Periodic Reporting for period 2 - HolyEarth (A holistic approach to understand Earth formation)
Période du rapport: 2023-07-01 au 2024-12-31
This interdisciplinary project aims to develop a novel and holistic approach to Earth’s origin based on dynamical and geochemical modelling that accounts for the largest set of constraints ever simultaneously considered: (a) the global architecture of the Solar System, (b) the isotopic properties of Earth, Mars and meteorites, (c) their chemical similarities and differences, (d) their formation timescales as provided by radionuclide chronometers, (e) the observational evidence that some original planetesimals survived basically intact (e.g. Vesta) while others were catastrophically disrupted in collisions (e.g. iron meteorite parent bodies), (f) the preservation or regeneration of sub-millimetre grains (chondrules or chondrule precursors) for a few million years, as required to form the last generation of planetesimals, namely chondrite parent bodies. The overall goal of HolyEarth is to identify, for the first time, a unique scenario of Earth formation that simultaneously and self-consistently satisfies all these observables.
With isotopic analyses of Earth and meteorites, we demonstrated that our planet accreted most of its mass (~95%) from local material, the rest being accreted from carbonaceous material formed in the outer Solar system. But, for a volatile element like Zn, Earth accreted 30% of its Zn budget from carbonaceous material, because the latter is much reacher in Zn than the local, volatile depleted material. Mars accreted basically no carbonaceous material, because no carbonaceous signature is detected in martian Zn. Together, these results prove that the terrestrial planets did not form by accreting dust drifting radially towards the Sun from the outer part of the protoplanetary disk (a process dubbed pebble accretion), because this model would predict a large fraction of carbonaceous material in all terrestrial planets.We have shown that pebble accretion was inefficient for the terrestrial planets because dust particles were small and the flux of dust from the outer part of the disk was blocked by the presence of Jupiter
We have studied the dynamics of dust in the early phases of the protoplanetary disk, characterized by a viscous radial expansion as the disk was accreting material from the protosolar cloud. We have shown that radial expansion of the disk favors the accumulation of dust at the snowline (where water condenses) and at the silicate line (where silicates condense). Dust accumulation at these two locations favors the formation of planetesimals. The ring of icy planetesimals is favorable to start forming the core of Jupiter, while the ring of silicate planetesimals is potentially the birthplace of terrestrial planets.
Thus, we have modeled in detail the process of terrestrial planet formation, starting from a ring of 100km planetesimals placed at 1 AU. We have shown that this ring generates rapidly a system of planetary embryos of masses comparable to the mass of Mars. This system tends to spread and migrate, unless the surface density distribution of the gas of the protoplanetary disk also peaks at about 1 AU. In this case, the system of planetary embryos remains confined and embryos tend to coagulate at the center of the ring, forming protoplanets of 3 - 4 Mars masses by the time the gas is removed from the system. The subsequent evolution of the system leads to the final formation of Venus and Earth, i.e. forming a planetary system that reproduces the observed mass distribution of the real planets (with small Mercury and Mars at the extremes and large Venus and Earth at the center). The timing of the giant planet instability has a weak impact on terrestrial planet formation. The simulations show a large range of possible formation timescales for Earth, ranging from ~20 to 150 My. But a formation timescale of ~ 60 My is preferred because it allows explaining the mass of material accreted by our planet after the Moon-forming event (called the Late Veneer) and the dynamical excitation of the terrestrial planet system (quantified by the so-called " angular momentum deficit" ).
We have studied the dynamics of dust in the early phases of the protoplanetary disk, characterized by a viscous radial expansion as the disk was accreting material from the protosolar cloud. We have shown that radial expansion of the disk favors the accumulation of dust at the snowline (where water condenses) and at the silicate line (where silicates condense). Dust accumulation at these two locations favors the formation of planetesimals. The ring of icy planetesimals is favorable to start forming the core of Jupiter, while the ring of silicate planetesimals is potentially the birthplace of terrestrial planets.
Thus, we have modeled in detail the process of terrestrial planet formation, starting from a ring of 100km planetesimals placed at 1 AU. We have shown that this ring generates rapidly a system of planetary embryos of masses comparable to the mass of Mars. This system tends to spread and migrate, unless the surface density distribution of the gas of the protoplanetary disk also peaks at about 1 AU. In this case, the system of planetary embryos remains confined and embryos tend to coagulate at the center of the ring, forming protoplanets of 3 - 4 Mars masses by the time the gas is removed from the system. The subsequent evolution of the system leads to the final formation of Venus and Earth, i.e. forming a planetary system that reproduces the observed mass distribution of the real planets (with small Mercury and Mars at the extremes and large Venus and Earth at the center). The timing of the giant planet instability has a weak impact on terrestrial planet formation. The simulations show a large range of possible formation timescales for Earth, ranging from ~20 to 150 My. But a formation timescale of ~ 60 My is preferred because it allows explaining the mass of material accreted by our planet after the Moon-forming event (called the Late Veneer) and the dynamical excitation of the terrestrial planet system (quantified by the so-called " angular momentum deficit" ).
The isotopic analyses of Zn had never been done before.
We have identified for the first time the conditions in disk's evolution that lead to the formation of two rings of planetesimals, one of which rocky (potentially source of terrestrial planets).Before, models could form only planetesimals at the snowline
Our simulations of terrestrial planet formation from a ring are the first to start from a population of self-interacting planetesimals in the presence of gas, while all previous works imposed in the initial conditions the existence of planetary embryos, thus missing the crucial phase of formation of such embryos.
We are now investigating the chemical and isotopic properties of an Earth accreting from such a ring, to compare it with the constraints provided by the Earth's mantle chemistry (BSE).
We will also investigate the implantation of ring planetesimals into the asteroid belt and the dynamical and collisional evolution of the latter.
Cosmochemical analyses will focus on components of chondrites, to identify the carriers of isotopic anomalies that characterize the local and carbonaceous planetesimals. This will allow refining the disk evolution and planetesimal formation model.
We have identified for the first time the conditions in disk's evolution that lead to the formation of two rings of planetesimals, one of which rocky (potentially source of terrestrial planets).Before, models could form only planetesimals at the snowline
Our simulations of terrestrial planet formation from a ring are the first to start from a population of self-interacting planetesimals in the presence of gas, while all previous works imposed in the initial conditions the existence of planetary embryos, thus missing the crucial phase of formation of such embryos.
We are now investigating the chemical and isotopic properties of an Earth accreting from such a ring, to compare it with the constraints provided by the Earth's mantle chemistry (BSE).
We will also investigate the implantation of ring planetesimals into the asteroid belt and the dynamical and collisional evolution of the latter.
Cosmochemical analyses will focus on components of chondrites, to identify the carriers of isotopic anomalies that characterize the local and carbonaceous planetesimals. This will allow refining the disk evolution and planetesimal formation model.