An origin story for a family of oddball meteorites
Study suggests the rare objects likely came from an early planetesimal
with a magnetic core

Date:
July 24, 2020
Source:
Massachusetts Institute of Technology
Summary:
Most meteorites that have landed on Earth are fragments of
planetesimals, the very earliest protoplanetary bodies in the
solar system. Scientists have thought that these primordial bodies
either completely melted early in their history or remained as
piles of unmelted rubble. But a family of meteorites has befuddled
researchers since its discovery in the 1960s.

The diverse fragments, found all over the world, seem to have
broken off from the same primordial body, and yet the makeup of
these meteorites indicates that their parent must have been a
puzzling chimera that was both melted and unmelted. A new study
suggests a family of rare meteorites likely came from an early
planetesimal with a magnetic core.



FULL STORY ==========================================================================
Most meteorites that have landed on Earth are fragments of planetesimals,
the very earliest protoplanetary bodies in the solar system. Scientists
have thought that these primordial bodies either completely melted early
in their history or remained as piles of unmelted rubble.


==========================================================================
But a family of meteorites has befuddled researchers since its discovery
in the 1960s. The diverse fragments, found all over the world, seem to
have broken off from the same primordial body, and yet the makeup of
these meteorites indicates that their parent must have been a puzzling
chimera that was both melted and unmelted.

Now researchers at MIT and elsewhere have determined that the parent body
of these rare meteorites was indeed a multilayered, differentiated object
that likely had a liquid metallic core. This core was substantial enough
to generate a magnetic field that may have been as strong as Earth's
magnetic field is today.

Their results, published in the journal Science Advances, suggest that
the diversity of the earliest objects in the solar system may have been
more complex than scientists had assumed.

"This is one example of a planetesimal that must have had melted and
unmelted layers. It encourages searches for more evidence of composite planetary structures," says lead author Clara Maurel, a graduate
student in MIT's Department of Earth, Atmospheric, and Planetary Sciences (EAPS). "Understanding the full spectrum of structures, from nonmelted to
fully melted, is key to deciphering how planetesimals formed in the early
solar system." Maurel's co-authors include EAPS professor Benjamin Weiss, along with collaborators at Oxford University, Cambridge University,
the University of Chicago, Lawrence Berkeley National Laboratory, and
the Southwest Research Institute.



========================================================================== Oddball irons The solar system formed around 4.5 billion years ago
as a swirl of super-hot gas and dust. As this disk gradually cooled,
bits of matter collided and merged to form progressively larger bodies,
such as planetesimals.

The majority of meteorites that have fallen to Earth have compositions
that suggest they came from such early planetesimals that were either
of two types: melted, and unmelted. Both types of objects, scientists
believe, would have formed relatively quickly, in less than a few million years, early in the solar system's evolution.

If a planetesimal formed in the first 1.5 million years of the solar
system, short-lived radiogenic elements could have melted the body
entirely due to the heat released by their decay. Unmelted planetesimals
could have formed later, when their material had lower quantities of
radiogenic elements, insufficient for melting.

There has been little evidence in the meteorite record of intermediate
objects with both melted and unmelted compositions, except for a rare
family of meteorites called IIE irons.



========================================================================== "These IIE irons are oddball meteorites," Weiss says. "They show
both evidence of being from primordial objects that never melted,
and also evidence for coming from a body that's completely or at least substantially melted. We haven't known where to put them, and that's what
made us zero in on them." Magnetic pockets Scientists have previously
found that both melted and unmelted IIE meteorites originated from the
same ancient planetesimal, which likely had a solid crust overlying a
liquid mantle, like Earth. Maurel and her colleagues wondered whether
the planetesimal also may have harbored a metallic, melted core.

"Did this object melt enough that material sank to the center and formed a metallic core like that of the Earth?" Maurel says. "That was the missing
piece to the story of these meteorites." The team reasoned that if the planetesimal did host a metallic core, it could very well have generated
a magnetic field, similar to the way Earth's churning liquid core produces
a magnetic field. Such an ancient field could have caused minerals in the planetesimal to point in the direction of the field, like a needle in a compass. Certain minerals could have kept this alignment over billions
of years.

Maurel and her colleagues wondered whether they might find such minerals
in samples of IIE meteorites that had crashed to Earth. They obtained
two meteorites, which they analyzed for a type of iron-nickel mineral
known for its exceptional magnetism-recording properties.

The team analyzed the samples using the Lawrence Berkeley National
Laboratory's Advanced Light Source, which produces X-rays that interact
with mineral grains at the nanometer scale, in a way that can reveal
the minerals' magnetic direction.

Sure enough, the electrons within a number of grains were aligned in a
similar direction -- evidence that the parent body generated a magnetic
field, possibly up to several tens of microtesla, which is about the
strength of Earth's magnetic field. After ruling out less plausible
sources, the team concluded that the magnetic field was most likely
produced by a liquid metallic core. To generate such a field, they
estimate the core must have been at least several tens of kilometers wide.

Such complex planetesimals with mixed composition (both melted, in the
form of a liquid core and mantle, and unmelted in the form of a solid
crust), Maurel says, would likely have taken over several million years
to form -- a formation period that is longer than what scientists had
assumed until recently.

But where within the parent body did the meteorites come from? If the
magnetic field was generated by the parent body's core, this would mean
that the fragments that ultimately fell to Earth could not have come
from the core itself. That's because a liquid core only generates a
magnetic field while still churning and hot. Any minerals that would
have recorded the ancient field must have done so outside the core,
before the core itself completely cooled.

Working with collaborators at the University of Chicago, the team ran
high- velocity simulations of various formation scenarios for these
meteorites. They showed that it was possible for a body with a liquid
core to collide with another object, and for that impact to dislodge
material from the core. That material would then migrate to pockets
close to the surface where the meteorites originated.

"As the body cools, the meteorites in these pockets will imprint this
magnetic field in their minerals. At some point, the magnetic field
will decay, but the imprint will remain," Maurel says. "Later on, this
body is going to undergo a lot of other collisions until the ultimate collisions that will place these meteorites on Earth's trajectory."
Was such a complex planetesimal an outlier in the early solar system,
or one of many such differentiated objects? The answer, Weiss says,
may lie in the asteroid belt, a region populated with primordial remnants.

"Most bodies in the asteroid belt appear unmelted on their surface," Weiss says. "If we're eventually able to see inside asteroids, we might test
this idea. Maybe some asteroids are melted inside, and bodies like this planetesimal are actually common." This research was funded, in part,
by NASA.


========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Clara Maurel, James F. J. Bryson, Richard J. Lyons, Matthew R. Ball,
Rajesh V. Chopdekar, Andreas Scholl, Fred J. Ciesla, William
F. Bottke and Benjamin P. Weiss. Meteorite evidence for partial
differentiation and protracted accretion of planetesimals. Science
Advances, 2020 DOI: 10.1126/sciadv.aba1303 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200724141349.htm

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