October 15, 2019
(Arizona bird song)
[0:05] Narrator: I'm standing on the rim of the Barringer Meteor Crater in northern Arizona. It's an impressive sight. 55 stories deep and a mile wide, you could fit 20 football fields in the bottom of it. From a bird's eye view, the crater looks almost perfectly round; like a hole punched into the desert by an angry giant's fist.
The crater was made fifty thousand years ago by a rock from space. Jeff Beal, head tour guide of Meteor Crater, describes the event.
Jeff Beal: When the meteor hits the planet it's moving very, very fast. Somewhere between maybe seven and 11 miles per second, and when it hits it explodes like a bomb. It's exploding as it's crushing its way through the surface of the planet, and it actually peels open the layers of rock, and debris blasts out from the crater, we think about seven miles in every direction. And the blast wave coming out from the crater would've been a lot like a nuclear explosion, it would've just incinerated everything around the crater for at least 10 to 15 miles in every direction.
[1:02] Narrator: At the time of impact, victims of this cataclysm would have included Mammoths and large ground sloths that roamed forests of pinyon and juniper. Arizona's climate is hotter and drier today, and that's helped keep the crater intact - in fact, Meteor Crater is the best-preserved impact crater in the world.
Jeff Beal: It's just such a great example of a crater in good condition. Most of the craters fill up with water almost immediately, and then you have vegetation, trees, it's just a natural erosion process that destroys the crater. So this crater is in excellent condition, and we get geologists who come study the crater, also it's great for the public. One time there was a guy who actually had a tear in his eye, because he had traveled from the other side of the world to come see the crater.
[1:43] Narrator: Craters made by asteroid impacts are both fascinating and terrifying. We worry about the random, destructive power that could end our existence, and yet ironically, humans are the biggest threat to the survival of Meteor Crater.
Jeff Beal: In about 80 or so years, we did 5,000 years-worth of erosion to the rim of the crater, and that's simply people stepping on the ground and softening up the ground a little bit. The wind blows the sand away, and then the person after that, and the person after that. Over time, it does a really serious amount of erosion to the crater, so today we don't let people hike in or around the crater. We just have observation decks and we have guided tours.
Unfortunately, there's also a fair amount of vandalism out there as well from people carving their name in the soft rocks. Most of the vandalism is from the 1930s until the 1980s. And even on the guided tours, when we have the bigger groups we do occasionally catch people in the background trying to carve their name in the rocks. One time several years ago I had a group ... if you've heard of the Red Hats? It's a group of senior citizen ladies who travel together. A guy started carving his name in the rocks, and I asked him to stop and he argued with me, and then the ladies just let him have it. He didn't realize he had about 30 grandmothers on the tour with him, so it was not his finer moment for sure. (laughter)
[3:20] Narrator: We're "On a Mission," a podcast of NASA's Jet Propulsion Laboratory. I'm Leslie Mullen, and this is Season 2, Episode 2: Impact!
Our planet has nearly 200 impact craters that serve as mute testimony to past devastation. But we should have many more. If you look at the Moon, you'll see it's completely covered in impact craters. The Moon's heavily pockmarked face tells us that asteroids zoom through our region of space all the time. In fact, Earth should have even MORE impact craters than the Moon, because we're a bigger target with more gravity.
The reason we don't is because Earth has a great beauty regimen.
Unlike the Moon, Earth is active, with molten lava bubbling up from below to resurface the land. The land itself gets rearranged as tectonic plates shift around on Earth's hot mantle. This cosmic cosmetology is complemented by Earth's atmosphere. Rain, wind, snow and other forms of weather act like refreshing scrub brushes on our planet's face.
When astronauts were training for the Apollo missions to the Moon, they knew they'd encounter a lot of impact craters. So they visited a few places on Earth with a lunar-like landscape, including Meteor Crater.
[4:35] Jeff Beal: Eugene Shoemaker was our geologist from the '60s until the '90s, and during the operations for going to the Moon he trained all the astronauts out here for the Apollo missions. This was the best likeness to a crater on the Moon, so he trained them how to identify the different rock layers and explained the geology, so when they were actually on the Moon they could pick up samples on the side of a crater without having to go into a crater, which of course is very dangerous.
The astronauts didn't really have a background in geology, and Eugene Shoemaker unfortunately was not able to go with them to the Moon because of some sort of medical issue that he had at that time. They trained mostly just hiking in and out of the crater, but they trained one time in their space suits, and Buzz Aldrin actually ripped open his spacesuit hiking in the crater. It's the site of an explosion, so there's a lot of sharp rocks out here, and he rubbed up against some sharp limestone. They had to go back in and redesign the spacesuits to make them tougher.
