  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 EUV fluxes 100 to 400 times higher than today with escape rates as high
as $\approx 1.5\times 10^{28}$ 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 Myr
after the formation of the solar system. If Titan's atmosphere originated that
early, it could have lost between $\approx 0.5 - 16$ 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 NH$_3$-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 NH$_3$-ices is
consistent 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 cometary
ammonia, some of Titan's nitrogen might have originated from refractory
organics.
