INTERVIEWS BY JESSE PEARSON
Mark Reid is Senior Radio Astronomer at the Harvard-Smithsonian Center for Astrophysics. He specializes in what many consider to be the Alpha of astrophysical doomsday boogiemen—the black hole.
Jesse Pearson: I’ve always wanted an astrophysicist to explain black holes to me as if explaining them to a 10-year-old. Use the simplest terms possible.
Mark Reid: It’s something that’s got so much mass in such small volume, such a high density—but I don’t know if that’s a good word for 10-year-olds—that nothing can escape its gravitational grasp. Light doesn’t come out of it. Objects falling into it don’t come out.
And how are black holes created?
There are two types in the universe as far as we know. One type we call “stellar mass” black holes, meaning they came from an exploding star, like a supernova. When a star explodes, a lot of stuff goes out, but the stuff on the inside implodes—it gets so dense that it becomes a black hole. These are presumably all around the galaxy. There are probably tens of thousands of them. The other type of black hole is what we call supermassive. Those are always at the centers of galaxies—the Milky Way has one.
It’s called Sagittarius A, right?
Yes. And these are usually between about one million and one billion times the mass of a star like our sun. We don’t really know the details of how they form. Presumably there would have been a very massive early star that exploded, and then it accreted matter.
How can a black hole gather matter?
Dust and gas can fall into it. If a star gets too close, it can be stretched by the black hole, and then shredded, and the gas that remains will fall in. And the other possibility for the growth of black holes has its roots in the early universe. It’s thought that galaxies like the Milky Way grow by taking little sub-galaxies, little groups of stars, and merging them together. You’d have two groups of stars and gas that might have a million solar masses in it and those two might collide, merge, and become a two million solar mass cloud. If you do a lot of that, especially hierarchically, like you get two two millions and you get a four, it goes pretty quickly.
It’s an exponential growth kind of a thing.
Yeah. So that’s sort of how people think galaxies grow. And you might merge black holes in the centers of other sub-galaxies as they’re merging. So you’d get two, five, or ten million solar mass black holes in two different sub-galaxies and as they merge, you’d double the mass of the black hole.
And the location of our own supermassive black hole, Sagittarius A, is known by the behavior of things around it, right? There seems to be a gravitational force that stars around the constellation of Sagittarius are sort of circling?
Yeah, but there are sort of two ways of locating Sagittarius A. If you’re a radio astronomer, like me, it’s actually a very strong radio source that you look for. There’s this source of radio waves that we can easily detect, and we can tell that it’s sitting exactly at the center of the Milky Way, and it’s not moving. So that’s pretty clear evidence that it’s a supermassive black hole. If it weren’t massive, it’d be moving around very quickly. If you’re an infrared astronomer, you can see that some stars are orbiting something that isn’t really visible, and you can figure out the center of the orbit, and then the center of the orbit gives the position of Sagittarius A. And then if you compare the radio and the infrared, you can tell that the two positions are the same.
So if there are these stars orbiting something that isn’t visible, it’s a good guess that there’s a black hole there.
Right. At infrared it’s mostly invisible, but basically the infrared observations, which give you the orbits of stars, tell you that there’s something about 4 million times the mass of the sun at the center of these orbits.
There’s a pop culture and pop science obsession with black holes as terrifying things.
Yeah, I have to explain to my mother all the time that we’re not going to fall into one. She’s really worried about that. Every time I mention black holes, she grimaces and worries.
Where do you think that reputation originates?
I think it’s the name, you know? Like the Black Hole of Calcutta. It sounds pretty scary, right?
It does. Can you tell me the potential catastrophic scenarios that surround supermassive black holes? Are they any really realistic ones?
Well, I don’t know about supermassive. I would say the scenario that would be scarier—and more realistic even if the odds are astronomically low—is that we’d run into a black hole that’s floating around the Milky Way. There are a large number of them, we don’t know the exact number, floating around the Milky Way unseen. You could think of them as giant asteroids, only much more massive. If we were to get very unlucky and have a collision with one, then yeah that would destroy the Earth [laughs].
What would the nature of the destruction be? How would it function?
If it was a direct hit—which is almost impossible—the Earth would be stretched and shredded, and then all the materiality would fall into the black hole and never be seen again.
And is there any sort of quantification of the likelihood of this ever happening?
