Suprathermal chemical reactions driven by fast hydrogen atoms in cometary comae

We have investigated the role that energetic hydrogen atoms, produced in cometary comae by the photodissociation of water molecules, could have in driving chemical reactions that are endothermic, or possess activation energy barriers. We have developed a model of the density and energy spectrum of these atoms in the coma and have incorporated a number of reactions driven by fast H atoms into our existing coma model. We find that, in high-activity comets close to the Sun, such reactions are competitive with direct photodissociation as the principal destruction mechanism for molecules with long lifetimes in the solar radiation field. We show that measurements of the CH₂OH : CH₃O ratio may be used to assess the importance of suprathermal reactions in the coma. We also confirm that these reactions are probably unable to account for the observed HNC : HCN ratios.