A More Violent Moon Birth?

It took an even bigger cataclysm to form the moon than researchers had thought, a new study suggests. The moon is thought to be the legacy of a jarring collision between a small planet and the early Earth, which lofted debris that later coalesced into the moon. Now a detailed computer simulation described in tomorrow's issue of Nature suggests that the incoming object, originally pictured as about the size of Mars, might have been twice that large in order to kick up enough debris.

The giant impact scenario replaced an earlier picture in which Earth and the moon both formed at about the same time. But moon rocks brought back by the Apollo missions of the early 1970s contained surprisingly little iron compared to rocks on Earth, indicating that the two were born in vastly different circumstances. So planetary scientists proposed that a collision between the early Earth and another protoplanet ejected material from both protoplanets' outer layers into orbit around Earth. Because most of Earth's iron had already sunk to the core, the lofted material would contain lighter elements. Eventually, the theory says, the material cooled and clumped together to form the moon.

Details of how the storm of orbiting silicate vapor resulting from the impact settled out to give a single, massive moon had been hazy, however. To fill in the story, a group of three researchers at the University of Colorado and the Tokyo Institute of Technology constructed a detailed computer model that simulates the gravitational interactions between some 2000 orbiting globs of debris and Earth.

The calculations revealed that a smallish moon (and sometimes two) would form quite readily from the debris, typically in less than a year. But making a massive moon proved considerably harder. "The process is very inefficient," says Glen Stewart, a team member at the University of Colorado. "Typically three-quarters of the stuff falls back to Earth." Once a dominant clump starts to form, he says, its gravitational tug can slow down other bits of matter, sending them on a slow trajectory back to Earth. So to create today's moon, the simulations demanded that the initial collision kick up at least twice as much debris as required by previous computer models. And that could require a bigger impacting body.

But other modelers aren't ready to accept the conclusion. "These are very, very preliminary results," cautions Alastair Cameron, an astronomer at Harvard University and the man behind many models of the impact itself.