One night in April, 2019, the skies above La Palmera, a village in northern Costa Rica, started to glow as a motorcycle-sized meteorite broke apart and scattered chunks of hot space rock over the rain forest below. It was just one of thousands of meteorites that hit the Earth every year, but this particular one, later dubbed Aguas Zarcas, caused a frenzy among experts. To the untrained eye, its fragments look like unassuming gray rock. But packed inside are a menagerie of organic molecules and space dust that predate the birth of our solar system.
>
Aguas Zarcas is among the most pristine examples ever discovered of a class of meteorites known as carbonaceous chondrites. It’s a deeply unsexy name, but these ancient space rocks are like time machines that provide windows into the universe as it existed billions of years ago. They’re unique geological records that detail the formation of amino acids in space, which some scientists believe may have been the abiotic grist that kick-started the evolution of life on Earth. They’re a rarity among rarities, prized by collectors and scientists, and are often worth more than their equivalent weight in gold.
Carbonaceous chondrites play a starring role in Meteorite, a new book by the University of Bristol cosmochemist Tim Gregory. But these bizarre extraterrestrial visitors are just one of a seemingly endless variety of weird and wonderful space rocks, and Gregory’s passion for his subject drips from every page. Meteorite is a mix of science and history that’s filled with anecdotes of close calls and happy accidents. Gregory strikes a good balance between hard science and the hard-to-believe, but he promises everything between the covers is true.
WIRED caught up with Gregory at home in Nottingham, England, to learn more about the book and why the best place to find a meteorite is at the end of the Earth. The following interview has been lightly edited for clarity and length.
WIRED: You work as a ‘cosmochemist.’ What is cosmochemistry, and how did you get into it?
Gregory: I've always loved rocks, and I've always loved space, as well. I discovered a couple of years into my undergraduate degree that there's a discipline that combines both of them—rocks and space—and that's cosmochemistry. It uses the same tools as geochemistry, but it just happens to be on rocks from outer space instead of the Earth.
What makes space rocks different from Earth rocks?
There are a few things that distinguish meteorites from Earth rocks. The most obvious one is their age. Almost all meteorites we’ve discovered come from asteroids, and they cooled down very quickly after they formed. The Earth has an internal heat engine through the decay of radioactive isotopes that is still powering volcanic and tectonic processes. So the Earth is still geologically active, whereas the geological processes on these asteroids was very short-lived. So the rocks that come from these places, the meteorites, haven't changed much at all in the last four and a half billion years. They’re far older than the oldest Earth rocks.
How can you tell a meteorite from any other rock on Earth unless you see it fall to the ground?
Meteorites look exactly like Earth rocks, so we have to go into the chemistry and look at their isotope composition. There are very subtle chemical differences that sort of prove their extraterrestrial origin. They come from fundamentally different worlds, which inherited a slightly different blend of chemicals when they formed. With the meteorites, there's no way that you can find that sort of chemical fingerprint on Earth unless it came from another world.
Where do scientists find their meteorites?
We've got about 60,000 meteorites in the worldwide collection, and most of them came from Antarctica. There are a few reasons for that. The first one is really obvious: Generally, meteorites are really dark when they land on the surface, and ice is white. So they stand out like a sore thumb on the ice sheet.
But there's another really curious property in Antarctica that I go into some detail about in the book. Antarctica has this flowing ice sheet that acts like a natural conveyor belt. Meteorites pitter-patter down on it at the same rate they do all over the planet, but unlike the rest of the planet, Antarctica’s ice flows from the center out toward the sea, and it takes the meteorites with it. And if the subsurface land beneath the Antarctic ice sheet is just the right topography, these mountains below the ice sheet can sort of boost the ice off and cause it to stop flowing. If that coincides with the place where there's really high velocity winds, all the ice is stripped away from the surface, leaving behind the meteorites. So you get these “accumulation zones” in Antarctica where meteorites pile up in phenomenal numbers—many hundreds of times the normal density of meteorites that you might expect elsewhere on the planet. So since the ‘70s there have been regular expeditions there, and they're still finding them all the time.
You’ve studied the chemistry of meteorites for your PhD, but you said a lot of the history in Meteorite was new to you. What was the most surprising thing you learned from digging into the history of space rocks?
One of the most surprising things to me was how quickly the scientific establishment came around to accepting these things as real. Nobody really believed that meteorites were real in 1800, and then five years later, it was pretty much universally accepted that they were real. It so happened that asteroids started being discovered around the same time, and that was amazing timing. Edward Howard did his chemical analyses of a meteorite in 1802, and within a year or two there were a handful of asteroids that were discovered, and it all just seemed to knit together beautifully.
Is it true that Edward Howard’s meteorite almost crushed somebody?
Yeah. The meteorite happened to fall in a field when people were out plowing. And it happens that the guy who owned the field was like a Kardashian of the day, which is an amazing coincidence. Because if it didn't happen to fall on his land, it wouldn't have ended up in London on display, it wouldn't have come to the attention of the Royal Society, and the chemical experiments of Howard wouldn't have gone ahead. The history of meteorites is just a wonderful series of coincidental events.
What do you think is the biggest meteorite mystery today?
There are so many unanswered questions, but the first one that springs to mind is the problem of chondrules. There are these meteorites called ordinary chondrites that come from asteroids that never melted, and so they preserve this sort of primitive dust from which the asteroids, comets, and planets coalesced. Ordinary chondrites are particularly interesting because they're made almost exclusively from one type of dust that we call chondrules. The word comes from the Greek chondros, meaning grain. Chondrules are these little round circles and you can see them with the naked eye. They're a major building block of meteorites, and by extension the planets, but we actually have no idea how they formed. What we do know is these little motes of dust clumped together and somehow, that dust became molten. The big mystery is how? There was something going on in the early solar system that was melting these chondrules in the trillions.
When I first learned about chondrules, one of my professors said that he remembers going to conferences in the 1980s where people would stand up and shout at each other in the conference sessions about how chondrules formed because everyone had their favorite idea. I've actually witnessed it as well myself. People are still shouting about it in conference sessions. Nobody knows. But there are some really good ideas. Some people think that within a few light years of our own solar system there were other solar systems forming as well, and these stars burned through their fuel incredibly quickly, went supernova, and shockwaves from the explosion rippled through our solar system, flash heating the dust. There are loads of ideas, but there isn't a single one that explains all of the observations.
What does the future of meteorite research look like?
I can’t wait for the meteorite samples we’ll get from the human and robotic exploration of Mars and the moon. Actually, the Apollo astronauts found meteorites on the moon and the Mars rovers have found meteorites on the surface of Mars. But there must be meteorites littering the surface of these bodies that we’ve never seen before. And if we’ve learned anything from meteorites, it’s that we often find unexpected things inside of them.