Although the Earth has long been studied in detail, some fundamental questions still need to be answered. One of these concerns the formation of our planet, whose origins researchers are still unclear about. An international research team led by ETH Zurich and the National Center of Competence in Research PlanetS is now proposing a new answer to this question based on laboratory experiments and computer simulations. The researchers have published their research in the journal Nature Astronomy.
An inexplicable discrepancy
“The prevailing theory in astrophysics and cosmochemistry is that the Earth is formed from chondritic asteroids. These are relatively small, simple blocks of rock and metal that formed early in the solar system,” explains the study’s lead author, Paolo Sossi, professor of experimental planetology at ETH Zurich. “The problem with this theory is that no mixture of these chondrites can explain the exact composition of the Earth, which is much poorer in light, volatile elements like hydrogen and helium than we expected.”
Several hypotheses have been put forward over the years to explain this discrepancy. For example, it was postulated that the collisions of the objects that later formed the Earth produced enormous amounts of heat. As a result, the light elements evaporated and the planet remained in its current composition.
However, Sossi is convinced that these theories become implausible once you measure the isotopic composition of the different elements of the Earth: “The isotopes of a chemical element all have the same number of protons, albeit different numbers of neutrons. Isotopes with fewer neutrons are lighter and should therefore be able to escape more easily. If the heating evaporation theory were correct, we would find fewer of these light isotopes on Earth today than in the original chondrites. But that’s exactly what the isotope measurements don’t show.”
A cosmic melting pot
Sossi’s team therefore looked for a different solution. “Dynamic models simulating the formation of planets show that the planets in our solar system have formed progressively. Tiny grains grew into planetesimals the size of a kilometer over time by accumulating more and more material through their gravitational pull,” explains Sossi. Like chondrites, planetesimals are also small bodies of rock and metal. But unlike chondrites, they are heated enough to differentiate into a metallic core and a rocky mantle. “In addition, planetesimals that formed in different regions around the young sun or at different times can have very different chemical compositions,” adds Sossi. The question now is whether the random combination of different planetesimals actually results in a composition that corresponds to that of the Earth.
To find out, the team ran simulations in which thousands of planetesimals in the early solar system collided. The models were designed to reproduce over time celestial bodies corresponding to the four rock planets Mercury, Venus, Earth and Mars. The simulations show that a mixture of many different planetesimals could actually lead to the effective composition of the Earth. Moreover, the composition of the earth is even the most statistically likely outcome of these simulations.
A blueprint for other planets
“Although we suspected it, we found this result quite remarkable,” recalls Sossi. “Not only do we now have a mechanism that better explains the formation of the Earth, but we also have a reference to explain the formation of the other rock planets,” said the researcher. For example, the mechanism could be used to predict how Mercury’s composition differs from that of the other rock planets. Or how rocky exoplanets from other stars might be composed.
“Our study shows how important it is to consider both dynamics and chemistry when understanding planet formation,” Sossi noted. “I hope our findings will lead to closer collaboration between researchers in these two areas.”
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