by Kim Luke
A team of physicists from the Massachusetts Institute of Technology and the University of Toronto have found new clues in the magnetic fields of ancient meteorites about conditions at the beginning of the solar system.
"Some surprising things were happening with planetisimals during the formation of the solar system" said Professor Sabine Stanley of U of T´s Department of Physics, one of the co-authors of the study, which was published in Science on Oct. 31.
Planetesimals are the chunks of rock that collided to form planets.
"We showed that it is likely that some planetesimals were melted, differentiated into core, mantle and crust and had active dynamos -- the mechanism by which planets generate magnetic fields -- for short periods of time in the early solar system. This means that planetesimals may have been more like mini-planets than just chunks of rock flying about.
"We´ve long known that the dust and rubble in a disk around the sun collided and stuck together forming ever-larger rocks and eventually the planets we know today. What´s new here is that we found that even objects that are much smaller than planets -- just 160 kilometers across or so -- could melt almost completely and have cores capable of generating magnetic fields," Stanley said.
This total melting of planetesimals caused their constituents to separate out, with lighter materials including silicates floating to the surface and eventually forming a crust, while heavier iron-rich material sank down to the core, where it began swirling around to produce a magnetic dynamo. The crust recorded the planetesimal´s magnetic field as it cooled and pieces of this crust eventually became meteorites which fell to Earth. The researchers identified strong magnetic fields produced by that dynamo in a group of ancient meteorites called Angrites. Some of these rocks formed just three million years after the birth of the solar system 4,568 million years ago.
"This goes a long way to solving the mystery of magnetism in meteorites," Stanley said.
The Science paper´s lead author is Benjamin Weiss from MIT. Other MIT co-authors include Linda Elkins-Tanton, Mitsui Career Development Assistant Professor of Geology, research scientist Eduardo Lima, post-doctoral researcher Laurent Carpozen and student James Berdahl.