CO2-ice in the context of planet formation

行星形成的过程始于微米大小的尘埃颗粒中固体的凝固。通常情况下，硅酸盐团聚体的生长一开始是有效的，但在几毫米到几厘米的团聚体大小时就会停滞不前，因为这里的团聚体相互反弹而不是粘在一起。原行星盘显示出温度梯度，靠近恒星的地方温度较高，而离恒星较远的地方温度较低。在一定距离上，也就是所谓的雪线，水冰开始形成。由于其极性结构，水冰粒比硅酸盐更有效地粘在一起。由于粘着行为有利于生长，预计水冰团将生长到更大的尺寸。在更远的原行星盘中，温度进一步下降，因此存在其他冰种。在这里，CO和CO2是最重要的物种。更多的折射率物质为挥发性物质形成凝结种子，因此颗粒表面以挥发性最强的物质为主，以固体物质的形式存在。因此，二氧化碳-冰线的形成标志着该地区的外部边界，在那里，碰撞主要是由水冰的表面特性决定的。虽然对二氧化碳冰的初步研究已经存在，但系统的研究仍然缺失，迄今为止只研究了一组非常明确的参数（团聚体大小和速度）。该项目的目标是研究二氧化碳-冰团块在不同粒径（< 100微米-几厘米）和碰撞速度（几厘米/秒- 7米/秒）范围内的碰撞行为。通过这些实验，研究了co2团聚体的自洽生长及其孔隙度的演化。所提出的实验参数范围将涵盖从粘着、弹跳、传质生长到最终侵蚀、靶体完全破坏的碰撞结果的全部范围。虽然这个数据库将适合于在圆盘模型中包括二氧化碳碰撞，但将进行额外的实验来测量二氧化碳颗粒之间的接触力。

The process of planet formation begins with the coagulation of solids from micrometer size dust grains. Typically, growth of silicate aggregates is efficient at first, but stalls at an agglomerate sizes of few millimeters to centimeter, as here agglomerates rather bounce off each other than stick together. Protoplanetary disks show a temperature gradient with higher temperatures close to the star and cooler regions further out. At a certain distance, the so-called snowline, water ice begins to form. Due to its polar structure, water ice grains stick more efficiently together than silicates. Water ice agglomerates are expected to grow to larger sizes as the sticking behavior favors growth. Further out in the protoplanetary disk temperatures drop further, so other ice species are present. Here, CO and CO2 are the most important species. More refractive materials form the condensation seed for the more volatile materials, so the surface of particles is dominated by the most volatile species, which is present as solid material. The CO2-iceline forms therefore marks the outer boundary of the region, where collisions are dominated by the surface properties of water ice. Although first studies with CO2-ice exist, a systematic study is still missing and only a very distinct set of parameters (agglomerate sizes and velocities) has been investigated so far.The goal of the proposed project is to investigate the collision behavior of CO2-ice agglomerates in a wide range of agglomerate sizes (< 100 µm – few cm) and collision velocities (few cm/s – 7 m/s). With these experiments the self-consistent growth of CO2-agglomerates and the corresponding evolution of their porosity will be studied. The parameter range of the proposed experiments will cover the complete range of collision results, from sticking, bouncing, growth via mass transfer to erosion and total destruction of target bodies, finally. While this database will be suitable to include CO2-collisions in disk models, additional experiments will be performed to measure the contact forces between CO2-grains.