Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001dps....33.2307d&link_type=abstract
American Astronomical Society, DPS Meeting #33, #23.07; Bulletin of the American Astronomical Society, Vol. 33, p.1081
Astronomy and Astrophysics
Astronomy
Scientific paper
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 and Ar/H2O ratios of comets that no more than 15% of the Earth's water can be of cometary origin. Carbonaceous chondritic material containing up to 10 wt % water 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 Earth's Primitive Upper Mantle (PUM) is distinct from that of 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 to 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 of PUM composition. Taken together, these observations lead to the conclusion that no primitive material similar to Earth mantle material is currently represented in our meteorite collections and that the ``building blocks" of Earth are ``Earth chondrite" or ``Earth achondrite." All of these materials have been incorporated in Earth, ejected into the Sun or out of the inner solar system, or remain unsampled.
Drake Michael J.
Righter Kevin
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