Did the Earth Once Have Two Moons?

A new paper makes the claim Earth once has two moons.

(PhysOrg.com) -- As early as Sept. 8th, NASA's GRAIL mission will blast off to uncover some of the mysteries beneath the surface of the Moon. That cratered gray exterior hides some tantalizing things – even, perhaps, a long-lost companion.

If a paper published recently in the journal Nature* is right, two moons once graced our night skies. The proposition has not been proven, but has drawn widespread attention.
"It's an intriguing idea," says David Smith, GRAIL deputy principal investigator at the Goddard Space Flight Center. "And it would be a way to explain one of the great perplexities of the Earth-Moon system – the Moon's strangely asymmetrical nature. Its near and far sides are substantially different."

The Moon's near side, facing us, is dominated by vast smooth 'seas' of ancient hardened lava. In contrast, the far side is marked by mountainous highlands. Researchers have long struggled to account for the differences, and the "two moon" theory introduced by Martin Jutzi and Erik Asphaug of the University of California at Santa Cruz is the latest attempt.

Scientists agree that when a Mars-sized object crashed into our planet about 4 billion years ago, the resulting debris cloud coalesced to form the Moon. Jutzi and Asphaug posit that the debris cloud actually formed two moons. A second, smaller chunk of debris landed in just the right orbit to lead or follow the bigger Moon around Earth.

"Normally, such moons accrete into a single body shortly after formation," explains Smith. "But the new theory proposes that the second moon ended up at one of the Lagrange points in the Earth-Moon system."

Lagrange points are a bit like gravitational fly traps. They can hold an object for a long time--but not necessarily forever. The second moon eventually worked its way out and collided with its bigger sister. The collision occurred at such a low velocity that the impact did not form a crater. Instead, the smaller moon 'went splat,' forming the contemporary far side highlands.

In short, the lunar highlands are the lost moon's remains.