Formation of the Solar System: news on the origin of Mars and the Earth

Some cosmochemists suspected that the Earth and Mars would have formed by accreting significant amounts of matter from the regions where the giant planets were born. By showing that this should not be the case, other cosmochemists discovered that these two planets would have accreted matter from a vanished population of celestial bodies that left no direct trace in the form of meteorites in the belt. main asteroids.

The term cosmochemistry is generally attributed to Harold Urey. The Nobel Prize chemistry, past master in quantum theory of atoms and molecules, as one of his works published in 1930 shows, had turned in the early 1950s in the direction of a theory of the origin of Moon and more generally of the planets of Solar system in one book published in 1952 and that we can rightly consider as one of the starting points of cosmochemistry.

In the years to follow, together with colleagues, Urey conducted a very extensive analysis for his time of the abundances of chemical elements, in particular non-volatile, in meteorites, by making comparisons with the compositions of theatmosphere from Soleil and rocks on Earth, and in order to derive a probable determination of the abundances of chemical elements on a cosmic scale.

The article he will publish on this subject in 1956 with Hans Eduard Suess (the grandson of geologist Austrian Eduard Suess) served as one of the bases monumental article published in 1957 through E. Margaret Burbidge, GR Burbidge, William A. Fowler, and F. Hoyle. This foundational work explained for the first time how the stars could synthesize chemical elements heavier thanhydrogen, until do and beyond from the theory of nucleosynthesis stellar.

The Solar System is a laboratory for studying the formation of giant planets and the origin of Life that can be used in conjunction with the rest of the Universe, observable for the same purpose. Mojo: Modeling the Origin of JOvian planets, that is to say, modeling the origin of the Jovian planets, is a research project which gave rise to a series of videos presenting the theory of the origin of the Solar System and in particular of the gas giants by two renowned specialists , Alessandro Morbidelli and Sean Raymond. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on « Subtitles » and finally on « Translate automatically ». Choose « French ». © Laurence Honnorat

Cosmochemistry, a key to the cosmogony of the Solar System

Cosmochemistry will unite over the coming decades with analytical models and digital of the formation of planets resulting from celestial mechanics and the kinetic theory of gases within the framework of the theory ofaccretion initially developed by researchers like Russian Viktor Safronov and the United States George Wetherill. This resulted in a scenario of the formation of the planets of the Solar System which is broadly accepted today, being further supported by observations of young planetary systems in formation. A good presentation is given in the video above which is part of a series on this subject. There is also an excellent book on physics and chemistry of the solar system that we can consult.

However, this scenario ramifies into several possible sub-scenarios depending on whether one seeks to understand, for example, the origin of rocky planets or gas giants, so there is still work to be done to really understand how the planets formed. With the new data available, the pendulum swings between these sub-scenarios as they become more precise. We see a new example today with a publication in the journal  » Science Advances »A work that we owe to researchers from the University of Münster (Germany), the Côte d’Azur Observatory (France), California Institute of Technology (United States), Natural History Museum Berlin (Germany) and the Free University of Berlin (Germany). It is based on a new and more exact determination of the composition of the rocky planets of Earth and Mars.

In a statement, cosmochemist Christoph Burkhardt of the University of Münster, first author of the study, explains the initial goal of the research team: “ We wanted to know if the building blocks of Earth and Mars came from the outer or inner Solar System. « . This required them to obtain a determination of the abundances of isotopes from metals rare in the outer layers rich in silicate of the two planets that are the titanium, the zirconium and the molybdenum by examining them like never before in Martian meteorites in particular.

The cosmogonic model of the formation of the Solar System involves a gradient chemical and temperature in the protoplanetary disc rich in gas and dust where the planets were formed from the cooling of this matter that the process ofcollapse gravitational of a cloud dusty molecular which gave birth to the proto-Sun surrounded by this disc. Close tostar central, it is the rocky bodies rather refractory and containing few volatile elements which have condensed, i.e. silicates and metals.

Beyond a line called ice or snow, it is dust surrounded by a coat ice, mainly water, which will condense, with comets and carbonaceous meteorites such as the famous chondrite d’Allende. It is therefore in the internal part of the protoplanetary disc that the rocky planets were born and in the external part that the gas giants were born. Jupiter and Saturn, and the ice giants Neptune and Uranus.

The chemical gradient is found at the level of the abundances of certain isotopes, so that it can be used as tracers to assess in which chemical reservoir at a given distance from the Sun the material composing a meteorite, itself sometimes a fragment produced during a collision of a large body such as a embryo planet or a planetesimals (see details on these terms in the video above).

There are, however, signs of a turbulent mixing process in the protoplanetary disk and other processes suggesting that material was being exchanged between the inner and outer parts of this disk. It is on the existence and the importance of these material transfer processes that specialists debate a lot today. Not long ago, a study even advanced the thesis that there was a discontinuity between these two parts of the disc, blocking chemical transfers.

The work of Christoph Burkhardt and his colleagues seems compatible since it leads to admit that the Earth and Mars were formed mainly from internal materials of the Solar System and only a few percent of the constituent elements of these two planets at most would originate from ‘beyond the’orbit of Jupiter, in good agreement with a scenario already advanced by George Wetherill.

Remember that we have good reasons to believe that the majority of meteorites that we find on Earth come from asteroid belt main between Mars and Jupiter. This belt itself has been populated throughout the history of the Solar System by small celestial bodies from both the inner Solar System and its outer part, in particular due to the gravitational influence of Jupiter. The non-carbonaceous chondrites reflect the composition of the outer protoplanetary disc beyond Jupiter and the non-carbonaceous chondrites that of the inner part below.

Meteorites whose memory has not been preserved in the main asteroid belt

It is possible to model the distribution of the abundances of titanium, zirconium and molybdenum isotopes in the protoplanetary disc as well as the way in which the matter was accreted to form rocky planets and therefore to go back to the origin of the matter in the beltasteroids and the one making up the crust of Mars and Earth. These models were combined with data from new measurements of these isotopes in Martian rocks, the case of Earth being already well known.

Cosmochemists then discovered that the outer rock layers of Earth and Mars have little in common with the carbonaceous chondrites of the Outer Solar System at the level of newly determined isotopes, which can only be explained if these planets formed. by being largely isolated from material inputs in the form of materials common to carbonaceous chondrites, unlike a scenario which involved a significant accretion of « pebbles » (pebbles in English, as shown in the diagram below and Sean Raymond’s explanations in the video above) formed in the outer part of the protoplanetary disk.

The second discovery revealed by the isotopes is that the composition of Mars and the Earth is also not understood by limiting itself to the chemical reservoirs of the known carbonaceous and non-carbonaceous chondrites. On the other hand, it can be explained well from a third reservoir described by modeling of the chemical gradient in the protoplanetary disc, a reservoir corresponding to the innermost region. Small rocky bodies present in this part are not found in the main asteroid belt, but it does seem that part of them were accreted on Earth and Mars.

We can think that it is the same for Venus and Mercure even if these discoveries do not upset the classic scenario of the formation of terrestrial planets from rocky planet embryo collisions described by Sean Raymond in the previous video.

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