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We apply a 1D upper atmosphere model to study thermal escape of nitrogen overTitan's history. Significant thermal escape should have occurred very early forsolar EUV fluxes 100 to 400 times higher than today with escape rates as highas $\approx 1.5\times 10^{28}$ s$^{-1}$ and $\approx 4.5\times 10^{29}$s$^{-1}$, respectively, while today it is $\approx 7.5\times 10^{17}$ 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 to 1000 Myrafter the formation of the solar system. If Titan's atmosphere originated thatearly, it could have lost between $\approx 0.5 - 16$ times its presentatmospheric mass depending on the Sun's rotational evolution. We alsoinvestigated the mass-balance parameter space for an outgassing of Titan'snitrogen through decomposition of NH$_3$-ices in its deep interior. Our studyindicates that, if Titan's atmosphere originated at the beginning, it couldhave 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 surviveuntil present-day, implying later outgassing or an additional nitrogen source.An endogenic origin of Titan's nitrogen partially through NH$_3$-ices isconsistent with its initial fractionation of $^{14}$N/$^{15}$N $\approx$ 166 -172, or lower if photochemical removal was relevant for longer than the last$\approx$ 1,000 Myr. Since this ratio is slightly above the ratio of cometaryammonia, some of Titan's nitrogen might have originated from refractoryorganics.

Escape and evolution of Titan's N$_2$ atmosphere constrained by $^{14}$N/$^{15}$N isotope ratios