CONTINTAL VS OCEANIC ASTEROID IMPACTS

The present work was motivated by a question posed by Dallas Abbott: Could there possibly be a systematic difference between oceanic and continental impacts in the amount of energy coupled into the mantle? If there were, it might help explain the antipodal phenomena observed by Hagstrom (this session).

We have used the hydrocode Autodyn(TM) to calculate the normal impact at 20 km/s of a 4 km-diameter olivine asteroid on both continental and oceanic crusts. The ocean was modeled as 5 km of water over 5 km of basalt over 70 km of olivine mantle. The continent was modeled as 30 km of granite over 50 km of olivine mantle. In both sets of calculations, the extreme pressure regimes of the asteroid and the uppermost ten km of the impact region were modeled with Tillotson and Sesame equations of state. The mantle was modeled with Hugoniot data for Transvaal dunite. The lower 20 km of the continental crust was modeled with a shock wave equation of state that incorporated the hysteretic phase changes of quartz and feldspars to stishovite-like and hollandite-like phases.

To date, our calculations have shown that substantially more (tentatively, about twice as much) energy is coupled into the mantle for oceanic impacts than for continental impacts. This result, in agreement with Swegle's calculation of smaller scale events (1), is a consequence of high shock attenuation in the continental crust caused by the hysteretic phase changes of quartz and feldspar.

The results presented here are specific to the particular input conditions and models chosen. The difference between oceanic and continental impacts would be smaller for larger impacts, e.g., of a 20 km diameter asteroid.

We thank Chris Quan and Richard Clegg of Century Dynamics for technical assistance. PSD thanks Century Dynamics for supporting his use of Autodyn(TM).

(1) Swegle, J. W., (1990), J. Appl. Phys. 68, 1563-1579