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.
Flying in formation around the Moon, NASA's twin GRAIL spacecraft makes precise measurements of the lunar gravitational field.
"By probing the Moon's gravity field, GRAIL 'sees' inside the Moon, illuminating the differences between the near and far sides."
GRAIL's twin spacecraft around the Moon for several months while a microwave ranging system precisely measures the distance between the two spacecraft. By watching that distance expand and contract as the pair fly over the lunar surface, researchers map the Moon's underlying gravity field.
These measurements tell us a lot about the distribution of material inside the Moon, and give us pretty definitive information about the differences in the two sides of the Moon's crust and mantle. If the density of crustal material on the lunar far side differs from that on the near side in a particular way, the finding will lend support to the 'two moon' theory."
But this information is just one "piece of the jigsaw puzzle." To prove a sister moon ever existed, other pieces are needed. NASA's Lunar Reconnaissance Orbiter has already provided key information on the Moon's surface topography. Scientists can also refer to lunar surface chemistry data and look at old seismic information from Apollo for clues.
But what's really needed, said Smith, is a sample return mission to the far side 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."
The Daily Galaxy via http://science.nasa.gov
Image credit: sciencewsblog.blogspot.com