Scientists working on a primitive meteorite known as Semarkona have found evidence that the protoplanetary disk of the early Solar System was shaped by extremely strong magnetic fields that drove a massive amount of gas into the Sun within just a few million years.
Magnetic field lines weave through the cloud of dusty gas surrounding the young Sun; in the foreground are asteroids and chondrules, the building blocks of chondrite meteorites. Image credit: Hernan Canellas / MIT Paleomagnetism Laboratory.
It may seem all but impossible to determine how our Solar System formed, given it happened about 4.5 billion years ago. But making the Solar System was a messy process, leaving lots of construction debris behind for planetary scientists to study.
Among the most useful pieces of debris are the oldest, most primitive type of meteorites, called the chondrites.
Chondrite meteorites are pieces of asteroids that have remained relatively unmodified since they formed at the birth of the Solar System. They are built mostly of small stony grains, called chondrules, barely a millimeter in diameter.
Chondrules themselves formed through quick melting events in the dusty gas cloud – the Solar Nebula – that surrounded the young Sun. Patches of the nebula must have been heated above the melting point of rock for hours to days.
Dustballs caught in these events made droplets of molten rock, which then cooled and crystallized into chondrules.
As chondrules cooled, iron-bearing minerals within them became magnetized like bits on a hard drive by the local magnetic field in the gas.
These magnetic fields are preserved in the chondrules even down to the present day.
The chondrule grains whose magnetic fields were measured by Roger Fu from Massachusetts Institute of Technology and his colleagues came from Semarkona, a space rock that crashed in northern India in 1940.
In their experiments, the scientists extracted individual chondrules from a small sample of the meteorite, and measured the magnetic orientations of each grain to determine that, indeed, the meteorite was unaltered since its formation.
They then measured the magnetic strength of each grain, and calculated the original magnetic field in which those grains were created.
They determined that the early Solar System harbored a magnetic field as strong as 5 to 54 microteslas – up to 100,000 times stronger than what exists in interstellar space today.
Such a magnetic field would be strong enough to drive gas toward the Sun at an extremely fast rate.
“Modeling for the heating events shows that shock waves passing through the Solar Nebula is what melted most chondrules. Depending on the strength and size of the shock wave, the background magnetic field could be amplified by up to 30 times,” said team member Dr Steven Desch of Arizona State University.
“Given the measured magnetic field strength of about 54 microtesla, this shows the background field in the nebula was probably in the range of 5 to 50 microtesla.”
There are other ideas for how chondrules might have formed, some involving magnetic flares above the Solar Nebula, or passage through the Sun’s magnetic field. But those mechanisms require stronger magnetic fields than what is measured in the Semarkona samples.
This reinforces the idea that shocks melted the chondrules in the Solar Nebula at about the location of today’s asteroid belt, which lies some two to four times farther from the Sun than Earth now orbits.
“This is the first really accurate and reliable measurement of the magnetic field in the gas from which our planets formed,” concluded Dr Desch, who is a co-author of the paper published in the journal Science.
_____
Roger R. Fu et al. Solar nebula magnetic fields recorded in the Semarkona meteorite. Science, published online November 13, 2014; doi: 10.1126/science.1258022
