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Atmospheric Erosion Induced by Oblique Impacts Meteoritics & Planetary Science 44, Nr 8, 1095–1105 (2009) Abstract available online at http://meteoritics.org Atmospheric erosion induced by oblique impacts V. SHUVALOV Institute of Geosphere Dynamics, 38 Leninsky prosp. Bld.1, 119334 Moscow, Russia *Corresponding author. E-mail: [email protected] (Received 31 March 2009; revision accepted 27 May 2009) Abstract–Previous investigations of impact-induced atmospheric erosion considered vertical impacts only. Numerical simulations of oblique impacts presented in this paper show that the loss of air strongly depends on trajectory inclination and it increases as the impact angle decreases. The results of numerical simulations over the wide range of impact parameters (projectile sizes from 1 to 30 km, impact velocities from 15 to 70 km/s, escape velocities from 5 to 11.2 km/s, projectile densities from 1 to 3.3 g/cm3, normal atmospheric densities varying by three orders of magnitude) can be approximated by simple analytical formulae. INTRODUCTION the cratering process on air ejection. The significance of these factors was outlined by Vickery (1994) and Svetsov The idea of planetary atmospheric erosion resulting from (1996). Detailed numerical simulations (Shuvalov and asteroid impacts was suggested by Cameron (1983). It is Artemieva 2000, 2002) taking into account both the based on the assumption that a considerable mass of shock- projectile interaction with the atmosphere and the cratering heated and upward accelerated atmospheric air can reach process have shown that vertical impacts of both comets and velocities exceeding uesc = 11.2 km/s (i.e., escape velocity for asteroids with diameters exceeding 1 km could not the Earth) and can be ejected to space. The first estimates of considerably erode the Earth’s atmosphere. The ratio of impact induced atmospheric erosion were obtained by using a escaping mass to the impactor mass in vertical impacts is simplified sector blow-off model (Vickery and Melosh 1990). less than predicted by both blow-off and Newman’s models. This approximation uses the Zel’dovich and Raizer (1967) It does not exceed the value of 2–3%, which is significantly solution for expansion of a vapor plume and momentum less than the mass of volatiles delivered by impactors balance between the expanding gas and the mass of the (Chyba 1990) and released during the impact (Pierazzo et al. overlying atmosphere. The model predicts a cone-shaped 1998). escape region with the angle of taper depending on impact Nemtchinov and Shuvalov (2003) and Svetsov (2007) energy. The blow-off model was widely used to study the have shown that vertical impacts of comets 100 to 300 m in evolution of planetary atmospheres (see, e.g., Zahnle et al. diameter could provide strong erosion of the Earth’s 1990, 1992). atmosphere. In such impacts, the ratio of escaping mass to the Later Newman et al. (1999), using a more sophisticated impactor mass reaches up to a few tens percent. Thus they analytical model based on the solution of Kompaneets (1960) could strongly influence the evolution of the atmosphere. and a numerical model with simplified initial conditions, However, the existence of such small comets and their concluded that the mass of escaping air is significantly less number are still being questioned. At least, small comets are (by a factor of about five for Chicxulub-scale impacts) than not considered to be a major type of Earth’s impactors predicted by the blow-off model. In these simulations, the (Kuzmitcheva and Ivanov 2008). authors did not consider a real impact (penetration and Most previous papers considered atmospheric erosion excavation stages). Instead, they assumed that the initial after vertical impacts only. However, preliminary results vapor cloud had a pancake or pillbox shape with density (Shuvalov and Artemieva 2002) obtained in simulations with approximating that of lithospheric material. low resolution have shown that the escaping mass in oblique However, both models did not take into account the impacts could be much higher than in vertical ones. The aim influence of atmospheric perturbations generated during the of the present study is to calculate the loss of air in oblique impactor flight through the atmosphere and the influence of impacts of large (1 to 30 km) asteroids and comets. 