Our solar system contains planets, dwarf planets, asteroids and comets — but only one world is known to harbor life. Scientists have long debated whether Earth is truly unique. Perhaps our planet just happened to have the right combination of ingredients, conditions and timing to allow life to emerge.
But a pinch of grit from a distant asteroid collected by a NASA spacecraft holds hints that our planet may not be so special. A team of researchers reported in the journal Nature on Wednesday that the asteroid, known as Bennu, contains a wealth of organic molecules, including many crucial building blocks of life. The chemistry that produced them might be going on today on the ice moons of Jupiter and Saturn.
“Our odds of finding life elsewhere are increasing,” said Daniel Glavin, a senior scientist for sample return at NASA’s Goddard Space Flight Center and a co-author of the two papers.
In 2016, NASA launched OSIRIS-REx, a robotic probe, to Bennu in order to gather clues to the birth of the solar system. Some 4.5 billion years ago, our solar neighborhood started as a cloud of dust and ice. Planets gradually emerged from the cloud, each developing down a different path in the billions of years that followed. Jupiter became a gas giant, for example, while Venus ended up with a rocky, scorched landscape.
But some of the primordial rubble continued to orbit the sun, becoming today’s asteroids. For decades, scientists were able to study asteroids only when a fragment has fallen to Earth as a meteorite. One of the most important of these landed in 1969 near the town of Murchison in Australia. Researchers who inspected it were surprised to find amino acids, the building blocks of proteins. (Our cells use 20 amino acids to make thousands of proteins.)
The discovery raised the possibility that objects from space might have delivered amino acids and other ingredients for life to early Earth. Chemical reactions might have taken place in ponds or deep-sea vents to turn these compounds into the first cells.
But meteorites can offer only a blurry record of the early solar system. Before scientists can look at them, they take a scorching, shattering journey through the atmosphere. They then sit on the ground — in some cases for millions of years — before being discovered. In that time, chemical reactions with Earth’s atmosphere can alter meteorites even more.
By traveling to Bennu, NASA researchers reasoned, a probe could gather pristine material. The OSIRIS-REx probe arrived at the 1,850-foot-wide asteroid in 2020, scooped up rock and dirt, and then jetted back to Earth.
On Sept. 24, 2023, the OSIRIS-REx return capsule parachuted down to a Utah desert. NASA researchers immediately stored the pristine Bennu samples in nitrogen so that they would not react with Earth’s atmosphere.
Dr. Glavin and his colleagues then began to catalog the compounds inside. They found 16,000 kinds of organic molecules. Among the most remarkable were 16 amino acids our cells use to make proteins. Our DNA, on the other hand, is built from four units called nucleobases; Bennu’s rocks contained all four. To make a protein, our cells copy a gene from DNA to a similar molecule called RNA, which uses three of DNA’s nucleobases plus one of its own, called uracil. Bennu contains uracil, too.
Bennu’s minerals offered crucial clues to how the asteroid formed — and how its amino acids and nucleobases developed along the way.
The scientists concluded that the asteroid was a relic of a much bigger object — a mix of rock and ice that measured perhaps 60 miles wide. It formed in the outer solar system, beyond the orbit of Jupiter.
Despite its great distance from the sun, Bennu’s parent body remained warm because it contained radioactive elements. Dr. Glavin and his colleague estimate that its interior may have reached room temperature.
The ice melted into a salty brine inside Bennu’s parent object. It may have filled hidden chambers and sloshed in underground tunnels. These conditions allowed ammonia and other compounds to turn into amino acids and nucleobases.
Bennu’s parent body may have remained in that state for a few million years. Eventually the radioactive heat ran out, but the ammonia may have acted like antifreeze, helping keep the brine in liquid form as it cooled.
Eventually, the researchers suspect, a gravitational disturbance from Jupiter flung the parent body out of its original orbit. It ended up between Mars and Earth, where an impact later blasted it to bits. Some time in the last 65 million years, a little of its debris drifted back together into a floating rubble pile — which we know today as Bennu.
David Deamer, an astrobiologist at the University of California, Santa Cruz, who was not involved in the papers, said they offered a new level of insight into the chemistry of the early solar system. “These are going to be classics,” he predicted.
Mark Schneegurt, an astrobiologist at Wichita State University, agreed. “There could hardly be any study more important to our understanding of the origins of life in the solar system,” he said.
The new findings hint that the conditions were right across much of the early solar system for making the molecules required for life. “It doesn’t take something like a planet or a big moon,” said Tim McCoy, the curator of meteorites at the Smithsonian National Museum of Natural History and a co-author of the studies. “These are run-of-the-mill, small bodies in the outer part of the solar system.”
The Bennu team is continuing to look at unstudied material from the asteroid sample to see if even more complex compounds are lurking inside. Some amino acids could have possibly been combined into primitive, protein-like molecules. It’s conceivable that reactions combined the nucleobases into short chains — primitive forerunners of our genes.
While the researchers are ready for more surprises from Bennu, they do not anticipate finding any evidence of full-blown life in its grit. With just a few million years of warmth, that icy world probably would not have had enough time to generate primitive cells.
“I don’t think it went that far,” Dr. McCoy said. “I think it went somewhere down the path towards life.”
But the same chemistry might have had more opportunity to lead to life on other icy worlds. Ceres, a 580-mile-wide dwarf planet in the asteroid belt, still has brine sloshing through its interior. Enceladus, a 310-mile-wide moon of Saturn, has an icy shell encasing a salty ocean with many of the same minerals as Bennu. In October, NASA launched a probe to Jupiter’s moon Europa, which has more water than all of Earth’s oceans combined.
“These are absolutely going to be important targets,” Dr. Glavin said.
Nilton Rennó, a planetary scientist at the University of Michigan who was not involved with the research on Bennu, said that the findings also opened up more exotic possibilities that scientists should explore seriously. “It opens our eye to think more broadly about life,” he said.
If a vast swarm of briny little worlds produced biological precursors, it could have mixed them together as they crashed into one another. The heat of the impacts could have fueled more chemistry, giving rise to even more complex molecules in their interiors, and perhaps even living cells.
“Could life have started there?” Dr. Rennó asked. “I’m open to it. I like crazy ideas.”
Carl Zimmer has reported on the origin of life and is the author of “Life’s Edge: The Search for What It Means to be Alive.”
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