行星起源新见解

Meteors that have crashed to Earth have long been regarded as relics¹ of the early solar system. These craggy chunks² of metal and rock are studded with chondrules -- tiny, glassy, spherical³ grains that were once molten droplets⁴. Scientists have thought that chondrules represent early kernels⁵ of terrestrial planets: As the solar system started to coalesce⁶, these molten droplets collided with bits of gas and dust to form larger planetary precursors⁷. However, researchers at MIT and Purdue University have now found that chondrules may have played less of a fundamental role. Based on computer simulations, the group concludes that chondrules were not building blocks, but rather byproducts of a violent and messy planetary process.

 

The team found that bodies as large as the moon likely existed well before chondrules came on the scene. In fact, the researchers found that chondrules were most likely created by the collision of such moon-sized planetary embryos⁸: These bodies smashed together with such violent force that they melted a fraction of their material, and shot a molten plume⁹ out into the solar nebula¹⁰. Residual¹¹ droplets would eventually cool to form chondrules, which in turn attached to larger bodies -- some of which would eventually impact Earth, to be preserved as meteorites¹².

 

Brandon Johnson, a postdoc in MIT's Department of Earth, Atmospheric¹³ and Planetary Sciences, says the findings revise one of the earliest chapters of the solar system.

 

"This tells us that meteorites aren't actually representative of the material that formed planets -- they're these smaller fractions of material that are the byproduct of planet formation," Johnson says. "But it also tells us the early solar system was more violent than we expected: You had these massive sprays of molten material getting ejected out from these really big impacts. It's an extreme process."

 

Johnson and his colleagues, including Maria Zuber, the E.A. Griswold Professor of Geophysics and MIT's vice¹⁴ president for research, have published their results this week in the journal Nature.

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