Life’s ingredients discovered in samples Nasa probe returned from an asteroid

Samples returned from an asteroid contain a surprising abundance of the basic ingredients of life. They were discovered to be rich in carbon, nitrogen and ammonia, with over 30 kinds of amino acids and the five nucleobases found in RNA and DNA.¹ The asteroid, Bennu, was targeted by a Nasa mission that returned a capsule to Earth in September 2023.

Bennu is about 500m in diameter and is a loosely bound accumulation of boulders, gravel and dust. It is barely held together by its own gravity. ‘Its surface is darker than coal, so we intuited that it would be rich in organic materials since carbon is known to make materials dark,’ says Dante Lauretta, a planetary scientist at the University of Arizona, who led the Osiris-Rex asteroid return mission to Bennu that launched in 2016.

The evidence now suggests that Bennu is comprised of fragments of a world ‘probably 100 km across, or larger’ that was destroyed around a billion years ago, says Lauretta, which formed far out in the solar system where temperatures allowed ammonia, carbon dioxide and methane to accrete as ice.

‘The big surprise for us was the ammonia,’ says Lauretta. Enrichment of the samples with nitrogen-15 pointed to ammonia and other nitrogen-containing soluble molecules forming in a cold molecular cloud or protoplanetary disc in the early solar system.

It had been thought that Bennu formed in the main asteroid belt, between Mars and Jupiter, but the samples reveal a new history. The discovery of salt-rich minerals, including sodium-bearing phosphates and sodium-rich carbonates, sulfates, chlorides and fluorides, suggest that they likely formed during the evaporation of a brine rich in ammonia.²

‘The range of evaporites means that you had to have a large body of salt water, such as a lake or ocean, to build up that high salinity,’ says Lauretta. ‘So we’re looking at a very wet world with water that ultimately evaporated away, leaving behind the salts.’

Ammonia and formaldehyde were identified in the samples and are starting materials for the synthesis of amino acids and other soluble organics. A total of 33 amino acids were present in the Bennu samples, including 14 of the 20 amino acids that make up proteins in terrestrial biology. At least 23 different N-heterocycles were identified, including all five of the nucleobases of DNA and RNA.

‘You’ve got all these interesting minerals that are precipitating all these salt crystals and their surfaces become areas where important reactions can be catalysed,’ says Lauretta. ‘One that I am most excited about is phosphorus because it is a critical component of so many important biomolecules.’

‘We’ve been analysing the organic content of meteorites for decades, but they had fallen through the atmosphere, been heated and interacted with our terrestrial environment and water,’ says Queenie Chan, a planetary scientist at the Royal Holloway University of London. ‘What’s very special about this sample from Bennu is that it came directly back to the lab, without any contamination,’ she says. This gives scientists confidence that the compounds are not terrestrial in origin.

Another discovery was that there were both left-handed and right-handed amino acids in the asteroid sample – life only uses left-handed ones. ‘This is quite exciting because it means that the left-handedness of life can’t be traced back to what came in from asteroids,’ says Chan.

The implications go beyond life on Earth. ‘It is likely this kit came not only to Earth, but because it came from an asteroid from the outer solar system, they were delivered to Venus, to Mars, to the moons of Jupiter and Saturn and other icy worlds throughout the solar system,’ says Lauretta. ‘They all had a chance for the origin of life.’

The key question is ‘what had to happen on Earth to go from a rock full of organic goo into something that’s actually alive’, he says.