article: About 4.45 billion years ago, 150 million years after the solar system formed, Earth was hit by a Mars-size object called Theia. The collision created the moon, but debate has raged exactly what happened during this event - and a mystery has persisted on why the moon and Earth are so similar in their composition. Now, conflicting studies have provided two different theories - one that Theia was similar to Earth, and another that Earth and the moon were showered by debris following the collision. Three studies have provided different theories for what happened after Earth was struck by a Mars-sized object called Theia, 4.45 billion years ago. What is agreed on, though, is that the impact actually happened (artist's illustration shown). This is known as the giant impact hypothesis . The two theories appear in the journal Nature across three studies - two in favour of the ‘similar to Earth’ theory, and the other favouring the ‘shower of debris’ model. In the latter theory, a team of scientists at the University of Maryland said the impact of Theia with Earth was so violent, the resulting debris cloud mixed thoroughly before settling down and forming the moon. Many researchers believe the moon formed after Earth was hit by a planet the size of Mars billions of years ago. This is called the giant impact hypothesis. The hypothesis claims the moon is debris left over following an indirect collision between our planet and an astronomical body approximately 4.5 billion years ago. The colliding body is sometimes called Theia, after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. But one mystery has persisted, revealed by rocks the Apollo astronauts brought back from the moon - why are the moon and Earth so similar in their composition? Several different theories have emerged over the years to explain the similar fingerprints of Earth and the moon. Perhaps the impact created a huge cloud of debris that mixed thoroughly with the Earth and then later condensed to form the moon. Or Theia could have, coincidentally, been isotopically similar to young Earth. A third possibility is that the moon formed from Earthen materials, rather than from Theia, although this would have been a very unusual type of impact. This cloud would have been composed of some Earth material, explaining the similarity between Earth and the moon, and other material. ‘This means that, right after the moon formed, it had exactly the same isotopic composition as Earth's mantle,’ said Dr Richard Walker, a professor of geology at UMD and co-author of the study. Both bodies have a similar isotopic ‘fingerprint,’ notably similar types of Tungsten, suggesting they had a similar origin. One difference, though, is that Earth has proportionally much less of a lighter isotope known as Tungsten-182. The team said this is because after both bodies formed, Earth swept up more debris that had little Tungsten-182. ‘We still need to work out the details, but it's clear that our early solar system was a very violent place,’ added Dr Walker. The team said their theory largely rules out the idea that the Mars-sized body Theia was of similar composition, or that the moon formed from only material contained in the pre-impact Earth. The other studies, though, disagree. Scientists at the Israel Institute of Technology and the Southwest Research Institute in Colorado said that Theia actually had a similar composition to Earth. Dr Alessandra Mastrobuono-Battisti and colleagues simulated collisions between protoplanets and compared the composition of each surviving planet with the composition of its last giant impactor. In one theory, a team of scientists at the University of Maryland said that the impact of Theia with Earth was so violent, the resulting debris cloud mixed thoroughly before settling down and forming the moon. Shown is an artist's illustration of the young Earth and moon . The University of Maryland team examined the tungsten isotopic composition of two moon rocks collected by the Apollo 16 mission, including sample 68815, seen here. When corrected for meteoritic additions to Earth and the moon after formation of the moon, the two bodies had identical Tungsten isotopic compositions . Supporting evidence includes the Earth's spin and Moon's orbit having similar orientations. Moon samples indicating the surface of the moon was once molten also provide evidence,  as does the moon's relatively small iron core. The moon also has a lower density compared to Earth, while there is evidence of similar collisions in other star systems (that result in debris disks). Giant collisions are also consistent with the leading theories of the formation of the solar system. And finally, the stable isotope ratios of lunar and terrestrial rock are identical, implying a common origin. They found that 20 to 40 per cent of impactors had similar compositions to the planets they impacted, whereas planets that formed in the same simulation tended to have distinct compositions. The authors suggest their results may explain the similarities between Earth and the moon - and why their compositions differ from those of other planets in the solar system. ‘We find that different planets formed in the same simulation have distinct compositions, but the compositions of giant impactors are statistically more similar to the planets they impact,’ the researchers wrote. They said their theory has a 20 per cent probability of correct. Whichever theory turns out to be correct though, both are in agreement that the formation of the moon remains a confusing mystery that is yet to be fully solved. ‘This is interesting stuff and no one has all the answers regarding the formation of the moon - we certainly don't claim to,’ Dr Walker, one of the authors of the former study, told MailOnline. However, the other theory says that Earth (left) and the moon (right) are so similar because the initial Mars-sized object, Theia, actually had a similar composition to Earth. Dr Alessandra Mastrobuono-Battisti and colleagues simulated collisions between protoplanets to make their finding .
highlights: The moon formed when a Mars-sized body called Theia hit it early on .
  But no one is sure why the moon and Earth have a similar composition .
  University of Maryland researchers say cloud of debris rained on worlds .
  But Israel Institute of Technology says impactor was similar to Earth .
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