An artist’s rendering of a protoplanetary impact. Early in the impact, molten jetted material is ejected at a high velocity and breaks up to form chondrules, the millimeter-scale, formerly molten droplets found in most meteorites. These droplets cool and solidify over hours to days.
Image: NASA/California Institute of Technology
New study finds meteorites were byproducts of planetary formation, not building blocks
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 journal Nature.
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, 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.
Jennifer Chu | MIT News Office
Source: MIT News
