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We experimentally investigated thermal modifications of porous dust aggregates composed of micrometersized grains by furnace, electrical discharge, and laser radiation heating. In the furnace, porous SiO2 aggregates of 95% porosity at first underwent surface diffusion sintering, which led to progressively increasing necking between adjacent particles. Subsequently, viscous flow dissolved the particulate structure of the still porous sample, and finally melting occurred. Exposing aggregates of various grain types to electrical discharges dispersed most of the sample and left it thermally unprocessed. Nevertheless, some material was thermally processed to sintered aggregates and a tiny fraction to solidified melt spherules with diameters of less than 180 μm and most with interior bubbles. In comparison, radiative laser heating turned out to be a much more efficient process to produce melt spherules of chondrule size, and voids were rarer than in discharge heating. Besides providing material data for further applications, our work also allows a direct conclusion to be drawn on chondrule formation. Low energetic efficiency and aggregate destruction exclude chondrule formation from loosely-bound aggregates inside hypothetical nebular lightning channels. However, radiative heating of whatever origin, including possible lightning, remains a candidate process of chondrule formation.

Thermal metamorphoses of cosmic dust aggregates: Experiments by furnace, electrical gas discharge, and radiative heating