Nitrogen isotope fractionation and its consequence for Titan’s atmospheric evolution

Abstract

The first determination of the 15N/ 14N ratio in Titan’s atmosphere comes from Earth-based millimetric wavelength spectroscopic observations of HC 15N/HC 14N. These measurements indicate that the bulk nitrogen is enriched in the heavy isotope 15N by about 4.5 times relative to the Earth value. A 15N/ 14N anomaly on Mars of about 1.6 times the terrestrial value has 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, electron impact dissociation of N 2 molecules, nonthermal exothermic ionosphere-related photochemical reactions, atmospheric sputtering via solar wind and magnetospheric particles, solar wind pick-up and the loss of 14N to 14C via cosmic rays as possible sources of nitrogen isotope fractionation in Titan’s atmosphere where this molecule is the principal constituent. Using a Monte Carlo method we have shown that electron impact dissociation and dissociative 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 15N isotope is slightly smaller than the necessary escape energy, or in the case of dissociative recombination, close to Titan’s escape energy. We show that the isotope fractionation for the other more efficient escape processes like atmospheric sputtering is negligibly small, since the energy of both newly released nitrogen isotopes is much greater than Titan’s escape energy. We found that diffusive separation of 15N/ 14N according to their atomic mass is very important in the solution of this isotope anomaly. Further indications of a much greater particle output or much higher solar wind mass flux of the early Sun during the first half billion years are presented which could explain the observed enrichment of 15N. We found that 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 nitrogen anomaly. Our study indicates that the mass of Titan’s early atmosphere 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 reservoir required to produce the present Titan atmosphere. In situ measurements and confirmation of the Earth-based 15N/ 14N isotope anomaly observations will be possible with the Gas Chromatograph and Mass Spectrometer (GCMS) instrument on board of the Huygens probe. They will be of great importance for understanding the formation and evolution of atmospheres around bodies in the solar system.