COSMIC DUST SYNTHESIS BY ACCRETION AND COAGULATION

The morphology of grains grown by accretion and coagulation is reveale d by a new laboratory method of synthesizing cosmic dust analogs. Subm icron carbon particles, grown by accretion of carbon atoms from a gas, have a spherical shape with a cauliflower-like surface and an interna l micro-structure of radial columns. This shape is probably common for grains grown by accretion at a temperature well below the melting poi nt. Coagulated grains, consisting of spheres that collided to form irr egular strings, were also synthesized. Another shape we produced had a bumpy non-spherical morphology, like an interplanetary particle colle cted in the terrestrial stratosphere. Besides these isolated grains, l arge spongy aggregates of nanometer-size particles were also found for various experimental conditions. Grains were synthesized using ions t o sputter a solid target, producing an atomic vapor at a low temperatu re. The ions were provided by a plasma, which also provided electrosta tic levitation of the grains during their growth. The temporal develop ment of grain growth was studied by extinguishing the plasma after var ious intervals.