The Effects of Early Collisional Evolution on Amorphous Water Ice Bodies

TL;DR

This study uses impact simulations to assess how collisional processes during Solar System formation affected the survival of amorphous water ice in icy bodies, revealing that survival is highly stochastic and depends on impact velocities and collisional history.

Contribution

The paper introduces a combined simulation approach to quantify how impact velocities influence the crystallization or preservation of primordial amorphous water ice in icy bodies.

Findings

Impact speeds during planet migration likely crystallized most primordial AWI.

Survival of AWI is highly stochastic and depends on impact velocities.

Multiple impacts could have fully converted AWI to crystalline ice.

Abstract

Conditions in the outer protoplanetary disk during Solar System formation were thought to be favorable for the formation of amorphous water ice (AWI),a glassy phase of water ice. However, subsequent collisional processing could have shock crystallized any AWI present. Here we use the iSALE shock physics hydrocode to simulate impacts between large icy bodies at impact velocities relevant to these collisional environments, and then feed these results into a custom-built AWI crystallization script, to compute how much AWI crystallizes/survives these impact events. We find that impact speeds between icy bodies post-planet migration (i.e., between trans-Neptunian Objects or TNOs) are too slow to crystallize any meaningful fraction of AWI. During planet migration, however, the amount of AWI that crystallizes is highly stochastic: relatively little AWI crystallizes at lower impact velocities (less than ~2 km/s), yet most AWI present in the bodies (if equal sized) or impactor and impact site (if different sizes) crystallizes at higher impact velocities (greater than ~4 km/s). Given that suspected impact speeds during planet migration were ~2-4 km/s, this suggests that primordial AWI's ability to survive planet migration is highly stochastic. However, if proto-EKB objects and their fragments experienced multiple impact events, nearly all primordial AWI could have crystallized; such a highly collisional proto-EKB during planet migration is consistent with the lack of any unambiguous direct detection of AWI on any icy body. Ultimately, primordial AWI's survival to the present day depends sensitively on the proto-EKB's size-frequency distribution, which is currently poorly understood.