WHERE DID EARTH'S WATER COME FROM?

A long standing question in planetary science is, "what is the Earth made of?". For historical reasons, volatile-depleted primitive material similar to current chondritic meteorites was considered to be the "building blocks" of the terrestrial planets. Volatile species such as water were thought to be late cometary or asteroidal additions at the tail end of accretion. This view has persisted in spite of modern theories of planetary accretion. It is now clear from measurements of the D/H ratio and Ar content of comets that no more than 15% of the Earth’s water can have been added to the Earth at the end of accretion. Carbonaceous chondritic material containing up to 10 wt % water (CI chondrites) has also been proposed as the source of the Earth’s water. However, mass balance indicates that delivery of sufficient carbonaceous material to account for the minimum mass of water in the Earth, one Earth ocean mass, is inconsistent with the mass of the "late veneer" thought to be responsible for the highly siderophile elements in the Earth’s mantle. The Os isotopic ratio of the Earth’s Primitive Upper Mantle (PUM) is distinct from that in carbonaceous chondrites, but is consistent with ordinary chondrites. Ordinary chondrites, however, are low in water and cannot account for even the minimum Earth water budget if they are also responsible for the late veneer. The Mg/Si ratio of PUM is high compared enstatite, ordinary, and carbonaceous chondrites, leading some workers to propose sequestering Si in the lower mantle or core. There is, however, a continuum of planetary materials increasing in Mg/Si and Al/Si ratios in the order enstatite, ordinary, carbonaceous chondrites, PUM. If the upper mantle and the lower mantle have the same composition, then an interpretation is that the Earth was made of primitive materials with its current characteristics. Taken together, these observations lead to the obvious conclusion that no existing form of primitive material is similar to Earth mantle material, that Earth accreted "wet", and that the "building blocks" of Earth are "Earth chondrite" or "Earth achondrite", and that the "late veneer" is "Earth veneer". All of these materials have been incorporated in Earth or ejected into the Sun or out of the inner solar system