Escape and evolution of Titan’s N2 atmosphere constrained by 14N/15N isotope ratios

Abstract

ABSTRACT We apply a 1D upper atmosphere model to study thermal escape of nitrogen over Titan’s history. Significant thermal escape should have occurred very early for solar extreme ultraviolet (EUV) fluxes 100–400 times higher than today with escape rates as high as ≈1.5 × 1028 s−1 and ≈4.5 × 1029 s−1, respectively, while today it is ≈7.5 × 1017 s−1. Depending on whether the Sun originated as a slow, moderate, or fast rotator, thermal escape was the dominant escape process for the first 100–1000 Myr after the formation of the Solar system. If Titan’s atmosphere originated that early, it could have lost between $\approx0.5\,\, \mathrm{ and}\,\, 16$ times its present atmospheric mass depending on the Sun’s rotational evolution. We also investigated the mass-balance parameter space for an outgassing of Titan’s nitrogen through decomposition of NH3-ices in its deep interior. Our study indicates that, if Titan’s atmosphere originated at the beginning, it could have only survived until today if the Sun was a slow rotator. In other cases, the escape would have been too strong for the degassed nitrogen to survive until present day, implying later outgassing or an additional nitrogen source. An endogenic origin of Titan’s nitrogen partially through NH3-ices is consistent with its initial fractionation of 14N/15N ≈ 166–172, or lower if photochemical removal was relevant for longer than the last ≈ 1000 Myr. Since this ratio is slightly above the ratio of cometary ammonia, some of Titan’s nitrogen might have originated from refractory organics.