The age and origin of the moon have been studied with the use of the recently developed short-lived hafnium-tungsten chronometer (Hf-182-W-182, half-life of nine million years). The tungsten isotopic compositions of 21 lunar samples range from chondritic to slightly radiogenic (epsilon(W) = -0.50 +/- 0.60 to +6.75 +/- 0.42). This heterogeneity may have been inherited from material excavated from Earth and the putative impactor, but it is more likely the result of late radioactive decay within the moan itself; in this case, the moon formed 4.52 to 4.50 billion years ago, and its mantle has since remained poorly mixed.