Particle growth beyond the CO2 snowline – dynamic and mechanical properties of CO2 ice

The initial process of planetesimal formation is the coagulation of micrometer-sized solid particles. In the inner regions of a protoplanetary disk, silicate dust is the dominant material. Further out beyond the corresponding snow lines, various other volatile compounds condense and serve as additional building material for planetesimals. The most frequent compounds are H₂O, CO₂ and CO.

Since the coagulation behavior largely depends on the properties of the building material, an understanding of these properties is essential to comprehend the first steps in planetesimal formation. To provide a systematic analysis on this, we conducted experiments on the collision behavior and the mechanical properties of CO₂-ice agglomerates, consisting of micrometer-sized particles.

Collisions of agglomerates with sizes of 10 µm to 150 µm are analyzed to investigate the collision behavior. The coagulation is studied at temperatures around 100 K and a pressure of 1 mbar with impact velocities of up to 3.4 m/s. The collisional outcome is analyzed regarding the events of sticking, bouncing or fragmentation. Below impact velocities of 0.1 m/s sticking is observed. Above 0.1 m/s the particles bounce inelastically, with a maximum coefficient of restitution of 0.5 only. For velocities higher than 1 m/s fragmentation of the agglomerates sets in. The three velocity ranges are shown in fig. 1. The experiments indicate that the collision behavior of CO₂-ice agglomerates is similar to that of silicate agglomerates with a comparable grain size distribution [Fritscher M., Teiser J., 2021, ApJ, 923, 134].

Fig. 1: Coefficient of restitution ϵ for sticking (lighter blue dots) and bouncing events (darker blue dots) and fragmentation strength μ (red triangles) depending on the impact velocity [Fritscher M., Teiser J., 2021, ApJ, 923, 134].

 

To examine the mechanical properties of CO₂ ice, the well-established method of the Brazilian test is used. For varying volume fillings between 0.35 and 0.54, the splitting tensile strength is determined. As can be seen in fig. 2 it follows a power law, with values between 2·10⁴ Pa and 10⁵ Pa. The effective surface energy is derived from the measured splitting tensile strength as 0.060±0.022 J/m². The surface energy depending on the volume filling is shown in fig. 3 [Fritscher M., Teiser J., 2022, MNRAS, 512, 3754].

 

Fig. 2: Tensile strength σ depending on the volume filling Φ [Fritscher M., Teiser J., 2022, MNRAS, 512, 3754].

Fig. 3: Surface energy γ depending on the volume filling Φ [Fritscher M., Teiser J., 2022, MNRAS, 512, 3754].

For silicates simulation methods and models exist, describing aggregate collisions and the evolution of dust in protoplanetary disks. The collision experiments show that these models can also be applied to collisions of CO₂-ice agglomerates, when scaled properly with the surface energy. Therefore, we conclude that the growth processes known for silicate dust also apply for CO₂ ice, including growth barriers as the bouncing barrier or the fragmentation barrier.

 

References

Fritscher, M. and Teiser, J. (2021). The Astrophysical Journal, Volume 923, Issue 2, id. 134, 9 pp. DOI: 10.3847/1538-4357/ac2df4

Fritscher, M. and Teiser, J. (2022). Monthly Notices of the Royal Astronomical Society, Volume 512, Issue 3, pp. 3754-3759. DOI: 10.1093/mnras/stac676