How did the Earth form? This is a question still debated. Because there are certain chemical anomalies in the composition of the earth’s crust which suggest that our Planet was not formed simply by accretion of planetesimals.
And the number of exoplanets discovered continues to increase, examples with characteristics close to those of the Earth remain very rare. Is this apparent rarity linked to the difficulties in detecting these small rocky planets? Or does the Earth have certain specificities in the way it was formed?
It is true that the processes leading to the formation of planets are still poorly understood. Several recent studies also point to the fact that the current composition of the Earth does not correspond exactly with what one would expect if the planet had only formed by accretion of primitive rocky meteorites (chondrites).
A birth in the chaos of collisions between planetesimals
Until now, the preferred model for explaining the formation of rocky planets has in fact mainly involved collisions. And it started with the collapse of the solar disk, composed of dust and gas. Besides the Sun, many small planetary bodies, called planetesimals, have formed by accretion of this material. By colliding, these many planetesimals gave rise to larger and larger objectsuntil the formation of the different rocky planets that characterize the inner part of the Solar System.
This formation process requires a phenomenal number of collisions. A mechanism favored by the growing increase in the gravitational attractiveness of protoplanets, in connection with their growth. This is the time of giant impacts, such as the one that led to the formation of the moon. This period of intense bombardment from young planets was critical in shaping the Solar System as we know it today.
But what was the impact of these collisions on the chemical composition of rocky planets? For Paul Frossard of the Magmas and Volcanoes Laboratory in Clermont-Ferrand and his colleagues, the chemical composition of the primordial earth’s crust would have changed over time in response to the erosion created by these multiple cosmic impacts.
An anomaly in chemical composition due to erosion from cosmic impacts
Since 2005, we have known that there is a significant discrepancy between the composition of the earth’s crust and the composition of chondrites, these rocky meteorites which participated in the formation of the Earth. In particular, the Earth seems enriched in 142Nd (neodynium) compared to 144Nd. Neodynium is a heavy element, but incompatible with the composition of the mantle, which therefore tends to be “expelled” towards the surface and to end up in the composition of the crust. Several hypotheses have been proposed to explain this anomaly. The first suggests that our observations are biased by the fact that we can only analyze the surface layers of the Earth and that the composition of the more internal levels would fall back on the expected composition which is that of the chondrites. Another hypothesis suggests that the composition of the solar disk was not uniform and that it was the planetesimals themselves that were depleted in 142Nd.
But Paul Frossard nevertheless proposes another solution. The primordial crust of the Earth would have been continuously abraded by meteorite impacts during its formation, resulting in the loss of elements preferentially present in the crust, such as 144Nd. 142Nd being produced by the radioactive decay of 146Sm (samarium), an element present in the crust but also in the mantle, this erosion process would explain the final excess of 142Nd compared to 144Nd.
20% of the Earth torn off during the impacts
According to the results published in the journal Science, more than 20% of the Earth would have been “ripped off” by collisions during its formation. This hypothesis would also make it possible to explain other “abnormalities” of composition, in particular the depletion of certain rare earths such as uranium, potassium and thorium.
Is this mechanism of planetary erosion usual during the formation of rocky planets? Or is it a terrestrial specificity? New research might make it possible to define to what extent this mode of formation might have impacted the capacity of the Earth to support life.
