Formation of Refractory Grains in Shoemaker–Levy 9 Fireballs

The composition, size, and number of the most refractory grains to condense in Shoemaker–Levy 9 fireballs are discussed. Chemical heterogeneity in the fireball may have led to the formation of two principal types of refractory particles. Small particles consisting of a mixture of Al2O3and silicates are expected to form in those portions of the fireball where C/O < 1. In carbon-rich portions of the gas where C/O > 1, carbon may have condensed heterogeneously on preexisting SiC grains. Homogeneous formation of soot through the production and agglomeration of polycyclic aromatic hydrocarbons (PAHs) does not occur. Calculations based on kinetic models indicate that the mean particle radii would be less than 0.1 μm for both types of chemical composition and that both types of particle would have been produced in sufficient numbers to have been visible as they rose into sunlight from behind Jupiter's limb. It is suggested that they may have provided sites for the heterogeneous nucleation of more volatile species after they came to rest in the jovian stratosphere and therefore may have comprised the cores of the dark aerosol debris particles seen suspended above the impact sites.