(Apollo 12 excerpt)
Alan Bean: This is a very small crater, Houston, probably about three feet in diameter, and looks like it was made (by a) not very fast moving or energetic or heavy projectile. Yet right in the middle of the hole is glass-covered rock fragments. I'm putting them all in a sample bag.
[5:47] Narrator: The astronauts who walked on the Moon, including Apollo 12's Alan Bean and Pete Conrad, gathered hundreds of pounds of lunar rocks from different impact craters.
(Apollo 12 excerpt)
Pete Conrad: All right, Al, where do you want to grab a sample here?
Alan Bean: Right here. I'd like to grab that rock right there, because it's got kind of a sharp edge on it, and you don't see many like that around here.
Pete Conrad: Which one? The big one?
Alan Bean: The big one.
Pete Conrad: Ho, ho, ho!
Alan Bean: This rock is different, Houston, in the way it's shaped, and it's partly rounded and got some oblique angles on it. The thing that was giving it that unusual shape was the dirt that was adhering to it. (they laugh). That's okay, we'll take it back with us. It's a good rock.
[6:20] Narrator: Those Apollo rock samples told us a stunning story about our Moon: that it is the result of a planet-shattering impact.
Earth itself formed about 4.6 billion years ago, when jumbles of dust and rocks surrounding the young Sun crashed into each other. Over hundreds of millions of years, small pebbles grew into large boulders and eventually, planets. Evidence suggests our Moon formed during this chaotic time, when another planet smacked right into Earth. (impact sound)
The Moon is thought to be a mixture of molten material thrown out by that impact. The solar system is far less cluttered these days, and so we'll probably never get hit by another planet again. But smaller asteroids still spin around the Sun, and sometimes they cross our path.
Aaron Cavosie, a geologist and planetary scientist at Curtin University in Australia, investigates some of the largest asteroid impacts in Earth's history.
[7:21] Aaron Cavosie: My job is to study what the end of the world looks like, or at least what it's looked like in the past.
What a lot of people don't realize is pretty much every day on Earth is a day of Russian Roulette. The hazards that exist from space are ever present. In the last couple years, there have been dramatic examples, such as the Chelyabinsk airburst that happened over Russia in 2013. That was a relatively small event, but that caused extensive property damage and a lot of people got hurt. And so there's reason to take pause and wonder about things from above.
When you're standing at the rim of Meteor Crater, it looks vast and impressive, and it's hard to really wrap your head around how quickly such a big hole in the ground forms because we're really talking about a matter of seconds to minutes. But Meteor Crater is actually quite small. Some of the larger basins that I've worked on include the Vredefort structure in South Africa. It's a little over an hour south of Johannesburg. Estimates for that feature are on the order of 300 kilometers in diameter. You could fit maybe 70,000 Meteor Crater size impacts within the single Vredefort basin in South Africa.
I used to teach at the University of Puerto Rico, and I would show maps to my students that the entire island of Puerto Rico could fit inside this impact basin. There's a couple of towns inside the structure. People live in this impact basin, they farm in it. And when you're in the middle of it, you do not have the sense of what you are in. You can see distant hills far away that just look like mountains to most people. But from above, you realize that those are remnants of the crater walls and other rocks that were uplifted violently during the impact process.
Vredefort, that occurred about two billion years ago. There certainly was life on Earth at the time, just not life that had spines and bones, much simpler forms of life, and it would have had an effect on them at the time.
[9:25] Narrator: The Vredefort impact happened so long ago, it's hard to say exactly what went down. The story of Earth is written in the rocks, but there are smudged pages, pages with writing scribbled all over them, even entire chapters ripped out. It's the job of a geologist to try to piece the book back together.
Aaron Cavosie: Geologists have to play a little bit adventurer and a little bit detective. Most of geologic studies are effectively forensic science.
Scientists go to a known or suspected impact structure to collect some rock samples, look at the minerals in the rocks carefully under the microscope and look for the telltale signs of damage that is only caused by high pressure shock waves. When such evidence is found, it's proof-positive evidence that an impact had occurred in the geological past.
[10:21] Zircon grains shatter, sometimes they recrystallize, sometimes they even transform into new minerals. When you subject a mineral to extreme pressures it can rearrange the atoms inside a crystal. You can think of it as if you have a packed room at a party full of 25 people, it's like shoving an additional 25 people in the same space. When an impact event happens, all the atoms get squished and have to squeeze together. And that can cause a mineral to recrystallize into something that's denser. Some of the earliest evidence for impact processes of any kind on Earth are these ejecta deposits: material that gets ejected up into the atmosphere and can travel around the planet, and rains down as little droplets. This would have been a pretty miserable thing to have land on you. These materials fall down to the surface and represent effectively an event layer. So when you find them and determine the age of deposition, they date an impact event at some point in the past. Now people have found such ejecta horizons as far back as about 3.4 or 3.5 billion years ago. But we still have 1 billion years of Earth's history before 3.5 billion years ago. And in a way, that's when a lot of the action was happening. And so, how does one go about going further back in time?
