COLLISIONS OF CO₂ ICE GRAINS IN PLANET FORMATION

Abstract

In protoplanetary disks, CO 2 is solid ice beyond its snow line at ∼ 10AU. 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 ∼ 100 µm. 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.

ICE GRAIN COLLISIONS IN COMPARISON: CO2, H2O, AND THEIR MIXTURES

“…Depending on the impact velocity, sticking or bouncing can be observed. In an earlier work, we also observed fragmentation for CO 2 (Musiolik et al 2016). In Figure 3 we show samples of the largest aggregates for each data set of different ices (not analyzed collisionwise).…”

“…Other approaches try to describe the COR by heuristic functions, like Higa et al (1996Higa et al ( , 1998 did for water ice spheres. For the COR resulting from the observed collisions, we use our model deduced in the earlier work on pure CO 2 collisions (Musiolik et al 2016) …”

“…The experimental setup is equal to the former experiment from Musiolik et al (2016). Details can be found there.…”

“…In earlier experiments, we showed that it is necessary to distinguish the different ices while studying their growth potential (Musiolik et al 2016). We suggested that collisional growth should be most efficient between the water and the CO 2 iceline or between 2.0 and 9.3 AU according to the minimum mass solar nebula model.…”

“…Planet formation in general, but especially collisional outcomes, are tied to these icelines and the physics of the prevailing kind of ice. (Aumatell & Wurm 2011Ali-Dib et al 2014;Blum et al 2014;Deckers & Teiser 2016;Musiolik et al 2016).…”