Cratering of the terrestrial planets by Apollo objects

Abstract

Abstract— Absolute and relative cratering rates on the terrestrial planets have been calculated using the same asteroidal collision model and Monte Carlo program used for previous studies of the terrestrial meteorite flux, the steady‐state number of Apollo‐Amor objects, and the orbital distribution of both meteorites and Apollo‐Amor objects. The most straightforward result is that projectiles from the asteroid belt appear to provide about one‐third the observed present‐day production of terrestrial craters larger than 10 km in diameter. When uncertainties in the calculations and observations are included, it cannot be excluded that the entire terrestrial cratering flux is asteroidal. On the other hand, assumption of an additional Apollo‐Amor source of extinct comets, in the same quantity permitted by Apollo‐Amor observations, provides better agreement with the observed cratering rate. In addition, a significant (e.g., ∼30%) terrestrial contribution from active long and short period comets is acceptable within the uncertainties of the assumptions required.The ratios of the cratering rates on the different terrestrial planets are somewhat sensitive to the assumed source. A purely asteroidal source predicts a martian cratering rate per unit area about four times that on Earth, whereas the difference is reduced to about a factor of two for the mixed asteroid‐extinct comet source. The opposite effect is found for Mercury. As discussed by previous authors, the predicted lunar cratering rate is significantly higher than that observed. It is not clear whether this is a result of scaling to impacts on a body considerably smaller than Earth, or if it indicates an increase in the cratering flux during the Phanerozoic.