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 lightning 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 differs from previous models in two important aspects. First, we identify a new, powerful charging mechanism that is based on the differences in contact potentials between particles of different composition, a form of triboelectric charging. In the presence of fine silicate grains and fine iron metal grains, large silicate particles (the chondrules) can acquire charges ≳+10⁵e. Second, we assume that the chondrule precursor particles are selectively concentrated in clumps ~100 km in size by the turbulent concentration mechanism described by J. N. Cuzzi et al. (1996, in Chondrules and the Protoplanetary Disk, pp. 35-43, Cambridge Univ. Press). The concentration of these highly charged particles into clumps, in a background of negatively charged metal grains, is what generates the strong electric fields. We calculate that electric fields large enough to trigger breakdown easily could have existed over regions large enough (~100 km) to generate very large discharges of electrical energy (~10¹⁶ erg), assuming a lightning bolt width ≲10 electron mean-free paths. The discharges would have been sufficiently energetic to have formed the chondrules. We place constraints on the generation of lightning and conclude that it could not be generated if the abundance of ²⁶Al in chondrules was as high as the level in the calcium-aluminum-rich inclusions (CAIs). This conclusion is consistent with isotopic analyses of chondrules. This possibly implies that ²⁶Al was nonuniformly distributed in the solar nebula or that the chondrules formed several million years after the CAIs.