Moon's origin confirmed in early Earth collision

Look up at the moon and you are looking at the aftermath of a collision between the Earth and a planet the size of Mars, scientists have confirmed.

Since the 1970s, astronomers have suspected that such a planet-sized impact roughly 4.5 billion years ago at the dawn of the solar system led to the moon's formation. The "giant impact" idea explains why Earth has such a large moon, one-quarter as wide as the planet.

In a new Nature journal study led by Randal Paniello of Washington University, St. Louis, a look at lunar chemistry finds fresh evidence confirming a big smash-up as part of the explanation for the moon. The study examined the zinc seen in the lunar soil and asked how it differed from the same element as seen on Earth, Mars and asteroids.

In particular, the authors report, the moon appears conspicuously loaded with a heavier variety of zinc that doesn't vaporize as easily at high temperatures. The results suggest that the moon formed not from the left-over crust of the Mars-sized planet that hit the young Earth, but from a debris disk of material thrown up by both planets. Volatile elements in that dust ring cooked off before snowballing together to form the moon.

The idea represents "an entirely new model for Moon formation, in which the impactor hits a rapidly spinning proto-Earth," says planetary scientist Tim Elliott of the United Kingdom's University of Bristol, in a commentary accompanying the study.

"It's nice to see how many aspects and variants of the giant impact idea are still alive and kicking, 38 years after we gave the first paper on the idea," says planetary scientist William Hartmann of the Planetary Science Institute in Tucson. Hartmann says that an "impact trigger" explanation where the moon formed from a debris disk might be a better name for what happened than a "giant impact" one where the moon was quickly carved from another planet.