Erosion of planetary atmosphere due to surface waves induced by giant impact

Introduction: The idea of erosion of planetary atmosphere by giant impact was introduced by early works of Arrhenius et al. [1], Benlow and Meaows[2], Ringwood [3] and Cameron [4]. This idea was further expanded by other researchers to study the blow-off of atmosphere by other mechanisms due to the impactor [5-7]. Ahrens [8] described the degree of atmosphere erosion due to the surface wave induced by a giant impact on planetary surface. He also calculated the amount of erosion achievable at different surface wave velocity. Chen and Ahrens [9] further simulated the amplification of particle velocity in the atmosphere column perturbed by the planetary surface with 2 km/s velocity using a gravity-added Lagrangian hydrodynamic code, WONDY [10]. With this simple model, Chen and Ahrens demonstrated that the motion of the planetary surface is sufficient to launch the air particles to escape velocity. Ni and Ahrens [11] constructed the propagation of surface wave due to a giant impact with 10 J energy on a hypothetical, homogenous planet about the same size as the Earth. Their main result is a series of the surface velocities at several stations of equal distances (10o separation) from the impact point. In this contribution, we report our effort in further modification of the WONDY code so that the surface velocities determined by Ni and Ahrens can be imported to WONDY as a time sequence of boundary conditions. Simulation Methods: We modified WONDY, a one-dimensional, finite-difference, compressible fluid dynamics, Lagrangian code for the simulation. The code was modified from its original form in the following ways: 1. Earth gravity was added to the initialization and the equations of motion. Before impact, the atmosphere is isothermal at 300K and gravitationally stable, its density decays exponentially with altitude. A scale height of 8 km is used for the Earth’s atmosphere. 2. Re-meshing criterion was changed from stress to mass. 3. The ground velocity at the atmosphere-ground boundary can be imported as a time sequence. 4. Ground velocities between two time sequence points were interpolated using a simple linear interpolation algorithm. The geometry of the Earth-atmosphere in this study is a 6400 km radius shell with density of 2500 kg/m. On top of the shell is a 128 km thick atmosphere (air density at the shell-atmosphere interface is 1 kg/m and pressure is 10 pa). The gravity is 9.8 m/s at the ground-atmosphere boundary. Data from Ni and Ahrens [11] consisted of 19 time sequences which represent the ground motions at stations separated 10o from each other in a half sphere along the great circle and the other half is symmetrical due to the normal impact geometry in their calculation. These data were imported into the WONDY code as the ground –atmosphere boundary conditions at different time as the simulation proceeded. We scaled Ni and Ahrens’ data to simulate the ground velocity profiles with impact energy varying from 5.5 x 10 J to 1.4 x 10 J. Results and Discussions: Our simulation results are summarized in Table 1 and 2.