Comet fragment discovered inside meteorite gives clues to the origin of the solar system

An international team including researchers from the Institute of Space Studies of Catalonia at the Institute of Space Sciences (Spanish National Research Council-CSIC) has discovered a pristine comet fragment inside a meteorite. This finding demonstrates that the class of meteorites known as carbonaceous chondrites contains clues to the composition of more fragile objects that formed in regions distant from the Sun, more than 4,560 million years ago. The results are published in the journal Nature Astronomy.

An illustration showing how a sliver of cometary building block material was swallowed by an asteroid
and preserved inside a meteorite, where it was discovered by a Carnegie-led team of scientists
[Credit: Larry Nittler, NASA]

After a three-year study of the carbonaceous chondrite La-Paz 02342, from NASA's Antarctic collection, researchers have come to the conclusion that the comet fragment, of about one hundred microns, is composed of an unusual mixture of organic materials, amorphous and crystalline silicates, sodium sulphates, sulphides, and presolar grains; the latter synthesised in stars that enriched the original materials of our Solar System. Among other instruments, a secondary ion mass spectrometer (nano-SIMS) of the Carnegie Institution for Science (USA) has been used for its analysis, has been used for its analysis, which allows studying at a nanometric scale the composition of the meteorite at an isotopic and elemental level.

"This fragment, technically known as xenolith, has unusual characteristics that we think were produced from the incorporation of primitive materials embedded in ice", says IEEC-CSIC researcher Josep Maria Trigo-Rodríguez, who works at ICE and co-leads the study. "Many objects in the Solar System have a very different composition than the meteorites available in terrestrial collections. Carbonaceous chondrites, such as La Paz 02342, constitute a fossil legacy of the creation of the planetesimals in their interior and are capable of preserving unique samples of other objects much richer in organic and volatile matter, known as comets", explains Trigo-Rodríguez.

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

As the researcher points out: "The asteroid progenitor of this carbonaceous chondrite underwent aqueous alteration, but fortunately, it was neither extensive nor homogeneous. This led to the preservation of the unique properties of this cometary dust speck, among which the richness in tiny mineral grains formed in stars of the same environment in which the Sun was born."

Carbonaceous chondrites come from transitional bodies, a category falling between asteroids and comets. Given their sizes typically smaller than a few hundred kilometres, such bodies never melted or suffered internal chemical differentiation as occurred to the planets. The materials that make up these objects are usually fragile and do not usually survive the transit of tens of millions of years that transport them from their parent bodies to the Earth orbit. In case they do, they fragment and volatilise when entering into the atmosphere at hypersonic velocities. Precisely because of this reason, ultracarbonaceous materials such as those discovered are extremely rare and have only been identified as micrometeorites.

The carbon-rich fragment of the material comets are built from is colored red in this scanning electron microscope image.
 The scale bar shows its size [Credit: Larry Nittler, Carnegie Institution for Science]

The search for primordial materials among the most primitive meteorites can be carried out at ICE, given that it is the only international repository of NASA's Antarctic meteorites in Spain. The samples studied by the IEEC-CSIC scientific team come from NASA's Johnson Space Center. Hence, researchers have access to unique specimens, being able to select those that have not undergone thermal metamorphism or extreme aqueous alteration.

Source: Spanish National Research Council (CSIC) [April 15, 2019]