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We present numerical simulations of the thermal and dynamical histories of solid particles (chondrules and their precursors--treated as 1-mm silicate spheres) during passage of an adiabatic shock wave through a particle-gas suspension in a minimum-mass solar nebula. The steady-state equations of energy, momentum, and mass conservation are derived and integrated for both solids and gas under a variety of shock conditions and particle number densities using the free-molecular-flow approximation. These simulations allow us to investigate both the heating and cooling of particles in a shock wave and to compare the time and distance scales associated with their processing to those expected for natural chondrules. The interactions with the particles cause the gas to achieve higher temperatures and pressures both upstream and downstream of the shock than would be reached otherwise. The cooling rates of the particles are found to be nonlinear but agree approximately with the cooling rates inferred for chondrules by laboratory simulations. The initial concentration of solids upstream of the shock controls the cooling rates and the distances over which they are processed: Lower concentrations cool more slowly and over longer distances. These simulations are consistent with the hypothesis that large-scale shocks, e.g., those due to density waves or gravitational instabilities, were the dominant mechanism for chondrule formation in the nebula. Key Words: meteorites; Solar System origin; solar nebula; thermal histories.

Lightning is a viable mechanism for chondrule formation within the range of expected plasma conditions. A considerable mass fraction of primitive meteorites is represented by chondrules which appear to be the products of intensive transient heating events. The effects of 'lightning' in the early Solar system are modelled. The alteration processes by which corundum transforms to other phases is also studied.

Studies of a newly discovered ferromagnesian chondrule from the Allende metasite reveal the presence of intact Ca, Al-rich inclusions (CAI). The finding reinforces the concept of CAIs coexisting with ferromagnesian chondrule precursor materials. It is also possible to constrain the timing of the formation events of CAIs and ferromagnesian chondrules.

Numerical simulations aid the investigation of the reaction of a one dimensional radiative gas dynamic shock wave with a solar dust nebulae cloud. The numerical simulations involve the calculation of energy and momentum transfer to the shock-heated gas during gas penetration into the dust cloud. Grain heating caused by molecular interactions and rays from shocked gas are also obtained. The dust cloud is constituted by silicate grains of 1 millimeter radius.

The process by which chondrules, the largest constituents of primitive meteorites, are formed, is investigated. The thermal mechanism required to convert precursor dust aggregates into chondrules depends on the texture and composition of the chondrules. Shocks inside a nebula are capable of creating chondrules. The shocks may originate from episodic accretion on the solar nebulae of low mass dumps of gas.

The aerodynamic heating of meteoric particles on entering the atmosphere of a planet is explained by a theoretical model. The effects of melting, vaporization and heat conduction into the interior of the particle are incorporated in the model. The model is used for the interpretation of the properties of chondrule rims. The fractionation of the low-melting-temperature component in the outer part of the chondrule, before the rim formation, causes the formation of true melt rims by atmospheric entry. The different encounter conditions and chondrite types are indicated by the existence of thermal alteration effects in UOC chondrites.

Studies of a chondrule obtained from the Allende meteorite contain an unmelted precursor grain that may be a Ca, Al-rich inclusion. The grain is MgAl2O4 and includes many refractory silicates and measures 200-mu M. The evidence points to chondrule formation in the nebula by collision and retention as nebular dust.