Is there an isotopic signature from vaporizing collisions during planet formation?

Abstract:

During terrestrial planet formation, collision velocities surpass the thresholds that lead to partial vaporization of all the major chemical components: ices, iron alloys, and silicates. Vaporization is commonly associated with mass-dependent fractionation of isotopes; however, only small isotopic variations are observed in moderately volatile elements such as potassium even though the concentrations observed in planetary bodies and meteorites vary by orders of magnitude. Here, we examine the question of whether or not impact-induced vaporization should be accompanied by large isotopic fractionation of vaporized components.

The preservation of an isotopic fractionation during vaporization requires separation of the vapor from the condensed components. We find that the evolution of the impact-produced vapor is different for collisions between planetesimals versus collisions between protoplanets. In the case of accretionary collisions between protoplanets, the generated vapor is bound by the gravitational well of the growing planet. Thus, the isotopes are not fractionated because the vapor is not separated. In the case of vaporizing collisions between planetesimals, the impact velocities generally exceed the catastrophic disruption threshold and the partially vaporized material is dispersed in an impact vapor plume. We examined the thermodynamic path and opacity of plumes expanding into vacuum. We find that most of the vapor is produced at high pressures and temperatures, which limits the magnitude of the initial isotopic fractionation, and most of the silicate mass cools to the triple point within the expanding liquid-vapor mixture. Hence, much of the vapor in impact ejecta recondenses before physical separation from the liquid component and mass-dependent isotopic fractionation will be limited. In general, large mass-dependent isotopic fractionation is not expected from vaporizing collisions during planet formation.