Single-impact US moon theory claims to clarify planet diversity

Scientists have come up with the best explanation yet of how the moon was formed. The new theory should help researchers understand how Earth-like planets form around other stars.

The lunar landings 30 years ago showed the moon is made up of the same material as Earth. This led to a radical theory that the moon was gouged out of Earth after a colossal impact.

Physicists tried to calculate what kind of impact could eject enough material to make the moon. They suggested it would take a planet the size of Mars to produce the energy needed to form the moon.

The Southwest Research Institute and the University of California at Santa Cruz, have,

published a new theory in the science journal, Nature. The study shows the feat could have been done with a smaller body, perhaps just a 10th the size of Earth and smaller than the moon.

"It is now known that giant collisions are a common aspect of planet formation, and the different types of outcomes from these last big impacts might go a long way towards explaining the puzzling diversity observed among planets," explained Prof Erik Asphaug of UCSC.

Earlier theories suggested a collision doubled the earth's angular momentum and needed a second impact to reduce the momentum to what we see today. A double impact was considered improbable but it could account for the creation of the moon.

Another theory said the moon-forming impact happened when Earth was still growing in size as it swept up dust and rubble floating in its orbit. But this theory allowed the moon to achieve too high a density as it also accumulated iron-rich material from space.

The latest theory accounts for all these problems. It holds that an impact of a 10th the size of Earth hit the planet late in its formative stage, when it would be fairly plastic.

It was a glancing blow but enough to eject low-density surface material to form the moon and throw it into permanent orbit.

The researchers believe their theory is superior because it involves a single impact, but accounts for the iron question and also matches the existing angular momentum of the Earth-moon system.