This question started bothering me about 10 years ago. But I'd never visited an impact crater. I knew nothing about shock deformation, nothing about planetary science. And I sat there looking at these zircons wondering, could any of these have been involved in early impacts on Earth? I asked myself that question one day and said, "Well, you don't even know what a shocked zircon looks like, my friend. There's no way you're going to find them until you teach yourself what they look like."
And so I bought a ticket to South Africa, and my wife and I went on a fun trip. We visited the giant Vredefort impact basin in South Africa and stayed at some lovely cottages up in the foothills, the mountains around Vredefort. I grabbed a spoon and a Ziploc bag, and I went down into the Vaal River that flows across the impact structure, and literally scooped out wet sand out of the river. I'm sure that people are wondering what I was doing. I took a few samples of the sand back to the laboratory and I had my students there carefully go through and separate out the minerals. One of the mineral phases that we looked at in great detail was zircon, and lo and behold, almost every single zircon grain in the sand that is eroding from the giant Vredefort impact crater today, all of these zircons are shocked. Almost every single one of them.
The great thing about studying the end of the world is if you find a single sand grain that has this micro-structural record of shock deformation, this is effectively a single grain ambassador of an event that could have been a cataclysmic event for anything on Earth at the time that it happened.
[13:33] Narrator: In the tiniest grain of sand, you can read a tale of one of the most massive asteroid impacts on Earth.
But if the shocked zircon sand grains from the Vredefort crater compose a story that's short on specific details, we have an entire volume about the smaller asteroid impact that ended the age of the dinosaurs. And that saga is one of complete devastation.
Sean Gulick, a researcher at the University of Texas at Austin, has been studying what happened when a six-mile-wide asteroid hit a shallow sea, 66 million years ago, over what is now Mexico.
Sean Gulick: So it's obviously a bad day in the Gulf of Mexico. I mean you're hitting with something that's equivalent of 10 billion Hiroshimas worth of energy.
[14:13] Narrator: The asteroid drilled down eighteen miles into the ground, sending trillions of tons of debris and molten material skyward. The Earth's crust briefly rebounded into a peak higher than Mt. Everest.
Sean Gulick: It's an enormous amount of energy. So that creates all the fracturing and all the ejection and all the earthquakes that are generated and the tsunamis, I mean you name it. There's an enormous amount of damage and deformation that occurs in the Gulf of Mexico.
If you're in the Gulf of Mexico, you're being killed by the impact itself, but that doesn't explain a global catastrophe, doesn't explain 75 percent of life going extinct everywhere on the planet. So that has got to be caused by something in the atmosphere.
[14:58] Narrator: A six-mile-wide asteroid isn't that big compared to the size of Earth, but the impact led to a chain of devasting effects. Super-heated air within fifteen hundred miles of the impact site baked everything to a crisp. Red-hot blobs of glass created by the impact rained down over the Western Hemisphere, igniting forest fires. Molten material was thrown all the way up into space, and when that came back down, it set the world on fire and cooked the ocean surface.
Earth was enveloped in a thick curtain of dust and soot for months. Because sunlight couldn't filter through this suffocating haze, most plants died, including plankton in the ocean. The loss of photosynthesizing plants sunk oxygen levels, while the vaporized rock and water had added new elements to the atmosphere.
[15:46] Sean Gulick: We know it released an enormous amount of water vapor. It's a limestone target so it released a lot of CO2. And it had a bunch of evaporated ocean sediments, and those have a lot of sulfur in them, so it released also a whole lot of sulfur into the atmosphere, orders magnitude more than any volcanic eruption ever could. It's sort of the 300 gigatons of sulfur kind of number; it's a huge amount. And then there was a paper that did a global climate model using just an input of 100 gigatons of sulfur, and they came up with a global temperature drop of about 25 degrees C everywhere. So much of the planet would've been in freezing temperatures for years, maybe 15 years or something like that.
The idea that dust, soot from wildfires and sulfur from the crater itself, combining in the atmosphere to basically reduce photosynthesis everywhere and crash the food chain makes a lot of sense. And it explains how the surface oceans went extinct, but the deep oceans, which didn't depend on sunlight, made it through.
