Rock and dust samples retrieved by NASA from the asteroid Bennu exhibit some of the chemical building blocks of life, according to research that provides some of the best evidence to date that such space rocks may have seeded early Earth with the raw ingredients that fostered the emergence of living organisms.
The U.S. space agency's robotic OSIRIS-REx spacecraft in 2020 collected the samples from the near-Earth asteroid, a rocky remnant of a larger celestial body that had formed near the dawn of the solar system roughly 4.5 billion years ago. The samples were delivered to Earth in 2023 by parachute inside a capsule released by OSIRIS-REx that landed in the Utah desert.
Two analyses of the samples were published on Wednesday.
One, in the journal Nature Astronomy, found that the samples contained a diverse mixture of organic compounds. And the other, in the journal Nature, found that the samples contained minerals formed when brine - salty water - evaporated on Bennu's parent body, the type of wet environment where prebiotic organic chemistry may have brewed.
Present in the samples were 14 of the 20 organic compounds called amino acids that are used to make proteins - complex molecules that play indispensable roles in the structure, function and regulation of living organisms. Present also were all five nucleobases - the genetic components of DNA and RNA in all life on Earth.
In the early solar system, planets including Earth and various moons were pelted by asteroids and other space debris that carried water and chemicals including organic compounds.
"The detection of these key building blocks of life in the Bennu samples supports the theory that asteroids and their fragments seeded the early Earth with the raw ingredients that led to the emergence of life," said astrobiologist Danny Glavin of NASA's Goddard Space Flight Center in Maryland, lead author of one of the studies.
"In addition, the fact that these chemical building blocks of life can be formed in space and are widespread throughout the solar system increases the chances that life could have started beyond Earth," Glavin added.
Organic compounds have one or more carbon atoms that are bound to other elements, usually hydrogen, oxygen, nitrogen and sulfur. All life on Earth is based on carbon and is built from organic compounds including the amino acids used to create proteins and nucleobases.
A nucleobase is a nitrogen-containing compound that stores genetic information. DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are biomolecular cousins that are fundamental molecules in cell biology. DNA contains an organism's genetic code. RNA carries genetic information it receives from the DNA, putting this information into practice.
"All biology is comprised of organic compounds. The origin of life is related to organic chemistry, some of which is preserved in these rocks from 4.5 billion years ago," said astrobiologist and study co-author Jason Dworkin, the OSIRIS-REx project scientist.
The Bennu organic compounds all have been identified previously in meteorites that have landed on Earth. But there have been lingering questions because these meteorites could have been contaminated by terrestrial sources. The Bennu samples were obtained directly from an asteroid and were kept pristine.
"We can trust these results," Glavin said.
Bennu's icy parent body, perhaps about 60 miles (100 km) in diameter, appears to have formed in the outer solar system and was later destroyed, possibly 1-2 billion years ago. The fragments then formed Bennu and other "rubble pile" asteroids - loose amalgamations of rocky material rather than solid objects.
Early in its history, some of the ice inside the parent body apparently melted and formed brine. The minerals formed in the evaporation of this brine had never previously been detected in meteorites that landed on Earth.
"The brines provide an environment in which elements and simple organics could have combined to form more complex prebiotic organics on the pathway to life," said geologist Tim McCoy, curator of meteorites at the Smithsonian Institution's National Museum of Natural History in Washington and lead author of one of the studies.
The researchers did not find evidence of actual DNA or RNA in the samples.
"The suite of simple protein amino acids and nucleobases that were found in Bennu are a long way from anything that could be considered 'living,' for example, a more complex self-sustaining chemical system that can replicate and evolve which is comprised of much larger polymers - proteins and nucleic acids - that are found in cells," Glavin said.
