Japanese Hayabusa spacecraft found some features of boulders, such as clustering, lining up and size-sorting, on asteroid (25143) Itokawa. These features might be formed by granular convection and migrations triggered by micro body impacts. We also performed impact cratering experiments in the microgravity environment using a drop tower in order to establish the scaling law on crater size. Since the result is somewhat controversial, the conclusion is still not obtained. Understanding of particle and powder behavior in the microgravity and high-vacuum environments is necessary for elucidation of these phenomena. The microgravity experiment may be a new useful method for planetary sciences to elucidate surface processes on small bodies.
This paper reviews studies on dust motion above the surface of asteroids. Photoelectron emission effect makes asteroidal surface charged-up positively resulting in a thin electron sheath above the surface. At the same time, a dust grain on or above the surface is also charged-up positively or negatively due to the balance between photoelectron emission and injection of solar wind and electron sheath electron. Thus motion of dusts above the surface is affected not only by gravitational acceleration but also electric force from the electric field above the surface. Under some special condition, a dust levitates above surface for a longtime, which might result in characteristic morphology such as “pond” on Eros. If dust size is smaller than a threshold value, the dust can escape from the gravity of the asteroid due to electric repulsion. Such an electric escape can be one of source mechanisms of interplanetary dust grains.
Field-induced rotation and translation caused by static field below 1T was studied for sub-mm sized diamagnetic particles using a short term μg condition. Magnetic motions of ordinary solid, free of spontaneous moment, has not been reported at such low field before. In a given field distribution, the motions were mainly determined by intrinsic magnetization of the material; they were independent to mass of particle. This was because the motions were induced by magnetic volume force. The above translation is applicable in identifying the μm-sized grains or regolith collected in various regions of the solar system. The magnetic rotation provides quantitative information to solve the mechanism of dust alignment observed in various interstellar regions. The above motions were observable in a short μg duration produced by a compact drop-shaft; here the observation was realized by adopting a small NdFeB magnetic-circuit. The established technique to observe the motions of sub-mm sized sample is a step to detect the magnetic motions of μm & nm grains.
The gas evaporation method, which is a typical smoke experiment, has a history of almost half century. Nevertheless, there has been almost no report concerning nucleation in a smoke in view of crystal growth. Here, we observed a nucleation process of thermally evaporated manganese vapor in an argon gas using a Mach-Zehnder type interferometer and showed that nanoparticles homogeneously condense only in very highly supersaturated environments. Condensation occurred at 660-785 K well below the equilibrium temperature and the degree of supersaturation was as high as ~5×104. Based on the condensation temperature and size of the condensed particles, which were measured by transmission electron microscopy, we determined the surface free energy and sticking coefficient for nucleation of Mn at 1106 ± 50 K to be 1.57 ± 0.35 J/m2 and 〖0.42〗_(-0.21)^(+0.42), respectively, by means of a semi-phenomenological (SP) nucleation theory. The large errors in these two parameters will be decreased by microgravity experiments.