Much of what scientists know about the early solar system comes from meteorites – ancient rocks that travel through space and survive a fiery plunge through Earth’s atmosphere. Among meteorites, one type – called carbonaceous chondrites – stands out as the most primitive and provides a unique glimpse into the solar system’s infancy.
The carbonaceous chondrites are rich in water, carbon and organic compounds. They’re “hydrated,” which means they contain water bound within minerals in the rock. The components of the water are locked into crystal structures. Many researchers believe these ancient rocks played a crucial role in delivering water to early Earth.
Before hitting the Earth, rocks traveling through space are generally referred to as asteroids, meteoroids or comets, depending on their size and composition. If a piece of one of these objects makes it all the way to Earth, it becomes a “meteorite.”
From observing asteroids with telescopes, scientists know that most asteroids have water-rich, carbonaceous compositions. Models predict that most meteorites – over half – should also be carbonaceous. But less than 4% of all the meteorites found on Earth are carbonaceous. So why is there such a mismatch?
In a study published in the journal Nature Astronomy on April 14, 2025, my planetary scientist colleagues and I tried to answer an age-old question: Where are all the carbonaceous chondrites?
Sample-return missions
Scientists’ desire to study these ancient rocks has driven recent sample-return space missions. NASA’s OSIRIS‑REx and JAXA’s Hayabusa2 missions have transformed what researchers know about primitive, carbon‑rich asteroids.
Meteorites found sitting on the ground are exposed to rain, snow and plants, which can significantly change them and make analysis more difficult. So, the OSIRIS‑REx mission ventured to the asteroid Bennu to retrieve an unaltered sample. Retrieving this sample allowed scientists to examine the asteroid’s composition in detail.
Similarly, Hayabusa2’s journey to the asteroid Ryugu provided pristine samples of another, similarly water-rich asteroid.
Together these missions have let planetary scientists like me study pristine, fragile carbonaceous material from asteroids. These asteroids are a direct window into the building blocks of our solar system and the origins of life.
Carbonaceous near-Earth asteroid Bennu as seen from NASA’s OSIRIS-REx sample-return spacecraft.
NASA
The carbonaceous chondrite puzzle
For a long time, scientists assumed that the Earth’s atmosphere filtered out carbonaceous debris.
When an object hits Earth’s atmosphere, it has to survive significant pressures and high temperatures. Carbonaceous chondrites tend to be weaker and more crumbly than other meteorites, so these objects just don’t stand as much of a chance.
Meteorites usually start their journey when two asteroids collide….
