By Johnny Gibbons
What was happening (geologically speaking) on Earth way back when it was a mere babe and being showered with meteorites? Until a time machine is invented, it might be hard to tell….that is, unless you’re Cari Corrigan, geologist at the Smithsonian’s National Museum of Natural History.
To get a better picture of how Earth’s nascent rocky surface reacted each time it was hit, Corrigan turns to a 4.5 billion-year-old kaleidoscope of elements known as a ureilite (YUR-a-lite) meteorite. Ureilites are made up of olivine and pyroxene (elements containing different levels of magnesium and iron), and carbon―all things also found on Earth.
Ureilites, however, weren’t just calmly floating around in space until they one day made their way to Earth. No, they were knocking, bumping and smashing into each other in the asteroid belt between Mars and Jupiter, and that’s when something really cool happened.
“When these asteroids collided, which happened a lot during the early parts of our solar system’s history, we think a shockwave was sent through them with pressure so great that it transformed their carbon into graphite and diamonds. Yes, diamonds in space,” Corrigan says.
“And because olivine, pyroxene, and carbon also occur here, it’s probable that the same process was happening when meteorites were smacking into Earth. These ureilites allow us to study our planet’s geological history in way that would otherwise be impossible,” she adds.
To look back 4.5 billion years through a ureilite lens, Corrigan takes a slice from a meteorite 30 microns thick (about the width of a human hair) and puts it under a petrographic microscope. Things look pretty routine so far―the specimen is kind of dark with some clear-ish crystals, like one would expect from a rock.
But when Corrigan hits it with polarized light, the ureilite transforms. Bright blues, yellows, magentas and greens appear, each separated by a busy network of the black graphite and diamonds―all looking like an avant-garde stained glass window.
The colors revealed by the polarized light are influenced by the different levels of magnesium and iron in the olivine and pyroxene crystals. The shattered appearance reflects how the asteroid smashed from an impact, the heat from which caused the crystals to fuse back together with the carbon filling in the spaces in between.
In addition to hinting at what may have been happening on Earth billions of years ago, ureilites have a much bigger story that they help tell.
“There is a lot of information in ureilites,” Corrigan says. “They help us understand what type of materials are out there in the asteroid belt and the kind of activity happening. It’s not as simple as rocks banging into each other―it takes a lot of pressure to turn carbon into diamonds!”
Because asteroids in the asteroid belt never formed into planets, they are very primitive (and valuable) resources for scientists. If an asteroid collision results in broken pieces being shot off their trajectory and into Earth’s gravitational pull and onto its surface, they are delivering a unique opportunity.
“They’re time capsules from billions of years ago,” says Corrigan. “They show us the very building blocks of our solar system when it started.”