Oh, I’ve never calculated it. I’d hate to put a number on it without calculating. My guess is it might happen once in the universe. Or, no, maybe more than that. But there is another scenario. If we got close enough to one, even without a direct collision, if one came into the solar system, it could be like having another star here, and that would probably eject most of the planets in the solar system out into space and then we wouldn’t have a star at all anymore.
At which point we would become a frozen rock.
Exactly. And it’s also possible that the planets could exchange orbits—instead of orbiting the sun, we would start orbiting the black hole as a star, and the black hole isn’t going to do you any good because there’s no light coming off of it.
Are there other astrological doomsday scenarios that are more likely than those concerning black holes?
The only super real one would be a big asteroid hitting us.
But we can rest easy in terms of black holes?
Yeah. The odds of a meteorite problem are infinite times more likely than those of a black hole.
What are some of the current specific topics that are being discussed amongst people who specialize in black holes?
Well, Sagittarius A is as well established as a supermassive black hole, I’d say, in terms of astrophysicists, as anything. Physicists who don’t look at all the information might be a little more skeptical, but it’s pretty certain. And if Sagittarius A is not a supermassive black hole, it’s something even more interesting. It would have to be some material we don’t know about, some type of matter that acts like a black hole, but isn’t. That would be almost weirder. There’s one group in South Africa that was suggesting that it might be what’s called a Fermion Ball, like a big cloud of electrons or something like that. Then there are even more exotic things called boson stars. Bosons can be as compact as a black hole, basically. From a physical point of view, if you took all these elementary particles called bosons and crammed them into a very small space, you could get something that would have about the right mass, if you could get it into that volume.
Astrophysics is an interesting science because so much of it has to be hypothesis and theory.
Yeah, we can’t do experiments. Basically, we do observations.
Do black holes have sort of a life span? I mean, are there ends to black holes?
The types of black holes we’ve been talking about should keep growing. They’ll run into material, either as they orbit the Milky Way or at the center of a galaxy, and they will accrete matter and grow.
So if that’s the case, then in billions of years do black holes consume the universe?
It would be more than billions of years.
The sun will die way before that, then.
Oh yes, absolutely. We know that within about 4 or 5 billion years the sun will become a red giant and we will be very close to being inside it.
I wish I could be around for that.
It’s kind of sobering. There’s an episode of Dr. Who, the newest version, where he takes his girlfriend to see the end of the Earth, the day the sun becomes a red giant.
Yeah. But anyway, that’s a sure thing. There’s an old joke about this. An astronomer is giving a talk to a community group and he says that in four or five billion years the sun will expand and engulf the Earth. This woman in the back faints. They revive her and he says, “Well gee, it’s a long way off. Don’t worry.” And she says, “Well, what did you say?” And he says, “Four or five billion years.” And she says, “Oh! I thought you said million.”
Gamma Ray Bursts
Jonathan Katz is a professor of astrophysics at Washington University in St. Louis and the author of the book The Biggest Bangs, which concerns gamma ray bursts.
Jesse Pearson: Please tell me just what a gamma ray burst is.
Jonathan Katz: They were discovered almost 40 years ago, and they are brief bursts—brief means anything from a few tenths of a second to several minutes—of what we call gamma rays, which we can think of as basically somewhat higher energy versions of what comes out of your dentist’s x-ray machine, coming from distant astronomical sources. For several decades, these sources were unidentified. Now we know that they come from very distant galaxies, or from components of very distant galaxies, usually from 90 percent of the way across the universe. When they’re going off, gamma ray bursts are more powerful than the whole regular light of a galaxy. Of course, they’re only going off for, typically, tens of seconds. About a dozen years ago it was discovered that they are followed by an emission of visible light, which fades over some hours or days. But for a period of hours or a day or two, the visible light is about as bright as a whole galaxy.
How could these possibly be a threat to us?
The question that people ask is what would happen if one went off in our own galaxy. Well, the first thing to remember is that it doesn’t happen very often. In a good sized galaxy like the one we’re in, there’s roughly one of these every million years. On the one hand, that means they hardly ever happen; on the other hand, our galaxy is about ten billion years old, so it means there’s been about 10,000 of them in the age of the galaxy. The age of our solar system is about half that, so there have been about 5,000 of these in its lifespan. Really, the numbers are very poorly known, but we can say some thousands. These of course are closer to home, and so one might ask if they are going to have any effect on us.
