MASSACHUSETTS INSTITUTE OF TECHNOLOGY: A twist on planetary origins
New study finds meteorites were byproducts of planetary formation, not building blocks. Jennifer Chu | MIT News Office
Meteors that have crashed to Earth have long been
regarded as relics of the early solar system. These craggy chunks of metal and
rock are studded with chondrules — tiny, glassy, spherical grains that were
once molten droplets. Scientists have thought that chondrules represent early
kernels of terrestrial planets: As the solar system started to coalesce, these
molten droplets collided with bits of gas and dust to form larger planetary
precursors.
However, researchers at MIT and Purdue University have
now found that chondrules may have played less of a fundamental role. Based on
computer simulations, the group concludes that chondrules were not building
blocks, but rather byproducts of a violent and messy planetary process.
The team found that bodies as large as the moon likely
existed well before chondrules came on the scene. In fact, the researchers
found that chondrules were most likely created by the collision of such
moon-sized planetary embryos: These bodies smashed together with such violent
force that they melted a fraction of their material, and shot a molten plume
out into the solar nebula. Residual droplets would eventually cool to form
chondrules, which in turn attached to larger bodies — some of which would
eventually impact Earth, to be preserved as meteorites.
Brandon Johnson, a postdoc in MIT’s Department of Earth,
Atmospheric and Planetary Sciences, says the findings revise one of the
earliest chapters of the solar system.
“This tells us that meteorites aren’t actually
representative of the material that formed planets — they’re these smaller
fractions of material that are the byproduct of planet formation,” Johnson
says. “But it also tells us the early solar system was more violent than we
expected: You had these massive sprays of molten material getting ejected out
from these really big impacts. It’s an extreme process.”
Johnson and his colleagues, including Maria Zuber, the
E.A. Griswold Professor of Geophysics and MIT’s vice president for research,
have published their results this week in the journalNature.
High-velocity
molten rock
To get a better sense of the role of chondrules in a
fledgling solar system, the researchers first simulated collisions between
protoplanets — rocky bodies between the size of an asteroid and the moon. The
team modeled all the different types of impacts that might occur in an early
solar system, including their location, timing, size, and velocity. They found
that bodies the size of the moon formed relatively quickly, within the first
10,000 years, before chondrules were thought to have appeared.
Johnson then used another model to determine the type of
collision that could melt and eject molten material. From these simulations, he
determined that a collision at a velocity of 2.5 kilometers per second would be
forceful enough to produce a plume of melt that is ejected out into space — a
phenomenon known as impact jetting.
“Once the two bodies collide, a very small amount of
material is shocked up to high temperature, to the point where it can melt,”
Johnson says. “Then this really hot material shoots out from the collision
point.”
The team then estimated the number of impact-jetting
collisions that likely occurred in a solar system’s first 5 million years — the
period of time during which it’s believed that chondrules first appeared. From
these results, Johnson and his team found that such collisions would have
produced enough chondrules in the asteroid belt region to explain the number
that have been detected in meteorites today.
Falling into
place
To go a step further, the researchers ran a third
simulation to calculate chondrules’ cooling rate. Previous experiments in the
lab have shown that chondrules cool down at a rate of 10 to 1,000 kelvins per
hour — a rate that would produce the texture of chondrules seen in meteorites.
Johnson and his colleagues used a radiative transfer model to simulate the
impact conditions required to produce such a cooling rate. They found that
bodies colliding at 2.5 kilometers per second would indeed produce molten droplets
that, ejected into space, would cool at 10 to 1,000 kelvins per hour.
“Then I had this ‘Eureka!’ moment where I realized that
jetting during these really big impacts could possibly explain the formation of
chondrules,” Johnson says. “It all fell into place.”
Going forward, Johnson plans to look into the effects of
other types of impacts. The group has so far modeled vertical impacts — bodies
colliding straight-on. Johnson predicts that oblique impacts, or collisions
occurring at an angle, may be even more efficient at producing molten plumes of
chondrules. He also hopes to explore what happens to chondrules once they are
launched into the solar nebula.
“Chondrules were long viewed as planetary building
blocks,” Zuber notes. “It’s ironic that they now appear to be the remnants of
early protoplanetary collisions.”
Fred Ciesla, an associate professor of planetary science
at the University of Chicago, says the findings may reclassify chondrites, a
class of meteorites that are thought to be examples of the original material
from which planets formed.
“This would be a major shift in how people think about
our solar system,” says Ciesla, who did not contribute to the research. “If
this finding is correct, then it would suggest that chondrites are not good
analogs for the building blocks of the Earth and other planets. Meteorites as a
whole are still important clues about what processes occurred during the
formation of the solar system, but which ones are the best analogs for what the
planets were made out of would change.”
This research was funded in part by NASA.
