Amorphous water ice (AWI) is a metastable “glassy” phase of water ice thought to have formed within the proto-planetary disk and accumulated within the primordial Kuiper Belt. Subsequent collisional evolution, particularly during planet migration, could have shock-heated any AWI present. In previous work, we found that the amount of AWI that survives these impacts depends sensitively on impact speed, varying from nearly all surviving at slower speeds (1 km/s) to nearly all crystallizing at higher impact speeds (4–5 km/s). However, this work was limited to simulating non-porous objects; “real” icy bodies are known to be highly porous.
In this work, we conduct additional numerical experiments to explore how porosity affects AWI's survival. As before, we use the iSALE hydrocode to simulate impacts between icy bodies, using tracer particles to record the temperature each parcel of material experienced throughout the impact process. We then use this tracer information in a Gibbs Free Energy crystallization model to compute the degree to which each parcel of material crystallizes throughout the impact process; this data is then aggregated to compute the fraction of an object's AWI that is impact-crystallized.
We simulated 3 km/s impacts between identical 1 km nuclei (spheres of ), and found that the amount of AWI that survives the impact process is sensitive to the porosity of the colliding objects, with porosity generally reducing the amount of material that crystallizes. Furthermore, the crystallized material moves closer to the surface surrounding the impact site compared to non-crystallized; buried material is protected from additional shock heating by the more surficial materials, which must first have their pore spaces crushed out (absorbing considerable energy). We discuss how this mechanism affects the survival of AWI to the present day in high-porosity objects (including all comets with densities determined via in situ techniques).