The galaxy is like a disc. A lot of them are across on the other side, and some of them are on our side. Out of these 10,000 or so, there will of course be a few that are comparatively close. But comparatively close in this case means the closest ever was, probably, a few hundred light years away. To set the scale of things, the closest star—other than the sun—is about four light years away. So what would it have been like when that went off? Well, it would have been the brightest thing in the sky. It would have been brighter than Venus, but it wouldn’t have been as bright as the moon. It’s something you sure would have noticed, but it’s not something that was going to dazzle you.
How long would it last in the sky?
It would fade in a couple of days. There are things called supernovae, which are exploding stars that also can be brighter than Venus. There were two of them about 500 years ago, visible in the daytime, and at night visible for months to the naked eye. This was all before telescopes. In fact, there have been none of them in our galaxy since the invention of the telescope, which is a bit of a pity. It gives you an idea of how often they go off nearby, too.
So, suppose we have one of these gamma ray bursts as close as it’s ever been in the history of our galaxy. We’d probably see it in the daytime. It wouldn’t be as bright as the moon but it’d be pretty bright. Now, what about the gamma rays? Well, the Earth’s atmosphere absorbs them, mostly in the top one percent of the atmosphere. People who study gamma rays have to put their instruments up on satellites. In the early days they put them up on balloons that floated very near the top of the atmosphere. Anyway, in the event of a gamma ray burst we wouldn’t be irradiated to the ground. If it were really close, it would make some extra ozone in our upper atmosphere. But that also happens when the sun erupts in solar flares.
Right. Those are also something that people fear as potential bringers of doom.
It’s an ordinary thing, nothing terribly extraordinary. The other things people sometimes worry about are very energetic particles, which we call cosmic rays. It is possible that some of these are produced in gamma ray bursts. Nobody really knows. These can be very penetrating—we get cosmic rays that hit the ground—and in fact much of the regular radiation dose that people get comes from cosmic rays. The rest of it, almost all of it, comes from natural radioactivity in rocks.
Oh, that’s interesting.
Yeah. And if you take a long high altitude airplane flight you get a measurable extra dose of cosmic rays. If you live in Colorado you get a measurable extra dose.
So pilots and air stewards are getting constant doses of cosmic rays?
Oh yeah, they are. They have a total radiation exposure that’s quite a bit bigger than that of those of us who live on the ground and only go up in the air a few times a year. It doesn’t seem to hurt them, but it’s certainly measurable.
There’s a hypothesis that one of the earth’s minor extinctions—the Ireviken-Silurian—could have been the result of a gamma burst.
Yeah, people have said that, but it’s hard to see how it could have killed things off. And let me point out something: supposedly the rays from a gamma ray burst all come at once, and they’re all coming in one direction. So the other side of the Earth, so to speak, the side that’s pointing away from such a burst, is completely shielded. It isn’t affected at all. If you want my favorite cosmic catastrophe, it’s the comet, asteroid or meteorite.
That’s what our black hole expert told us as well.
It’s what killed off the dinosaurs 65 million years ago. Sooner or later, something will hit the Earth. It’s like blindfolding yourself and throwing darts at a dartboard. Every now and again you’re going to hit it.
The real cosmic catastrophe I’d worry about is the comet, more than the asteroid, for the following reason: you can track asteroids a long way in advance. But comets don’t become detectable until a few months before they’re in the inner solar system. Asteroids are on orbits that go out to Mars and maybe partway to Jupiter and so on, so they’re never that far from the sun, and so they’re never incredibly dim, and so you can observe them using modern telescopes. Gradually you make a more and more complete catalogue of smaller and smaller ones and measure the orbits more and more accurately, and eventually—we’re not there yet—but eventually we’ll have all the ones that are big enough to really endanger us catalogued and we’ll know in advance which ones might be a danger and we’ll track those more carefully.
But comets don’t have such small orbits.
Comets come from way, way out in the solar system, far beyond the orbit of Pluto, and in a completely unpredictable way. At that distance they’re so dark they’re completely unobservable, because there’s not much sunlight out there. And so we don’t discover them, and they can come from any direction.
So with a comet, it enters our inner solar system and we say, “Oh, we’ve got about two months”?
