A new look at the chemistry of planets
It is generally believed that planets formed from the same material as the central star, originally produced by nucleosynthesis, ending in supernovae cosmic events. Such material is assumed to undergo condensation-photoevaporation phases depending on the distance from the protostar and the moment of its evolution. Such processes would determine the bulk chemical composition of the material in the regions where planets form—this is the common explanation for the compositional differences of planets. However, this model cannot clarify all the geochemical differences between planets. A research team (LRS, Sorbonne Université / CNRS) has examined the chemical composition of the Earth's upper crust to compare it with the chemical composition of the sun and also with measurements on lunar samples and meteorites, in-situ analyses from Mars and Venus, and spectral data for Mercury. In an article published on January 14 in the Astrophysical Journal, they revealed semi-log correlations between the ionization potential of a given element and its relative abundance at the surface of a given planetary body. Moreover, these correlations depend on the distance from sun.
In order to interpret the observed correlations, equations have been developed by Dr. Hervé Toulhoat, emeritus scientist at LRS, from principles of statistical physics. This theory assumes that at some early stage of protoplanetary disc evolution, the gaseous atomic material was ionized by X-rays emitted by the protostar, a very hot black body, while falling confined in the vicinity of the ecliptic plane, down this star's gravitational potential and normal to its magnetic field lines. In consequence, Lorenz forces prevailed, causing capture of charged atoms in stable orbits, i.e. a plasma state. The ionization probability of an atomic element depends on both its ionization potential and the local ionic temperature. It is then shown that the radial temperature profile along the protoplanetary disk determines a distance-dependent chemical differentiation of the protoplasma, which will predate and imprint the later condensation stages, showing up until present times as a fossil differentiation of planets. This theory was successfully tested on the previously mentioned available chemical data for the solar system. It should be generalizable to any planetary system involving exoplanets.
Among other consequences and predictions, the model situates the Earth at a very particular distance from the star, where the local temperature at the early ionization stage reached its lower limit, the cosmic background temperature. The electronic temperature was simultaneously minimal, and as a consequence, at this distance, the chemical differentiation with respect to the primitive material was maximal. It was negligible by contrast for both the star corona and the most distant planets.
The model also predicts a very high initial content of hydrogen: 83% by weight. Most of this hydrogen certainly escaped to space from local gravitational potentials due to the Jeans effect, but some fraction could have been stored inside of the Earth chemically bonded in form of hydrides. Dr. Viacheslav Zgonnik, the second author of the paper, explains that multiple works have proposed that the Earth's interior could store some important quantitates of hydrogen. Last year he published a comprehensive review on natural hydrogen, which demonstrated that deep-seated hydrogen is likely the largest source of molecular hydrogen in nature. The conclusion that Earth's depths are hydrogen-rich is based on the analysis of geophysical data on the density of the Earth's core and the laboratory experiments on the stability of hydrides at the core conditions. However, there was a lack of plausible mechanism of delivery of such large quantities to the planets' interior, which is covered by the present work.
The paper discusses further the marked chemical differentiation induced radially by this significant flux of hydrogen, showing up as actual compositions of planetary surfaces departing from the predicted bulk composition: oxygen and halogens being the most strongly bound as hydrides were driven towards the surface by the hydrogen flux, forming ultimately the salty oceans of our planets. All other elements position themselves preferentially the closer to the surface the higher their affinity for oxygen: for example, lanthanides and actinides mostly in the crust and late transition metals mostly in the core).
The proposed model presents new and alternative insights on the chemical differentiation of planets and shows that they all were not formed from the same material. It opens a door for a reappraisal of Earth bulk composition, reconsideration of geochemical paradoxes, explanation by a different way the origin of volatiles (including water), reevaluation of the energetic resources of the planet, and much more. It might also help to identify closer analogs to our Earth among newly discovered distant exoplanets
More information:
Hervé Toulhoat et al, Chemical Differentiation of Planets: A Core Issue, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac300b
Provided by CNRS
