Evidence of a primordial isotopic gradient in the inner region of the solar protoplanetary disc

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: mah@mpia.de
² Earth Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan
³ Origins Research Institute, Research Centre for Astronomy and Earth Sciences, 15–17 Konkoly Miklós Thege utca, 1121 Budapest, Hungary
⁴ Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
⁵ Laboratoire Lagrange, Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, 06304 Nice, France
⁶ Bayerisches Geoinstitut, Universität Bayreuth, 95447 Bayreuth, Germany
⁷ Department of Lithospheric Research, University of Vienna, 1090 Vienna, Austria
⁸ Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309-0399, USA

Received: 16 December 2021
Accepted: 22 February 2022

Abstract

Not only do the sampled terrestrial worlds (Earth, Mars, and asteroid 4 Vesta) differ in their mass-independent (nucleosynthetic) isotopic compositions of many elements (e.g. ε⁴⁸Ca, ε⁵⁰Ti, ε⁵⁴Cr, ε⁹²Mo), the magnitudes of some of these isotopic anomalies also appear to correlate with heliocentric distance. While the isotopic differences between the Earth and Mars may be readily accounted for by the accretion of mostly local materials in distinct regions of the protoplanetary disc, it is unclear whether this also applies to asteroid Vesta. Here we analysed the available data from our numerical simulation database to determine the formation location of Vesta in the framework of three planet-formation models: classical, Grand Tack, and Depleted Disc. We find that Vesta has a high probability of forming locally in the asteroid belt in models where material mixing in the inner disc is limited; this limited mixing is implied by the isotopic differences between the Earth and Mars. Based on our results, we propose several criteria to explain the apparent correlation between the different nucleosynthetic isotopic compositions of the Earth, Mars, and Vesta: (1) these planetary bodies accreted their building blocks in different regions of the disc, (2) the inner disc is characterised by an isotopic gradient, and (3) the isotopic gradient was preserved during the formation of these planetary bodies and was not diluted by material mixing in the disc (e.g. via giant planet migration).