

chondritic
meteorites, but icy planetismals may have contributed components of the
atmosphere andhydrosphere. By analogy with the evolution of the moon,
the growth of the terrestrial mass and volumeresulting from meteorite
impact and accretion occurred exponentially over a period of about 50
millionyears (Ouyang ZY et al., 2002). After 4.0 Ga the number of
meteorite impacts decreased very rapidly,and the augmentation of the
Earth's mass since that time, caused by the impacts, has been only
1025grams, 1/600 of its total mass. This implies that accretion of the
Earth effectively ceased at about 4.0 Ga,and the mass and volume of the
Earth has since remained essentially unchanged.