Scientists have been studying Earth for so long, yet a few questions remain unanswered. An international research team led by ETH Zurich and the National Centre of Competence in Research PlanetS is proposing a new answer to the question- how the Earth formed.
The prevailing theory suggests that the Earth is formed from chondritic asteroids. These are relatively small, simple rock and metal blocks that formed early in the solar system. However, 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 such as hydrogen and helium than expected.
Over the years, numerous theories have been proposed to explain this disparity. For instance, it was proposed that the objects that subsequently became the Earth collided and produced tremendous heat. The light components were vaporized; as a result, leaving the planet with its current makeup.
The study’s lead author, Paolo Sossi, Professor of Experimental Planetology at ETH Zurich, said, “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 theory of vaporization by heating were correct, we would find fewer light isotopes on Earth today than in the original chondrites. But that is precisely what the isotope measurements do not show.”
Scientists, in this new study, looked for another solution.
Sossi explains, “Dynamic models with which we simulate the formation of planets show that the planets in our solar system formed progressively. Small grains grew over time into kilometer-sized planetesimals by accumulating more and more material through their gravitational pull.”
“Similar to chondrites, planetesimals are also small bodies of rock and metal. But unlike chondrites, they have been heated sufficiently to differentiate into a metallic core and a rocky mantle.”
“What’s more, planetesimals formed in different areas around the young Sun or at different times can have very different chemical compositions. The question is whether the random combination of different planetesimals results in a composition that matches that of Earth.”
The scientists ran simulations in which tens of thousands of planetesimals collided in the early solar system to find out. The models were created in a fashion that allowed for the gradual replication of the four rocky planets, Mercury, Venus, Earth, and Mars. The simulations demonstrate that the makeup of the Earth might result from a combination of numerous planetesimals. Furthermore, the models’ most statistically likely result is the makeup of the Earth.
Sossi recalls, “Even though we had suspected it, we still found this result very remarkable. We now not only have a mechanism that better explains the formation of the Earth, but we also have a reference to explain the formation of the other rocky planets.”
“The mechanism could be used, for example, to predict how Mercury’s composition differs from that of the other rocky planets. Or how rocky exoplanets of other stars might be composed.”
“Our study shows how important it is to consider both the dynamics and the chemistry when trying to understand planetary formation. I hope our findings will lead to closer collaboration between researchers in these two fields.”
- Sossi, P.A., Stotz, I.L., Jacobson, S.A. et al. Stochastic accretion of the Earth. Nat Astron (2022). DOI: 10.1038/s41550-022-01702-2