Meteorites
that are among the oldest rocks ever found have provided new clues about the
conditions that existed at the beginning of the solar system, solving a
longstanding mystery and overturning some accepted ideas about the way planets
form.
The ancient
meteorites, called angrites, still contain magnetic records about the very
early history of planets, like disk drives salvaged from an ancient computer, new
research by MIT planetary scientist Benjamin P. Weiss indicates.
The results
of the study, which was by a grant from the National Science Foundation's
Instrumentation and Facilities Program, are detailed in the Oct. 31 issue of
the journal Science.
The
analysis showed that surprisingly, during the formation of the solar
system, when dust and
rubble in a disk around the sun collided and stuck together to form
ever-larger rocks and eventually the planets we know today, even objects much
smaller than planets — just 100 miles (160 kilometers) across or so — were
large enough to melt almost completely.
This total
melting of the planet-forming chunks of rock, called planetesimals, caused
their constituents to separate out, with lighter materials including silicates
floating to the surface and eventually forming a crust, while heavier iron-rich
material sank down to the core, where it began swirling around to produce a
magnetic dynamo. The researchers were able to study traces of the magnetic
fields produced by that dynamo, now recorded in the meteorites that fell to
Earth.
"The
magnetism in meteorites has been a longstanding mystery," Weiss said, and
the realization that such small bodies could have melted and formed magnetic
dynamos is a major step toward solving that riddle.
Until
relatively recently, it was commonly thought that the 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," Weiss said. "Now we're realizing that many of
the things that were forming planets were mini-planets themselves, with crusts
and mantles and cores."
That could
change theorists' picture of how the planets themselves took
shape.
If the
smaller bodies were already molten as they slammed together to build up larger
planet-sized bodies, that could "significantly change our
understanding" of the processes that took place in the early years of the
nascent planets, as their internal structures were forming, Weiss said. This
could have implications for how different minerals are distributed in the
Earth's crust, mantle and core today, for example.
"Events
happened surprisingly fast at the beginning of the solar system," Weiss
said. Some of the angrite meteorites in this study formed just 3 million years
after the birth of the solar system itself, 4,568 million years ago, and show
signs that their parent body had a magnetic field that was 20 to 40 percent as
strong as Earth's today.
"We
are used to thinking of dynamo magnetic fields in rocky bodies as uncommon
phenomena today," Weiss said. "But it may be that short-lived
planetesimal dynamos were widespread in the early solar system."
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