Written by Sara Russell and Timothy J. McCoy
On the evening of March 31, 1965, a dazzling fireball blazed across the sky above spruce forest, mountains, and glaciers in British Columbia, Canada, close to the town of Revelstoke.Beaver trappers found pieces of this meteorite that had fallen over a lake. They were shielded from the depths by a layer of ice, which also gave astronomers a glimpse into the solar system’s early history.
A sample of an asteroid called Bennu, which rained boulders over Revelstoke, was brought back from space by NASA’s OSIRIS-REx mission almost 60 years later. A chemical examination of those rocks, released by our research team, sheds light on how some of the elements necessary for life would have initially reached Earth.
The two of us were born in the years surrounding the fall of the Revelstoke meteorite and have worked in the meteorite collections at the Natural History Museum in London and the Smithsonian Institution in Washington, D.C. We have fantasized about examining samples taken by a spaceship from an asteroid similar to Revelstoke.
Then, over twenty years ago, we started making those aspirations come true. In order to launch a spacecraft to gather and bring back an asteroid sample to Earth, we joined NASA’s OSIRIS-REx mission team. We were able to delve into a story of rock, ice, and water that suggests how life might have originated on Earth once those samples arrived on September 24, 2023.
The CI chondrites and asteroid Bennu
We began by studying meteorites in order to learn more about an asteroid, which is a rocky or metallic object in orbit around the Sun.
Bennu is one among the rocky or metallic asteroids that circle the Sun. The fragments of asteroids and other naturally occurring alien objects that make it through the scorching descent to the Earth’s surface are known as meteorites.
Our goal was to investigate an asteroid that resembled a group of meteorites known as chondrites, which originated billions of years ago in a cloud of gas and dust at the beginning of the solar system.
The Revelstoke meteorite belongs to a class of meteorites known as CI chondrites. The elements delivered by convection from the Sun’s interior and measured in the outermost layer are nearly the same as the compositions of CI chondrites tested in laboratories, with the exception of hydrogen and helium. They resemble chemically unaltered time capsules of the early solar system since their constituent parts were produced billions of years ago.
Therefore, the final reference standard for geochemistry is the chemical compositions of CI chondrites. From Earth rocks to other chondrites, they are able to compare their compositions. The same mechanisms that created planets and asteroids would have produced any variations from the composition of CI chondrites.
Clay-rich CI chondrites were created when ice melted on an old asteroid, changing the rock. They also contain a lot of organic prebiotic compounds. The building blocks of life are some of these kinds of molecules.
Because of this mix of rock, water, and organics, OSIRIS-REx decided to examine the organic-rich asteroid Bennu, which contains organic chemicals and water that are crucial to the genesis of life.
Evaporites the legacy of an ancient brine
We and our colleagues on four continents have been investigating the Bennu samples for hundreds of hours since they were returned to Earth on September 24, 2023.
When the OSIRIS-REx spacecraft drew close to the asteroid Bennu, its instruments observed reflected light that showed the most abundant minerals and organics. Our laboratory studies revealed that these observations were consistent with the compositions of the samples.
The majority of the samples are clay that is rich in water, containing minerals such as sulfide, carbonate, and iron oxide. These minerals are also present in Revelstoke and other CI chondrites. However, we were both taken aback when uncommon minerals were found in the Bennu samples. Many of these minerals are unknown to us, even after decades of studying meteorites.
We discovered sodium-dominated minerals, such as potassium chloride, magnesium phosphate, carbonates, sulfates, chlorides, and fluorides. These minerals are not formed only by the reaction of rock and water. When water evaporates, they are created.
Many of these sodium-rich minerals are occasionally discovered in dried-up lake beds on Earth, such as Searles Lake in California, but we have never encountered them in meteorites.
Bennu’s rocks originated on a bigger parent asteroid 4.5 billion years ago. It was a dirty, damp asteroid. The evaporite minerals we discovered in the sample were the result of water evaporating beneath the surface in pockets that might have been just a few feet broad. The ancient lake beds where these minerals can be found on Earth were also created by the same evaporation mechanism.
Bennu’s parent asteroid most likely broke off between one and two billion years ago, with some of the pieces assembling to create the mound of rubble that is today known as Bennu.
The outer solar system’s cold bodies also contain these minerals. Sodium carbonate is found in bright concentrations on the largest body in the asteroid belt, the dwarf planet Ceres. The same material was detected in plumes on Enceladus, Saturn’s moon, by the Cassini mission.
We also discovered that even with the very small amount of water present in the air, these minerals—which are created when water evaporates—disappear when exposed to water again. Researchers preserved some of the Bennu samples in air after examining them and their minerals. We use meteorites in this way.
Sadly, humanity lost these minerals since they dissolved due to the moisture in the Earth’s atmosphere. However, that explains why meteorites that have lived on Earth for decades or even centuries do not contain these minerals.
Thankfully, the majority of the samples have been transported and preserved in nitrogen, shielding them from airborne water traces.
This collection of minerals has never been observed in a meteorite until scientists were able to use a spacecraft to perform a controlled sample return and meticulously select and preserve the samples in nitrogen.
An unexpected discovery
The OSIRIS-REx mission observed Bennu for more than two years before returning the samples. Researchers discovered that the asteroid’s surface is covered in stony pebbles as a result of their two years of effort.
We observed veins of white carbonate a few feet long, which were left behind by ancient liquid water, and we could see that the asteroid is rich in carbon and water-bearing clays. However, the rarer minerals were not seen from these observations.
One small grain at a time, we went through the returning sample using a variety of methods. We were able to examine the rock at a scale that was not feasible on the asteroid thanks to techniques like CT scanning, electron microscopy, and X-ray diffraction.
Cooking up the ingredients for life
From the salts we identified, we could infer the composition of the briny water from which they formed and see how it changed over time, becoming more sodium-rich.
It would have been perfect for the formation of organic molecules and new chemical processes to occur in this salty water.
While our team characterized salts, our organic chemist colleagues were busy identifying the carbon-based molecules present in Bennu. They found unexpectedlyhigh levels of ammonia, an essential building block of theamino acidsthat form proteins in living matter. They also found all five of the nucleobases that make up part ofDNA and RNA.
Based on these results, we d venture to guess that these briny pods of fluid would have been the perfect environments for increasingly complicated organic molecules to form, such as the kinds that make up life on Earth.
When asteroids like Bennu hit the young Earth, they could have provided acomplete packageof complex molecules and the ingredients essential to life, such as water, phosphate and ammonia. Together, these components could have seeded Earth s initially barren landscape to produce a habitable world.
Without this early bombardment, perhaps when the pieces of the Revelstoke meteorite landed several billion years later, these fragments from outer space would not have arrived into a landscape punctuated with glaciers and trees.
Timothy J. McCoy is a Supervisory Research Geologist at the Smithsonian Institution. Sara Russell is a Professor of Planetary Sciences at the Natural History Museum.
