Particle growth beyond the CO2 snowline &#8211; dynamic and mechanical properties of CO2 ice
Authors: Fritscher Miriam , Teiser Jens 
Published: September 2022

&lt;p&gt;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 H&lt;sub&gt;2&lt;/sub&gt;O, CO&lt;sub&gt;2&lt;/sub&gt; and CO.&lt;/p&gt;
&lt;p&gt;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 CO&lt;sub&gt;2&lt;/sub&gt;-ice agglomerates, consisting of micrometer-sized particles.&lt;/p&gt;
&lt;p&gt;Collisions of agglomerates with sizes of 10 &amp;#181;m to 150 &amp;#181;m 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 CO&lt;sub&gt;2&lt;/sub&gt;-ice agglomerates is similar to that of silicate agglomerates with a comparable grain size distribution [Fritscher M., Teiser J., 2021, ApJ, 923, 134].&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; width=&quot;461&quot; height=&quot;304&quot; /&gt;&lt;/p&gt;
&lt;p&gt;Fig. 1: Coefficient of restitution &amp;#1013; for sticking (lighter blue dots) and bouncing events (darker blue dots) and fragmentation strength &amp;#956; (red triangles) depending on the impact velocity [Fritscher M., Teiser J., 2021, ApJ, 923, 134].&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;To examine the mechanical properties of CO&lt;sub&gt;2&lt;/sub&gt; 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&amp;#183;10&lt;sup&gt;4&lt;/sup&gt; Pa and 10&lt;sup&gt;5&lt;/sup&gt; Pa. The effective surface energy is derived from the measured splitting tensile strength as 0.060&amp;#177;0.022 J/m&lt;sup&gt;2&lt;/sup&gt;. The surface energy depending on the volume filling is shown in fig. 3 [Fritscher M., Teiser J., 2022, MNRAS, 512, 3754].&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; width=&quot;481&quot; height=&quot;318&quot; /&gt;&lt;/p&gt;
&lt;p&gt;Fig. 2: Tensile strength &amp;#963; depending on the volume filling &amp;#934; [Fritscher M., Teiser J., 2022, MNRAS, 512, 3754].&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; width=&quot;507&quot; height=&quot;336&quot; /&gt;&lt;/p&gt;
&lt;p&gt;Fig. 3: Surface energy &amp;#947; depending on the volume filling &amp;#934; [Fritscher M., Teiser J., 2022, MNRAS, 512, 3754].&lt;/p&gt;
&lt;p&gt;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 CO&lt;sub&gt;2&lt;/sub&gt;-ice agglomerates, when scaled properly with the surface energy. Therefore, we conclude that the growth processes known for silicate dust also apply for CO&lt;sub&gt;2&lt;/sub&gt; ice, including growth barriers as the bouncing barrier or the fragmentation barrier.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;References&lt;/p&gt;
&lt;p&gt;Fritscher, M. and Teiser, J. (2021). The Astrophysical Journal, Volume 923, Issue 2, id. 134, 9 pp. DOI: 10.3847/1538-4357/ac2df4&lt;/p&gt;
&lt;p&gt;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&lt;/p&gt;