Dissipation of the primordial terrestrial atmosphere due to irradiation of the solar EUV

The escape of a primordial Earth's atmosphere due to heating by solar radiation is studied by integrating numerically hydrodynamic equations for steady and spherically- symmetric outflow of hydrogen molecules and helium atoms. As heating sources, we take account of (1) the solar EUV radiation which is expected to be very strong during the T Tauri stage, (2) the solar visible light and (3) the release of gravitational energy of accreting planetesimals. The effect of solar wind is neglected but the condition of this neglect is estimated. The results show that the primordial atmosphere, having existed in the early stage of the Earth's history, is dissipated within a period of 5 x 108 y, which is the upper limit imposed from the origin of the present terrestrial atmosphere, as far as the solar EUV flux is more than 2 x 102 times as large as the present one. (author)