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Study: Colliding planets don't always stick together
Scripps Howard News Service


January 13, 2006
Friday AM

The inner solar system's rocky planets are largely products of a cosmic bumper-car game lasting tens of millions of years, with collisions sometimes merging two bodies but often ending up as hit-and-run affairs, researchers report in a new study.

A team led by Erik Asphaug at the University of California-Santa Cruz used computers to run simulations of various scenarios, from grazing encounters to direct hits involving planets of comparable sizes to Earth, Mars, Venus and Mercury.




Over the years, scientists had mostly viewed the collisions in terms of the planet that got hit and primarily whether material was added (accretion) or pulled away, as many think is the case with Earth's moon after a collision with a Mars-sized object within 100 million years of the sun's formation.

The inner planets began from a disc of gas and dust around the sun that gradually clumped together until the inner solar system hosted as many as 100 moon- to Mars-sized planets, Asphaug said.

Gravitational interactions with each other and with Jupiter, the hydrogen giant of the outer part of the system, tossed those protoplanets out of their circular orbits, setting off an era of giant impacts that went on for 30 million to 50 million years.

But the new study, published Thursday in the journal Nature, points out that when planets collide, they don't always stick together. "It's actually not easy for planets to merge," Asphaug said. "Our calculations show they have to be moving fairly slowly and hit almost head-on in order to accrete."

About half the time, a planet-sized impactor that hits another planet-sized object bounces away, but such bumps have dire consequences for the smaller planet, the modeling showed.

"You end up with planets that leave the scene of the crime looking very different from when they came in. They can lose their atmosphere, crust, even the mantle, or they can be ripped apart into a family of smaller objects," Asphaug said.

Remnants of those shattered impactors can be found throughout the asteroid belt and in meteorites, fragments of other planetary bodies that have landed on Earth.

Even the planet Mercury may have once been a hit-and-run body whose collision with something bigger left it with a relatively large core and thin crust and mantle, Asphaug said, although he cautions that this scenario needs more study.

After Asphaug and postdoctoral researcher Craig Agnor ran the computer scenarios, earth-sciences professor Quentin Williams analyzed the simulations to determine what the compositions and states of remnant objects might have been.

The researchers observed that even close encounters between two big objects in space can severely affect the smaller object, a finding that scientists will need to consider if Earth is ever on a near-miss course with a large asteroid in the future.

"As two massive objects pass near each other, gravitational forces induce dramatic physical changes - decompressing, melting, stripping material away and even annihilating the smaller object," Williams said.

While planets are held together under the enormous pressure of their own gravity, the gravitational pull from a larger object passing nearby can make that pressure drop precipitously, with explosive effects.

"It's like uncorking the world's most carbonated beverage," Williams said. "What happens when a planet gets decompressed by 50 percent is something we don't understand very well as this stage, but it can shift the chemistry and physics all over the place."

A variety of hit-and-run collisions and near-misses may explain why asteroids come in many different shapes and compositions. In particular, Williams said, sudden depressurization could explain why some asteroids appear to have melted at some point in their past.

"If pressure drops by a factor of two, you can go from something that is merely hot to something molten," he said.


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Contact Lee Bowman at BowmanL(at)
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