5:15 PM - 6:45 PM
[U11-P02] Development of a numerical model for atmospheric hydrodynamic escape in early Earth driven by celestial impacts
Keywords:Earth, atmosphere
The primordial atmosphere of early Earth was likely formed from hydrogen-based solar nebula gas and water vapor and carbon dioxide ejected from the Earth's interior by celestial collisions and other events. The atmospheric gases were lost by hydrodynamic escape from the early Earth (Yoshida et al., 2020). About 3.8 billion years ago, when life emerged, the Earth was in the middle of or just after the Late Heavy Bombardment Period (LHBP) with frequent celestial impact events. Although many studies addressed atmospheric escape due to a single large-scale impact event during LHBP (Shuvalov et al., 2013), there has been no study for atmospheric escape due to frequent and small celestial impact events. Therefore, the total contribution of the celestial impacts to early Earth's atmospheric escape has yet to be quantified. In this study, we combined a numerical model of hydrodynamic escape due to solar X-ray - Ultraviolet (XUV) heating proposed by Yoshida et al. (2021) with our newly developed model for the atmospheric heating by small-scale frequent celestial impact events as an energy source, with which we assess the effect of celestial impact events on the Earth's atmospheric environment when life began. Based on the impact flux distribution measurement for the diameter of craters formed in the lunar Nectaris basin (Marchi et al., 2012) and the scaling law between the crater and impactor diameters (Morbidelli et al., 2018), we obtained a relation between the impactor diameter and impact flux during the late heavy bombardment. With the obtained impactor diameter-flux relation and an analytical model for the kinetic energy of an impactor entering the atmosphere (Collins et al., 2005), we derived the altitude distribution of the atmospheric heating rate by the impactors during celestial impact events over about 200 million years. The frequent and small celestial impacts during the late heavy bombardment were found to heat the atmosphere with a rate of ~10-10[J/m3/s] at altitudes of 0-500 [km]. This is comparable to atmospheric heating rates of ~10-8 -10-11[J/m3/s] at altitudes of 1000 - 190000 [km] due to the XUV heating during the same period. The instantaneous heating rate for a single small impact was ~103[J/m3/s] at altitudes of 0-500 [km]. We are implementing this impactor heating rate model in the hydrodynamic escape model of Yoshida et al. (2021). The current status will be presented in this presentation.