16/12 Alessandro Morbidelli - "The age of the Moon" ENS ampli L
According to the generally accepted scenario, the last giant impact on the Earth formed the Moon and initiated the final phase of core formation by melting the Earth’s mantle. A key goal of geochemistry is to date this event, but different model ages have been proposed. Some argue for an early Moon-forming event, 30 My after the condensation of the first solids, while others claim a date later than 50 My and possibly as late as ~100 My. We use a large number of N-body simulations to demonstrate a relationship between the time of the last giant impact on Earth-like planets and the subsequent mass accreted, called the late veneer. As the time of the last giant impact increases, the mass of the late veneer decreases in a quantitatively predictable fashion. This relationship is model-independent and holds across a large number of initial disc and dynamical conditions. The mass of the late veneer – or at least an upper limit on the mass – is constrained by the abundance of the highly siderophile elements in the Earth's mantle. We use current measurements to determine a range of plausible Moon formation ages. Only certain initial and dynamical conditions produce giant impacts in the determined age-range and match simultaneously other constraints such as the rapid (2 My) formation of Mars.
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