CHONDRULE FORMATION IN LIGHTNING DISCHARGES

Chondrules represent a significant mass fraction of primitive meteorites. These millimeter-sized glassy droplets appear to be the products of intensive transient heating events. Their size distribution, chemical and mineral composition, texture, and isotope composition suggest that chondrules were produced as a result of short duration melting followed by rapid cooling of solid precursor particles. Gas dynamic heating, magnetic reconnection, and electrostatic discharges are thought to be the leading candidates to explain chondrule formation. In this paper we model the effects of ''lightning'' in the early Solar System. Differential settling of various sized dust particles toward the midplane of the nebula is suspected to build large scale charge separations that episodically relax via the electric breakdown of the nebular gas. The electrostatic discharge is analogous to lightning in the Earth's atmosphere. In this paper we do not discuss the complex physical processes that may lead to electric field generation. Instead, assuming that this may occur due to a number of inductive and noninductive processes, we use the expected initial conditions in such a discharge. We then follow the expansion of the initially energetic plasma column as it expands, cools, and recombines. We calculate the energy flux reaching the surface of an embedded dust grain and also its subsequent heating. We show that, within the range of expected initial plasma conditions, lightning is a viable mechanism for chondrule formation worthy of more complex theoretical and also laboratory investigations. (C) 1995 Academic Press, Inc.