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Rocks that may have originated deep within the Moon were spied on its surface by a Japanese probe.

The Kaguya (Selene) mission surveyed Earth's satellite until a year ago.

Scientists report in Nature Geoscience that it saw exposures of rocks rich in the mineral olivine in concentric rings around craters.

They suggest that large impacts could have penetrated the Moon's outer crust, bringing into view the mantle olivine stored just below the surface.

The observations are said to fit well with ideas about how the Moon formed.

Current theory holds that Earth was hit by a Mars-sized body early in the evolution of the Solar System, and that the debris thrown into space by this impact coalesced into the Moon.

The sequence of mineral crystallisation in this ball of molten rock would have seen olivine (a magnesium iron silicate) produced before more dense materials, causing it to sink deep into the interior.

But this then led to a gravitationally unstable body with those more dense materials eventually displacing the olivine. As a consequence, the Moon's mantle, it is hypothesised, underwent an "overturning", in which the olivine was transferred to just below the crust.

That being the case, one might expect to see olivine in places where a thin crust has been broken - such as at impact craters - say Satoru Yamamoto, of the National Institute for Environmental Studies in Tsukuba, and colleagues.

"These craters with olivine-rich sites are very large, with diameters of several 100km to a 1,000km," Dr Yamamoto told ´óÏó´«Ã½ News.

"In this case, the excavation depth during the formation of these craters is about 100km. Thus, we propose that olivine-rich sites found here originate from deep-seated lunar mantle (as deep as 100km), which were excavated by gigantic impacts of huge meteoroids."

In the journal, Dr Yamamoto's team reports Kaguya's observations of strong olivine signals at 34 sites, including three previously reported. The sites include the South Pole-Aitken, Imbrium and Moscoviense impact basins.

Dr Yamamoto added: "I think that our new data give us an important piece of knowledge on the inside development of the Moon's mantle. Although there is a lot of information on the surface of the Moon, little is known about below the crust.

"Therefore, the structure and origin of the Moon's mantle have been debated for a long time. On the other hand, our new data provide important constraints on any models that try to show how the early crust and mantle of Moon-like bodies form and evolve."

The Japanese space agency (Jaxa) probe was launched in 2007. It orbited the Moon for one year and eight months, and famously returned the first high-definition movies of the lunar surface.

It was intentionally crashed into the Moon on 10 June last year.