We investigate the evolution of molecular abundances in circumstellar disks around young stars via numerical simulations. The results are compared with the composition of comets and radio observations of protoplanetary disks. First, we consider the molecular evolution in the midplane of the disk. Because of ionization by cosmic-rays or decay of radioactive nuclei, the molecular evolution in the region R ≳ 10 AU bears some similarity to that in molecular clouds. Considerable amounts of CO and N2, which come from the cloud core, are transformed, however, into CO2, CH4, NH3, and HCN. In regions where the temperature is low enough for these products to freeze onto grains, they accumulate in ice mantles. In the inner warmer regions of the disk, these molecules are desorbed from the mantles and transformed by gas-phase reactions into less volatile molecules, such as larger hydrocarbons, which then freeze out. Molecular abundances both in the gas phase and in ice mantles crucially depend on the temperature and thus vary significantly with the distance from the central star. Molecular abundances in ice mantles show reasonable agreement with the composition of comets. Our model suggests that comets formed in different regions of the disk having different molecular compositions.
Secondly, we report two-dimensional (R, Z) distributions of molecules in circumstellar disks. In the Z-direction, which is perpendicular to the midplane, there is a steep gradient of density and radiation field. Since the density is lower in the regions removed from the midplane, there are significant amounts of gas-phase species in these regions, while most species are adsorbed onto grains in the midplane. Chemistry in the surface regions is also affected by the X-rays and UV radiation from the central star, and UV radiation from the interstellar field. The molecular abundances in these regions differ significantly from those in the midplane. Radicals such as CN are especially abundant near the disk surface. We obtain radial distributions of molecular column densities and averaged molecular abundances, which show reasonable agreement with the observational data at millimeter wavelengths.