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• W2007647560 abstract "Several hundred impact craters produced historically and at times as early as 1.9 × 109 years ago with diameters in the range 10−2 to 102 km are observed on the surface of the earth. Earth-based and spacecraft observations of the surfaces of all the terrestrial planets and their satellites, as well as many of the icy satellites of the outer planets, indicated that impact cratering was a dominant process on planetary surfaces during the early history of the solar system. Moreover, the recent observation of a circumstellar disk around the nearby star, β-Pictoris, appears to be similar to our own hypothesized protosolar disk. A disk of material around our sun has been hypothesized to have been the source of the solid planetesimals from which the earth and the other planets accreted by infall and capture. Thus it appears that the earth and the other terrestrial planets formed as a result of infall and impact of planetesimals. Although the present planets grew rapidly via accretion to their present size (in∼ 107 years), meteorite impacts continue to occur on the earth and other planets. Until recently meteorite impact has been considered to be a process that was important on the earth and the other planets only early in the history of the solar system. This is no longer true. The Alvarez hypothesis suggests that the extinction of some 90% of all species, including 17 classes of dinosaurs, is associated with the 1 to 150 cm thick layer of noble-element rich dust which is found all over the earth exactly at the Cretaceous-Tertiary boundary. The enrichment of noble elements in this dust is in meteorite-like proportions. This dust is thought to represent the fine impact ejecta from a ∼ 10km diameter asteroid interacting with the solid earth. The Alvarez hypothesis associates the extinction with the physics of a giant impact on the earth. Using finite-difference techniques, cratering flow calculations are used to obtain the spatial attenuation of shock pressure with radius, r, along the impact axis for the impact of silicate rock and iron impactors on a silicate half-space at speeds of 5 to 45 km/sec. Stress wave attenuation is found to be represented by two regimes, if the peak pressure, P, is fitted to expressions of the form P oc r2. At distances from 2.2 to 5.6 projectile radii into a silicate target, the constant, a, is on the order of −0.2. This low-attenuation rate impedance matching regime extends further into the target at the slower impact velocities. This occurs because of the slightly divergent flow associated with the penetration of a spherical projectile. For the near-field impact regime, an impact at 5 km/sec of an iron object with silicate surface will induce complete melting for silicate; the iron will remain solid. At 15 km/sec, partial vaporization occurs for both silicate and iron whereas at 45 km/sec, complete vaporization occurs in both materials. Similar calculations were conducted for a silicate meteoroid striking a silicate surface at velocities ranging from 5 to 45 km/sec. At greater radii in the far-field regime, the exponent, a, varies systematically from −1.45 to −2.15 for impacts of silicate onsilicate as the impact velocity is increased from 5 to 45 km/sec. For an iron projectile impacting at speeds of 5 to 45 km/sec, the exponent, a, varies from −1.67 to −2.95. Upon impact of a 10 to 30 km diameter silicate or water object onto a 5 km deep ocean overlying a silicate half-space planet at 30 km/sec, we find that from 12 to 15% of the incident energy is coupled into the water. In the gravity field of the earth, some 10 to 30 times the impactor mass of water is launched on trajectories which can achieve stratospheric heights. The amount of ejecta launched to stratospheric altitudes is similar to the 101 to 102 projectile masses which result from impact of objects on an ocean-free silicate half-space (land). In the case of impact directly onto a silicate-half-space, only ejecta launched on trajectories which would carry it to stratospheric heights, has an impactor to target mass ratio which matches the fraction (10−2 to 10−1) of extraterrestral material found in the platinum-metal-rich Cretaceous-Tertiary boundary layer. Oceanic impact results in impulsive-like giant tsunamis initially having amplitudes of ∼ 4km, representing the solitary waterwave stability limit in the deep ocean, and containing 10−2 to 10−1 of the energy of the impact. Calculation of the interaction of a ∼ 10km bolide with the atmosphere indicates that only some 8% of the energy is imparted to the air during initial passage through the atmosphere. However, upon impact with the earth ∼ 101to102 times the bolide mass of water or rock is ejected into the stratosphere, although, only ∼0.1 bolide masses are in <1μm particles. The vaporized, melted, and (<1mm) solid ejecta transfer up to ∼40% of their energy to the atmosphere. Using the results of a similarity solution for the flow of gas as a result of an explosion in exponential atmosphere it is found that the atmosphere above such a large energy source is entirely ejected at speeds exceeding the escape velocity of the earth. Using the similarity solution we have calculated the mass of atmosphere lost due to impacts of 1 to 5 km radius projectiles. No atmosphere is lost for surface sources with energies less than 1027 ergs. Impact of objects in the energy range 1027 to 1030 ergs causes gas losses of 1011 to 1014 kg or 10−8 to 10−5 of the total present atmospheric mass. Impact energies of greater than 1030 ergs cause little increase in atmospheric loss." @default.
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• W2007647560 date "1987-01-01" @default.
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• W2007647560 title "Impact on the earth, ocean and atmosphere" @default.
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• W2007647560 doi "https://doi.org/10.1016/0734-743x(87)90028-5" @default.
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