Something like that, right. And if it happens to be on a collision course with us, that’s probably not enough time. Even if we’re prepared, even if we have a rocket somewhere with a nuclear explosive on it and we’re all ready for this, it’s probably not going to give it a big enough push. Remember, the closer it is when you detect it, the bigger push you’ve got to give it.
OK, so just to confirm: As someone who knows a lot about gamma ray bursts, you’re telling me that near earth objects are a much more viable doomsday scenario.
They’re a much more serious problem, yes. Comets in particular. It’s probably about one chance in a hundred million in a year.
Was there an epiphanic moment when you decided you were going to become an astrophysicist?
I was an undergraduate at Cornell when these things called pulsars were discovered. They’re stars that are about the same mass as the sun, but only about ten kilometers across. So they’re very dense, very small, and they rotate very fast. They give out regular pulses of radio radiation. The analogy people always use, and it’s a good one, is it’s like a light on a lighthouse, a beam, going round and round.
So like a lighthouse it’s rhythmic and measurable—
Yeah, very periodic. This was discovered my first year in college, and it turned out that a number of the people who were doing theoretical astronomy were there at Cornell. I was actually interested mostly in physics at the time, but this was really exciting. Pulsars weren’t discovered at Cornell, they were discovered in England, but they were immediately studied with the world’s biggest radio telescope, which Cornell ran. It became a center for the theoretical study, as well as the experimental—operational study of these things. Being at the center of interpreting a major discovery was really exciting.
So you weren’t majoring in astrophysics at first?
I was majoring in physics. But with physics and astronomy, there’s a big overlap. It was mostly physicists who were working on this. I got drawn into it, and then ended up making my career in it. Now, I’m working on quite different applied physics problems today.
What’s your daily focus now?
At the moment I’m involved in a collaboration that’s doing an experiment on the flow of drilling mud through oil wells.
Oh, speaking of catastrophes and doomsday.
Yeah, it is rather topical. We were able to predict that when they first attempted to do what they called a “top kill” on the BP well, it wasn’t going to work because the stuff they injected would get spat out. And that’s precisely what happened. Now that they have the well capped, nothing can get spat out. It’s going to work, and it’s all under control.
Markus Aschwanden is the author of the book Physics of The Solar Corona. He is an adjunct professor of physics and astronomy at Rice University. From 2004-2005, he was a member of NASA’s Sun-Earth connection roadmap team.
Jesse Pearson: Can you give me a quick Solar Flares 101?
Markus Aschwanden: The sun doesn’t have a solid surface. It rotates at different speeds at different latitudes. That is one reason why it has much stronger magnetic fields than the Earth, and why such violent processes happen when the magnetic field gets crazy out there. Inside the sun, there is a dynamo that constantly creates new magnetic fields—very strong fields, which bubble to the surface and create these dipolar magnetic fields that pop up and look like beautiful loops. They then get constantly twisted and sheared, and once they get too twisted they break, at which point they create huge amounts of magnetic and electric energy. This is the reason for solar flares. The fields they create are so strong that particles get accelerated to really high energies, and some of them escape the sun and travel to the Earth. Astronauts are not supposed to get high doses of these high-energy particles. They can knock out satellites and GPS.
How frequently do they happen?
Well, they come in bunches. We’re just ramping up to the solar maximum, which comes around every eleven years. That’s how frequently the magnetic field reverses within the sun. It’s fascinating—the solar dynamo creates magnetic fields like crazy, and the field just kind of breaks apart and the South and North poles switch.
So once that polar shift happens on the sun, the cycle begins again and it rebuilds to the solar maximum?
Right. Also, the rotations inside the sun are so complex. Different layers rotate at different speeds. That gives it a lot of friction and that produces the electric field, so that’s the solar dynamo.
Is the dynamo sort of the battery of the sun?
Yeah, exactly, and it constantly charges itself as the different layers rotate at different speeds.
What do you think of the doomsday scenarios that have risen around solar flares? Is it just sensationalism or is there anything to it?
Sometimes there are really huge events. The worst that can happen is that maybe the power grid here on Earth would get disrupted or communications could break down for a couple hours.
Is it sort of like an EMP?
Yeah, because the particles enter these currents in the ionosphere and these currents charge up satellites.
