THE PRODUCTION AND ESCAPE OF NITROGEN-ATOMS ON MARS

We have computed the production rates and densities of odd nitrogen species in the Martian atmosphere using updated rate coefficients and a revised ionosphere-thermosphere model. We find that the computed densities of NO are somewhat smaller than those measured by Viking 1, but reasonable agreement can be obtained by assuming that the rate coefficient for loss of odd nitrogen in the reaction of N with NO is smaller at temperatures that prevail in the lower Martian thermosphere (about 130-160 K) than the standard value, which applies to temperatures of 200-400 K. We have also modeled the escape fluxes of N from the Martian atmosphere due to photodissociation, photodissociative ionization, and electron impact dissociative ionization of N2, ion-molecule reactions, and dissociative recombination of N2+. The magnitudes of the latter two sources depend strongly on the N2+ density profile, for which there are no in situ measurements. The revised ionospheric model, which incorporates the larger electron temperatures recently derived from Viking measurements (Hanson and Mantas, 1988) and a loss process for ions at high altitudes (Shinagawa and Cravens, 1989), differs substantially from those of previous models (Fox and Dalgarno, 1979; 1983; Fox, 1989). In dissociative recombination of N2+, sufficient energy for escape is available only when the products are N(4S) + N(2D), and the results are therefore sensitive to assumptions made about the product yields. Unfortunately, the experimental values for the yields of excited states of N in dissociative recombination (Queffelec et al., 1985) differ significantly from those obtained in recent ab initio calculations (Guberman, 1991). When the theoretical data are employed, the computed isotope ratio enhancement and the initial column density obtained are about 2.6 and 1.0 x 10(23) cm-2, respectively. The existence of a dense early atmosphere is consistent with this model. If the initial CO2 pressure is assumed to be 2 bars, exponential loss of the atmosphere with a time constant of about 2.2 x 10(16) s (7 x 10(8) years) reproduces the measured isotope ratio.