The Moon took quite a beating in its early days, more than previously believed, scientists report.
This surprising new view of the Moon comes from detailed gravity mapping by twin Nasa spacecraft, which slipped into orbit around the celestial body earlier this year to peer into the interior.
Researchers have long known that the moon and rocky planets - including the Earth - suffered heavy bombardment from asteroids and comets during their formative years, billions of years ago. Now they are just starting to realise the extent.
The Moon is "far more broken up and shattered than we've seen before," said mission chief scientist Maria Zuber of the Massachusetts Institute of Technology.
Measurements by the Nasa spacecraft called Ebb and Flow also found that the Moon's crust, or outermost layer, is much thinner than scientists thought - only about 40km thick.
Results were presented Wednesday at a meeting of the American Geophysical Union meeting in San Francisco and published online in the journal Science.
Though past missions have measured lunar gravity - about one-sixth Earth's pull - Ebb and Flow are the first spacecraft dedicated to this pursuit.
To collect data, the washing machine-size spacecraft flew in formation, orbiting about 46km above the Moon's surface. Their positions allowed them to look deep inside.
The resulting gravity maps revealed an exceptionally smooth lunar interior - consistent with it being pulverised by impacts.
Fracturing extended deep into the crust and may have penetrated the mantle, scientists said.
The maps also exposed numerous lunar features in greater detail than before including volcanoes, basins and craters.
A provocative paper published last year hypothesized that Earth once had two moons that collided early on in the solar system's history to create the orb that graces the sky today. But Zuber said high-resolution mapping by the spacecraft did not find evidence for that.
The mission is scheduled to end later this month when Ebb and Flow crash into the Moon.
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