And so things that depended directly on grazing of some form, whether it's in the oceans and you need to eat photosynthetic plankton basically, but those had gone extinct, so your food's gone away; those are in trouble. Things that don't have to eat very often, like crocodiles, might've had a chance to make it. So they're eating carrion for awhile, and then they're basically starving but they don't die because of it. There's a lot of different ideas about survival strategies. And we know the specialists were going extinct in many cases and the generalists were the ones that made it, which is true with most mass extinctions.
[17:17] Narrator: More than 99 percent of all living beings at the time died, and 75 percent of Earth's species were forever extinguished. In the blink of an eye, our planet had become a toxic wasteland.
But we only learned about all of this recently. The first hints about the end of this ancient world were concealed in a layer of clay so thin, it could have easily been overlooked. Here's Aaron Cavosie again.
Aaron Cavosie: The way that discovery was made is astonishing. It started with the curious nature of a very thin clay layer at a site in Italy called Gubbio in the 1970s. The funny phenomena about this centimeter clay layer in otherwise normal sedimentary rocks is it had a high abundance of an unusual element called iridium.
The conclusion that made the most sense at the time was that to generate that amount of iridium, it had to rain down from the sky. And the only event they could think of that would do that would be a large meteorite impact. The way that connection was made is that certain rocks from space have a high level of iridium relative to rocks on Earth. And if you slam a big rock from space into the Earth, it vaporizes, and the liquid rock rain that falls down to the Earth thus has a high level of iridium.
This hypothesis was published in Science in 1980. In my opinion, it's one of the more foundational papers published in the 20th century because of what it ultimately led to: a great change in understanding. It kicked off an arms race to find the site of where this feature may have been made. And so lot of criticism against the hypothesis was that if that crater existed, we would have found it by now.
[19:04] Narrator: Such a big asteroid - or meteorite, which is what you call an asteroid when it smacks into our planet - such a big rock causing world-wide extinction had to have left a big hole when it hit the ground. But it turns out this impact crater was hidden: buried half under the Yucatan Peninsula, and half under the Gulf of Mexico. Sean Gulick is co-chief scientist on the impact crater's drilling project. He describes how the crater got buried, which made it so hard to find.
Sean Gulick: You make a hole on Earth, you know, even if the hole is not super deep, any hole gets filled in very quickly. So in this case it was a hole in a marine environment. There's no crater rim, because the rim would've actually been made of water and so it left the crater open to the world's ocean. So after the crater formed the ocean was able to come back in right away. And so as we bring in sediments with the oceans, as storms happen, as the rain of dying organisms in the oceans occurs, that hole filled in fairly rapidly, probably within a few tens of millions of years it was mostly buried.
(explosion with ocean rushing in)
The crater had actually been discovered decades before by the Mexican oil company, Pemex. They saw a low circular gravity anomaly and they'd even drilled it thinking it was a volcano. They were looking for oil, and they had this mistaken idea that the heat from a volcano would actually maybe enhance the possibility of oil. So there's all kinds of things wrong with that concept, including the idea that it was a volcano at all, given where it was. They found some very unusual rocks, didn't understand, and they kind of said, "Forget it." It was just lost to time. Core is put up on a shelf.
As people were studying more and more of the boundary sections around the world, they realized the thickness of the layer that represented the event was getting bigger as you moved towards the Gulf of Mexico. So people started investigating more deeply in the Gulf of Mexico area, thinking somewhere around here, it's getting thicker this way, it's got to be the source. Sure enough, it was discovered to be in the Yucatan.
[21:05] Narrator: The impact crater is named after the Chicxulub Pueblo, a town located at the crater.
Sean Gulick: Chicxulub or Chicxulub, depending if you're Mayan or not. The Mayans would make it an SH. But most Westerners say Chicxulub. Either one is fine. Narrator: Sean has been studying this impact crater since 2002. Because the crater is on the coast and buried, on average, under a kilometer of land and water, it hasn't been easy. A recent project to get rock samples from the crater took place in 2016.
[21:36] Sean Gulick: So we had to put feet down for a drilling platform that would then lift the platform up out of the water so we'd be isolated from the ocean conditions. It was only a 220-foot-long platform. And on that we had to have an entire drilling rig. The whole crew... we might've had 60 people on board. And you're living on it, six to a cabin, which is tight, right? And our offices and our labs were in shipping containers, and there was like a whole suite of shipping containers laid out on the deck. One was analyzing the cores as they were split, another one was doing geochemistry on them, another one was measuring their physical properties, like what's the density and things like that. Another one was doing the paleontology, because we were actually trying to figure out the age of the sediments as we got closer and closer to the boundary. It was actually a fairly small place to put all this on.
