A tiny, ancient rock has travelled millions of kilometres through space to deliver a profound message about life’s origins. The asteroid Bennu, a relic from the dawn of the solar system, has yielded a treasure trove of organic molecules and minerals that could have played a role in shaping the chemistry of life as we know it.
The findings, published in the journal Nature and Nature Astronomy, stem from United States’ National Aeronautics and Space Administration’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission, which successfully returned samples from Bennu to Earth in 2023.
Scientists have now revealed that these rocks contain key components of life: Amino acids, nucleobases and traces of ancient saltwater — the essential ingredients for biological processes.
Bennu contained a diverse range of organic molecules — the specimens contain the greatest concentration of life-supporting extraterrestrial compounds ever brought to Earth. But more importantly, the samples were unaltered when they reached Earth. When meteorites enter Earth’s atmosphere, they undergo intense heating and pick up contamination from the planet’s molecules.
In contrast, the Bennu samples were transported in a securely sealed canister, shielded from heat, and examined in an ultra-clean laboratory environment under inert gas conditions.
“What makes these results so significant is that we’re finding them in a pristine sample,” co-author Daniel Glavin, an astrobiologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, stated in an article in Nature.
Asteroids like Bennu are remnants from the formation of the solar system, offering a snapshot of conditions that existed more than 4.5 billion years ago. Unlike meteorites that have fallen to Earth, these pristine samples were collected in space, avoiding contamination from the planet’s atmosphere.
“NASA’s OSIRIS-REx mission already is rewriting the textbook on what we understand about the beginnings of our solar system,” Nicky Fox, associate administrator for NASA’s Science Mission Directorate, said in a statement. “Asteroids provide a time capsule into our home planet’s history, and Bennu’s samples are pivotal in our understanding of what ingredients in our solar system existed before life started on Earth.”
Among the most striking discoveries in the Bennu samples are 14 of the 20 amino acids that life on Earth uses to build proteins, as well as all five nucleobases needed for genetic material like DNA and RNA. However, its amino acids show something unusual. Life on Earth mostly uses ‘left-handed’ amino acids, but Bennu has nearly equal amounts of both left- and right-handed types. This challenges the idea that similar asteroids may have helped start life on Earth.
The presence of ammonia and formaldehyde — compounds that can react to form more complex molecules — further strengthens the case for asteroids as potential incubators for prebiotic chemistry.
Beyond the molecular building blocks, Bennu’s samples also contain traces of water in the form of mineral salts. Scientists detected 11 different evaporite minerals, including calcite, halite, sylvite, and trona — the latter being identified in an extraterrestrial sample for the first time. These minerals suggest that Bennu once harboured briny water that evaporated over thousands, if not millions, of years.
Similar salty water reservoirs have been detected elsewhere in the solar system, such as beneath the icy crust of Saturn’s moon Enceladus and on the dwarf planet Ceres. The presence of these minerals on Bennu further supports the idea that watery environments, which could facilitate chemical reactions leading to life, were once widespread.
“These papers really go hand in hand in trying to explain how life’s ingredients actually came together to make what we see on this aqueously altered asteroid,” said Tim McCoy, curator of meteorites at the Smithsonian’s National Museum of Natural History.
While Bennu’s samples contain an abundance of organic molecules, a curious question remains unanswered: Why did life on Earth develop a preference for left-handed amino acids? The samples retrieved from Bennu contain an equal mix of left- and right-handed amino acids, suggesting that early Earth may have started out with this balance before a preference emerged. The reason for life’s eventual “left turn” remains one of the biggest puzzles in the study of life’s origins.
“Data from OSIRIS-REx adds major brushstrokes to a picture of a solar system teeming with the potential for life,” said Jason Dworkin, the mission’s project scientist. “Why we, so far, only see life on Earth and not elsewhere, that’s the truly tantalising question.”
For all the answers Bennu has provided, the journey to understanding the origins of life is far from over. But with each new discovery, we inch closer to uncovering whether life’s ingredients are scattered across the cosmos – and whether they could have taken root elsewhere.