You know, a lot of the more extreme survivalist types believe in the possibility of a solar flare shutting down the power grid of the Western world and society being thrown into chaos as a result of that.
There are a lot of security systems that rely on our power grid. And it is rare that a ripple will shut something down, but it happens occasionally. Five years ago there was a ripple on the East Coast and it wiped out a third of the electric grids. But that was not caused by a solar flare.
I live in Manhattan and was here for that blackout. People simply dealt with it. Society did not collapse. Looking back, it was all drum circles. The city devolved into Burning Man for a night.
Certainly it’s not life threatening. It’s more an annoyance. But the point is that we are vulnerable because we have so many electronics that are controlled from spacecraft. Twenty years ago we were not so dependent. There was hardly a telecommunications satellite up there. But now GPS’s, cell phones… everything depends on satellites.
I’m getting the sense from talking to various astrophysicists about their areas of expertise that we don’t have much to fear from these astrophysical phenomena.
No, they are all so far away.
Do you know when you decided to become an astrophysicist?
When I was a teenager, a friend and I would build telescopes together. We were curious to see what was out there. It was fascinating, how a poor man with $100 could build a telescope and see the moon 100 times bigger, could see every crater.
What’s your relationship with science fiction like?
When I was a teenager, I was reading all of it that I could get. It was very inspiring. But then later, when I become a scientist, I also became more interested in the real thing. Though my children still favor fiction over real astronomy. I tell them, “I could show you footage of a real solar flare,” and they, “Oh no, we prefer Star Trek.”
Near Earth Objects
Dave Williams work at NASA’s Goddard Space Flight Center in Maryland. He is a part of the National Space Science Data Center, which is NASA’s deep archive for spacecraft data. Dave is in charge of planetary and lunar data.
Jesse Pearson: I’m starting to get the sense that asteroids and comets are the most feasible space-generated doom bringers.
Dave Williams: Yeah, asteroids seem to be the one doomsday scenario that’s at least on the radar. Some of these other things are sort of pie in the sky. The difference between all those things and asteroids is that asteroids have hit Earth in the past—and they’re all around us today.
Isn’t the Chesapeake Bay partially an asteroid crater?
There is a large asteroid crater in the Chesapeake Bay. It didn’t form it, but it did probably influence its shape and size.
And, of course, one of the most accepted theories for the extinction of the dinosaurs is an asteroid strike.
Exactly right. It’s definitely something that has happened in the past and will happen in the future. There are asteroid strikes every day, but asteroids can be anything down to the size of a piece of dust, technically. There’s no real cutoff. And anytime you see a meteor or a meteor shower, you’re basically seeing material coming into Earth’s atmosphere. Luckily, most of that stuff is tiny.
And then, once in a great while, something significant hits.
Sure. For example, there is a roughly 30-mile diameter crater in Tajikistan. It’s called Kara-kul. The estimated age is roughly five million years. Now, that’s a long, long time when imagined by humans, but in geologic time, it’s nothing. It’s yesterday. In fact, the impact that caused the Chesapeake Bay crater was about 36 million years ago and it’s about 90-kilometers in diameter. That’s relatively ancient, at least.
When we talk about the catastrophic effects of something like this, we’re talking mostly about the debris it sends into the atmosphere. Those effects are more lethal in the long-term than the impact.
Yes, definitely. Whether it hits on land or in the ocean—because it’s more likely to hit in the ocean just because more of the Earth is ocean—it will throw up a tremendous amount of material. Also, when you think of the size of some of these large asteroids like the dinosaur-extinction-type, if it hits in the ocean, it can plow right through to the bottom. In that case, it sends debris way up into the stratosphere, and it stays there for a really long time. Every ecosystem on Earth would be dramatically affected.
I’ve been told that comets are scarier than asteroids as near-Earth objects because they’re unpredictable.
Asteroids are mostly concentrated in the main belt between Mars and Jupiter. That’s pretty close. Every once in a while, usually when Jupiter perturbs an asteroid, it will change its orbit. But they basically keep an elliptical orbit that’s not really eccentric. In other words, it’s an oval but not too squashed. So you can count them and you can keep track of them. Comets come from way out beyond Neptune, and they can come in at any angle. That’s why every once in a while there’s all this excitement because you discover a comet like Hale-Bopp. No one knew it existed. It was way out there and then all of a sudden it comes in. We’re not going to find a huge asteroid, all of a sudden, out of nowhere. In fact, we’re putting a lot of effort into cataloguing the orbits for all of the asteroids that are any threat to the Earth. There’s no chance that we could catalogue every comet. One could appear at anytime from somewhere way out in the solar system. There could be something with a 10,000-year orbit or a 100,000-year orbit.
