A chronometer for Earth's age

Abstract

Simulations of Earth's growth show a correlation between the timing of the Moon's formation and the amount of mass that Earth accreted afterwards. This relationship provides a way of measuring the age of our planet. See Letter p.84 The age of the Moon has been a focus for geochemists for at least the past three decades. A number of chronometers have been used to address the question but the results differ from method to method, in part because of the varying assumptions required in the calculation of the so-called model ages. Seth Jacobson et al. have used an alternative approach. They run a large number of numerical simulations, some based on early Moon-forming events, others later events. They then arrive at a model-independent correlation between the formation age of the Moon and the amount of mass accreted by the Earth since then, the so-called Late Veneer. The concentration of highly-siderophile (iron-loving) elements observed in the Earth's mantle provides a constraint on the timing and rules out an early Moon-forming event. Instead, the authors calculate that the Moon-forming impact must have occurred at least 40 million years after formation of the Solar System.