A brilliant fireball streaked throughout the sky above mountains, glaciers and spruce forest close to the city of Revelstoke in British Columbia, Canada, on the night of March 31, 1965. Fragments of this meteorite, found by beaver trappers, fell over a lake. A layer of ice saved them from the depths and allowed scientists a peek into the beginning of the photo voltaic system.
Almost 60 years later, NASA’s OSIRIS-REx mission returned from area with a pattern of an asteroid named Bennu, much like the one which rained rocks over Revelstoke. Our analysis staff has revealed a chemical evaluation of these samples, offering perception into how among the elements for all times might have first arrived on Earth.
Born within the years bracketing the Revelstoke meteorite’s fall, the 2 of us have spent our careers within the meteorite collections of the Smithsonian Establishment in Washington, D.C., and the Pure Historical past Museum in London. We’ve dreamed of learning samples from a Revelstoke-like asteroid collected by a spacecraft.
Then, almost twenty years in the past, we started turning these desires into actuality. We joined NASA’s OSIRIS-REx mission staff, which aimed to ship a spacecraft to gather and return an asteroid pattern to Earth. After these samples arrived on Sept. 24, 2023, we acquired to dive right into a story of rock, ice and water that hints at how life may have fashioned on Earth.
On this illustration, NASA’s OSIRIS-REx spacecraft collects a pattern from the asteroid Bennu.
NASA/Goddard/College of Arizona
The CI chondrites and asteroid Bennu
To study an asteroid – a rocky or metallic object in orbit across the Solar – we began with a examine of meteorites.
Asteroids like Bennu are rocky or metallic objects in orbit across the Solar. Meteorites are the items of asteroids and different pure extraterrestrial objects that survive the fiery plunge to the Earth’s floor.
We actually wished to review an asteroid much like a set of meteorites known as chondrites, whose parts fashioned in a cloud of fuel and dirt on the daybreak of the photo voltaic system billions of years in the past.
The Revelstoke meteorite is in a bunch known as CI chondrites. Laboratory-measured compositions of CI chondrites are basically equivalent, minus hydrogen and helium, to the composition of components carried by convection from the inside of the Solar and measured within the outermost layer of the Solar. Since their parts fashioned billions of years in the past, they’re like chemically unchanged time capsules for the early photo voltaic system.
So, geologists use the chemical compositions of CI chondrites as the final word reference normal for geochemistry. They’ll evaluate the compositions of all the pieces from different chondrites to Earth rocks. Any variations from the CI chondrite composition would have occurred by means of the identical processes that fashioned asteroids and planets.
CI chondrites are wealthy in clay and fashioned when ice melted in an historical asteroid, altering the rock. They’re additionally wealthy in prebiotic natural molecules. A few of these varieties of molecules are the constructing blocks for all times.
This mixture of rock, water and organics is one motive OSIRIS-REx selected to pattern the organic-rich asteroid Bennu, the place water and natural compounds important to the origin of life may very well be discovered.
Evaporites − the legacy of an historical brine
Ever because the Bennu samples returned to Earth on Sept. 24, 2023, we and our colleagues on 4 continents have spent a whole lot of hours learning them.
The devices on the OSIRIS-REx spacecraft made observations of mirrored mild that exposed essentially the most considerable minerals and organics when it was close to asteroid Bennu. Our analyses within the laboratory discovered that the compositions of those samples lined up with these observations.
The samples are principally water-rich clay, with sulfide, carbonate and iron oxide minerals. These are the identical minerals present in CI chondrites like Revelstoke. The invention of uncommon minerals inside the Bennu samples, nevertheless, shocked each of us. Regardless of our many years of expertise learning meteorites, we’ve got by no means seen many of those minerals.
We discovered minerals dominated by sodium, together with carbonates, sulfates, chlorides and fluorides, in addition to potassium chloride and magnesium phosphate. These minerals don’t type simply when water and rock react. They type when water evaporates.
