科研之友
微信
新浪微博
Facebook
分享链接
摘要
Quantifying the atmospheric mass loss during planet formation is crucial for understanding the origin and evolution of planetary atmospheres. We examine the contributions to atmospheric loss from both giant impacts and planetesimal accretion. Giant impacts cause global motion of the ground. Using analytic self-similar solutions and full numerical integrations we find (for isothermal atmospheres with adiabatic index gamma = 5/3) that the local atmospheric mass loss fraction for ground velocities v(g) less than or similar to 0.25 v(esc) is given by chi(loss) = (1.71 v(g)/v(esc))(4.9), where v(esc) is the escape velocity from the target. Yet, the global atmospheric mass loss is a weaker function of the impactor velocity v(imp) and mass m(imp) and given by X-loss similar or equal to 0.4x + 1.4x(2)-0.8x(3) (isothermal atmosphere) and X-loss similar or equal to 0.4x + 1.8x(2) - 1.2x(3) (adiabatic atmosphere), where x = (v(imp)m/v(esc)M). Atmospheric mass loss due to planetesimal impacts proceeds in two different regimes: (1) large enough impactors m greater than or similar to root 2 rho(0)(pi hR)(3/2) (25 km for the current Earth), are able to eject all the atmosphere above the tangent plane of the impact site, which is h/2R of the whole atmosphere, where h, R and rho(0), are the atmospheric scale height, radius of the target, and its atmospheric density at the ground. (2) Smaller impactors, but above m > 4 pi rho(0)h(3) (1 km for the current Earth) are only able to eject a fraction of the atmospheric mass above the tangent plane. We find that the most efficient impactors (per unit impactor mass) for atmospheric loss are planetesimals just above that lower limit (2 km for the current Earth). For impactor flux size distributions parametrized by a single power law, N(> r) proportional to r(-q+1), with differential power law index q, we find that for 1 < q < 3 the atmospheric mass loss proceeds in regime (1) whereas for q > 3 the mass loss is dominated by regime (2). Impactors with m less than or similar to 4 pi rho(0)h(3) are not able to eject any atmosphere. Despite being bombarded by the same planetesimal population, we find that the current differences in Earth's and Venus' atmospheric masses can be explained by modest differences in their initial atmospheric masses and that the current atmosphere of the Earth could have resulted from an equilibrium between atmospheric erosion and volatile delivery to the atmosphere from planetesimal impacts. We conclude that planetesimal impacts are likely to have played a major role in atmospheric mass loss over the formation history of the terrestrial planets.
- 出版日期2015-2
- 单位MIT
