Laboratory Formation and Properties of Carbonaceous Particles for Cometary Outflows

R. J. Cody (NASA Goddard Space Flight Center)

During the Giotto spacecraft flyby of Comet Halley, particles were observed composed of carbon, hydrogen, oxygen, and nitrogen and have been labeled as the "CHON" particles. Within a decade the STARDUST mission will collect cometary grains, which will be used to deduce properties of the primitive solar nebula and the interstellar grain population. Recent laboratory results suggest that not all of the grains collected from the outflow will date from the early solar system. In fact, these experiments suggest that some fraction of the carbonaceous grains in a cometary outflow may be produced in situ from the photolysis and subsequent reaction of organic molecules, released from the ice and entrained in the escaping water vapor.

Small carbonaceous particles are formed by photolysis of a gas mixture of parts per million of small aromatic molecules (benzene, toluene, ethylbenzene, xylenes) and water in a carrier gas at atmospheric pressure. The photolysis source is an excimer laser at 193 nm. Since the flow area of the gas is larger than that of the laser beam, a mixture of irradiated and unirradiated gases are producing the particles. A visible laser (He-Ne at 633 nm) is used to monitor the particles by scattering. Nitrogen, Argon or Helium are used as carrier gases in flowing as well as static gas streams. Initially, the laser beam penetrates 10 - 40 cm of the 150 cm long flow tube; therefore, a particle formation front is established. Differences are noted in particle density as well as resistance to UV destruction. The size range of the particles is submicron according to the amount of time needed to settle by gravity. This research is being supported by the Infrared, Submillimeter, and Radio Astronomy Program of NASA.