Scientists studying asteroid grains scooped from asteroid Bennu and returned to Earth by NASA’s OSIRIS-REx spacecraft, have found so many precursors to life that they have a new question: why didn’t life form on Bennu billions of years ago, when conditions seemed ripe for it?
“What did Bennu not have that Earth did have?” asked OSIRIS-REx project scientist Jason Dworkin at a NASA press conference elaborating on findings released shortly before in Nature and Nature Astronomy. “This is a future area of study for astrobiologists to ponder, looking at Bennu as a place that had all the ‘stuff’ but didn’t make life,” he says.
Bennu itself isn’t currently hospitable to life. It’s just a rubble pile that coalesced from fragments of an ancient asteroid smashup a billion or two years ago, says Tim McCoy, curator of meteorites at the Smithsonian Institution’s National Museum of Natural History in Washington, D.C.
But the rubble, McCoy says, comes from an asteroid that was once warm enough to have liquid water in its interior. That water, he says, reacted with rock to produce a brine that would have been highly suitable for life as we know it. Evidence of that brine now appears in the samples as salt crystals akin to those found on dry lakebeds on Earth.
Not that Bennu’s parent body was likely to have had lakes. More likely, McCoy says, it had “something like a muddy surface,” with pockets or veins of fluid underneath. “It was within those that the evaporation occurred,” McCoy says. “Water was lost and these minerals were left behind.”
From the diversity of salts in the samples, it was also clear that “the briny fluids underneath the surface of Bennu’s parent body were crammed full of bio-essential elements like phosphorus and sulfur,” added Sara Russell, a cosmic mineralogist at the Natural History Museum, London.
The samples were also rich in ammonia, says Nicky Fox, associate administrator of NASA’s Science Mission Directorate in Washington, D.C. That’s important, she says, because “on Earth ammonia is important to biology because it can react with formaldehyde, which was also found in the sample, to form complex molecules such as amino acids.”
In fact, she noted, the team found 14 of the 20 amino acids used by Earth life to make proteins, as well as all five of the nucleobases life on Earth uses to encode genetic information in DNA and RNA.
All told, says Danny Glavin, a senior scientist for sample return at NASA’s Goddard Space Flight Center, Greenbelt, Maryland, the scientists found more than 10,000 different organic chemicals. “These samples provide a really unique opportunity to explore the prebiotic organic chemistry that occurred in the Solar System prior to the emergence of life,” he says.
This does not mean, however, that the scientists found any evidence that Bennu’s parent body actually hosted life: and before doing other analyses, the scientists indeed looked for it. “We have looked through the Bennu sample at a very fine level—a sort of micron level, which is a millionth of a meter—and we don’t see any cellular structures that you might expect if there were fossils there,” Russell says. Nor, she says, were there any “chemical fossils,”
What the findings do suggest, Fox says, is that conditions necessary for the emergence of life were widespread across the early Solar System. They also support the theory, she and Dworkin says, that asteroids like Bennu were among the sources that delivered water and chemical building blocks for life to the infant Earth—and other worlds like Mars, Europa, Ceres, and Enceladus—early in the history of the Solar System. It is a process, Fox says, that could have “seeded” Earth and other worlds “with all the ingredients they needed to kickstart life.”
Meanwhile, there are mysteries. In addition to the 14 amino acids used by earthly life, Dworkin says, there are many others, probably hundreds, in the sample not used by Earth life. “This tells us a lot about the chemistry that happened to form these different compounds,” he says, “but if I knew why life picked these 20 as opposed to other ones that were available, I’d be booking my flight to Stockholm.”
Meanwhile, the current analyses only looked at about 30 percent of the 120-gram sample returned by OSIRIS-REx. The rest remains in storage, including 7.5 grams that will be archived at -80°C for 50 years, awaiting the development of analytical techniques currently unheard of.
“It’s very early in the process,” Humberto Campins, an asteroid researcher at the University of Central Florida who is part of the OSIRIS-Rex science team but not part of the sample-analysis team told Cosmos. “We’re just scratching the surface.”
