News on the formation of the Earth and all the planets of the Solar System

If the geological archives allow us to go back in time to the first times in the history of our planet and thus to understand its evolution, there are very few traces testifying to the very formation of the earth.

The data at our disposal are necessarily indirect and often lead to various hypotheses. If it is clear today that the Earth was formed from the material available in the nebula solar and only small rocky bodies of types comets and asteroids participated in its growth, the details of this process are still poorly known.

An earth composition that is not explained by the current theory

Until now, the dominant theory assumed that the Earth was formed by accretion small compound bodies of rocks and metalnamed chondritesformed very early in the history of the Solar system. The problem is that this theory does not explain the exact composition of the Earth, which appears to be particularly poor in light elements such ashydrogen and thehelium. Scientists have tried to explain this ” anomaly » of composition by various hypotheses such as, for example, the fact that late collisions with the Earth produced enormous amounts of heat having literally vaporized the light elements. An unconvincing explanation for Paolo Sossi of ETH Zurich who led a new study published in Nature Astronomy.

A team of researchers has just proposed a new scenario, based on results obtained from laboratory experiments and numerical simulations.

An accretion of towering planetesimals instead of a shower of small asteroids

The dynamic models obtained suggest that the planets of the Solar System would have formed very gradually and not by a continuous “rain” of small asteroids. For the researchers, the small grains first accumulated to form a multitude of larger bodies, a few kilometers in diameter, called planetesimals. These planetesimals would then have accreted to each other to form embryos planets, including that of the Earth. This theory diverges from that currently in force by the fact that unlike chondrites, planetesimals would have, during their formation, stored enough heat internal to initiate a process of differentiation, the heavy and metallic elements migrating towards the center to form a core, the lighter elements remaining on the outer part to form a rocky mantle. This differentiation does not exist in chondrites, which are far too small bodies to undergo this kind of process.

A model that explains the composition of the different rocky planets

Moreover, not all planetesimals would have been similar to each other. Their composition would indeed have varied greatly depending on their training area and in particular the remoteness of the young Sun. Numerical simulations show that these thousands of planetesimals would then have collided to form the different rocky planets of the Solar System : Mercury, Venus, Earth and Mars. The composition of the Earth would be particularly well explained by the collision of planetesimals coming from various zones of the Solar System.

This study makes it possible in particular to explain why the composition of Mercury, the planet closest to the Sun, differs from the three other rocky planets. It should also prove very useful in predicting the composition ofexoplanets rocks orbiting aroundstars distant.

Should we review the theory of the formation of the Earth?

The dominant theory of the formation of the Earth is that of homogeneous accretion. In recent years, new elements have prompted planetary scientists to rehabilitate, at least in part, a competing theory, that of heterogeneous accretion. A recent publication goes in this direction.

Article of Laurent Sacco published on May 17, 2010

The theory of planet formation is not just a matter of celestial mechanics and physicalit is also a matter of chemistry – cosmochemistry to be precise. In this context, the formation of the Earth had been studied essentially using two models during the 20th century. These are based on what we know about the differentiated structure of the planet (with a ferrous core, a mantle, a crusta hydrosphere and an atmosphere) and the data from the analyzes of meteorites and moon rocks.

Within the framework of the theory of accretion, one initially starts from a cold protosolar nebula rich in gas and in dust, like those currently observed Herschel. A proto-Sun forms, surrounded by a disc in which embryos of planets appear by accreting matter. Within this disk there is a gradient chemical and thermal, the materials condensing all the more far from the Sun as they are volatile. This is why ice can only exist far from the Sun.

For the continuation of the history of the formation of the Earth, two theories clash: that of homogeneous accretion and that of heterogeneous accretion. Within the framework of the second, the structure of the Earth is almost as old as the planet itself because it is first the heaviest elements and the less volatile materials which agglomerate first. This is why the heart formed essentially of iron and nickel appear first, followed quickly by a silicate mantle and, soon after, an atmosphere.

According to the opposing theory, the Earth first formed from rather homogeneous chondritic material, i.e. meteorites and planetesimals. It only differentiates afterwards, in less than a hundred million years. This model of homogeneous accretion quickly became dominant after the end of the missions Apollo.

In recent years, however, some have proposed that the accretion cannot be entirely homogeneous. Indeed, they explain, the differentiation of the small celestial bodies subsequently accreted during these hundred million years also occurred during a certain duration. Moreover, the model of degassing of the mantle supposed to explain the origin of the Earth’s atmosphere and oceans has been losing ground in recent years. The water of the oceans would thus have come fromspace.

Money proof

A recent publication in Science has just brought one more piece to the file of a rehabilitation, at least partial, of the model of heterogeneous accretion. Richard Carlson and his colleagues Maria Schönbächler, Erik Hauri, Mary Horan and Tim Mock looked at small variations in the abundances of silver isotopes in meteorites and terrestrial rocks.

Silver occurs in the form of two stable isotopes. One, silver 107, is the trace of an extinct radioelement, the palladium 107. Very unstable, it turned into silver 107 in less than 30 million years at the beginning of the formation of the Solar System. However, palladium is less volatile than silver and therefore tends to bond more easily with iron. We can therefore use the traces left by this radioactivity extinct to estimate how rich the material from which the Earth originated was in volatile elements.

Gold, terrestrial mantle and primitive meteorites show reports ¹⁰⁷Ag/¹⁰⁹Ag very close. A non-negligible part of the Earth therefore comes from the accretion of a material containing non-negligible quantities of volatile elements whereas this is no longer the case today (the Earth lacks hydrogen, carbon and D’nitrogen for example).

This relationship is above all in contradiction with the estimate of the dating of the formation of the Earth’s core with the isotopes of tungsten and hafnium which indicate a duration of between 30 and 100 million years. Indeed, silver isotopes lead to an age of 5 to 10 million years.

For cosmochemists, a good way to reconcile these data is to assume that 85% of the mass of the planet comes from the accretion of materials poor in volatile elements, which accumulated first. This phase would have been followed by a period of accretion of materials rich in volatile elements, similar to those of primitive chondrites but not necessarily identical. This therefore amounts to reviving the theory of heterogeneous accretion.

In fact, these observations would be compatible, according to the researchers, with the collision between the proto-Earth and Theiaat the origin of the formation of the Moon in the currently popular theory. Théia would have brought most of the materials rich in volatile elements in one go.