A hidden surprise: Meteorite hides a fragment of an ancient comet

A tiny piece of the building blocks from which comets form has been found inside a primitive meteorite — offering clues to the formation of the solar system.

A tiny piece of the building blocks from which comets formed has been discovered inside a primitive meteorite — by a Carnegie Institution of Science-led team. The finding could offer clues to the formation, structure, and evolution of the solar system.

The meteorite — named LaPaz Icefield 02342 — was found in Antarctica’s LaPaz Icefield and belongs to a class of primitive carbonaceous chondrite meteorites that have undergone minimal changes since they formed more than 4.5 billion years ago — likely beyond the orbit of Jupiter.

The arrow in this view of the LaPaz meteorite points to where the scientists found the carbon-rich cometary fragment. The colours are produced polarized light shining through a thin slice of the meteorite; the grid lines are spaced one millimetre apart. ( Carles Moyano-Cambero, Institute of Space Sciences, Barcelona)

Jemma Davidson, a research scientist with ASU’s Center for Meteorite Studies in the School of Earth and Space Exploration, explains: “Primitive meteorites provide a snapshot of the early solar system that we can study in the lab.

“The LaPaz meteorite is a nice example since it has experienced minimal terrestrial weathering.”

Meteorite, asteroid and comet comparisons.

Asteroids and comets both formed from the disk of gas and dust that once surrounded the young Sun — aggregating at different distances from it. This varying distance affected their chemical makeup. Compared to asteroids, comets contain larger fractions of water-ice and far more carbon a result of them forming farther from the Sun where the environment was colder.

Meteorites were once part of asteroids — breaking away from these larger bodies as a result of collisions in space and taking a trip through Earth’s atmosphere. Their composition can vary substantially from one meteorite to the next — reflecting their origins in diverse parent bodies that formed in different parts of the solar system.

By studying a meteorite’s chemistry and mineralogy, researchers such as the paper’s lead author, Carnegie’s Larry Nittler, can unlock details about its formation and how much heating and other chemical processing it experienced during the solar system’s formative years.

A hidden treat

The carbon-rich fragment of the material comets are built from is coloured red in this scanning electron microscope image. The scale bar shows its size. (Nittler)

Inside the LaPaz meteorite, Nittler’s team found a very carbon-rich slice of primitive material. It 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 to 3.5 million years after the solar system formed — while the Earth was still evolving — this tiny object — about one-tenth of a millimetre across — was trapped by the growing asteroid from which the meteorite originated.

Davison notes that meteorites like LaPaz are great places to hunt for presolar grains — microscopic pieces of stardust formed by stars that predate the solar system. Yet, no-one in the team expected also to find evidence for a surviving cometary building block inside a meteorite.

Davison confirms her surprise at the discovery: “When Larry and Carles showed me the first electron images of the carbon-rich material. I knew we were looking at something very rare. It was one of those exciting moments you live for as a scientist.”

Interrogating an ancient survivor

the Kuiper Belt — the place where many comets originate.

By undertaking sophisticated chemical and isotopic analysis of the material, Nittler and his colleagues were able to show that the encased material likely originated in the icy outer solar system along with objects from the Kuiper Belt — the place where many comets originate.

Nittler explains: “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.

“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.”

The existence of this primitive material captured inside the meteorite suggests that due to the drag caused by the surrounding gas, particles like it migrated from the outer edges of the solar system, where comets and Kuiper Belt objects formed, to the closer-in area beyond Jupiter, where the carbonaceous chondrites formed. This reveals details about how our solar system’s architecture took shape during the early stages of planet formation.

Concludes Davison: “Discoveries like this demonstrate how important it is to retrieve precious meteorites like LaPaz from Antarctica.

“We never know what secrets they’ll reveal.”

Original research: http://dx.doi.org/10.1038/s41550-019-0737-8