Planetary 'Eggbeaters' Stirred Up Early Solar System
(From the September/October 2005 issue of StarDate magazine)
The solar system today is relatively tranquil. But the young solar system was turbulent and chaotic. And although it quickly settled down, there may have been one final bout of violence almost four billion years ago.
Evidence of this violent period is found in the Moon's dark features, which are "seas" of volcanic rock. Most of them formed when the solar system was no more than 700 million years old, when the Moon was pounded by a barrage of giant asteroids. This same barrage hit Earth and the other planets of the inner solar system. There's no evidence on Earth, though, because wind, rain, and the motions of the crust have erased such ancient features.
A team of astronomers recently reported that this bombardment may have taken place because the giant outer planets stirred things up.
The young solar system contained lots of leftover "planetesimals" -- the balls of rock and ice that merged to form the planets. Gravitational interactions with the giant outer planets pushed around these leftovers, hurling many of them into orbits far from the Sun. These interactions altered the orbits of the planets, too, stretching them and moving them around.
An international team of astronomers recently proposed that when the solar system was about 700 million years old, these orbits carried the giant planets into the realm of the leftover planetesimals. This scattered the planetesimals like chocolate chips caught in the blades of an eggbeater, sending some plunging toward the Sun where they could smack into the Moon and planets.
Over time, the gravitational encounters smoothed out the orbits of the outer planets, ending their turbulent churning of the region of the planetesimals. -- Damond Benningfield
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