Formation of Organic Compounds in Solid Bodies by Solar and Cosmic Proton Bombardment

Abstract

The organic matter present in meteorites is generally considered to have been incorporated in the bodies at the time they were formed. Except for the possibility of biological origin, little consideration has been given to processes which might produce organic compounds after termination of gas-phase interactions in the solar nebula. It has been found that measurable quantities of both hydroxyl ions and hydrocarbons can be produced in solids by proton irradiation. Proton and deuteron energies between 0.5 and 1.5 MeV and total doses of approximately 1017 particles were used in the experiments. When solids such as diamond or silicon carbide are irradiated with protons or deuterons, some of the characteristic infrared absorption bands for hydrocarbons appear in their spectra. Hydroxyl ions and water can be produced in silicates by similar bombardment, and there is no reason to doubt that protons can react with nitrogen to produce N-H bonds. Thus far, experimental work has been limited to irradiation of pure crystalline substances. In meteorites, however, mixtures of carbon-, oxygen- and nitrogen-containing substances might react more readily under proton bombardment to produce complex organic molecules.

Meteorites and other small solid bodies which orbit the sun are presently exposed to high-energy particles in quantities large enough to produce measurable effects within a few million years. If the radiation flux in the past were substantially larger, or if the carbonaceous chondrites were located at or near the surface of the parent body, such solid-state reactions may have played a very important role in the production of organic compounds. Furthermore, organic molecules produced inside solids are not subject to photolysis by solar ultra-violet and would tend to be retained, whereas the gas phase reaction products would tend to be destroyed.