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 2SiO 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.

Temperature and Density on the Forsterite Liquid‐Vapor Phase Boundary

“…(b) Measured apparent temperatures and specific entropies of states on the liquid‐vapor curve and their associated shock states. The blue curve, with 1‐ σ error envelope, is the experimentally determined forsterite principal Hugoniot (Davies et al., 2020; Root et al., 2018). The yellow (ANEOS‐C, Canup et al., 2013) and orange (ANEOS‐G, Ćuk & Stewart, 2012; Nakajima & Stevenson, 2014) lines are different forsterite ANEOS models previously used in giant impact simulations.…”

“…The material-specific us(up) equations can be transformed to pressure, specific volume and specific internal energy via the Rankine-Hugoniot conservation equations. References: (1) Root et al (2018), (2) Davies et al (2020), (3) Knudson and Desjarlais (2013), (4) Root et al (2015). on the detector.…”

“…The high pressure measurements on the principal Hugoniot (Davies et al, 2020;Root et al, 2018), preliminary analyses of the experiments presented in this work, and constraints on the critical point, now published in Townsend et al (2020), were used to constrain a revised forsterite ANEOS-based model (labeled New ANEOS). The ANEOS code package was updated to include a user adjustable heat capacity in the liquid and was focused on fitting high pressure Hugoniot data and the liquid and vapor regions of the phase diagram, see Stewart et al (2020).…”

“…The ANEOS models for forsterite used in previous giant impact studies were developed without experimental constraints for the vapor curve at high pressures and without measurements of the shock response above about 200 GPa. Recently, the shock states for forsterite were measured at pressures from 200 to 950 GPa, to determine the relationships between pressure (P), density (ρ), temperature (T), and specific entropy (S) (Davies et al, 2020;Root et al, 2018). Stewart et al (2020) found substantial differences between previous ANEOS models for forsterite and these new data, and developed improvements to the ANEOS code package to improve the fit to experimental temperatures and entropies.…”

“…Giant impacts between Earth-sized planets can generate pressures and temperatures in excess of 500 GPa and 15,000 K. Subsequent decompression melts and vaporizes significant fractions of the constituent materials. Recent work has shown that vaporizing collisions are relatively common during planet formation in that almost all impacts onto large embryos create vapor (Davies et al, 2020;Kraus et al, 2012Kraus et al, , 2015Stewart & Ahrens, 2005). Wide-ranging EOS models are required that span the extreme conditions achieved during giant impacts as well as the subsequent decompression to the liquid-vapor phase boundary.…”