Shock effects from a large impact on the moon

Abstract

The shock and shear deformation induced internal energy distribution is calculated for a major basin-forming hypervelocity meteorite impact on the moon. Using the Hageman and Walsh formulation of the axisymmetric two-dimensional conservation equations in finite difference form, the flow field induced upon impact of an iron meteorite traveling at 15 km/sec with a gabbroic anorthosite lunar crust is calculated for sequential time steps over a grid of hoop-shaped zones fixed in space. A grid of massless tracer particles follows the outline in detail of the deformed meteorite and lunar surf ace in the cratering region. Assuming an energy of 5 x 10^(32) ergs for the projectile energy required to excavate a basin of Imbrium size yields a flow field some 210 km in radius, ~19 sec after impact. At this point the maximum stress level has decayed to pressures less than 200 kbar and left a zone of shock-melted rock immediately surrounding the meteorite at stresses below 100 kbar. However, portions of the meteorite are still moving downward at speeds of 2 km/sec. At this time some 7.2 and 4.2 meteorite masses of lunar surf ace material has been partially or completely melted or vaporized, respectively. Using recent estimates of ejecta volume for Imbrium-sized craters and the present results implies that the ejecta from a single such event will contain only 0.03-8% melt depending upon the scaling relation used. This implies that for impacts in the present velocity range, multiple events are required to produce the (larger) mass fractions of glass seen in the lunar breccias and soils. Comparison with predictions of the Gault and Heitowit formulation indicates that this earlier model is approximately valid. It predicts too rapid a decay of shock pressure with distance and hence, energy densities which are too high in the mass of surf ace material immediately surrounding the impacting meteorite.

Additional details