Surprisingly salty asteroid Bennu contains the building blocks of life
These amazing discoveries would not have been possible without NASA's OSIRIS-REx mission.
Pristine samples of asteroid Bennu have revealed something entirely unexpected — not only does the asteroid contain the chemical building blocks of life, but it also originated from an ancient, long-gone parent body that apparently had briny liquid water on its surface.
In September 2023, NASA's OSIRIS-REx spacecraft dropped off a special delivery that scientists had been anticipating for nearly 3 years. Secured inside that delivery was over 120 grams of material collected directly from 101955 Bennu, a large near-Earth asteroid that NASA ranks as one of the biggest potential impact threats we know about.
Asteroid Bennu, as imaged by the OSIRIS-REx spacecraft's camera. (NASA Goddard)
Collected in October of 2020, this sample was the very first NASA ever returned from an asteroid for study by scientists back here on Earth.
Now, two new papers have been published detailing the results of that research.
The first paper, which appears in the journal Nature, revealed something completely unexpected — the rocks from Bennu contain various types of salt crystals known to form when briny water slowly evaporates over time. One of these is sodium carbonate, which we find here on Earth in a white powdery form after salty lakes like Searles Lake, in the Mojave Desert, dry up.
"We're seeing minerals in Bennu samples that we have never seen before in a meteorite or any extraterrestrial sample," Tim McCoy, curator of meteorites at the Smithsonian's National Museum of Natural History and lead author of the research paper, said in a University of Arizona press release.
Salt minerals from the Bennu samples are shown here that were deposited as a result of brine evaporation from the asteroid’s parent body. The needles of hydrated sodium carbonate (in purple in this false-colored image) would likely have been lost if the samples had been exposed to water in Earth’s atmosphere. (Rob Wardell/Tim McCoy/Smithsonian Institution/Heather Roper/University of Arizona)
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Bennu may look like one solid mass from afar, but it's not. Instead, it is loose collection of boulders, rocks, and gravel that all clumped together due to their mutual gravitational pull on each other. If you were able to instantly (and safely) transport Bennu from space to the surface of Earth, it would collapse into a giant heap. Thus, Bennu is often referred to as a 'rubble pile' asteroid.
It is believed that Bennu formed when there was a massive collision between asteroids, 1-2 billion years ago, and fragments from the parent body then collected together to form the asteroid we see today.
The discovery of these salts in the Bennu samples means that whatever that original parent body was, it has water on its surface, possibly in a liquid briny form.
"We didn't even predict this based on what we observed remotely at the asteroid," says Dr. Kim Tait, the Senior Curator of Mineralogy at the Royal Ontario Museum, who is a co-author of the study. "That's the reason NASA has missions and why we study the samples so carefully in our Earth-based laboratories, all for the chance to find the unexpected."
Meteorites can tell us a lot about the nature of asteroids. However, meteorites experience intense heat and pressure when they enter Earth's atmosphere, and they are exposed to water and oxidation between the time when they fall and when they are picked up and taken to a lab. Due to those changes, we lose a certain amount of what they could tell us.
Some meteorites have been found mere moments after they've fallen. To learn as much as we can, though, scientists need pristine samples that haven't been exposed to any of those changes, like those collected by OSIRIS-REx.
The OSIRIS-REx sample containment pod is shown here, shortly after touching down in October 2023. (NASA)
"The only way this could be discovered is because of controlled sample return by a spacecraft, careful curation to limit exposure to the Earth's atmosphere, and state-of-the-art instruments used to study the samples," Tait explained. "These minerals are unstable in Earth's atmosphere, so studying meteorites we would unlikely see these minerals."
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The second study, published in Nature Astronomy, found something even more profound.
The Bennu samples contain 14 of the 20 amino acids that are essential for life on Earth to build proteins. Also found were all five of the basic building block for life — Guanine, Cytosine, Adenine, Thymine, and Uracil, the key components of both DNA and RNA.
These five nucleotide bases, which combine to form DNA (Cytosine, Adenine, Thymine, and Guanine) and RNA (Cytosine, Adenine, Guanine, and Uracil), were all found in samples of asteroid Bennu (NASA Goddard/OSIRIS-REx)
"These samples from Bennu are an incredible discovery, showing that the building blocks of life were widespread across the early solar system," said Dante Lauretta, a co-author of both papers from the University of Arizona's Lunar and Planetary Laboratory who led the OSIRIS-REx sample return mission. "By studying how these ingredients interacted in environments like those on Bennu and in places inferred for the early Earth — such as salty ponds similar to those Darwin once imagined — we can better understand how life might emerge and where to search for it beyond our planet."
Launched in September 2016, the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft began orbiting Bennu on New Years Eve 2018, collected a sample from its surface in October 2020, and returned that sample in September 2023.
Bennu was chosen because it is one of the largest potentially hazardous asteroids known. Its overall odds of hitting Earth are fairly low. The greatest probability is on Sept. 24, 2182, with a 1 in 2,700 chance. While that's still a 99.963 per cent chance of a miss, given Bennu's size, at roughly half a kilometre wide, any chance of an impact is worth keeping an eye on.
This graphic shows asteroid Bennu's closest potential pass by Earth, in Sept. 2182, with just a 0.037 per cent chance of impact. The exact distance from Earth on that date apparently depends on how close the asteroid gets during an encounter on Sept. 25, 2135, and how much its orbit is diverted by Earth's gravity. (NASA Goddard)
"OSIRIS-REx has been a highly successful mission," Jason Dworkin, OSIRIS-REx project scientist at NASA Goddard and co-lead author on the Nature Astronomy paper, said in a NASA press release. "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."
Having completed its first mission, OSIRIS-REx has a new goal, and a new name — OSIRIS-APEX (APEX = APophis EXplorer) — as it will rendezvous with asteroid Apophis just after the close encounter with Earth in April 2029.