Meteorites are messengers from the depths of primordial time—cast-off fragments of asteroids and comets that formed alongside our sun from raw materials predating our star itself. But their messages are often muddled by their final, fateful encounter with Earth—charred in their fiery plunge through our planet’s atmosphere and contaminated by our world’s ever-shifting environmental tumult. And unlike a typical piece of lost mail, they don’t come with a return address to reveal their provenance. But what if the scientists wishing to be historians of our solar system’s earliest days could sidestep these problems? Rather than relying solely on the random, scattered chapters of cosmic history from meteorites, wouldn’t it be better to directly visit space’s most ancient archives—the asteroids and comets—to bring back entire geologic books to read?
NASA’s Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft did just that in 2020, when it dove down to the surface of the near-Earth asteroid Bennu and retrieved some rocks dating back 4.5 billion years before bringing them back to Earth last September in dramatic fashion. It’s not the first (or second) spacecraft to burgle from an asteroid. But it retrieved the largest sample to date: a whopping 121.6 grams of pristine material from the solar system’s dawn.
Almost immediately after the sample return capsule landed on Earth, scientists began their forensic examinations. And earlier this month at the Lunar and Planetary Science Conference in The Woodlands, Tex., they presented their first in-depth findings for all the world to see. Their analyses are preliminary, but it seems that Bennu’s original form was shockingly familiar across the vast gulf of eons. Billions of years ago Bennu was apparently part of a water-soaked world now long lost and otherwise forgotten, one with a beating geologic heart and an abundance of prebiotic organic material. In many respects, this nameless world could have borne a passing resemblance to the early, lifeless Earth.
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“Bennu literally carries the building blocks of life within its minerals,” says Louisa Preston, an astrobiologist at University College London.
Firmer conclusions are still to come, but already it’s clear that these precious pieces of Bennu harbor immense potential. “What we’re trying to do with these samples is understand how Earth was formed—not just its water, not just its prebiotic compounds but how Earth itself formed,” says Harold Connolly, a geologist at Rowan University and mission sample scientist on OSIRIS-REx.
And it’s not all about our blue-green marble. Some of the sample’s microscopic grains reveal that Bennu’s odyssey began before the sun’s first fires burned, meaning that planetary scientists can use it as a toehold in their ascent toward answering one of their field’s most enduring and monumental queries. “What was that starting mineralogy of the solar system? Where did that dust come from? Did it all come from just one star or multiple generations of stars or different types of stars?” says Ashley King, a meteoriticist at London’s Natural History Museum and an OSIRIS-REx science team member.
Thanks to the mission’s daring raid on Bennu’s ancient archives, “we’re putting that all together,” Connolly says.
Presolar Prelude
The “Origins” in OSIRIS-REx’s full name refers to the genesis and history of Bennu as a proxy for all other carbon and water-rich asteroids that have circled the sun these last few billion years. It’s a mammoth undertaking. “We’ve looked at 1 percent of the sample” so far, Connolly says. But this is sufficient to begin to test a wish list of hypotheses the team has about Bennu’s life.
A top-down view of the contents of the OSIRIS-REx sample return capsule, revealing a treasure trove of material from the earliest epochs of our solar system’s history.
Credit: NASA/Erika Blumenfeld & Joseph Aebersold
A key question: What went into making Bennu’s original (or “parent”) body? Clues reside within its presolar grains, crystals that condensed before the sun existed—“basically, the building blocks of our solar system,” says Pierre Haenecour, a cosmochemist at the University of Arizona and OSIRIS-REx team member.
So far they’ve identified at least two broad categories of presolar grains. Plenty have the chemical signatures of intermediate-to-low-mass stars that were in the latter stages of their life; such stars produce potent stellar winds as they age, expelling much of their atmosphere into deep space to create clouds of gas and dust that can be recycled into a newborn star. Other grains hint at a more violent origin. “We do have some presolar grains that appear to have compositions … more consistent with what we find in supernovae,” Haenecour says. Altogether, this supports the long-standing suspicion that our solar system was seeded and enriched by the explosive deaths of a diverse range of thermonuclear furnaces.
