"It's also possible that Theia was essentially made of ice, something from the Kuiper belt in the outer solar system."
"This would have provided the energy of impact, but with the ice evaporating away leaving Earth material to eject into space and form the moon."
Another option suggested by Zhang and colleagues involved the proto-Earth spinning much faster than previously thought, allowing a greater degree of mixing.
"However we don't really think Earth had spun rapidly enough for that to happen," says Carter.
"And then you have the problem of slowing down the Earth's rotation afterwards."
"Zhang and colleagues suggest a gravitational resonance effect between the Earth, moon and sun may have provided the forces needed to slow down the Earth's spin rate," says Carter.
BLOG: Oxygen Discovery Raises Hopes for Saturn Moon LifeBLOG: Oxygen Discovery Raises Hopes for Saturn Moon Life Despite the problems, Carter believes the paper makes a good point about the need for revision of the current theory.