Evrim Yazgin
Cosmos science journalist
The first samples returned from an asteroid to Earth have revealed that one – Bennu – had an ancient brine with minerals crucial to life.
Research published today in Nature shows that water evaporated on Bennu leaving behind a briny broth which would have allowed the ingredients for life to mix and create more complex structures.
These minerals date to the early formation of the solar system.
“We now know from Bennu that the raw ingredients of life were combining in really interesting and complex ways on Bennu’s parent body,” says co-lead author Tim McCoy, curator of meteorites at the Smithsonian’s National Museum of Natural History in the US. “We have discovered that next step on a pathway to life.”
Bennu’s parent asteroid formed about 4.5 billion years ago at the dawn of the solar system. This body appears to have had pockets of liquid water which evaporated and left behind brines similar to the salty crusts of dry lakebeds on Earth.
NASA launched the OSIRIS-REx mission to asteroid 101955 Bennu in 2016. It reached the asteroid in 2018 with about 120g of material from the surface sent back to Earth, arriving in 2023.
Bennu has sparked interest from astronomers and astrobiologists because of its near-Earth orbit and its carbon-rich composition.
It may help scientists understand how the organic compounds which eventually formed life, reached Earth.
Analyses of the samples from NASA’s OSIRIS-REx mission are being carried out across the world. The Smithsonian’s state-of-the-art scanning electron microscope allowed McCoy’s team to spot microscopic features less than a micrometre across – a hundred times smaller than the width of a human hair.
They found traces of water-bearing sodium carbonate compounds, commonly known as soda ash, in the sample. These compounds have never been directly observed on any other asteroid or meteorite.
Such sodium carbonate compounds occur naturally on Earth in evaporated lakes.
Bennu’s brine is chemically different to those found on Earth. The asteroid’s compounds are rich in phosphorous. It is lacking in boron, which is common in soda lakes on Earth.
“We now know we have the basic building blocks to move along this pathway towards life, but we don’t know how far along that pathway this environment could allow things to progress,” McCoy says.
A second paper published in Nature Astronomy includes additional findings from the analysis, including the identification of protein-building amino acids in the Bennu samples. There are also 5 nucleobases that make up the molecular composition of RNA and DNA.
Some of these compounds have never been seen in meteorites.
“This is the kind of finding you hope you’re going to make on a mission,” McCoy adds. “We found something we didn’t expect, and that’s the best reward for any kind of exploration.”
