Particle Accretion in Microgravity. I. Particle Sizes and Bulk Densities

Abstract

The precise mechanisms related to the accretion of protoplanetesimals from dust in protoplanetary disks (PPDs) are poorly understood. In order to better understand the early stages of planet formation and the growth of millimeter–centimeter agglomerates, we conducted a series of aggregation experiments in reduced gravity. Our objectives are to determine the effects of particle size, bulk density, and composition on the aggregation potential of dust-scale grains. This work builds upon previous microgravity experiments initially performed by astronaut Don Pettit on board the International Space Station, where free-floated bags of finely grained materials showed the aggregation of the cohesive materials. We expand upon these experiments using material more analogous to the PPD such as olivine, enstatite, ordinary chondrite, carbonaceous chondrite, and a carbonaceous chondrite simulant. We find that particle size is the most important driver of aggregate growth, with bulk density showing little dependence. The largest aggregate size ratios were formed from the finest particle size distributions, while aggregate sizes approach the mean grain size for particles greater than 1 mm. We measured aggregate sizes spanning roughly 2 orders of magnitude (∼0.2–20.0 mm), but the results show 5 orders of magnitude variations in aggregate size ratios (∼1–1 × 10 ^5 ). This could suggest that protoplanetesimals have an increased likelihood of accreting from micron-sized grains in particle size distributions with initially steeper slopes. Our results further the body of work that points to grain size and grain properties constraining the growth potential of protoplanetesimals, with implications for the timescale and initial conditions of accretion in the disk.

Keywords

• Planet formation
• Accretion
• Planetesimals
• Microgravity