Collisions of small ice particles under microgravity conditions (II): Does the chemical composition of the ice change the collisional properties?

TL;DR

This study investigates whether the chemical composition of small ice particles affects their collisional behavior under microgravity, finding that at 5% methanol or formic acid, composition does not influence collision outcomes.

Contribution

It provides experimental evidence that minor chemical variations in ice do not alter collisional properties under specific conditions, highlighting surface roughness as the key factor.

Findings

Chemical composition at 5% does not affect collisional outcomes.

Coefficients of restitution show no correlation with impact velocity or temperature.

Surface roughness dominates the variation in collision behavior.

Abstract

Context: Understanding the collisional properties of ice is important for understanding both the early stages of planet formation and the evolution of planetary ring systems. Simple chemicals such as methanol and formic acid are known to be present in cold protostellar regions alongside the dominant water ice; they are also likely to be incorporated into planets which form in protoplanetary disks, and planetary ring systems. However, the effect of the chemical composition of the ice on its collisional properties has not yet been studied. Aims: Collisions of 1.5 cm ice spheres composed of pure crystalline water ice, water with 5% methanol, and water with 5% formic acid were investigated to determine the effect of the ice composition on the collisional outcomes. Methods: The collisions were conducted in a dedicated experimental instrument, operated under microgravity conditions, at relative particle impact velocities between 0.01 and 0.19 m s^-1, temperatures between 131 and 160 K and a pressure of around 10^-5 mbar. Results: A range of coefficients of restitution were found, with no correlation between this and the chemical composition, relative impact velocity, or temperature. Conclusions: We conclude that the chemical composition of the ice (at the level of 95% water ice and 5% methanol or formic acid) does not affect the collisional properties at these temperatures and pressures due to the inability of surface wetting to take place. At a level of 5% methanol or formic acid, the structure is likely to be dominated by crystalline water ice, leading to no change in collisional properties. The surface roughness of the particles is the dominant factor in explaining the range of coefficients of restitution.