It is the general picture that planetesimals, the km-sized precursors of planets, are formed by inelastic collisions at low velocities and sticking by interparticle attraction of micron-sized dust particles (aggregation / coagulation process) within accretion disks surrounding young stars (Weidenschilling & Cuzzi, 1993). Although many laboratory investigations were performed in the past years (Blum & Wurm, 2008) many questions concerning this first stage of planet formation remain unsolved so far.
In contrast to previous laboratory experiments to investigate the first phase of planet formation, which were performed at room temperature, we study in this project the collision and sticking behavior of dust aggregates at elevated temperatures in the range of 300 to 1.300 K. Due to effects like e.g. sintering and eutectic melting, which can occur under the influence of high temperatures, we expect considerable changes in the effective growth process of colliding dust aggregates. Within this project we will perform laboratory studies on the mechanical and thermal properties as well as the collisional behavior of high-porosity dust aggregates.
Fig. 1: Dust sample within plastic tube with impacted glass sphere.
In order to calibrate the SPH codes for the simulation of collisions between large protoplanetary dust aggregates (project B4), we performed compression experiments with high-porosity dust samples. The dust samples we have analyzed were high-porosity dust aggregates consisting of micron-sized spherical SiO2 monomers. They were produced by the random ballistic deposition (RBD) method and have a volume filling factor of Φ = 0.15 (Blum & Schräpler, 2004). To determine the compaction behavior we dropped a single glass sphere into a dust sample cutout within a piece of plastic straw (Fig. 1).
The x-ray micro-tomography of the sample mounted on a stepwise rotating sample carrier (Fig. 2) allows a three-dimensional density reconstruction. The impact experiments were carried out under vacuum conditions at a height of fall of ∼ 75 mm with two different sphere diameters of ∅ ∼ 1 & 3 mm for sintered and non-sintered dust aggregates.
Fig. 3: Volume filling factor of dust sample with impacted glass sphere.
The diagram in Fig. 3 shows an example of a porous dust sample impacted by a sphere in terms of a cross section of the spatial distribution of the volume filling factor. The data represents the result of a cylinder-symmetric density average of the reconstruction data with the vertical axis through the sphere center, mirrored at the vertical center line of the diagram. Marked by the yellow-green color one can clearly see the compaction area underneath the sphere.
Fig. 4(a) plots a Gaussian-shaped distribution of the volume filling factors within an uncompressed dust sample with a mean value of approximately 15%. In Fig 4(b) .we analyzed the distribution of volume filling factors within volume intervals normalized by the sphere volume occurring in the compacted area underneath the glass sphere (∅ ∼ 1 mm) for two experiments. In both curves the most prominent volume filling factor is around 23%. The increase of the left side of the curve (square symbols) shows the transition to the volume filling factor distribution of the uncompressed dust. Fig. 4(c) represents the amount of compacted volume as a fraction of the sphere volume which corresponds to a volume filling factor greater than a certain value. From both curves we can conclude that the compaction area due to an impacting sphere with a velocity of ∼ 1.2 m/s has a volume of ∼ 0.8 … 1.2 sphere volumes.
Fig. 4: Volume filling factor distributions of (a) a normal dust sample, (b) & (c) compressed area of two experiments.
We developed a novel non-invasive method to measure the thermal conductivity of protoplanetary dust samples. The investigation of the heat transfer of these dust samples will provide us with knowledge about the ability to melt or sinter by radioactive decay of short-lived isotopes, like e.g. 26Al. Thereby we intend to measure the radial temperature distribution of a dust sample heated by a laser beam. As a non-invasive measuring method the temporal and spatial temperature distribution is recorded by an IR camera. This method allows us to derive the thermal conductivity of such a highly porous material without changing the mechanical microstructure at the contact points of sensors used in other thermal conductivity measuring methods, which would probably lead to wrong measuring results.
To investigate dust collisions at temperatures up to 1.300 K, we are constructing an experimental setup, which enables the observation of colliding mm-sized dust grains by a high-speed camera and an IR camera. Using a two-particle release mechanism we will achieve collision velocities up to 1 m/s. A wide parameter study varying the temperature, grain material, grain size and collision velocity is intended.