The Earth may once have had two moons. If so, what happened to the other one?
NASA’s GRAIL mission launched a lunar probe in 2011 to try to find remnants of a second moon, buried beneath the surface of the moon we see at night.
The Daily Galaxy website quotes NASA’s David Smith as saying, "It’s an intriguing idea, 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."
Scientists agree that when a Mars-sized object crashed into the Earth about 4 billion years ago, the resulting debris cloud formed the moon. Researchers Martin Jutzi and Erik Asphaug think the debris cloud actually formed TWO moons, and that the second, smaller one landed in just the right orbit to lead or follow the bigger moon around the earth.
Smith says, "Normally, such moons accrete into a single body shortly after formation, but the new theory proposes that the second moon ended up in a Lagrange point of the Earth-Moon system."
The Daily Galaxy explains that, "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. ”
Flying in formation around the Moon, NASA’s twin GRAIL spacecraft makes precise measurements of the lunar gravitational field. "
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."
But Smith says that what’s really needed is a mission to the far side of the moon to return a sample from the far side, in order to determine the ages of rocks there.
"The smaller moon, if there was one, was about 1/3 the size of our current moon. So upon collision it would have cooled down faster, and the rocks on the far side, where its remains are thought to have spread, would be older than the ones on the near side."