Hafnium-Tungsten Chronometry of Planetary Accretion and Differentiation

摘要

The formation and earliest history of many planetary bodies in the inner solar system involved heating and melting of planetary interiors, causing differentiation into silicate mantles and metal cores. A record of these earliest stages of planetary evolution is preserved in the W isotopic composition of meteorites as well as lunar and terrestrial rocks. Hafnium-tungsten chronometry reveals that the early thermal and chemical evolution of asteroids is controlled by the decay of ~(26)Al. This nu-clide was sufficiently abundant to melt early-formed planetesimals (iron meteorite parent bodies) but could not raise the temperatures in the late-formed chondrite parent asteroids high enough to cause differentiation. Larger bodies such as the Earth grew over longer timescales and the energy required for differentiation was mainly delivered by large impacts. Formation of the Earth was largely completed by the impact of a Mars-sized body that led to the formation of the Moon between 4.53 and 4.47 billion years ago.