A BEEP sounded in Katherine Joy’s earpiece and a light flashed on her handlebar display. The metal detector dragging behind her snowmobile had found something buried in the thick Antarctic ice. She dismounted. Could this finally be it?
A convoluted story had brought Joy and her fellow treasure hunters here, 700 kilometres south of the British Antarctic Survey’s Halley VI research station. You might say the story started 4.5 billion years before, as a result, probably, of a massive star going supernova. Its rumbling shock wave caused a cloud of dust and gas laced with heavier elements to begin to collapse in on itself, eventually forming the sun and the planets, moons, asteroids and, eventually, other components of our solar system, like us.
For decades, researchers have been hunting for pristine material from these turbulent times to better understand exactly how these processes occurred. Joy and her colleagues had ventured out into the Antarctic wilderness following a hot lead to fill a crucial gap in the tale: the mystery of the missing meteorites. What they found, however, wasn’t one mystery, but two.
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Meteorites are time capsules from the solar system’s birth. They are mostly fragments of asteroids that orbit between Mars and Jupiter, plus the occasional unsullied piece of the moon or Mars that has come unstuck and crossed Earth’s orbital path. These fragments have existed more or less untouched since bits of rock first began to accrete from smaller dust particles as they whirled around the infant sun. With their chemistry unadulterated by the tectonics, volcanism and other violent processes of Earth, they preserve vital clues as to how the solid parts of the solar system formed.
We know lots of individual details about what must have happened as bits of rock crashed together and aggregated to form larger bodies, or sometimes split apart again in the maelstrom of the early solar system. But we lack a convincing, unifying picture. “We don’t have an equivalent of, say, evolution by natural selection in planetary science right now,” says Luke Daly at the University of Glasgow, UK. “We don’t have a good theory that takes us all the way from gas and dust to planetary systems.”
Iron pilings
Meteorites come in a wide variety of shapes and sizes, from the 60-tonne Hoba meteorite, discovered in Namibia in 1920, to the tiny specks of dust constantly raining down on our planet that present their own difficulties to researchers (see “Dusting The Rooftops”).
The vast majority of meteorites found worldwide are stony, made mainly of dull-looking silicate rock of the sort that makes up the bulk of a planet like Earth. Then there are the stony-iron meteorites, formed of a mixture of rock and metal. A striking example of these is the Imilac meteorite that was found in the Atacama desert in Chile hundreds of years ago. look like dazzling slices of extraterrestrial stained glass.
The third category comprises the iron meteorites, made of a mix of iron and nickel. The Hoba meteorite is the largest known example of this type, which is particularly crucial to understanding the solar system’s origins. For a lump of iron and nickel to have formed, it must at some point have been part of a space rock that grew so large that its innards melted, allowing heavy metals to sink to its core and less dense rock to rise.
A similar process of differentiation created Earth’s dense, partially liquid iron-nickel core surrounded by a mantle and crust of silicate rock. The moving liquid metal in these early space rocks would have generated a magnetic field, just as Earth’s churning core does today. Iron meteorites are fragments of these early cores. By looking at preserved traces of the magnetic fields now frozen in them, we can work out how large their parent rock must have been – and so how, and how quickly, the process of planet formation progressed.
“They’re a really enigmatic and interesting group to study because they’re the only way we can actually think about planetary interiors,” says Claire Nichols at the Massachusetts Institute of Technology. “The more samples we can possibly get our hands on, the more we can learn about how planets work.”
Whatever type of meteorite you are interested in, Antarctica is far and away the best place on Earth to look for them. Its frozen surface is largely pristine and untouched for millennia. Interior glaciers act like a conveyor belt, slowly carrying any rocks that fall on them down from the high points of the East and West Antarctic ice sheets towards the Transantarctic mountains, a range rearing out of the ice that divides the two regions as it slices across the continent. As the glaciers butt up against the mountain rock, they force ice and any space rocks trapped within it up to the surface. Each Antarctic summer, from October to February, teams from around the world head south to harvest them. The continent provides almost two-thirds of verified meteorite finds made on Earth.
“Antarctica provides two-thirds of all meteorite finds on Earth”
But here’s the thing: discoveries of iron and stony-iron meteorites, rare at the best of times, are almost non-existent in Antarctica. Why this should be is a mystery that has stumped many meteorite hunters, and one that hampers our attempts to construct a convincing story of the early solar system (see “Precious metal”).
Geoffrey Evatt never intended to get mixed up in this problem. A mathematician at the University of Manchester, UK, he began his career developing models of the dynamics of glaciers mainly because he liked mountain climbing and the outdoors. He first heard about the missing meteorites by chance at a scientific meeting. “At that point, I wouldn’t have known a meteorite if it had hit me on the head,” he says.
Things really kicked off in 2012 when Joy, an old friend and climbing buddy of his, moved to the University of Manchester. She just happened to be a meteorite expert. Discussing the mystery, Evatt and Joy hit upon a possible solution to the problem.
It went like this. Metal meteorites are much better at absorbing heat than stony ones. As the glaciers pushed them upwards at the base of the mountains, they would begin to absorb more sunlight, warm up, melt the ice around them and sink again. Unlike other types, metal-rich meteorites would remain perennially trapped a few centimetres beneath the surface. In other words, they aren’t missing at all – just hiding.
