Dust collisions on Earth could shed light on planet formation

17 Nov 2018 Hamish Johnston

On target: time sequence of a glass projectile striking a dust target. (Courtesy: Hiroaki Katsuragi and Jürgen Blum/Phys. Rev. Lett.)

By dropping tiny projectiles into clumps of dust, physicists in Germany and Japan may have uncovered important clues about how dust particles stick together in space to form planets. The work has been carried out by Hiroaki Katsuragi at Nagoya University and Jürgen Blum at the Technical University of Braunschweig, who looked at how a single particle collides with clumps of soft porous dust and also with clumps of hard beads.

The experiments were carried out by dropping a projectile from inside the top of a 1.5 m tall tower held in a vacuum to simulate conditions in space. The projectile, which accelerates under gravity, travels towards a target held in a cup at the bottom of the drop tower. Just before it reaches the target, however, the cup is rapidly removed so that the target material also begins to freefall. But because the projectile is falling much faster than the target, it quickly catches up and collides with the target.

Falling camera

The researchers studied a total of 64 impacts – 36 with porous-dust targets some 1-1.6 mm in size and 24 with glass-bead targets that were 1 mm in diameter. The soft, porous particles were created by sieving together irregularly-shaped pieces of silicon dioxide about 0.1-10 µm in size that tended to stick together.

A range of different projectiles was used, including glass beads of diameter 4 mm, 6 mm and 10 mm; lead shot at 4.5 mm; and plastic beads at 6 mm. By varying the release times of the projectile and targets, the velocity of an impact could be set within the range 0.045-1.6 m/s. The researchers captured the collisions using a high-speed camera falling outside the tower that keeps pace with the projectile.

Slowly does it

By inspecting the high-speed images of a collision, the team determined the deceleration of the projectile as it transfers kinetic energy to the target. When they analysed data from the 64 collisions, they found that a universal scaling law describes the relationship between the impact velocity, the projectile deceleration and the diameter of the projectile. This law applied regardless of the impact velocity of the type of projectile and target used.

Further analysis revealed that in every collision about 5% of the kinetic energy of the projectile is transferred to the target, with a further 80% of the projectile energy being dissipated by heat or the deformation of the target.

This universal behaviour in the collisions of particles with clusters could shed light on the processes whereby dust particles in space collide, clump together and eventually form a planet.

The research is described in Physical Review Letters.

16/11/2018from  physicsworld.com