A microscope’s view of micrometer-sized needles of hydrated sodium carbonate in a Bennu pattern. These minerals fashioned throughout evaporation of an historical sodium-rich brine greater than 4.5 billion years in the past.
Rob Wardell and Tim McCoy, Smithsonian Establishment
We’ve by no means seen most of those sodium-rich minerals in meteorites, however they’re typically present in dried-up lake beds on Earth, like Searles Lake in California.
Bennu’s rocks fashioned 4.5 billion years in the past on a bigger mother or father asteroid. That asteroid was moist and muddy. Below the floor, pockets of water maybe just a few ft throughout have been evaporating, leaving the evaporite minerals we discovered within the pattern. That very same evaporation course of additionally fashioned the traditional lake beds we’ve seen these minerals in on Earth.
Bennu’s mother or father asteroid probably broke aside 1 to 2 billion years in the past, and among the fragments got here collectively to type the rubble pile we all know as Bennu.
These minerals are additionally discovered on icy our bodies within the outer photo voltaic system. Brilliant deposits on the dwarf planet Ceres, the most important physique within the asteroid belt, include sodium carbonate. The Cassini mission measured the identical mineral in plumes on Saturn’s moon Enceladus.
We additionally discovered that these minerals, fashioned when water evaporates, disappear when uncovered to water as soon as once more – even with the tiny quantity of water present in air. After learning among the Bennu samples and their minerals, researchers saved the samples in air. That’s what we do with meteorites.
Sadly, we misplaced these minerals as moisture within the air on Earth precipitated them to dissolve. However that explains why we are able to’t discover these minerals in meteorites which were on Earth for many years to centuries.
Luckily, a lot of the samples have been saved and transported in nitrogen, protected against traces of water within the air.
Till scientists have been capable of conduct a managed pattern return with a spacecraft and thoroughly curate and retailer the samples in nitrogen, we had by no means seen this set of minerals in a meteorite.
An surprising discovery
Earlier than returning the samples, the OSIRIS-REx spacecraft spent over two years making observations round Bennu. From that two years of labor, researchers discovered that the floor of the asteroid is roofed in rocky boulders.
We may see that the asteroid is wealthy in carbon and water-bearing clays, and we noticed veins of white carbonate a couple of ft lengthy deposited by historical liquid water. However what we couldn’t see from these observations have been the rarer minerals.
We used an array of methods to undergo the returned pattern one tiny grain at a time. These included CT scanning, electron microscopy and X-ray diffraction, every of which allowed us to take a look at the rock at a scale not potential on the asteroid.
Sara Russell and Tobias Salge look at a Bennu pattern within the scanning electron microscope on the Pure Historical past Museum in London.
Innes Clatworthy, Pure Historical past Museum, London.
Cooking up the elements for all times
From the salts we recognized, we may infer the composition of the briny water from which they fashioned and see the way it modified over time, turning into extra sodium-rich.
This briny water would have been an excellent place for brand new chemical reactions to happen and for natural molecules to type.
Whereas our staff characterised salts, our natural chemist colleagues have been busy figuring out the carbon-based molecules current in Bennu. They discovered unexpectedly excessive ranges of ammonia, a vital constructing block of the amino acids that type proteins in residing matter. In addition they discovered all 5 of the nucleobases that make up a part of DNA and RNA.
Based mostly on these outcomes, we’d enterprise to guess that these briny pods of fluid would have been the proper environments for more and more sophisticated natural molecules to type, such because the sorts that make up life on Earth.
When asteroids like Bennu hit the younger Earth, they may have supplied a whole package deal of complicated molecules and the elements important to life, equivalent to water, phosphate and ammonia. Collectively, these parts may have seeded Earth’s initially barren panorama to supply a liveable world.
With out this early bombardment, maybe when the items of the Revelstoke meteorite landed a number of billion years later, these fragments from outer area wouldn’t have arrived right into a panorama punctuated with glaciers and timber.