Asteroid Bennu seems to have come from a long-lost world on the fringes of the solar system, where saltwater pooled and dried over thousands of years and life’s basic ingredients were widespread.
The discovery comes from new analysis of data gathered during NASA’s ambitious OSIRIS-REx mission. This saw a probe rendezvous with the 490-meter-wide (1,610-ft) space rock in 2018, swooping low enough to scoop up a 250-g (9-oz) sample of rock and soil from its surface. The sample was then flown back to Earth, touching down in Utah in 2023 – and scientists have been studying it intently ever since.
Asteroids are kind of like crumbs left over from the disk of dust and gas that formed the Sun, planets and moons 4.5 billion years ago. Fragments that fall to Earth as meteorites can be altered by heat during entry into the atmosphere and contaminated by local microbes and molecules. But samples taken directly out of an asteroid like Bennu, which has been floating around in the cold vacuum of space for billions of years, preserve a pristine ancient snapshot.
Water and a high carbon content have already been detected in Bennu, but now, more advanced analysis has revealed a fascinating history. Bennu’s parent body seems to have formed in the distant outskirts of the solar system, but at times it was warm enough for salty liquid water to pool in cracks and crevices beneath the surface. Over time, these brines evaporated, leaving behind traces of salty minerals called ‘evaporites.’
Some of these evaporites have previously been found in extraterrestrial samples, but never before has a complete set been detected. The 11 found here, including halite, sylvite, calcite and trona, allow scientists to piece together the story.
“The clues we’re looking for are so minuscule and so easily destroyed or altered from exposure to Earth’s environment,” said Danny Glavin, co-lead author of a study on the findings. “That’s why some of these new discoveries would not be possible without a sample-return mission, meticulous contamination-control measures, and careful curation and storage of this precious material from Bennu.”
These bodies of water, which would have been on the scale of meters, seem to have been remarkably similar to the ponds of ‘primordial soup’ from which life is thought to have originated here on Earth. And sure enough, many of the raw ingredients of life were detected on Bennu.
Amino acids are complex molecules that can link up to form proteins, which in turn perform most biological functions. Out of the 20 amino acids that life on Earth uses, 14 were found in the Bennu sample. In addition, all five nucleobases used in DNA and RNA were detected.
Intriguingly, other chemicals important for biology were picked up in the sample too. That includes formaldehyde and extremely high levels of ammonia, which react with each other to form amino acids under the right conditions.
While it’s unlikely that life itself ever arose on Bennu or its parent body, the team says the find does raise the chances that we might find it elsewhere in the solar system. The brines look similar to those thought to exist on dwarf planet Ceres and Saturn’s moon Enceladus, and finding them in the cold outer suburbs shows that they could have been more widespread than expected.
Analysis on Bennu samples are still ongoing, with some being sealed away long-term so they can be studied with more advanced technology in future. Meanwhile, samples from other asteroids like Ryugu are also being analyzed, which together could help us better understand how and where to look for extraterrestrial life.
“OSIRIS-REx has been a highly successful mission,” said Jason Dworkin, co-lead author on one newly published study. “Data from OSIRIS-REx adds major brushstrokes to a picture of a solar system teeming with the potential for life. Why we, so far, only see life on Earth and not elsewhere, that’s the truly tantalizing question.”
The research was published in two papers in the journals Nature and Nature Astronomy.
Sources: NASA, University of Arizona