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One early summer morning, I meet up with geologist Peter Schultz at a gun range. This isn’t your typical gun range—it’s a high-tech laboratory run by the NASA Ames Research Center out in California.
There aren’t any pistols or paper targets. Instead, in the middle of the room is a bright orange, two-story metal tube—a gigantic gun. When I arrive, Peter and his crew are cranking it into position.
Peter explains how the gun works. First it’s loaded with smokeless gunpowder. Lighting the gunpowder causes a plastic slug to tamp down on the barrel of the gun, pressurizing hydrogen gas that’s stored inside. That gas can reach up to a million times atmospheric pressure, and once it’s released, it expands quite rapidly. When the trigger is pulled, the gas careens down a tapered cylinder, launching a projectile at a maximum speed of 15,000 miles per hour.
“This sounds like a bad idea,” I laugh.
Peter’s using the gun to simulate impacts from space rocks like asteroids and meteorites.
“My background is in both astronomy and geology,” explains Peter, who's a professor emeritus of geological sciences at Brown University. “So there was another part of me that was interested in the objects flying around in space."
Imagine a meteorite slamming into the earth. The energy from the impact creates a curtain of dust and debris, plus an incredible amount of heat—thousands of degrees that melt soil, rock, and anything else nearby.
Out of all of that molten chaos—and under the right conditions—this process can create a certain type of rock called impact glass. Impact glass can come in all sorts of shapes and sizes: It can be shiny and smooth, or speckled with different colored pebbles. And sometimes it looks like ordinary gray rocks.
“If you study impact glass in detail, you’ll see that it’s twisted and folded, almost like molasses that you might make in your kitchen. It’s gorgeous,” Peter says describing a sample he has on the table.
This gun range is Peter’s test kitchen. Here, he will shoot a pea-sized Pyrex projectile out of this huge gun at nearly 11,000 miles per hour. That’s three miles per second. If all goes well, the Pyrex pea will hit a pumice-filled pit inside a steel capsule, generating temperatures on the order of thousands of degrees Fahrenheit. And that process will result, he hopes, in homemade impact glass.
As you might expect, it takes a while to load up a 20-foot gun—just about an hour. While the crew runs safety checks, Peter fills me in on what set him off in search of impact glass.
Peter is what you might call a crater hunter. But he started out as just a curious kid fascinated by the moon.
“Wouldn’t it be easy to find a big hole?” I ask Peter.
“Wouldn’t you think it would be easy to find?” he replies.
Turns out, it’s really hard to find ancient impact craters, because they’ve either been eroded or filled in with soil and sediments. Sometimes, these craters are located offshore, because, as Peter explains, “the oceans rose after the glacial periods, and they’ve eaten away the evidence.”
Impact glass can offer clues to where craters might be hiding.
After a meteorite hits the ground, pieces of this glass can violently fly out of the crater. Peter says that the asteroid that formed the Chicxulub crater, near the Yucatán Peninsula, scattered impact glass all the way up into present-day Colorado. Some space rocks can also explode in the atmosphere before hitting the ground, creating the conditions necessary to create the glass.
Piece together the trail of impact glass, and it can help tell the story of these impact events.
It was during one of his hunts for craters in the early 1990s that Peter would dig up a cache of impact glass. He had gotten word of a geologist named Marcelo Zárate, who was investigating a mysterious layer of rock in the Pampas region of Argentina. According to Marcelo, who’s now a researcher and a professor at the National University of La Pampa, these rocks looked like they could have been volcanic, except for one problem.
Peter suspected this strange rock striation could be made of impact glass, and he sent Marcelo a note. “We faxed back and forth because the computers weren’t that good,” Peter says.
They swapped ideas for months, but Peter wanted to see these rocks with his own eyes. So he took a trip to Argentina. “I was using my frequent flier miles just on this one lark to just go see if there was anything there,” says Peter. “I had no clue what I was going to find.”
Peter and Marcelo got to work collecting chunks of rocks, and not just from the sea cliffs. Marcelo had noticed this mysterious rock in different locations in the region.
They examined each piece of rock under a microscope to look for mineral features that would confirm that it was impact glass. They eventually went on to identify glass from seven potential impact events. And they also stumbled onto another discovery.
Peter and Marcelo spotted small blades of grass trapped in the rocks that had been scorched into existence.