The pair tested the idea by shining sunlight-simulating lamps on a stony and an iron meteorite in ice. Sure enough, the iron meteorite sank. “We’d gone from a bonkers theory to a point where we actually had evidence to explain the absence of the iron meteorites,” says Evatt. It was enough for him and Joy to secure a grant to put together to build some kit – a metal-detecting array commonly used to clear landmines, adapted and towed by a snowmobile – and go meteorite hunting.
By 2019, after a test in the Arctic and a first reconnaissance trip to Antarctica, the search was on in earnest. “Meteorite hunting is the best thing in the world,” says Joy. “You’re rocketing up and down over this quite bumpy surface, avoiding ice patches and looking around. And when you spot a meteorite, you kind of jump, the heart lifts a little bit.”
Things didn’t pan out quite as expected, however. The metal-detecting rig got badly bashed up by the bumpy ice, requiring frequent repairs. And while the researchers found lots of meteorites on the surface, about 130 of them all told, they didn’t discover any in the ice. Even that promising moment when Joy’s earpiece beeped was just one of many false alarms, caused by a metal screw that had fallen off her rig.
But there was an odd twist in the tale. Many of the rocks are still on a ship heading back to the UK for full analysis, but based purely on appearance, there are far more iron-rich meteorites among the surface finds than anyone expected. “It seems, in a very preliminary way, like we may well have found the lost iron meteorites of Antarctica,” says Evatt. “But for this particular area, they were right in front of our noses.”
Which means there are now two mysteries. The first is why the researchers failed to find any meteorites beneath the ice. It could be that their hypothesis and successful lab tests led them down the wrong path, or perhaps their equipment troubles meant they just hadn’t searched a wide enough area. The second mystery is why, comparatively, they found so many iron-rich meteorites on the surface when so many others before hadn’t.
“The lost meteorites may have been right in front of our noses”
Fluke find?
The Outer Recovery Ice Fields, as Evatt and Joy named their stomping grounds, hadn’t been searched before, and the discoveries could be a statistical blip: the small area they searched may have just had a larger than average concentration of iron meteorites. It might not say anything about Antarctica more generally. Alternatively, it could indicate some subtle shift in the ice dynamics, perhaps as a result of climate change – though Joy, for one, thinks that is unlikely, as the effects of global warming are only just being felt in the Antarctic interior.
Beginning to find answers will require a more conclusive analysis of the researchers’ haul. “We haven’t begun to ask these questions ourselves yet because we don’t know what we’ve found,” says Joy. Whatever the exact make-up of the meteorites turns out to be, however, planetary scientists are rubbing their hands. “Given that they’ve just been sitting on the ice, they’re highly pristine, so I would argue these are more valuable, probably, than a lot of what we have available at the moment,” says Nichols. James Bryson at the University of Oxford agrees. “I would totally be interested in getting hold of these meteorites once they have classified them,” he says.
The question then becomes whether the trove is a fluke or if we can find more of this precious cargo. That will mean venturing back into the field armed with whatever new information the samples provide to inform the search. Joy says that she is currently exploring sources of funding for a longer-term programme, and that other teams are welcome to replicate their search equipment and go foraging on the ice themselves.
If that happens, it won’t be for a few months yet. As Antarctica slips into the depths of its winter, no one is venturing out to the lonely interior glaciers for answers right now. But their slow movements will continue to churn more rocks from the dawn of our solar system down towards the mountains. When winter turns into summer once again, some more could be just waiting for someone to pick them up.
Dusting The Rooftops
Iron meteorites aren’t the only bits of space rock we have difficulty locating (see main story). Every day, Earth is bombarded by countless micrometeorites. Each is roughly the size of a full stop, but a day’s worth weighs in at an estimated 100 tonnes.
A phenomenon known as the Poynting-Robertson effect means these small particles get sucked in towards the sun, potentially crossing Earth’s orbit. Because of this, they come from a wider spread of space. They thus give a much broader feel for the chemistry of the early solar system than standard meteorites, which largely come from the asteroid belt between Mars and Jupiter. “The difference between looking at meteorites and micrometeorites is like looking at a piece of the jigsaw or looking at the whole puzzle,” says Matthew Genge at Imperial College London. But their size makes them hard to find.
Enter one of Norway’s most famous jazz guitarists, Jon Larsen. Whereas researchers typically go to pristine areas such as the bottom of the sea or Antarctica to collect micrometeorites, he was convinced he could gather them using a magnet wrapped in a plastic bag on the rooftops of Oslo, and contacted Genge for help.
Ideas similar to Larsen’s are popular among some amateur astronomers, and aren’t beyond the pale: micrometeorites do fall on urban rooftops. But then so does a lot of other dust in the form of pollution, making the search for them there seem like a fool’s errand. “My intention at first was to convince him this was a bad idea and to basically make him go away,” says Genge.
But Larsen persisted, scouring through three rubbish bins’ worth of metallic dust he had collected and sending Genge pictures of tiny spheres that looked promising. Some of them began to catch Genge’s eye. Eventually, the pair had amassed 500 of what they believed to be micrometeorites. that all 500 were probably the real deal.
These days, Larsen runs Project Stardust, a Facebook community of people who hunt for micrometeorites and share their finds. Every so often, a picture of a new type is posted. “It’s a bit like birdwatching,” says Genge. And the more we find, the more likely we are to find a truly exotic migratory specimen: by Genge’s reckoning, about 1 in 5000 micrometeorites must originate outside our solar system in interstellar space.
