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The Stochastic Impact-Induced Crystallization of Primordial Amorphous Water Ice in the Early Solar System

Presentation #410.09 in the session Dynamical Interactions in the Kuiper Belt (iPosters).

Published onOct 20, 2022
The Stochastic Impact-Induced Crystallization of Primordial Amorphous Water Ice in the Early Solar System

Amorphous water ice (AWI) is a glassy phase of water ice that is thought to have formed in the outer parts of the Sun’s protoplanetary disk during solar system formation, where low temperature and pressure conditions favored its formation. These AWI-rich grains likely accumulated in the proto-Edgeworth Kuiper Belt (proto-EKB), forming a large population of significantly AWI-rich objects. However, AWI is a meta-stable phase, and will irreversibly transform into ice I (crystalline ice) at a temperature-dependent rate. Thus, impact-induced shock heating could crystallize any primordial AWI present in these icy objects, especially during the significant collisional evolution of the proto-EKB population that occurred during planet migration in the early Solar System.

To investigate this process, we combine 2D axisymmetric iSALE hydrocode simulations with a custom-built AWI crystallization script, to compute the amount of AWI that crystallizes during relevant impact conditions. We find that the range of expected impact speeds between proto-EKB objects both before and after planet migration is much too slow to trigger significant AWI crystallization. However the expected impact speeds during planet migration (~2-4 km/s) corresponds to the range of speeds where the amount of AWI crystallization changes dramatically: whereas relatively little AWI crystallizes at slower impact speeds (~2 km/s or less), most AWI crystallizes at higher impact speeds (~4 km/s or greater). This suggests that the survival of primordial AWI is a highly stochastic process that requires the parent body to avoid both higher speed impacts and multiple such catastrophic impacts during planet migration, as each subsequent impact would further crystallize any surviving AWI. As such, primordial AWI’s survival depends sensitively on the size-frequency distribution and mass distribution of proto-EKB objects, which is currently poorly understood.

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