Scientists Find Cometary Fragment in Primitive Asteroidal Meteorite

A cometary building block has been discovered inside the LaPaz Icefield (LAP) 02342, a carbonaceous chondrite meteorite found in Antarctica in the 2000s.

An illustration showing how a sliver of cometary building block material was swallowed by an asteroid and preserved inside a meteorite. Image credit: Larry Nittler / NASA.

An illustration showing how a sliver of cometary building block material was swallowed by an asteroid and preserved inside a meteorite. Image credit: Larry Nittler / NASA.

Meteorites were once part of larger bodies, asteroids, which broke up due to collisions in space and survived the trip through the Earth’s atmosphere. Their makeup can vary substantially from meteorite to meteorite, reflecting their varying origin stories in different parent bodies that formed in different parts of the Solar System.

Asteroids and comets both formed from the disk of gas and dust that once rotated around our young Sun, but they aggregated at different distances from the Sun, affecting their chemical makeup. Compared to asteroids, comets contain larger fractions of water ice and far more carbon.

By studying a meteorite’s chemistry and mineralogy, planetary scientists can reveal details about its formation and how much heating and other chemical processing it experienced during the Solar System’s formative years.

A particularly primitive class of meteorites called carbonaceous chondrites are thought to have formed beyond Jupiter. One such meteorite, LAP 02342, is a particularly pristine example with minimal weathering since its landing on Earth’s surface.

Inside LAP 02342, Carnegie Institution researcher Larry Nittler and colleagues found a tiny carbon-rich slice of primitive material — about one tenth of a millimeter across — that bears some striking similarities to extraterrestrial dust particles that are thought to have originated in comets that formed near the Solar System’s outer edges.

Approximately 3-3.5 million years after the Solar System formed, but still long before Earth finished growing, this fragment was captured by the growing asteroid from which the meteorite originated.

By undertaking sophisticated chemical and isotopic analysis of the material, the team was able to show that the encased material likely originated in the icy outer Solar System along with objects from the Kuiper Belt, where many comets originate.

“Because this sample of cometary building block material was swallowed by an asteroid and preserved inside this meteorite, it was protected from the ravages of entering Earth’s atmosphere,” Dr. Nittler said.

“It gave us a peek at material that would not have survived to reach our planet’s surface on its own, helping us to understand the early Solar System’s chemistry.”

“Discoveries like this demonstrate how important it is to retrieve precious meteorites like LaPaz from Antarctica. We never know what secrets they’ll reveal,” said Dr. Jemma Davidson, a scientist at Arizona State University.

The discovery is reported in a paper in the journal Nature Astronomy.

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Larry R. Nittler et al. A cometary building block in a primitive asteroidal meteorite. Nature Astronomy, published online April 15, 2019; doi: 10.1038/s41550-019-0737-8

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