Silicate Melting and Vaporization during Rocky Planet Formation

Abstract

Collisions that induce melting and vaporization can have a substantial effect on the thermal and geochemical evolution of planets. However, the thermodynamics of major minerals are not well known at the extreme conditions attained during planet formation. We obtained new data at the Sandia Z Machine and use published thermodynamic data for the major mineral forsterite (Mg${}_2$SiO${}_4$) to calculate the specific entropy in the liquid region of the principal Hugoniot. We use our calculated specific entropy of shocked forsterite, and revised entropies for shocked silica, to determine the critical impact velocities for melting or vaporization upon decompression from the shocked state to 1 bar and the triple points, which are near the pressures of the solar nebula. We also demonstrate the importance of the initial temperature on the criteria for vaporization. Applying these results to $N$-body simulations of terrestrial planet formation, we find that up to 20 to 40% of the total system mass is processed through collisions with velocities that exceed the criteria for incipient vaporization at the triple point. Vaporizing collisions between small bodies are an important component of terrestrial planet formation.

arXiv comment15 pages, 8 figures, Accepted for publication in JGR: Planets

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• High Pressure Phase Diagram of Beryllium from \emph{Ab Initio} Free Energy Calculations
• The Role of Giant Impacts in Planet Formation
• Planetary Embryo Collisions and the Wiggly Nature of Extreme Debris Disks
• Multi-Messenger Measurements of the Static Structure of Shock-Compressed Liquid Silicon at 100 GPa
• Chondrules from high-velocity collisions: thermal histories and the agglomeration problem
• Impact-induced Vaporization During Accretion of Planetary Bodies
• New SiS destruction and formation routes via neutral-neutral reactions and their fundamental role in interstellar clouds at low and high metallicity values
• Episodic planetesimal disruptions triggered by dissipation of gas disk
• Re-accretion of Giant Impact Ejecta Can Drive Significant Atmospheric Erosion on Terrestrial Planets