Laboratory Observations of a New Grain Accretion Process: Implications for the Formation of Planetary Systems

Grain accretion is an essential yet poorly understood process in the building of planets from the early solar nebula. The spontaneous dipole-formation based linear aggregation behavior of small grains in a randomly charged, weak gravity environment, such as the early solar nebula or planetary regolith, has been documented. The disaggregation of grains when the environment develops a sufficient net charge has also been observed. Here, we examine the potential for these behaviors to contribute to the growth of planetesimals in such environments by observing the behavior of grains on a surface in a low discharge environment with a net overall charge. The resulting grains were accelerated, repelled from, attracted to, and reaggregated on surrounding surfaces. These accelerated monocharged grains could potentially provide ballistic mechanical energy to induce triboelectric dipole formation on less charged surrounding grains and thus accretion via inelastic collisions wherein the grains stick together post collision . We demonstrate this behavior through a series of experiments undertaken in a space simulating laboratory environment. Grains are readily accelerated by weak electron beams in the milliamp range in the presence of a moderate electric field of 500-1000 volts through the onset of a discharge. Initially neutral grains in the 20 micron size range extremely rapidly attain a charge to mass ratio that causes sufficient inter grain electrostatic repulsion to implant the grains on the vacuum chamber walls. Similar weak discharge processes in the solar nebula could thus enhance the relative velocities between solar nebula grains inside and outside of the discharge volume, giving rise to increased collisions and hence meeting a necessary conditions for increased accretion. This behavior would allow triboelectric transfer of charge from smaller grains to larger grains or surfaces and thus differential charge of these larger surfaces to create greater potential for dipole formation, ‘stickiness’ and a heterogeneous accretion model for planetesimals continuing to grow in size.