Material necessary for life on Earth may have been delivered by asteroids like Bennu

The secrets of the asteroid Bennu are being revealed.

Researchers studying samples from the asteroid have found minerals, salts, and protein-building components, some of which have never been seen in meteorites that have fallen to Earth.

NASA’s OSIRIS-REx mission undertook a seven-year journey, travelling hundreds of millions of kilometres to touch and collect material from Bennu. Since returning the sample home on the 24 September 2023, scientists have been carefully studying the material to see if it can offer clues as to how life formed on our planet.

Some of the material was brought to the Natural History Museum in London to be examined by our scientists who were part of a recent study published in the journal Nature.

The research shows that Bennu likely had both the environment and chemical ingredients necessary to produce molecules known to be associated with the evolution of life.

Professor Sara Russell, a cosmic mineralogist at the Natural History Museum who co-led the study, says, “There were things in the samples that completely blew us away. The combination of the molecules and minerals preserved are unlike any extraterrestrial samples studied before.”

“Comparing the sample return to meteorites in our Collection, using state-of-the-art analytical facilities here in the Museum, is invaluable in helping us understand our origins.”

What does the Bennu sample contain?

Bennu is thought to have come from a still unknown wet, salty parent body that formed at the dawn of the solar system.

The molecular composition of the asteroid suggests it came from the extremely cold regions of space, likely beyond the orbit of Saturn. But this latest analysis is shedding even more light on what its parent asteroid likely looks like.

It shows that Bennu’s parent asteroid was likely home to pockets of brine, or salt-saturated water, which evaporated leaving salts that resemble the dried-up lakebeds found on Earth. Brines are important in the search for life because they can foster chemical reactions and produce molecules needed for life to develop.

The researchers also found amino acids in the Bennu sample. Amino acids are the building blocks of proteins, which come together in living organisms to form everything from leaves to livers. Proteins form hormones and antibodies, support the structure of our cells and enable biological reactions in our bodies to take place.

Remarkably, all living things use only 20 different amino acids arranged in a limitless number of combinations to create millions of different proteins. Scientists identified 14 of these 20 amino acids in the Bennu sample.

Some amino acids can be built in two ways that mirror each other, like a pair of right and left hands. Life on Earth uses the left-handed variety of these amino acids. Scientists have previously found left-handed amino acids in similar meteorites that have fallen to Earth and so presumed that the Bennu sample would show the same.

“We were surprised to find Bennu has a mixture of left-handed and right-handed amino acids,” says Sara.

“What that means is that perhaps all meteorites contain some contamination. So the fact that Bennu is such a well-curated sample has helped us see what material is indigenous to the asteroid.”

Equal mixtures of mirror-imaged amino acids were also found in a similar asteroid, Ryugu, which was the focus of a separate sample return mission.

In addition to the salts and amino acids, the scientists also found nucleobases in the Bennu samples. Nucleobases are the individual building blocks of DNA, and Bennu contains all five main bases found in living organisms.

As Bennu shows a similar composition to other icy bodies observed in space, it suggests that these life-supporting materials may be widespread in the solar system.

Why did scientists want to sample Bennu?

Most of our knowledge of asteroids comes from studying samples that have fallen to Earth, such as the Winchcombe meteorite. But these rocks become tainted as soon as they enter Earth’s atmosphere and are exposed to oxygen and water.

Collecting samples directly from asteroids and keeping them contained prevents contamination from Earth and gives us a clearer picture of what they contain.

OSIRIS-REx was the first US mission to collect samples from an asteroid and return them to Earth. Bennu was chosen as the target because it is a carbon-rich asteroid that passes close to Earth around every six years, making it relatively easy to reach. It is also nearly 4.6 billion years old, which dates to around the time of the formation of our solar system.

It is this primitive quality of Bennu’s samples that is helping scientists learn how our planets and solar system formed and whether similar asteroids may have been the carriers of water and the chemical building blocks that helped life on Earth to develop.

“We have had extraterrestrial material at the Natural History Museum for 250 years, so we have a lot of expertise in what asteroids can tell us,” says Sara. “Because of that expertise, we were invited to be involved in this mission.”