Assume that during core formation the highly siderophile elements were completely removed from the terrestrial mantle. Today's mantle nevertheless contains 3.9 ppb of Os and 3.5 ppb of Ir. What is the proportion of late accretion in the form of chondrites with 460 ppb Os and 430 ppb Ir that has contributed to the modern mantle? Neglect the contribution of the continental crust. Assuming that the mantle is dry and that oceans were added by late accretion, what is the water content of the incoming projectiles? (Mass of Earth = 5.97 x 1024 kg; the mass of mantle = 4.0 x 1024 kg; the mass of ocean = 1.4 x 1021 kg).