1095 © The Meteoritical Society, 2009. Printed in USA. 1096 V. Shuvalov Table 1. Relative masses of escaping air, projectile and target material for different variants of asteroidal impacts D-V-A (D-projectile diameter (in km), V-impact velocity (km/s), A-impact angle), two values of escape velocity (11.2 and 5 km/s), and modern Earth’s atmosphere. Angle-averaged escaping masses are shown in brackets in lines for 45 degree impact angles. The values of dimensionless erosional power for escape velocities 11.2 km/s (ξ11.2) and 5 km/s (ξ5) are given in the last column. ma/M ma/M mpr/M mpr/M mt/M mt/M = = = = = = ξ ξ Variant (uesc 11.2) (uesc 5) (uesc 11.2) (uesc 5) (uesc 11.2) (uesc 5) 11.2 ( 5) 01-20-15 2e-5 0.04 0 0 0 3e-5 01 20-30 1E-3 0.5 0 0.08 0 5e-3 01-20-45 3E-4 (3e-4) 0.2 (0,2) 0 (0) 0.05 (0,03) 0 (0) 0.01 (7e-3) 5.1 (33) 01-20-60 1e-4 0.03 0 3e-3 0 0.01 01-20-90 1e-5 7e-3 0 1.5e-4 0 2e-3 03-20-15 0.02 1 4e-3 0.3 0 0.02 03-20-30 0.08 0.4 0.2 0.3 0.02 0.4 03-20-45 0.025 (0,029) 0.1 (0,25) 0.03 (0,04) 0.1 (0,2) 2e-3 (5e-3) 0.25 (0,2) 1.4e2 (9e2) 03-20-60 4e-3 0.05 1E-4 0.02 5E-3 0.18 03-20-90 1e-4 8e-3 0 7e-4 0 2e-3 10-20-15 0.15 0.3 0.3 0.4 0.03 0.09 10 20-30 0.03 0.06 0.3 0.4 0.07 0.6 10-20-45 0.01 (0,03) 0.025 (0,06) 0.04 (0,2) 0.15 (0,25) 0.03 (0,029) 0.3 (0,30) 5.1e3 (3.3e4) 10-20-60 5e-3 0.01 4e-3 0.02 6e-3 0.2 10-20-90 2e-4 2e-3 1e-4 0.015 2e-4 0.02 30-20-15 0.025 0.03 0.3 0.5 0.04 0.08 30-20-30 6e-3 0.01 0.3 0.5 0.07 0.6 30-20-45 2e-3 (5e-3) 5e-3 (0,01) 0.04 (0,1) 0.15 (0,25) 0.04 (0,04) 0.4 (0,3) 1.4e5 (9e5) 30-20-60 1e-3 0.01 4e-3 0.02 7e-3 0.2 30-20-90 1e-3 0.01 2e-3 0.02 2e-4 0.02 01-15-15 0 3e-3 0 0 0 0 01-15-30 1e-5 0.02 0 1e-4 0 0 01-15-45 2e-5 (1e-5) 0.04 (0,02) 0 (0) 0.04 (0,015) 0 (0) 0 (0) 1.9 (18) 01-15-60 2e-6 0.01 0 1e-4 0 0 01-15-90 2e-6 1e-3 0 1e-4 0 0 03-15-15 0 0.4 0 0.06 0 1.5e-3 03-15-30 0.025 0.3 1e-3 0.25 0 0.15 03-15-45 0.01 (0,01) 0.1 (0,16) 5e-4 (4e-4) 0.2 (0,15) 0 (0) 0.15 (0,1) 52 (4.8e2) 03-15-60 7E-4 0.05 0 0.02 0 0.05 03-15-90 2e-4 0.01 0 2e-4 0 0 10-15-15 0.03 0.4 6e-3 0.3 0 0.05 10 15-30 0.02 0.05 0.2 0.3 0.02 0.2 10-15-45 8e-3 (0,01) 0.02 (0,07) 0.06 (0,06) 0.2 (0,2) 0.04 (0,015) 0.4 (0,20) 1.9e3 (1.8e4) 10-15-60 3e-3 0.01 3e-3 0.02 5e-4 0.09 10-15-90 2e-4 2e-3 0 2e-4 0 7e-3 30-15-15 0.02 0.04 0.1 0.3 5e-3 0.05 30-15-30 5e-3 5e-3 0.2 0.3 0.02 0.2 30-15-45 2.5e-3 (4e-3) 4e-3 (0,07) 0.04 (0,07) 0.1 (0,2) 0.05 (0,02) 0.4 (0,2) 5.2e4 (4.8e5) 30-15-60 1.2E-3 3E-3 3E-3 0.02 3.4E-3 0.1 30-15-90 2e-4 1e-3 0 2e-4 0 1e-2 NUMERICAL MODEL transport equations consist of conservation equations of mass, momentum, and energy, which are solved for gas/liquid/solid. A 3D version of the SOVA multi-material hydrocode In the first step, the Lagrangian equations are solved using a (Shuvalov 1999) was used to model the interaction of the second-order difference scheme; in the second step, the data impactor with the atmosphere and solid Earth. The SOVA is is reinterpolated (remapped) from the Lagrangian grid to the an Eulerian material response code with some Lagrangian Eulerian grid with the second order accuracy. features. It allows considering strong hydrodynamic flows The target was considered to consist of granite with with accurate description of the boundaries between different density of 2.63 g/cm3.
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