Synthesis of Si02 spheres with magnetic cores: Implications for the primary accretion in the solar nebula

摘要

Current models used to explain how floating nebular dust accretes to planetesimals still encounter different kinds of problems. Since the magnetic force induced accretion model was first proposed by Nuth et al. (1994), it has been considered as an important route via which nebular dust accretes. In this paper, a system analogous to the primitive solar nebula has been created. A magnetic iron compound and silica were evenly dispersed in this system at high temperatures and their accretion behavior was investigated. Experimental results show that microspheres with magnetite and iron silicate cores and amorphous silica shells were formed. By analyzing the formation mechanism, we find that if the local temperature is under its Curie point, magnetite can aggregate spontaneously by magnetic interactions and can further attract charged silicon-bearing radicals via the Lorentz force. We have also found that the oxidation of iron to magnetite does not hinder the magnetic aggregation process, but facilitates the formation of widespread iron silicate. Therefore, our model provides an alternative view of "primary accretion" in the protoplanetary nebula.