Tumours switch to resist

Abstract

for the conditions of the earliest Solar System but not for the accretion of the Moon from a silicate-rich debris disk (Fig. 1). Paniello and colleagues' new Zn isotope data may also contribute to the reawakened discussion of the very mechanism by which the Moon formed. A theory known as the giant-impact hypothesis — according to which our natural satellite is the outcome of reassembled debris from a collision between the proto-Earth and another planetary body — has dominated recent thinking about lunar genesis. In particular, an oblique-impact scenario was shown 9 to reproduce many of the physical and chemical attributes of the Moon. However, this model predicts that the Moon should be comprised predominantly of mate-rial from the colliding impactor rather than the target proto-Earth. Instead, the Moon has proven to be embarrassingly similar to Earth in several isotopic characteristics 10–12 , seemingly requiring it to be derived almost wholly from Earth. Models have thus investigated mecha-nisms by which the isotopic composition of the Earth and Moon could have become homog-enized after the impact 13 . The notable differ-ences in Zn isotopic composition between Earth and Moon documented by Paniello et al. may be more readily reconciled with an entirely new model for Moon formation, in which the impactor hits a rapidly spinning proto-Earth 14 . This flurry of recent develop-ments emphasizes a waxing interest in our ever-puzzling satellite. ■