The surface motion from a large impact upon an attenuation-free fluid sphere was studied and numerically simulated. An analytic solution for the free-surface velocity shows that close to the source, the acoustic wave due to the free-surface interaction (a “quasi-surface wave”) is not separable from the direct wave. At >90°, the quasi-surface wave separates and has a larger amplitude than the direct body wave. Near the antipode the quasi-surface wave amplitude is much larger than the direct body wave and is comparable to the direct wave amplitude immediately near the source at 0°. The resulting solution covers both the wave interference range as defined in the asymptotic theory of near-surface wave propagation developed by Russian physicist V.S. Buldyrev reported in 1968, as well as in the geometric ray range. The geometric range theory was described in several papers in terms of multi-geometry reflection by R. Burridge, H. Jeffreys, and E.R. Lapwood in England in 1957 through 1963. For a large surface excitation (e.g., giant ∼10 27 J impact) a portion of the atmosphere above a plane tangent to Earth at the impact point is launched to velocities greater than the escape velocity. The resulting antipodal free-surface velocity achieved is ∼1.9 km/s, which is sufficient to launch a comparable fraction of the atmosphere to escape.

The low projectile component in tektites in contrast to the high projectile component in the Cretaceous-Tertiary (K-T) boundary clay has prompted a study of hypervelocity target-projectile mixing processes. Results from a 6.4-km/sec impact of a Fe-Ni-PGE alloy projectile (90% Fe) into a Mo target indicate that high-angle (55° to 75°), high-velocity (< 6 km/sec) melted ejecta is relatively projectile-rich, whereas low-angle (10° to 40°), low-velocity ejecta (< 1 km/sec) contains less projectile material and is more enriched in the target component. These results support theoretical predictions. Not predicted by theoretical calculation, but observed here, is a break in the compositional trend such that at angles of ejection between 50° and 70°, the projectile/target ratio in the melted ejecta decreases suddenly with increasing angle, only to rise to very high values at higher angles. It appears that for large-body terrestrial impacts, the composition of the high-angle, high-speed ejecta which reaches stratospheric heights will be critical to sudden changes in global climate and the induced environmental stresses. Application of these results to large impacts such as the K-T boundary event, are expected to provide new data pertinent to physical theories of extinction mechanisms.

The objective of this paper is to determine the thickness of the melt layer relative to the crater diameter for simple and complex craters. A numerical code was employed to calculate the amount of melting and the crater geometry. We used the code results and the scaling formalism of Holsapple and Schmidt (1987) to determine the scaling laws for the relative melt layer thickness. Simple crater dimensions are dominated by impact parameters and the planet’s strength, whereas complex crater dimensions are dominated by planetary gravity, strength, and the impact parameters. The volume of melt is proportional to impact energy for impact velocities and melt enthalpies of interest to planetary science. Crater geometry and dimensions scale with an exponent, μ, which is intermediate between momentum (μ = 1/3) and energy (μ = 2/3) scaling. For simple craters, the melt layer thickness/crater diameter, T / D , for a given planetary surface (constant melt enthalpy and mean impact velocity), is independent of the crater size. For complex craters, T / D , for a given planetary surface (constant melt enthalpy, impact velocity, and gravitational acceleration), increases with the size of the crater. For simple craters, at a fixed size, the relative melt layer thickness, T / D , increases slowly with increasing impact velocity, U, according to ∝ U 0.1 ), whereas, for complex craters (∝ U 0.22 ).

The mechanics of large-scale (~10-km diameter) asteroidal, cometary, and meteoroid swarm impact onto a silicate Earth covered by water and a gas layer (atmosphere) demonstrate that only ~ 15% to ~ 5% of the energy of 15 to 45 km/s bolides is taken up directly during the passage through the ocean and atmosphere, respectively. Upon impact with the Earth, ~ 10 to 10 2 times the bolide mass of water or rock can be ejected to the stratosphere: however, only ~0.1 bolide masses is in < 1 μ m particles. The vaporized, melted, and (< 1 mm) solid ejecta transfer up to ~40% of their energy to the atmosphere and possibly oceanic surface water, giving rise to a short, possibly lethal (to large animals) heating pulse. The initial high-speed ejecta that lofted to and above the stratosphere early in the cratering flow is enriched in bolide material and has concentrations of extraterrestrial material in the range of those measured (0.01 to 0.2) in the Cretaceous/Tertiary (C/T) boundary layer. We suggest that the origin of the C/T boundary layer is this ejecta, which is heavily shocked and in the < l- μ m range and, hence, once entrained in the stratosphere may be spread worldwide. Penetration of the atmosphere by the bolide creates a temporary hole in the atmosphere surrounded by strongly shocked air. The resultant inward and upward flow of the shocked atmosphere backward along the bolide trajectory lofts the vapor, fine-melted and solid ejecta to heights greater than 10 km. The larger, millimeter- to centimetersize, melt droplets that are lofted by this mechanism reenter the atmosphere and may represent microtektites and tektites. Sufficient impact-induced vapor, melted and comminuted silicate is ejected to stratospheric heights to markedly reduce the light levels at the Earth’s surface. The short-term effects of heating, followed by dust and possibly water-cloud deck induced worldwide cooling, provide several mechanisms to cause severe environmental stress to biota and possibly give rise to the varied and massive extinctions that occurred at the C/T boundary.