ON THE HYPOTHESIS OF HYPERIMPACT-INDUCED EJECTION OF ASTEROID-SIZE BODIES FROM EARTH-TYPE PLANETS

During the last two decades a number of facts have brought to life a s eemingly fantastic idea of ejection of large rocky fragments from plan ets into space, like for example SNC meteorites or many-km-size fragme nts of Vesta. The theoretical description of impact processes of this ejection lags behind. Considerable efforts have been spent to show the possibility of ejection of bodies several meters in size from large ( D similar to 30-100 km) impact craters on Mars. Taking into account th e rock strengthening up to similar to 100 kbar under the great confini ng pressures and jet-like pattern of the evaporated shock-heated matte r outflow from the crater, it can be shown that bodies as large as sim ilar to 1 km could be ejected into space from the Earth. The possibili ty of their gaining additional velocity in planet perturbations and su bsequent fall onto the Earth with increased energy after 1-3 Myr throw s light on the origin of SNC meteorites, near-Earth asteroids, shower bombardments of the Earth at the boundaries of geologic epochs etc. Fo r Mercury, having low escape velocity and large orbital velocity, the back infall of fragments with increased energy may initiate a kind of chain reaction of its self-destruction. This can explain the loss of M ercury's silicate mantle, the shrinkage of its orbit from the initial near-Venus path towards the Sun and other peculiarities of the evoluti on of Mercury-Venus system. In general, the possibility of impact self -destruction of inner planets may drastically alter traditional models of the origin of the Solar System. However, non-destructive gasdynami c ejection of large fragments from planets requires a mechanism for fa st conversion of shock-wave energy into heat. The extrapolation of dat a from laboratory impact experiments (similar to 10 kJ) and nuclear ex plosions (< 1 Mt TNT) in order to describe hyperimpact processes with 10(5)-10(6)Mt TNT energies can hardly be justified, that is why these calculations give relatively small gas production and, consequently, s mall velocities of fragment ejection from impact craters. It is predic ted that at such energies some instabilities may lead to formation of new dissipation channels, that would increase the part of the overheat ed gas fraction in the hyperimpact ejection products. This would elimi nate numerous contradictions in the impact history of planets, asteroi ds, meteorites etc.