The generation of lightning in the solar nebula

The process that melted and formed the chondrules, millimeter-sized glassy beads within meteorites, has not been conclusively identified. Origin by li ghtning in the solar nebula is consistent with many features of chondrules, but no viable model of lightning has yet been advanced. We present a model demonstrating how lightning could be generated in the solar nebula which d iffers from previous models in two important aspects. First, we identify a new, powerful charging mechanism that is based on the differences in contac t potentials between particles of different composition, a form of triboele ctric charging. In the presence of fine silicate grains and fine iron metal grains, large silicate particles (the chondrules) can acquire charges grea ter than or similar to +10(5) e. Second, we assume that the chondrule precu rsor particles are selectively concentrated in clumps similar to 100 km in size by the turbulent concentration mechanism described by J. N. Cuzzi et a l. (1996, in Chondrules and the Protoplanetary Disk, pp. 35-43, Cambridge U niv. Press). The concentration of these highly charged particles into clump s, in a background of negatively charged metal grains, is what generates th e strong electric fields. We calculate that electric fields large enough to trigger breakdown easily could have existed over regions large enough (similar to 100 km) to generat e very large discharges of electrical energy (similar to 10(16) erg), assum ing a lightning bolt width 10 electron mean-free paths. The discharges woul d have been sufficiently energetic to have formed the chondrules. We place constraints on the generation of lightning and conclude that it could not b e generated if the abundance of Al-26 in chondrules was as high as the leve l in the calcium-aluminumrich inclusions (CAIs). This conclusion is consist ent with isotopic analyses of chondrules. This possibly implies that Al-26 was nonuniformly distributed in the solar nebula or that the chondrules for med several million years after the CAIs. (C) 2000 Academic Press.