Much of what scientists understand about the early solar system has come from meteorites — ancient rocks that journey through space and survive a fiery descent through Earth’s atmosphere. Among these, carbonaceous chondrites stand out as particularly primitive, offering rare insight into the solar system’s origins. Rich in water, carbon, and organic material, these meteorites are thought to have played a crucial role in delivering water to early Earth. Curiously, although space observations suggest that many asteroids should be carbonaceous, fewer than 4% of meteorites found on Earth belong to this group — a puzzling discrepancy scientists are working to solve.
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To investigate, researchers have relied on cutting-edge sample-return missions. NASA’s OSIRIS-REx and Japan’s Hayabusa2 missions collected unaltered material from carbon-rich asteroids Bennu and Ryugu. These pristine samples, uncontaminated by Earth’s weather and biology, have helped planetary scientists study carbonaceous matter in its original state. Such missions are vital because meteorites on the ground are often altered by environmental exposure, making it difficult to understand their true nature.
To uncover why carbonaceous meteorites are underrepresented on Earth, researchers analysed nearly 8,000 meteoroid impacts observed by 19 international fireball networks. These global systems use low-cost digital cameras and sensors to detect and track meteors entering Earth’s atmosphere. By comparing the trajectories of meteoroids that burn up in the atmosphere with those that make it to the ground, scientists identified a key pattern: weaker, carbon-rich fragments are often destroyed before arrival. In fact, the Sun itself may be to blame — repeated heating and cooling during an asteroid’s orbit can cause fragile carbonaceous rocks to crack and fragment, removing them from the population before they even reach Earth.
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This phenomenon, known as survival bias, helps explain the low number of carbonaceous meteorites on the planet. Only around 30–50% of objects that survive thermal cracking in space manage to endure the intense pressures and heat of atmospheric entry. The findings suggest that the filtering doesn’t happen solely in the atmosphere, but begins much earlier, in space. As research advances, scientists aim to refine atmospheric models and improve detection methods to better understand the origins and make-up of these elusive space rocks — and possibly uncover more about the building blocks of life itself.
