Magnetic field of ancient meteorite holds clues to solar system formation

© MIT Paleomagnetism Laboratory

Magnified image of the section of the Semarkona meteorite used in this study. Chondrules are millimeter sized, light-colored objects.

By analyzing a meteorite that crash-landed in India eight decades ago, researchers have discovered the first experimental evidence suggesting that our solar system's protoplanetary disk was shaped by an intense magnetic field which propelled massive amounts of gas into the sun over the course of just a few million years.

In the study, MIT graduate student Roger Fu and colleagues from Cambridge University, Arizona State University and elsewhere studied a space rock known as a Semarkona, which fell to Earth in northern India back in 1940 and is said to be one of the most pristine relics of the early solar system. They extracted individual pellets known as chondrules from a small sample of the meteorite and measured the magnetic orientations of each grain.

As the study authors reported Friday in the journal , they found that the meteorite had not been altered since its formation. With that established, they then measured the magnetic strength of each chondrule and calculated the original magnetic field in which those grains were created. Their calculations revealed the early solar system's magnetic field was between five and 54 microteslas, or up to 100,000 times stronger than what currently exists in interstellar space.

While astronomers have long observed the process of protoplanetary disk evolution throughout the galaxy, the mechanism by which planetary disks evolved has remained a mystery to scientists for several decades, the researchers said. Based on their measurements, however, the researchers believe that the magnetic field would likely have played a major role in the formation of the solar system and of Earth-like planets.

© Science

Magnetic field lines (green) weave through the cloud of dusty gas surrounding the newborn Sun. In the foreground are asteroids and chondrules, the building blocks of chondritic meteorites. While solar magnetic fields dominate the region near the Sun, out where the asteroids orbit, chondrules preserve a record of varying local magnetic fields.

"Explaining the rapid timescale in which these disks evolve - in only a few million years - has always been a big mystery," Fu, a graduate student in MIT's Department of Earth, Atmospheric and Planetary Sciences, said in a statement Friday. "It turns out that this magnetic field is strong enough to affect the motion of gas at a large scale, in a very significant way."

The chondrules which comprise the Semarkona meteorite would have formed as a result of quick melting events in the solar nebula, the thick cloud of gas and dust that surrounded the newborn Sun, the researchers explained. Regions of the solar nebula would have had to have been heated above the melting point of rock for a period of several hours up to days, causing trapped dustballs to become molten rock, cool and crystallize.

As the pellets cooled, iron-bearing minerals contained within them would have become magnetized by the local magnetic field in the gas, and those properties have been preserved in the chondrules even today. Fu and his colleagues focused specifically on the embedded magnetic fields captured by "dusty" olivine grains that contain abundant iron-bearing minerals, which had a magnetic field of 54 microtesla - similar to that found at Earth's surface (which varied from 25 to 65 microtesla).

While previous measurements of meteorites suggested that they had magnetic fields of similar strength, it has now been realized that those data had detected magnetic minerals which had been contaminated by Earth's magnetic field or by hand magnets used by meteorite collectors. The new experiments, the study authors said, use chondrules that had never been measured before, and that they had become magnetized before becoming part of the meteorite.

"The measurements made by Fu and Weiss are astounding and unprecedented," said co-author Steve Desch of Arizona State University's School of Earth and Space Exploration. "Not only have they measured tiny magnetic fields thousands of times weaker than a compass feels, they have mapped the magnetic fields' variation recorded by the meteorite, millimeter by millimeter."

"The new experiments probe magnetic minerals in chondrules never measured before. They also show that each chondrule is magnetized like a little bar magnet, but with 'north' pointing in random directions," he added, noting that the shock waves traveling through the solar nebula was what melted most of the chondrules, amplifying the background field by as much as 30 times depending on their size and shape. "This is the first really accurate and reliable measurement of the magnetic field in the gas from which our planets formed."