Not long after the sun emerged, worlds began to coalesce around it under gravity’s influence, including Bennu’s unknown parent body. Bennu may exist as a midsize asteroid in a near-Earth orbit today, but the team suspects that, eons ago, its water-loaded parent first took shape beyond the snow line—a diffuse thermal circumstellar boundary that determines where more volatile substances, including water, can exist as ice around a star.
There is no agreement yet on just how far out Bennu’s protoworld formed. One hypothesis holds it wasn’t in the asteroid belt between Mars and Jupiter but somewhere further afield. Key to testing that notion will be the absence or presence of various ices and their residue within the sample; water ice can exist close to the sun, including within the asteroid belt, whereas frozen carbon monoxide needs to be more distant—somewhere in the realm of Neptune—to resist vaporization.
The array of temperamental chemicals already found in the sample is “consistent with an outer solar system origin,” says Kelly Miller, a cosmochemist at the Southwest Research Institute in San Antonio, Tex. Intriguingly, the detection of a soupçon of ammonia, an extremely volatile substance, was also announced at the conference. This could be associated with the asteroid’s organic matter. But if it came from ammonia ice, then “that would push [Bennu’s parent body] out even farther into the outer solar system,” Connolly says—perhaps in or beyond the realm of the ice giant planets Uranus and Neptune.
The Lost Water World
Wherever Bennu’s parent body formed, it was certainly not in stasis. The sample appears to be packed with clays and other mineral assemblages that are clear signs of dynamic transformations, such as being saturated in liquid water or even having some of that water evaporate to leave behind salts. “Bennu is dominated by materials that are altered by water,” says Sara Russell, a planetary scientist at London’s Natural History Museum and an OSIRIS-REx science team member.
Although the water wasn’t scorching hot, it was certainly warm and may have evolved in composition over time, which suggests that multiple hydrothermal systems were driven by melting ice. That ice melted, at least for a few million years, because the parent body had a toasty geologic heart heated by the decay of radioactive isotopes. Based on this, Bennu’s precursor was at least 10 kilometers wide, perhaps larger, Connolly says.
Back in February the mission team announced the surprising presence of phosphates in the sample. Beneath the icy carapace of Saturn’s moon Enceladus, a geologically tumultuous orb, is a warm liquid-water ocean that contains a range of ingredients essential to life, including phosphorous compounds. After finding phosphates within the Bennu sample, OSIRIS-REx principal investigator Dante Lauretta speculated that the asteroid “may be a fragment of an ancient ocean world.”
“I’m not willing to go there yet because we haven’t teased out enough of the petrology and petrography to put the story together,” Connolly says. But Bennu is decorated with features that may be linked to surprising geologic activity.
One of the types of rock the spacecraft observed on Bennu, which looks “cauliflowerlike”, is “a lot like a mélange,” Connolly explains—a smashed-up, squashed-together mess of a sediment-packed rock “that is typically formed in subduction zone areas” akin to those found at Earth’s continental margins and deep-sea basins. The thought of Bennu’s precursory world having an Earth-like shifting and tumbling of tectonic slabs is tantalizing, to say the least. But these rocks are chaotic and difficult to interpret. “It doesn’t mean that the parent body was tectonically active,” Connolly says.
Presently most are envisioning not so much a geologically hyperactive world but a waterlogged rock with a dynamic youth. “I like to think of it as a big mudball,” King says.
The Great Deliverer
That mudball eventually ended up in the asteroid belt, perhaps after being yanked out of a more distant orbit by the gravitational pull of Jupiter. One working hypothesis is that after about three billion years, this parent body was destroyed by a catastrophic collision, liberating the shard we know today called Bennu, which eventually made its way into near-Earth space.
