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The moon as solar system's Rosetta stone?

New research suggests that two distinct groups of objects battered the surface of the moon – and could give clues into the early days of Earth's solar system.

The full moon is shown in the early morning southwestern sky as seen from Tigard, Ore., Aug. 25.

Don Ryan/AP

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New observations of the moon's heavily cratered surface have moved planetary scientists a step closer to solving a cosmic "what done it": What were the objects involved in the intense assault on the moon and other bodies in the inner solar system some 4 billion years ago, and where did they come from?

An analysis of lunar craters published in Thursday's issue of the journal Science strongly suggests that two distinct groups of objects punished the lunar surface during a period known as the "late heavy bombardment."

One group, made up mainly of larger objects, appears to have dominated the initial phase of that bombardment, which began about 4.1 billion years ago. Smaller objects appear to have taken over the pummeling process around 3.8 billion year ago, and have been at it ever since.

This contrasts with a competing theory that the objects ricocheting through the solar system had the same source and size mix throughout the solar system's history, but that the numbers available for smacking planets has dwindled with time.

And while the driving force behind the shift in impactors remain unclear, the ability to spot the change allows scientists to begin hunting for the sources of each group and figure out what sent them on their collision course with the inner planets, explains James Head III, a Brown University planetary scientist who led the team reporting the results.

The possibilities include "large projectiles being moved out of the asteroid belt by late-stage adjustments in the orbits of the gas giants" such as Jupiter and Saturn, he said during a press briefing Thursday. "Maybe there were other populations – asteroids in some cases or comets in others."

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