Collisions of CO$_2$ Ice Grains in Planet Formation

TL;DR

This study experimentally investigates CO₂ ice grain collisions at 80 K, revealing thresholds for sticking, bouncing, and fragmentation, and compares their behavior to other ices, impacting planetesimal formation theories.

Contribution

First laboratory collision experiments with CO₂ ice particles at 80 K, providing new data on their collisional thresholds and behavior in protoplanetary disks.

Findings

Sticking threshold at 0.04 m/s for CO₂ ice particles.

Particles bounce at velocities below 1 m/s.

Fragmentation occurs at velocities above 1 m/s.

Abstract

In protoplanetary disks, CO$_2$ is solid ice beyond its snow line at $\sim 10 \rm AU$. Due to its high abundance, it contributes heavily to the collisional evolution in this region of the disk. For the first time, we carried out laboratory collision experiments with CO$_2$ ice particles and a CO$_2$-covered wall at a temperature of 80 K. Collision velocities varied between 0 - 2.5 m/s. Particle sizes were on the order of $\sim$ 100 $\mu$m. We find a threshold velocity between the sticking and the bouncing regime at 0.04 m/s. Particles with greater velocities but below 1 m/s bounce off the wall. For yet greater velocities, fragmentation occurs. We give analytical models for the coefficients of restitution and fragmentation strength consistent with the experimental data. Set in context, our data show that CO$_2$ ice and silicate dust resemble each other in the collisional behavior. Compared to water ice the sticking velocity is an order of magnitude smaller. One immediate consequence as example is that water ice particles mantled by CO$_2$ ice lose any "sticking advantage." In this case, preferential planetesimal growth attributed to the sticking properties of water ice will be limited to the region between the H$_2$O ice line and the CO$_2$ ice line.