And it’s heading back our way.
Or it’s been out there in the Kuiper Belt or the Oort cloud and it got perturbed and it’s on its way in right now.
The major asteroid that’s being tracked right now, the one that’s known as Apophis, is supposed to come very close to us in 2029 and then possibly impact us in 2036. Is that right?
That’s right. It is within statistical probability that it could hit the Earth, although the probability is really small. We’ll know a lot better in ten years or so. The thing about tracking asteroids and comets is, the more time that goes by and the longer of a baseline track you have, the better able you are to determine where it’s going to go next. It’s just like any equation: the more information you have, the closer you can get to solving it. It’s almost certain that as time goes on, this asteroid’s impact probability to impact Earth will diminish.
But however farfetched, that is threat number one right now—ruling out some comet that could get blasted out of the Oort Cloud at us.
Yeah, I guess that is a fair statement. As far as we know, Apophis is the one to watch.
Can you tell me a little about what your daily work is like?
In our archives, we’ve got data from back in the Apollo, Mariner, and the Viking days up to the Mars Rovers and the Messenger and everything that’s going on now. The nice thing about it for me is that I get to be a little bit involved in every planetary and lunar mission that comes along. We also put up information about these missions to aid in the study of asteroids and comets. Over the years, people have seen my name on these various web pages and sent me questions, and so I started doing these fact sheets just to say, “Take a look here, it’s all the information you need.”
How much energy and time do you think NASA spends on keeping up with potential near-Earth objects that could be catastrophic in nature
That’s a good question.
A lot of people think that worrying about asteroids is sheer alarmist paranoia.
It is a very hard question to answer because the odds of an asteroid large enough to do some damage hitting the Earth are really, really tiny. But then again, the consequences are so incredible that if the odds are not zero then we really should be paying attention. Right now the most important thing to do—and the most cost-effective thing—is to try to find everything. In other words, try to track every large asteroid that is anywhere near Earth’s orbit, and figure out what its orbit is, to be able to cross it off the list. “OK, this one’s not a threat. Oh, we found another one… and now this one’s not a threat.” The nice thing about that, too, is that it’s very scientifically useful because the distribution and size of asteroids can tell us a lot about how the solar system formed and how its dynamics work. Now, the idea of mitigation, and I get asked this a lot, like what is NASA going to do if they find an asteroid heading towards the Earth and it’s definitely going to hit the Earth? We have nothing in place right now. Maybe that makes sense. We may not need it for ten million years.
It would be hard to justify to taxpayers, I’m sure.
How far back does your interest in these things go?
When I was a kid I had a big poster of the solar system on my wall and I was always collecting rocks. I loved science. When I was an undergraduate I started doing physics and kind of decided that physics is really neat but after a while everything was just frictionless-this and mass-less that—all these perfect systems. I really like to apply things. My college had a geophysics program, so I switched over to that and I’ve sort of done that ever since—applying geophysics to other planets.
That’s interesting. It’s sort of theoretical and not theoretical.
Exactly. When you’re dealing with rocks and the crust and the mantle and gravity and things, all of a sudden everything gets a little messy, which to me is a lot more interesting and fun.
Since you know geophysics I may as well bring this up with you. After looking into all of these potential astrophysical doomsday scenarios, it seems that a more valid scenario for catastrophe is actually a terrestrial thing—the Yellowstone Caldera.
Yeah, that’s true. We know with plate tectonics that something like Krakatoa, for example, simply has to happen. There’s no way around it. And these things have to happen on the timescale of at least tens or hundreds of thousands of years. Yellowstone, at some point, is going to do something. No one knows the timescale, but that is a definite thing. And Yellowstone is just one. There are other possibilities. There could be a massive landslide on the slope of the big island of Hawaii. This would cause a tsunami to dwarf all tsunamis that we have had any experience with.
Something has to come due, basically, whether it’s earthbound or from space.
