks. Some of these smaller clumps did not make it into planets, and they still exist now, as meteorites. Studying the composition of meteorites, therefore, can give us a glimpse into the way rocky planets formed.

Two new studies published in Nature have used this to add to our understanding of this process; saying the composition of Earth may have come about from the evaporation of molten rock. 

Chondritic meteorits are stony, non-metallic meteorites that formed from the grains present in the early solar system. “Chondritic meteorites are our best approximation of the building blocks of planets,” said Dr Remco Hin, from Bristol University, and lead author of one of the papers. 

But when you look in more detail, it seems the composition of these meteorites does not match up with the rocky planets.

“We found that the samples from Earth, Mars and asteroid Vesta have higher magnesium isotope ratios than the primitive meteorites that are thought to represent the building blocks of our solar system,” said Hin.

Hin and his colleagues thought one possible explanation for this difference is that rock was vaporised during the growth of the Earth. Using models, they found 40 per cent of Earth’s mass could have been lost as vapour.

“When we calculated what the chemical consequences of this would be, we found that this matched very well the observed composition of the Earth,” said Hin.

The second study, published by researchers at Oxford University, backs this theory, experimentally. The researchers recreated processes that would have occurred during Earth’s formation by melting rock in a furnace. They found some elements could escape in vapour released from the molten rock in proportions similar to that observed within Earth. 

This is not the first time the idea of vapourised rock has been considered when it comes to planetary formation. But the two new papers provide a promising development for the models to be further explored.

“The physical chemistry of melting and evaporation could ultimately prove to be a key arbiter in competing models of planet formation,” said Edward Young from the University of California, who was not involved in either study.

In terms of next steps, “I will try to make a more realistic model of the physics of vapour loss from growing planets,” said Hin. “In our study, we made a start with understanding this, but the model we made can be made more realistic.”