Monkey Wrench In Solar System Evolution

A paper published today inthe August issue of Nature reports that the youngest known chondrules –the small grains of mineral that make up certain meteorites – have beenidentified in the meteorites known as Gujba and Hammadah al Hamra.

Researcherswho have studied chondrules generally agree that most were formed as asudden, repetitive heat, likely from a shock wave, condensed the nebulaof dust floating around the early Sun. Thinking that an analysis of thechondrules in Gujba and Hammadah al Hamra would be appropriate foraccurately dating this process, U of T geologist Yuri Amelin, togetherwith lead author Alexander Krot of the University of Hawaii, studiedthe chondrules’ mineralogical structure and determined their isotopicage. “It soon became clear that these particular chondrules were not ofa nebular origin,” says Amelin. “And the ages were quite different fromwhat was expected. It was exciting.”

Amelin explains that notonly were these chondrules not formed by a shock wave, but ratheremerged much later than other chondrules. “They actually post-date theoldest asteroids,” he says. “We think these chondrules were formed by agiant plume of vapour produced when two planetary embryos, somewherebetween moon-size and Mars-size, collided.”

What does this meanin the grand scheme of things? The evolution of the solar system hastraditionally been seen as a linear process, through which gases aroundthe early sun gradually cooled to form small particles that eventuallyclumped into asteroids and planets. Now there is evidence of chondrulesforming at two very distinct times, and evidence that embryo planetsalready existed when chondrules were still forming. “It moves ourunderstanding from order to disorder,” Amelin admits. “But I’m surethat as new data is collected, a new order will emerge.”

Financial support for this project was provided by NASA and the Canadian Space Agency.