Moon born in violent collision
3 Dec 2012 by Evoluted New Media
Planetary scientists have discovered evidence that the Moon was created when a body the size of Mars collided with the early Earth. The work was conducted at Washington University in St Louis and published in Nature.
The team found a tiny excess of a heavier variant of zinc in Moon rocks. They conclude that the enrichment probably arose because heavier zinc atoms condensed out of the cloud of vaporised rock (created by the collision) faster than the lighter zinc atoms. This sorting by mass is known as isotopic fractionation and scientists have been looking for this evidence since the Apollo missions first brought Moon rocks to Earth in the 1970s.
“When a rock is melted and then evaporated, the light isotopes enter the vapour phase faster than the heavy isotopes, so you end up with a vapour enriched in the light isotopes and a solid residue enriched in the heavier isotopes. If you lose the vapour, the residue will be enriched in the heavy isotopes compared to the starting material,” explained lead author, Frédéric Moynier.
To make sure the effect was global, the team analysed 20 samples of lunar rocks, including ones from Apollo 11,12,15 and 17 missions- which all collected samples from different locations on the moon- and a lunar meteorite.
“What we wanted were the basalts,” Moynier explained, “because they’re the ones that came from inside the Moon and would be more representative of the Moon’s composition.”
For comparison, the group also analysed Earth rocks and 10 Martian meteorites. Compared to terrestrial or Martian rocks, lunar rocks have much lower concentrations of zinc but are enriched in the heavy isotopes of zinc. The simplest explanation for the differences is that conditions during or after the formation of the Moon led to more extensive volatile loss and isotopic fractionation than was experienced by Mars or Earth.
The most likely large-scale event is wholesale melting during the Moon’s birth. The zinc isotope data supports the theory that a giant impact gave rise to the Earth-Moon system.
“The work also has implications for the origin of the Earth because the origin of the Moon was a big part of the origin of the Earth.”
