Asteroid fragments upend theory of how life on Earth bloomed

Samples from Bennu contain the chemical building blocks of life — but with a twist

Fragments of the asteroid Bennu, carefully collected and ferried to Earth by a robotic spacecraft, contain the building blocks for life, NASA announced today.

Not only does Bennu contain all 5 of the nucleobases that form DNA and RNA on Earth and 14 of the 20 amino acids found in known proteins, the asteroid’s amino acids hold a surprise. On Earth, amino acids in living organisms predominantly have a ‘left-handed’ chemical structure. Bennu, however, contains nearly equal amounts of these structures and their ‘right-handed’, mirror-image forms, calling into question scientists’ hypothesis that asteroids similar to this one might have seeded life on Earth.

The work appears in Nature Astronomy¹.

“What makes these results so significant is that we’re finding them in a pristine sample,” says co-author Daniel Glavin, an astrobiologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. When meteorites fall to Earth, they are heated in the atmosphere and contaminated by the planet’s molecules. The Bennu samples were ferried to Earth in a sealed canister, protected from the heat, and analysed in a super-clean laboratory space under inert gas.

The specimens contain the richest bounty of life-friendly extraterrestrial compounds ever brought to Earth.

A salty finding

But that’s not all they contain. In an accompanying paper published in Nature today², other researchers report that the material from Bennu is also rich in salts created billions of years ago, probably when watery ponds on Bennu’s parent asteroid evaporated and left behind a crust of minerals. Although no signs of life were spotted on Bennu, those salty ponds would have been a good environment to foster the chemistry that could lead to it. This might involve small, carbon-rich molecules such as methanal (formaldehyde) — a simple compound that was found in the Bennu samples.

“Having these brines there, along with simple organic stuff, may have kick-started [the process of] making much more complicated and interesting organics like the nucleobases,” says Sara Russell, a mineralogist at the Natural History Museum in London and an author of the Nature paper.

A scanning electron microscope image of the sodium carbonate crystalline needles found in samples of the asteroid Bennu.Credit: Rob Wardell, Tim Gooding and Tim McCoy/Smithsonian

The Bennu discovery helps to illuminate how, early in the Solar System’s history, asteroids and other planetary building blocks were not just “lumps of stone and ice, but active ‘living’ objects”, says Yasuhito Sekine, a planetary scientist at the Earth-Life Science Institute in Tokyo who wrote a News & Views article accompanying the Nature paper.

Both sets of results come from NASA’s US$1.2-billion mission OSIRIS-REx, which stands for Origins, Spectral Interpretation, Resource Identification, and Security — Regolith Explorer. The spacecraft launched from Florida in 2016, flew to Bennu, collected rocks and dust from the asteroid’s surface in 2020 and landed back on Earth in Utah in 2023.

Before the mission visited Bennu, scientists knew that the asteroid was probably carbonaceous — rich in carbon, and containing water molecules locked up in clay minerals. But until the samples came back to Earth and went into NASA’s curation facility in Houston, Texas, researchers didn’t know how much of a bounty they had.