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Prolonged global catastrophes from oblique impacts

By
Peter H. Schultz
Peter H. Schultz
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Donald E. Gault
Donald E. Gault
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Published:
January 01, 1990

Most impacts occur obliquely rather than vertically as typically modeled. Laboratory experiments permit documenting the effects of impact angle on energy partitioning and related phenomena. Three results have particular significance for understanding the possible global atmospheric and biospheric response to a major impact. First, at low impact angles (<30° from the horizontal) the original impactor disrupts and ricochets downrange at a significant fraction of the original impact velocity. Results for different projectile/target combinations lead to a general disruption law such that sufficiently low impact angles (<5°) can result in nearly intact ricochet of the projectile with velocities close to the original impact velocity. More probable impact angles (5 to 15°) result in disruption dominated by 5 to 10 large fragments retaining about 50 percent of the original impact velocity. Scaling relations incorporating strain rate and possible weakening with size indicate that a 10-km object impacting at 10° with a velocity of 20 km/s could ricochet numerous 0.1 to 1-km-diameter fragments at hypervelocities, thereby producing a global swarm of Tunguska-scale events and enhancing environmental stress. Craters produced by oblique impacts on the Moon and Mars exhibit many of the same features as observed in the laboratory, including downrange impacts by ricocheted fragments. This observation provides evidence that basic phenomena observed at laboratory scales can be extended to much broader scales. Second, our experiments reveal that energy partitioned to target heating surprisingly increases as cos2θ for impact angles between 45° and 15°. This contrasts with peak stress levels, which decrease as sin2θ, and reflects the effect of shear heating along the projectile/target interface. As a consequence, vaporization of easily volatized materials (water, carbonates) can occur without initially large energy densities, thereby potentially adding to—rather than escaping from—the atmosphere. Thus, nitrate production from such a swarm could greatly exceed that from a single vertical impactor not only due to the greater ionization efficiency by numerous small objects and their much longer cumulative atmospheric path lengths but also due to increased coupling with the atmosphere. Third, the ricochet component appears to be embedded in an expanding vapor cloud sufficient to drive gases away from the downrange trajectory. For a major collision, this process would substantially increase the zone and duration of biomass incineration in a downrange “fireline,” as well as providing a mechanism for inserting substantial material into orbit. A potential consequence of orbital insertion is the possible development of a temporary debris ring. Such a ring might substantially prolong the climatic response to an impact through the reduced solar constant and through strong thermal gradients created by the ring shadow. Moreover, impact debris, including the cosmic signature, may be titrated into the geologic record well after the initial collision (<10 m.y.).

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GSA Special Papers

Global Catastrophes in Earth History; An Interdisciplinary Conference on Impacts, Volcanism, and Mass Mortality

Virgil L. Sharpton
Virgil L. Sharpton
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Peter D. Ward
Peter D. Ward
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Geological Society of America
Volume
247
ISBN print:
9780813722474
Publication date:
January 01, 1990

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