An origin story for a family of rare meteorites

Most of the meteorites that have landed on Earth are fragments of planetesimals, the first protoplanetary bodies in the solar system. Scientists have thought that these primordial bodies either completely melted early in their history or were left as heaps of unmelted rubble.

But a family of meteorites has confused researchers since their discovery in the 1960s. The various fragments, found worldwide, appear to have detached from the same primordial body, and yet the composition of these meteorites indicates that his parents must have been a baffling chimera that melted and did not melt.

Now researchers at MIT and elsewhere have determined that the main body of these rare meteorites was in fact a multi-layered distinct object that likely had a liquid metallic core. This nucleus was substantial enough to generate a magnetic field that may have been as strong as Earth’s magnetic field today.

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

“This is an example of a planetesimal that must have had molten and non-molten layers. It encourages the search for more evidence of composite planetary structures,” says lead author Clara Maurel, a graduate student in the Department of Earth Sciences, Atmospheres and Planetary MIT (EAPS). “Understanding the full spectrum of structures, from unmelted to fully molten, is key to deciphering how planetesimals formed in the early solar system.”

Maurel’s co-authors include Professor EAPS Benjamin Weiss, along with collaborators from the University of Oxford, the University of Cambridge, the University of Chicago, the Lawrence Berkeley National Laboratory, and the Southwest Research Institute.

Odd Irons

The solar system formed about 4.5 billion years ago as a whirlpool of super hot gas and dust. As this disk gradually cooled, chunks of matter collided and merged to form progressively larger bodies, such as planetesimals.

Most of the meteorites that have fallen to Earth have compositions that suggest they came from planetesimals so early that they were of two types: molten and non-molten. Scientists believe that both types of objects would have formed relatively quickly, in less than a few million years, early in the evolution of the solar system.

If a planetesimal formed in the first 1.5 million years of the solar system, the short-lived radiogenic elements could have completely melted the body due to the heat released by its decomposition. Unmelted planetesimals could have formed later, when their material had lower amounts of radiogenic elements, insufficient to melt.

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

“These IIE irons are strange meteorites,” says Weiss. “They show evidence of being from primordial objects that never melted, and also evidence that they come from a body that is completely or at least substantially melted. We have not known where to place them, and that is what made us focus on them.” “

Magnetic pockets

Scientists have previously discovered that molten and unfused IIE meteorites originated from the same ancient planetesimal, which likely had a solid crust on top of a liquid mantle, like Earth. Maurel and his colleagues wondered if the planetesimal could also have housed a molten metal core.

“Did this object melt enough for the material to sink into the center and form a metal core like Earth’s?” Maurel says. “That was the missing piece in the history of these meteorites.”

The team reasoned that if the planetesimal harbored a metallic core, it could very well have generated a magnetic field, similar to the way the Earth’s liquid core produces a magnetic field. Such an ancient field could have caused the minerals in the planetesimal to point in the direction of the field, like a needle on a compass. Certain minerals could have maintained this alignment for billions of years.

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

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 magnetic direction of the minerals.

Indeed, the electrons within various grains were aligned in a similar direction, evidence that the parent body generated a magnetic field, possibly up to several tens of microteslas, which is the strength of Earth’s magnetic field. After ruling out less plausible sources, the team concluded that the magnetic field was likely produced by a liquid metal core. To generate this field, they estimate that the nucleus must have been at least several tens of kilometers wide.

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

But where do meteorites come from inside the parent’s body? If the magnetic field was generated by the core of the main body, this would mean that the fragments that ultimately fell to Earth could not have come from the core itself. This is because a liquid core only generates a magnetic field while it is stirred and hot. Any ore that the ancient field would have recorded must have done so outside the core, before the core itself has completely cooled down.

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

“As the body cools, the meteorites in these pockets will imprint this magnetic field on their minerals. At some point, the magnetic field will decompose, but the footprint will remain,” says Maurel. “Later, this body will suffer many other collisions until the final collisions that will place these meteorites in the path of Earth.”

Was such a complex planetesimal an atypical case in the early solar system, or one of the many differentiated objects? The answer, Weiss says, may lie in the asteroid belt, a region populated with primordial remains.

“Most of the bodies in the asteroid belt appear unmelted on their surface,” says Weiss. “If we could finally see inside asteroids, we could test this idea. Maybe some asteroids melt inside, and bodies like this planetesimal are really common.”