An animation showing the rotation of near-Earth asteroid Bennu, captured over a four-hour period on December 2, 2018 by NASA’s OSIRIS-REx spacecraft.
Credit: NASA/Goddard/University of Arizona
That inward migration speaks to a key chapter in the history of the solar system: the delivery of water and prebiotic organic material—carbon-based compounds used by biology—to rocky worlds.
“It’s a long-standing question: Where did Earth’s water come from?” says Richard Binzel, an asteroid expert at the Massachusetts Institute of Technology and OSIRIS-Rex co-investigator. “For a long time, we thought it [came from] comets, because they’re the most water-rich things we see.” But in recent years investigations of water ice on various comets have revealed its chemical fingerprints to be quite different from those for the water that fills Earth’s oceans.
Conversely, the water found within myriad soggy meteorites is a far closer match to that of our planet’s reservoirs. And what of Bennu? That grand reveal is still some time away, but regardless of whether Bennu has Earth-like water, that perennial query won’t be definitively answered—Earth’s seas and oceans were probably procured from a variety of cosmic sources. It’s also possible that their creation wasn’t reliant on asteroids at all; rather, Earth’s oceans may have been imprisoned within the planet as it formed before escaping to the surface via ancient volcanism.
Then there are the organic compounds. “Biology started off life as chemistry,” Preston says. Even in the exceedingly unlikely event that the OSIRIS-REx sample harbors fossilized alien microorganisms, Bennu won’t provide any concrete answers as to how life got started on Earth. But life couldn’t exist at all without a suite of carbon-bearing compounds such as amino acids. One idea is that these formed in the spaces between the stars before asteroids like Bennu brought these ingredients to Earth.
“We know [that asteroids] can deliver these things to the Earth. But the key step is: How did they become life? We need to know that inventory to be able to answer that,” King says. And already the team has identified a long list of organic molecules, including a suite of amino acids, present in the sample. “They even found uracil and thymine—uracil being one of the four nucleotide bases used in RNA and … substituted by thymine within DNA,” Preston says.
Some of these vital-to-life substances also have primeval inceptions. “Bennu contains organic matter that formed in the interstellar medium,” said Ann Nguyen, a planetary scientist at NASA and an OSIRIS-REx co-investigator, during a presentation at the conference.
Not all astrobiologists are fixated on amino acids. “I might be a bit of a heretic,” says Cole Mathis, an astrobiologist at Arizona State University. But he isn’t especially interested in abundances of organic matter in Bennu. “It’s not hard to make amino acids.” If you combine nitrogen, carbon and oxygen, he says, “these things are more or less unavoidable.” Asteroids may have delivered them to Earth, but much like the planet’s water, these compounds could have also easily formed on Earth without requiring a Bennu-like delivery.
Mathis wants to use Bennu to explore the boundary between chemistry and biology. “There are some molecules that are so complex that only life could have made them,” he says, offering vitamin B12 as an example. He isn’t expecting anyone to find anything like that in the sample. But he wants to find out which molecules can be made by both life and abiotic chemistry and which can only be made by life. “Where should that transition be?”
Bennu, he hopes, will offer hints as to where this boundary lies—because the more baroque an organic compound is, the trickier it is for chemistry alone to make it. Mathis’s query, then, is not about abundance but chemical convolution: “What’s the most complex individual molecules we can find in these materials?”
Answers to this question and many others are forthcoming. They are hidden within a small pouch of pristine asteroid material awaiting interrogation. Those grains may have cost $1.2 billion dollars to bring back home. But they are effectively priceless because they can add context to that famous aphorism: “we are all stardust.” Scientists are now beginning to find out the exact nature, and provenance, of this stardust—the stuff that went into making everything we see, including Earth and ourselves.
Hopes had been high when OSIRIS-REx scooped up that sample from Bennu. Already they have been surmounted. “The universe was smiling on us,” Connolly says.
