From the Mars-size object that slammed into our planet 4.5 billion years ago, forming the moon, to a bombardment that boiled off early oceans as
recently as 2.5 billion years ago, Earth has taken some massive stonings in its lifetime. Now scientists think they know where the rocks were coming
from. In a paper published online today in Nature, planetary dynamicists finger the now-depleted inner edge of the asteroid belt, located just
outside the orbit of Mars. Researchers had previously proposed that Jupiter and Saturn wandered toward the sun about 4 billion years ago,
gravitationally slinging asteroids toward Earth as they went. But new computer simulations suggest that these planets would have also flung some
innermost asteroids into inclined, but not perfectly stable, orbits. Slowly, these asteroids escaped from these orbits, pummeling Earth for billions of years to come.
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