A Two-Phase Model For The Evolution Of Planetary Embryos With Implications For The Formation Of Mars

Attempts to constrain the accretion history of Mars using geochemical observations have focused on the chronology of core formation. However, given the possibility of incomplete metal-silicate equilibration, most previous results cannot distinguish between the following two scenarios: (1) Mars was formed early through gradual accretion and remained isolated as a "planetary embryo" and (2) Mars underwent relatively protracted accretion involving collisions between planetary embryos. Here we use a two-phase model to demonstrate that, despite strong heating from the short-lived radionuclide Al-26, the mantle of a Mars-sized planetary embryo may stay effectively solid because the generation and migration of melts in relatively small fractions leads to efficient heat transport. In this scenario, the effectively solid mantle would have undergone substantial differentiation before the extinction of Al-26, and the disparate planetary building blocks would not have been homogenized within the embryo. Hence, features of very early silicate differentiation (within the first few million years of the solar system) and significant nucleosynthetic isotope heterogeneity would be expected to exist in stranded planetary embryos. In contrast, meteorite observations suggest that the crust-mantle system of Mars is homogeneous in nucleosynthetic isotope anomalies and the onset of silicate differentiation was no earlier than similar to 15 Myr after solar system formation. This discrepancy implies that Mars is not a stranded planetary embryo. Instead, we suggest that the accretion of Mars involved at least one collision between planetary embryos with comparable sizes that caused complete melting and homogenization during the giant-impact stage of planetary growth.Plain Language Summary The Earth is thought to be formed through the merging of multiple smaller bodies called planetary embryos. Mars is around one-tenth of the mass of the Earth, and it may either be a single stranded planetary embryo or composed of two or more planetary embryos. Here, we develop a physical model to investigate the internal evolution of a growing planetary embryo. We predict that the silicate portion of the planetary embryo is never completely melted as often assumed, but stays mostly solid during accretion. We also predict that a stranded planetary embryo would preserve isotopic heterogeneity inherited from its building blocks and exhibit signatures for extremely early silicate differentiation. These predicted features are not found in Martian meteorites. Therefore, we suggest that Mars likely experienced at least one giant impact that was energetic enough to cause complete melting and erase heterogeneity acquired from its component planetary embryos.