Like the fatbergs that haunt the sewers of large cities, most celestial objects arise by a process of accretion, whereby small particles clump together over time to form large bodies. While this model of development is widely accepted, scientists are divided over exactly which type of materials first amalgamated to form the Earth, although a new study in the journal Nature Astronomy may finally have the answer.
"The prevailing theory in astrophysics and cosmochemistry is that the Earth formed from chondritic asteroids,” explained study lead author Paolo Sossi in a statement. “These are relatively small, simple blocks of rock and metal that formed early on in the solar system."
"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 we would have expected."
To account for this discrepancy, researchers have argued that these lighter elements may have been vaporized by the high temperatures generated during the accretion process. Unfortunately, however, this theory simply doesn’t stick, as it fails to explain why the Earth remains abundant in light isotopes of chemical elements.
Sossi and his colleagues, therefore, suspect that the Earth may have originally formed not from chondritic asteroids, but from planetesimals. These are small bodies of rock that have been sufficiently heated to develop a differentiated metallic core and rocky mantle. Importantly, the chemical composition of planetesimals depends on the temperature at which they formed, thus providing scope for variation between planets according to their distance from the Sun.
To test their hypothesis, the study authors ran a series of simulations to observe the collisions between thousands of planetesimals in the early Solar System. According to their model, interactions between Jupiter and Saturn caused the former to move towards the Sun before retreating again, thus scattering planetesimals throughout the inner disk.
This allowed for high numbers of collisions between planetesimals of varying chemical compositions, thus giving rise to the Earth in its current form, as well as Mars, Venus, and Mercury. Strikingly, the simulations revealed that the Earth as we know it was actually the most likely outcome of these collisions.
Commenting on these findings, the researchers state that “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."