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If you think you've got problems, pal, spare a thought for these boffins baffled by 'oddball' meteorites

Chondritic and achondritic? Back to the drawing board, then

Meteorites discovered strewn across Earth after the 1960s may have broken off the same planetesimal body. What’s more, scientists have discovered signs this ancient body had a molten core, which turns some cosmological thinking on its head.

The boffins studied two of these specimens, named Colomera and Techado. The pair belong to the IIE iron meteorites, a group of 24 rocks, we're told, believed to have formed from a single parent, and ended up scattered across our planet. It's likely a boulder in the Solar System's asteroid belt smashed into something else, and shards from that cosmic prang, including Colomera and Techado, fell to Earth.

Scientists typically split stony meteorites into two types: chondritic, which haven't melted; and achondritic, which did melt at one point and contain a mishmash of minerals. The team believe that the parent planetesimal that birthed the IIE iron meteorites, however, is both chondritic and achondritic. It may even have had a liquid metallic core, a sign that planetesimals are more complex than previously thought.

“These IIE irons are oddball meteorites,” said MIT prof Benjamin Weiss, co-author of a paper into the rocks, published this month in Science Advances. “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.”

“Did this object melt enough that material sank to the center and formed a metallic core like that of the Earth?,” Clara Maurel, first author of the study and a graduate student at MIT, added. “That was the missing piece to the story of these meteorites.”

Magnetic clues to birth

The team analysed Colomera and Techado for signs of an iron-nickel-based mineral, known for its magnetism, using X-ray scattering. The scans revealed electrons within some of the mineral grains were aligned in the same direction, evidence that the parent planetesimal generated a magnetic field.

They speculated that the magnetic field was formed from a molten core. The planetesimal must have formed over several million years, with an inner achondritic layer and an outer chondritic layer. Simulations show that collisions with the planetesimal could have knocked material from its inner melted layer into pockets that formed on its outer unmelted layer.

“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 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,” Maurel concluded.

It’s possible that other asteroids and comets might be made up of a mixture of these types of materials too. “Most bodies in the asteroid belt appear unmelted on their surface,” Weiss said.

“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.” ®


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