Posted: 05 November, 2008
Magnetic records frozen into the cores of ancient meteorites have provided fresh insight into the planetary forming conditions at the beginning of the Solar System.
Benjamin Weiss of MIT and colleagues studied a group of the oldest known meteorites – angrites, basaltic rocks likely derived from main belt asteroids – to solve a longstanding mystery regarding the way planets form. The key result is that small planetary building blocks around 160 kilometres in diameter were still large enough to melt, separating out into a light crust and a heavier core. This heavy, iron-rich material began to turn over to produce a magnetic dynamo, the traces of which are still preserved in the meteorites that fell to Earth.
iPlanetary building blocks could have been differentiated into mini-planets with core, mantle and crust. Remnant magnetism from these planetesimals has been detected in ancient meteorites that fell to Earth. Image: NASA/JPL-Caltech.
"The magnetism in meteorites has been a longstanding mystery,” says Weiss. Indeed, until relatively recently, it was commonly thought that planetesimals — similar to the asteroids seen in the Solar System today — that came together to build planets were just homogenous, unmelted rocky material, with no large-scale structure. "Now we're realising that many of the things that were forming planets were mini-planets themselves, with crusts and mantles and cores."
The revelation has the potential to change key theories regarding the formation and evolution of planetary bodies in the early Solar System. Specifically, if the smaller bodies were already molten as they slammed together to build up larger planet-sized bodies, that could have implications for how different minerals are distributed in the Earth's crust, mantle and core today.
"In the last five or ten years our understanding of the early history of the Solar System has undergone a sort of mini-revolution, driven by analytical advances in geochemistry,” says Weiss. “In this study we used a geophysical technique to independently test many of these new ideas."
Events in the nascent Solar System took place at a fast rate and the fact that some of the angrite meteorites used in this study formed just three million years after the birth of the Solar System and show signs that their parent body had a magnetic field that was 20 to 40 percent as strong as Earth's today, has serious implications for the development of magnetic fields on planets.
"We are used to thinking of dynamo magnetic fields in rocky bodies as uncommon phenomena today,” says Weiss. “But it may be that short-lived planetesimal dynamos were widespread in the early Solar System." Because the magnetic record preserved in the angrite meteorites extends beyond the expected lifetime of the circumstellar disc, the magnetic fields must have been generated inside the body from which the meteorites were derived, possibly by an early magnetic dynamo in the planetesimal’s rapidly formed metallic core.
The results of the study are published in the 31 October edition of the journal Science.