It arrived there 4bn years ago

This is a cross-section through a grain from a well-travelled rock. It was brought to Earth from the Fra Mauro highlands of the Moon in 1971, by the crew of Apollo 14. Four billion years before that, though, it had made the journey in the opposite direction, according to an analysis by Jeremy Bellucci of the Swedish Museum of Natural History, published in Earth and Planetary Science Letters.

Fra Mauro is composed of ejecta from a celestial collision between an asteroid and the Moon, which excavated the biggest lunar impact basin, Mare Imbrium. Most of the samples returned by Apollo 14 are breccias created by this impact. Breccia is a type of rock formed by the higgledy-piggledy mixing of bits of other rock, and this two-gram grain was part of one such brecciated boulder.

Dr Bellucciís analysis of the minerals in the grain, particularly its zircon (Zr, in the picture) and quartz (Qtz), shows that they would have been unlikely to form in lunar conditions, but would easily have formed on Earth. The simplest explanation, therefore, is that Earth is where they came from.

Almost certainly, the grain arrived on the Moon as part of a larger rock blasted off Earthís surface by an impact similar to that which created Mare Imbrium. All this happened during a period of the solar systemís history called the late heavy bombardment, which lasted from 4.1bn to 3.8bn years ago. The Moon then being only a third as far away from Earth as it is now, travelling to the one from the other would have been an easy journey. The grain was then shifted again, by the Imbrium impact, to form part of the geological splatter now called Fra Mauro.

Terrestrial material this old is rare, so finding some on the Moon has been a useful addition to geologistsí collections. And this particular grain may not be unique. Apollo 14 brought back 42kg of rock. Other chips off the block of old Earth are probably hiding among them.