Formation of Venus, Earth and Mars: Constrained by Isotopes
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Formation of Venus, Earth and Mars: Constrained by Isotopes Helmut Lammer1 · Ramon Brasser2 · Anders Johansen3 · Manuel Scherf1 · Martin Leitzinger4
Received: 26 November 2019 / Accepted: 28 November 2020 © Springer Nature B.V. 2020
Abstract Here we discuss the current state of knowledge of terrestrial planet formation from the aspects of different planet formation models and isotopic data from 182 Hf-182 W, U-Pb, lithophile-siderophile elements, 48 Ca/44 Ca isotope samples from planetary building blocks, recent reproduction attempts from 36 Ar/38 Ar, 20 Ne/22 Ne, 36 Ar/22 Ne isotope ratios in Venus’ and Earth’s atmospheres, the expected solar 3 He abundance in Earth’s deep mantle and Earth’s D/H sea water ratios that shed light on the accretion time of the early protoplanets. Accretion scenarios that can explain the different isotope ratios, including a Moonforming event ca. 50 Myr after the formation of the Solar System, support the theory that the bulk of Earth’s mass (≥80%) most likely accreted within 10–30 Myr. From a combined analysis of the before mentioned isotopes, one finds that proto-Earth accreted most likely a mass of 0.5–0.6 M Earth within the first ≈3–4.5 Myr, the approximate lifetime of the protoplanetary disk. For Venus, the available atmospheric noble gas data are too uncertain for constraining the planet’s accretion scenario accurately. However, from the available imprecise Ar and Ne isotope measurements, one finds that proto-Venus could have grown to a mass of up to 0.85–1.0 M Venus before the disk dissipated. Classical terrestrial planet formation models have struggled to grow large planetary embryos, or even cores of giant planets, quickly from the tiniest materials within the typical lifetime of protoplanetary disks. Pebble accretion could solve this long-standing time scale controversy. Pebble accretion and streaming instabilities produce large planetesimals that grow into Mars-sized and larger planetary embryos during this early accretion phase. The later stage of accretion can be explained well Reading Terrestrial Planet Evolution in Isotopes and Element Measurements Edited by Helmut Lammer, Bernard Marty, Aubrey L. Zerkle, Michel Blanc, Hugh O’Neill and Thorsten Kleine
B H. Lammer 1
Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria
2
Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
3
Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Box 43, 22100 Lund, Sweden
4
Institute of Physics/IGAM, University of Graz, Universitätsplatz 5/II, 8010 Graz, Austria
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with the Grand-Tack model as well as the annulus and depleted disk models. The relative roles of pebble accretion and planetesimal accretion/giant impacts are poorly understood and should be investigated with N-body simulations that include pebbles and multiple protoplanets. To summarise, different isotopic dating methods and the latest terrestrial planet formati
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