The influence of the solar particle and radiation environment on Titan’s atmosphere evolution

Measurements of the ¹⁵N/ ¹⁴N isotope ratio in Titan's atmosphere by Earth-based millimetric wavelength spectroscopic observations of HC ¹⁵N/HC ¹⁴N indicate that the bulk nitrogen may be enriched in the heavy isotope ¹⁵N by about 3.9-4.5 times relative to the terrestrial value. For the explanation of this isotope anomaly, one finds that the mass of Titan's early atmosphere was at least 30 times larger than the present value. In our study we use recent astrophysical observations on radiative fluxes and stellar winds of solar-like stars with different ages as well as lunar and meteorite fossil records, which indicate that the early Sun underwent a highly active phase resulting in up to about 100 times higher X-ray and extreme ultraviolet (XUV) radiation fluxes and much higher solar wind mass fluxes 100-500 Myr after it arrived at the zero-age-main-sequence. Because the evolution of Titan's atmosphere can only be understood within the context of the evolving solar particle and radiation flux, we estimated the solar wind parameters and the XUV flux over Titan's history. Our study indicate that Saturn's magnetosphere, due to a higher solar wind pressure, was highly compressed during most of its lifetime to distances smaller than Titan's orbital radius of about 20 Saturn radii. Because the early Sun had a much denser solar wind mass flux up to 1000 times higher than today, we estimate the ion pick up and sputter loss rates on early Titan in the order of about 10 ²⁶ and 8 × 10 ²⁸ s ^-1, respectively, and found that both non-thermal atmospheric loss processes can not explain the observed enrichment in ¹⁵N isotopes. Therefore, atmospheric loss during hydrodynamic conditions is investigated, since it can be shown that the exospheric temperature may have been above the "blow-off" temperature of molecular nitrogen of about 8000 K during the early evolutionary stages of Titan's atmosphere. We found that due to the high XUV radiation input of the young Sun, Titan's nitrogen atmosphere could have been expanded up to about 3 Titan radii and a massive atmospheric evaporation with N ₂ loss rates in the order of about 5 × 10 ³⁰ s ^-1 may have occurred. The results of our study indicate that the ¹⁵N/ ¹⁴N isotope anomaly can be explained if Titan's upper atmosphere experienced hydrodynamic conditions during a time period of about 50 Myr after the satellites origin. In situ measurements by the Gas Chromatograph and Mass Spectrometer (GCMS) instrument on board of ESA's Huygens entry probe can be used as a confirmation of the Earth-based isotope anomaly observations and will be of great importance for understanding the formation and evolution of atmospheres around bodies in the solar system.