Inference on the nature and the mass of Earth's late veneer from noble metals and gases

Noble metals and gases are very sensitive to the late accretion to the Earth of asteroids and comets. We present mass balance arguments based on these elements that indicate that 0.7-2.7x10(22) kg of extraterrestrial bodies struck the Earth after core formation and that comets comprised less than 10(-3) by mass of the impacting population. These results imply that the dynamics of asteroids and comets changed with time and that biogenic elements and prebiotic molecules were not delivered to the Earth by comets but rather by carbonaceous asteroids.