Giant collision may have formed Moon from Earth

Moon could have indeed been born from the compositionally similar Earth, following a giant collision, new models have indicated. The giant impact theory � the idea that a catastrophic collision about 4.5 billion years ago between Earth and a protoplanet about half Earth's size created a disk of molten rock, gas and debris that consolidated to form the Moon � was first set forth in the mid-1970s. In the 1980s, computer modelling of the physics of such a collision suggested the Moon would have formed mainly from the remnants of the pulverised foreign body, not from pieces of Earth, 'Earth' magazine reported. However, newer analytical evidence has since shown that the Moon's chemical composition closely matches that of Earth's mantle. Whereas all other known extraterrestrial bodies in the solar system today have different isotopic ratios than Earth, the Moon has almost identical isotopic ratios. "This seemed to create a problem: the match of lunar and Earth material is too perfect," says William K Hartmann, senior scientist at the Planetary Science Institute (PSI). The perfect isotopic match called the giant impact theory into question and suggested the Moon likely formed from Earth material, but how remained unknown. Now, two new models suggest different ways a giant impactor could produce a Moon chemically similar to Earth. In one model, Robin Canup, an astrophysicist at the Southwest Research Institute in Boulder, suggests a larger impactor than previously considered possible, about the same size as the early Earth. A collision with a similar-sized impactor would leave in its wake a mixture of debris from Earth and the impactor. After the planet consolidated, the remaining debris would produce a chemically similar Moon, Canup reported. Although the collision of two similar-sized bodies, smaller than the modern Earth, was considered about 15 years ago, says Erik Asphaug, a planetary scientist at the University of California at Santa Cruz, the scenario was abandoned because the system resulted in too much angular momentum � which didn't match today's Earth-Moon system. However, the understanding of orbital dynamics has advanced since then. In another model published in the same issue of Science, astrophysicists Matija Cuk and Sarah Stewart of Harvard University suggested a chemically similar Moon could form if Earth was spinning much faster 4.5 billion years ago than it is today. Cuk and Stewart suggested a scenario in which a faster-spinning Earth was struck by a moderate-sized impactor, similar to those previously suggested. The higher rotational velocity would be sufficient, they report, to fling enough Earth material out into space to form a compositionally similar Moon.