Meteorite impacts can wipe out life in less than a second. But Peter and Marcelo realized that the resulting impact glass can actually preserve life, like a time capsule. Besides grass, Peter thinks this glass could trap atmospheric gases, soil, and other living material from bygone eras.
Back at the NASA gun range, Peter shows me a piece of impact glass that he brought back from Argentina. This sample is not the glossy variety—it’s dull and full of small holes. I inspect it under the microscope.
“If you look inside, you can see these striations,” says Peter.
“There is darker material surrounded by lighter material…,” I remark, my voice trailing off.
“That’s grass,” he says.
“That’s grass? How old is that grass?” I ask.
“This grass is 9 million years old. It was flash-heated and trapped inside the glass,” he tells me.
I can make out the outline of this single blade of grass that looks like the frayed end of an old shoelace. This is no imprint or fossil—this is the grass itself, the cellular structure.
It’s a relic from our prehistoric past, before the Pampas region was the windblown plains it is today—even before humans evolved. Somehow, this single blade of grass survived the fire and brimstone of a meteorite impact. And now I’m here, staring directly at it…9 million years later.
As I’m looking into this microscope, freaking out about how much history this piece of ancient grass holds, Peter hits me with an intriguing hypothesis.
Meteorites strike Mars and form impact glass there, too, he says. A catalogue of past life on the Red Planet could be hiding inside these dull rocks, preserved and waiting for something—or someone—to pick it up.
A few years ago, Peter, Marcelo, and their team published a study describing how they think impact glass could trap a piece of grass. The process is kind of like frying a French fry. When a meteorite strikes, the enormous energy that it produces causes the edges of grass to heat up really quickly. But the inner portion of the grass is shielded from the heat, and gets trapped as impact glass hardens around it.
To study this process, we’re now going to see if we can recreate it at the gun range.
Peter and his graduate student Stephanie Quintana set up the target. They smooth out the pumice-filled pit so it’s perfectly even, and Peter sprinkles on small pieces of Pampas grass likes he’s finishing off a birthday cake.
“If all goes well, we’ll see these pieces trapped inside the impact glass, trapped by the energy of the impact,” Peter tells me. “Everything happens within less than a microsecond.”
“How are you feeling?” I ask him.
“Nervous,” he replies. “It might not work, because we’re doing this with a little more atmosphere. I’m really curious to see what’s going to happen. But you always worry. Will it work? Sometimes it doesn’t. Sometimes the gun fails. It’s an experiment."
Different levels of atmosphere. Too much water in the grass. Even though Peter’s done this before, any tiny variable can throw the experiment off-track. We head to the control room and prepare for launch. The impact will happen in less than a second, so cameras inside the chamber will be filming at 25,000 frames per second.
“Okay, everyone’s ready,” the engineer says. The gun-firing alarm sounds. There’s an explosion like a metallic thud. We watch the playback in slow motion.
Peter narrates: “We did damage…The shot worked!”
The engineer announces that the projectile slammed into the pit at 5.8 kilometers per second.
After a few digs in the pumice-filled pit, Peter finds a small shard of freshly made glass. And under the microscope, you can see the outline of the shoelace-like fibers of the trapped grass.
There are still big unanswered questions about impact glass. How does this process scale up in the real world? Could impact glass preserve life forms smaller than grass, such as bacteria? How might this process work in the Martian sediments and atmosphere?
But the idea of impact glass as a preservation tool is catching on. Another team found tube-shaped burrows in a 15 million-year-old piece of glass. The researchers believe glass-eating microbes munched their way through the material shortly after it formed.
And using NASA data, researchers from Brown have detected impact glass in craters on the Red Planet. One of those craters could be the landing site for the Mars 2020 rover, which means that in a few years, we might be able to inspect the contents of that Martian impact glass.
Of course, Peter won’t be able to cash in his frequent flier miles for that trip. He’ll just have to wait back on Earth—with the rest of us—to find out if his hunch is right.
The text was updated on March 30, 2017 to reflect the following changes: Peter Schultz’s professional affiliation has been added; he’s a professor emeritus of geological sciences at Brown University. Also, the text originally stated that NASA detected impact glass on Mars. The text was amended to indicate that Brown researchers used NASA data to detect the glass.