Stochastic, Impact-Induced crystallization of Primordial Amorphous Water Ice

During Solar System formation, conditions in the outer proto-solar disk likely favored the formation of amorphous water ice (AWI). AWI is a metastable, non-crystalline glassy phase of solid water that crystallizes at sufficiently warm temperatures. AWI has the ability to trap other volatile species and release them upon crystallization, and can crystallize exothermically under certain conditions. These features have been invoked to explain observed activity amongst Centaurs and the abundance of hyper-volatiles (e.g., CO) in Centaurs and Jupiter Family Comets. These explanations require that this primordial AWI survive to the present day. However, Nice-style instabilities in the early Solar System likely caused the vast majority of these parent bodies to experience a catastrophic collision event, at a speed of ~2-4 km/s. The resulting shock-heating of the body could rapidly crystallize all or part of the object’s initial AWI content, permanently removing this AWI from the inventory of the ecliptic comet (TNOs, Centaurs, and JFCs).

To explore whether AWI could survive this early collisional evolution, we use the iSALE shock physics code to simulate relevant impact events, and use an AWI crystallization model to determine the degree of AWI crystallization that results from the thermodynamic conditions throughout these bodies during the impact process. We find that the amount of AWI that survives this Nice-style collisional evolution depends sensitively to the relative impact speed. At slower speeds, most AWI survives, but crystallizes during more energetic impact events. Nevertheless, the transition between these two regimes lies around ~2-4 km/s, which are typical impact speeds following Nice-style instabilities. Thus, AWI’s survival of this collisional evolution is highly stochastic. A combination of further impact studies, combined with detailed dynamical simulations of planet migration are required to identify how much primordial AWI survives to the present day.