A watery past may have spurred the formation of life’s basic molecules.
Life on Earth relies on molecular building blocks to make DNA and proteins. Scientists have long wondered how prevalent these precursors were at the birth of our solar system.
A sample of dust and rocks from an asteroid just took us closer to an answer.
Collected from Bennu, a space rock shaped like a spinning top, as it soared by Earth roughly five years ago, the samples were frozen in time by the vacuum of space. Essentially a time capsule of the earliest days of our solar system some 4.5 billion years ago—around the time when Earth was forming—they provide a peek into the chemical soup that may have kickstarted life.
Two new studies examining these extraterrestrial space grains found signs of life’s molecules preserved on the asteroid’s ancient surface. Dust and rocks from Bennu contained all five nucleobases—molecules that make up DNA and RNA—and 14 of the 20 amino acids in proteins.
These organic molecules had been found in other asteroids. But there’s a twist to Bennu’s chemical makeup. Whereas most Earthly amino acids exist in a left-handed form, samples from Bennu contain an almost equal amount of amino acids that are their mirror image. These right-handed amino acids aren’t naturally found on Earth.
Bennu also harbored telltale signs of saltwater, which could have been the soup that helped the molecules mingle and interact. The brine is similar in composition to dried lake beds on Earth.
To be clear, the teams didn’t find extraterrestrial life. But they did show that life’s precursor molecules—even “mirrored” ones—were widespread across the early solar system.
“Asteroids provide a time capsule into our home planet’s history, and Bennu’s samples are pivotal in our understanding of what ingredients in our solar system existed before life started on Earth,” said Nicky Fox, an associate administrator at NASA, in a press release.
The Mission
The samples were delivered by NASA’s OSIRIS-REx mission—the first US project to bring asteroid samples home. Bennu was an especially interesting target. Prior work had suggested asteroids have the organic molecules that form the basis of life on Earth. These molecules could have hitched a ride on asteroids and seeded the early planets or their moons to spark life.
On Earth, two critical components for life are nucleobases and amino acids.
Nucleobases are the molecular building blocks of DNA. They encode our bodies’ makeup, functions, and inheritance. RNA, which transmits the instructions contained in genes to the protein-making factories in cells, uses an additional nucleobase, which is also integral to some viruses. Beyond DNA and RNA, 20 amino acids link together to form proteins.
How these precursor ingredients spurred life remains a mystery, but asteroids may contain clues. A previous sample from 162173 Ryugu, a diamond-shaped asteroid, contained myriad organic compounds, including vitamin B3 and uracil, the additional nucleobase used in RNA.
Like Ryugu, Bennu is a carbonaceous asteroid. These space rocks are rich in carbon molecules that form the organic compounds critical for life. Bennu, a pile of rocks loosely held together by gravity, likely dates to the beginning of the solar system—some 4.5 billion years ago.
Thanks to the freezing vacuum of space, most organic molecules on Bennu have been preserved in their original state—locked in time—and could provide clues about the early solar system’s chemical makeup.
Bennu was also an attractive target because it skirts the asteroid belt, which circles the sun between Mars and Jupiter. At its closest, the asteroid is 200 million miles from Earth. While still a multi-year journey, the distance made it possible to land a space probe, map Bennu’s landscape, collect specimens, and shuttle the cargo back to Earth.
The probe was specifically designed to seal collected samples in a capsule to protect them from contamination when returning to and re-entering Earth’s atmosphere. As the capsule dropped back to Earth, the air was filtered to remove water vapor and dust particles. Upon landing in Utah, NASA immediately placed the capsule in a clean room and blasted it with nitrogen—a gas that doesn’t react with most other chemicals—to push out invading air.
“What makes these results so significant is that we’re finding them in a pristine sample,” Daniel Glavin, an astrobiologist at NASA and coauthor on a paper describing the work, told Nature.
These meticulous guidelines ensured the sample wasn’t contaminated by Earth’s natural chemicals. Weighing a little over four ounces—roughly a bar of soap—the collection of asteroid pebbles and dust is one of the largest to date.
Mirror, Mirror
One study in Nature Astronomy detected all five of the nucleobases present in genetic material on Earth and 14 of the 20 amino acids that make up proteins. The asteroid also contained 19 amino acids that don’t encode any proteins known to life on Earth.
Surprisingly, some of these amino acids exist in a mirror world. Amino acids on Earth are only left-handed. Synthetic biologists have begun genetically twisting these protein building blocks into a right-handed structure—which could benefit biomedicine in the form of longer-lasting medications. Some scientists have even proposed building fully “mirrored” lifeforms, a controversial and potentially risky endeavor scientists spoke out against last year.
Our early solar system may have even laid the groundwork. But how these molecules formed—and if they stuck around—remains a mystery.
The team also detected high amounts of ammonia and formaldehyde. The duo, prevalent on early Earth, is critical to the formation of complex molecules in the right conditions—basically providing a nutritious broth for ingredients like amino acids to simmer and chemically react.
Bennu may have once provided a compatible environment. Another study in Nature detected a cornucopia of minerals akin to brine on Earth—potentially a sign of water in the past. These salt-crusted spots, which usually occur due to freezing or evaporation, dot Earth’s landscapes in places like Badwater Basin in Death Valley and the Great Salt Lake in Utah.
Together, the samples form a snapshot of the asteroid’s multi-billion-year-long history, suggesting the space rock may have once harbored tiny pools of water friendly to life.
“Having these brines there, along with simple organic stuff, may have kick-started [the process of] making much more complicated and interesting organics like the nucleobases,” study author Sara Russell at the Natural History Museum in London told Nature.
A global coalition is still analyzing Bennu’s samples to learn more about the early solar system. In the meantime, the spacecraft—renamed OSIRIS-APEX—is gearing up for another mission to the asteroid Apophis as it skirts by Earth in 2029.
“Data from OSIRIS-REx adds major brushstrokes to a picture of a solar system teeming with the potential for life,” said study author Jason Dworkin. “Why we, so far, only see life on Earth and not elsewhere, that’s the truly tantalizing question.”
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