First we just penetrated without collecting core for 500 meters, and then we switched the drill bit out to one that has a hole in it that will collect the core. And then we're literally just drilling down, three meters at a time, pulling up three meters of rock, drilling ahead three meters, pulling up three meters of rock and so on. Penetrating maybe 30 or 40 meters in a day. And we drilled for two months.
We have all these ideas about what we're going to find, at what depth. And as we're drilling out there on the platform, that's what's running through your mind, are we going to get to what we need to get to, are we going to get clean enough recovery of the core so we can actually do the work, so on and so forth. And of course there's challenges where things go wrong for a week and you're just biting your nails and then we get going again.
[23:04] Narrator:The drilling cores help scientists figure out exactly what happened on the day of impact, and for many years after. Sean says the crater also has lessons for us today.
Sean Gulick: If we obviously got hit by a 10-kilometer asteroid we'd have a real problem, certainly, but I would twist this in the other way, which is the reason the dinosaur extinction occurred is because of an atmospheric event. And it's a sudden release of lots of volatile materials into the atmosphere. Well, we're doing that now. We're just doing it differently. It's not the same quantities, by any means, but it does just suggest that you do need to worry a little bit about what you're doing to your atmosphere. And so there is a lesson for us.
We really need to understand the sensitivity of our atmosphere to inputs. I don't think we fully get what particular chemical species are the problem, at what scales of input, over what timescales. These are areas where we have to get the atmospheric scientists and the planetary scientists and the geologists, all to kind of work together to be thinking about it more as a system than we often do.
[24:10] Narrator: Aaron Cavosie says studying craters made by asteroid impacts is one way that geologists come together with astronomers.
Aaron Cavosie: Geologists point downward and look at things on the ground. Astronomers point their telescopes upward and try to identify rocks that could hit us in the future. And so I think it's a nice complementary approach.
What makes me tick is learning new things and finding new ways that the world works, or the geological history has stories to tell. I can't claim a fairytale story, that I knew I wanted to be a geologist from age five. I got turned on to it by some really great instructors during my undergraduate education for which I'll be forever grateful.
When I was a college student at the University of Nebraska in Lincoln, I wasn't the fastest student to respond to things like registration. My first semester there I registered late, and my options for courses were quite limited. And I ended up being stuck with having to take a geology class at eight o'clock in the morning. As a college freshman, eight o'clock in the morning wasn't a very palatable time to attend a course. But to this day, I recall sitting in the chair in the back of the room, and being mesmerized by the instructor, and the remarkable stories about the Earth's history that hooked me in.
I'm hooked on discoveries. If there's about 200 impact structures that are known, there are several hundred more that are suspected, and they're just awaiting creative people to throw new investigative ideas at how we go about studying the materials there to see if we can tease out these evidences that will allow them to be confirmed as having an impact origin.
Google Earth has led to the discovery of some impact structures, but there are a lot of circular features on Google: volcanic phenomena, collapse craters, underground features, lakes, inflation basins. There's a lot of geological processes that can make a round feature on the surface of the Earth. And so if you find something that's round on Google Earth in some remote part of the world, I would say that's a first step approach. Then it requires a little bit look under the hood to see what the local geology is at that site. I've often contacted people and said, "Is it possible that you could go collect a rock sample for analysis? And we can check it out, and see what's going on with the rocks." It's a process that takes time, and commitment, and energy to go from a suspected site on a map or on an aerial image or satellite image, to being able to confirm this feature has an origin by a meteorite impact process.
If you're listening and you think space and science fiction is cool, the greatest thing about planetary science is that there's opportunities for you to join it. When I was young, I used to think that all of the great discoveries in science had already been made, and that my job was simply to learn about those discoveries and be knowledgeable and aware of them. I didn't understand until later, when I got deeper into this, that anyone that gets involved, you are the one in the driver's seat. There are oodles and oodles of fun and exciting discoveries that are waiting to be made, and the opportunities that science provides are endless.
[27:44] Narrator: As we heard in episode one, astronomers scan the skies every night looking for asteroids that come too close to Earth. So what can we do if we see another asteroid heading our way, one that's capable of ending the world as we know it?
Excerpt from Episode 3: Bracing for a Crash
Rob Landis: There's perhaps no greater gift that the space agencies of the world could do for humanity, then to know the time and place of an impact event, and prepare our response to it.
[28:05] Narrator: More on that, next time. If you like this podcast, please subscribe, rate us on your favorite podcast platform, and share us on social media. We're "On a Mission," a podcast of NASA's Jet Propulsion Laboratory.