Nitrogen isotope fractionation and its consequence for Titan's atmosphericevolution

The first determination of the N-15/N-14 ratio in Titan's atmosphere comes from Earth-based millimetric wavelength spectroscopic observations of (HCN) -N-15/(HCN)-N-14. These measurements indicate that the bulk nitrogen is enr iched in the heavy isotope N-15 by about 4.5 times relative to the Earth va lue. A N-15/N-14 anomaly on Mars of about 1.6 times the terrestrial value h as been established previously resulting from nonthermal atmospheric escape processes (e.g. dissociative recombination of N-2(+) ions). We investigated electron dissociative recombination of N-2(+) ions, electro n impact dissociation of N-2 molecules, nonthermal exothermic ionosphere-re lated photochemical reactions, atmospheric sputtering via solar wind and ma gnetospheric particles, solar wind pick-up and the loss of N-14 to C-14 via cosmic rays as possible sources of nitrogen isotope fractionation in Titan 's atmosphere where this molecule is the principal constituent. Using a Mon te Carlo method we have shown that electron impact dissociation and dissoci ative recombination in the low energy range of molecular nitrogen and N-2() ions could lead to an isotope fractionation since the energy of the newly released N-15 isotope is slightly smaller than the necessary escape energy , or in the case of dissociative recombination, close to Titan's escape ene rgy. We show that the isotope fractionation for the other more efficient es cape processes like atmospheric sputtering is negligibly small, since the e nergy of both newly released nitrogen isotopes is much greater than Titan's escape energy. We found that diffusive separation of N-15/N-14 according t o their atomic mass is very important in the solution of this isotope anoma ly. Further indications of a much greater particle output or much higher so lar wind mass flux of the early Sun during the first half billion years are presented which could explain the observed enrichment of N-15. We found th at atmospheric sputtering and pick-up caused by a high solar wind particle outflow during a Post T-Tauri phase could be responsible for the observed n itrogen anomaly. Our study indicates that the mass of Titan's early atmosph ere was at least 30 times greater than the present value. An explanation of this anomaly is important for enabling us to estimate the total nitrogen r eservoir required to produce the present Titan atmosphere. In situ measurem ents and confirmation of the Earth-based N-15/N-14 isotope anomaly observat ions will be possible with the Gas Chromatograph and Mass Spectrometer (GCM S) instrument on board of the Huygens probe. They will be of great importan ce for understanding the formation and evolution of atmospheres around bodi es in the solar system. (C) 2000 Elsevier Science Ltd. All rights reserved.