Planetary evolution: the early Earth as an example

For the inner planets of the solar system, surface features which reflect tectonism and the interplay of lithosphere with atmosphere (and hydrosphere, if present), versus the preservation of ancient impact features, is very much a function of planetary mass and distance from the Sun (Head & Soloman, 1981). Larger bodies such as Venus and the Earth have experienced geologically recognizable physiographic modification, whereas surficial activity declined or ceased on Mars, Mercury and the Moon several Ga ago. Smaller masses lose heat more rapidly, and are less able to retain gassy constituents as completely as larger bodies. Volatiles, particularly H2O, lower fusion temperatures because of solubilities in silicate melts, thus enhancing the planetary capacity for crystal–liquid–vapor fractionation. Partial melting, in turn, is probably related to thermally-created density instabilities in the mantle. Such convective flow, an important driving force for lithospheric plates, is a manifestation of the escape of internal heat within a gravitational field.

Isotopic data from meteorites and the Moon indicate that the solar system is approximately 4.5–4.6 Ga old (Patterson, 1956, Papanastassiou & Wasserburg 1971). Gravitational self-attraction of a locally dense cloud of interstellar gas resulted in condensation of the solar nebula which, to preserve angular momentum, initially formed a rotating disc (Safronov, 1972, Cameron, 1978). Accretion of planetesimals in progressively more peripheral regions of the disc gave rise to planetary bodies orbiting the newly-formed star.