Ingredients of Life Discovered on Near-Earth Asteroid Bennu – Sky & Telescope

In 2016, NASA launched a probe to collect samples from the asteroid 101955 Bennu in order to shed light on the solar system’s early environment. Now, scientists have found the ingredients of life in samples from the asteroid’s surface.

The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REX) mission captured a pristine sample of the near-Earth asteroid’s rock and dust, returning those bits and pieces to Earth in 2023. Now, analyses of the Bennu sample, published in Nature and Nature Astronomy, have revealed crucial molecules for life, including amino acids and the building blocks of DNA and RNA. The asteroid sample also contained traces of saltwater that may have helped mix molecules together to form more complex compounds.

Small, well-preserved asteroids like Bennu are useful probes of the early solar system’s composition. “Carbon-rich asteroids are leftovers of the early solar system that have been minimally altered for 4.5 billion years,” says Jason Dworkin (NASA Goddard Space Flight Center), a coauthor of both studies. “They preserve the chemistry that was happening at that time and was available at the origin of life.”

Scientists have also long thought that asteroids like Bennu could have delivered water and organic compounds to a primordial Earth and other solar system bodies. The findings demonstrate that the conditions for life to form might have been common in the early solar system — possible not just on Earth but also on other planets and moons.

Essential Molecules for Life

While scientists have found similar organic molecules in meteorite samples, Bennu provides a rare opportunity to study a sample from a remnant of the early solar system. Most meteorites become contaminated as they fall through Earth’s atmosphere.

“What makes Bennu samples special is that they are truly pristine extraterrestrial material,” says Dimitar Sasselov (Harvard University), who was not involved in the study. Preserving the integrity of the 120-gram sample of material — around the weight of a bar of soap — was essential to the OSIRIS-REx mission.

The team found that Bennu’s surface is rich in volatiles such as carbon, nitrogen, and ammonia. They also detected 33 amino acids, including 14 of the 20 that make up the proteins in terrestrial biology, and all five nucleobases that make up DNA and RNA.

Interestingly, while most amino acids found in life on Earth are shaped in a way that scientists dub “left-handed,” the team found an equal proportion of both left-handed and right-handed molecules in the Bennu sample. That finding raises questions about what produced the terrestrial asymmetry.

Bennu’s Briny History

Astronomers can also use the samples to trace Bennu’s history back to its parent asteroid, which probably broke into pieces — including Bennu — some billion years ago. Sample analysis revealed the presence of ammonia, methane, and other molecules, which suggests the parent body formed far enough from the Sun for those molecules to accrete as ices onto its surface.

Other evidence suggests the parent body hosted saltwater, which could have acted as a mixing agent to produce more complex organic elements. The asteroid sample shows a sequence of 11 salt-rich minerals that were left behind as water containing those dissolved salts evaporated. The array of minerals are similar to those found in the salty crusts in dried lakebeds on Earth; in fact, the sodium-rich carbonate known as “trona,” which is common on Earth, had never been detected in extraterrestrial samples.

These brines could have served as a primordial “broth,” allowing the building blocks of life to combine and intermingle. For example, ammonia might have reacted with formaldehyde, another molecule the team detected, to form the amino acids.

Brines also exist on other icy worlds of the outer solar system, so prebiotic chemistry might have happened there, too. The team hopes to re-examine the meteorite samples in the Smithsonian’s collection, though sample returns from comets, Mars, and Saturn’s moons Titan and Enceladus would be even more telling. These moons and planets can “tell us about later times in solar system history and might hold vast secrets about life,” Dworkin says.

The molecules and brines present on Bennu demonstrate that the ingredients to form life might have been ubiquitous in the early solar system. Asteroids could have delivered this life-starting kit not only to Earth, but to the inner terrestrial planets and icy worlds of the outer solar system, providing the opportunity for life to form. “This discovery [of brines] was completely unexpected, which is why we explore space,” says Timothy McCoy (Smithsonian Institution), who led the Nature study. “Not just to confirm what we think we know, but to make new discoveries.”