Yeah, statistically, it’s going to happen. But we’re probably not going to be surprised. The odds of something hitting the Earth in the next 20 years are very, very close to zero. The odds of something hitting the Earth, something large enough to really get our attention, in the next 100 million years are almost 100 percent. But whether that’s 200 years from now or 20 million years from now, there’s no telling.
A cynic would say that we’re a lot more likely to destroy each other before something else destroys us.
I would have to agree with that. The average person is more likely to die in a car crash or falling off a ladder. If you’re going to worry about things, you should eat better and get more exercise.
It’s interesting that often when science enters popular culture in terms of entertainment, it’s about Earth being laid to waste.
It’s too bad. And you know, there is actually a fairly new idea—meaning it was conceived within the last 20 or 30 years—that large impacts play a real role in the Earth. The idea before was basically that the planet evolved tectonically and geologically and also that life on earth evolved in this certain way but now we’re really thinking that there is this random component, that these asteroids—boom—come in and all of a sudden there’s a mass extinction and all these niches are changed and all these new life forms come in and take over. To me, it’s absolutely a fascinating thing.
So the idea is to look at asteroids as a contributing factor to evolution. Like maybe we wouldn’t be here without them.
Without asteroids, who knows. Maybe we’d have smart dinosaurs. We would have a very different world.
Paul Doss is a scientist at the Yellowstone Volcano Observatory and a professor of geology and physics at the University of Southern Indiana.
Jesse Pearson: I guess what I’d like to know first is the distinction between a caldera and a volcano.
Paul Doss: Volcano would be the very general term to describe any surface landform where magma from the Earth’s crust or deeper can get out onto the Earth’s surface. A caldera is basically one type of volcanic landform, just like shield volcanoes or strata volcanoes are other types of volcanic landforms.
And what is distinctive about calderas?
Most of the time when people conjure up an image of a volcano, they see an image of a high-peaked mountain, which is built up from the discharge of magma or lava. A caldera is where the eruptions tend to be large enough and significant enough that the chamber that was holding all of the magma underneath the Earth’s surface kind of empties out and all of the support for the crust is gone, so it collapses.
How rare are calderas?
On the scale of Yellowstone, very. But simply as landforms, they’re not very rare. A number of the volcanoes in the Aleutian Islands have caldera landforms. Crater Lake, another national park in Oregon, is actually a caldera. A lot of the volcanoes in Indonesia and other areas of the South Pacific have caldera forms. But a caldera of the scale of Yellowstone is essentially unheard of.
Please tell me a little bit about the scale of the Yellowstone caldera.
Uh… [laughs] It’s remarkable! It’s unique, it’s unparalleled, it’s breathtaking.
There is nothing like it on Earth.
That’s right. When you stand next to it or on it, it doesn’t have as striking an appearance as, say, Crater Lake does. That’s because it is such a big volcano. After its last catastrophic eruption, the caldera got filled up by the lava flows that followed. So the very dramatic basin or collapse crater that you might see in a normal caldera, you don’t see that out there because it’s so big that you simply can’t capture it all in your field of view and also because large parts of it have been filled up in the ensuing time.
Did that lack of a classic caldera shape make it hard to discover?
It did hamper its understanding, make it a bit more of a detective story. Some geologists in the first half of the 1900s hypothesized that there was a big volcano there. But the true scale of the beast, if you will, was figured out by an outstanding geologist named Bob Christensen, who recently retired from the US Geological Survey. He’s the one who, back in the 1960s, did most of the geologic mapping in Yellowstone. He found that it was the rocks that told the story, and he’s the one that weaved that story together.
Can you give me some sort of idea of the dimensions of the caldera?
It’s not circular, so you can’t give one dimension that precisely describes it. But it is 30 miles across, kind of, from north to south and 40-45 miles across, northeast to southwest. It’s got kind of an ovoid shape. In areas where there are segments of the caldera wall that are still exposed, you know you’re on the scale of 1,000 to a couple thousand of feet vertically that represent the collapse.
And is there any kind of way to measure or even hypothesize the volume of the caldera’s last eruption?
What has been done has been the quantifying of the material that was ejected from the volcano in its last eruption. It’s painstakingly difficult to do. You look at the rocks that were produced, and you measure how thick and widespread they are, and then you calculate volume.
But didn’t the surface area covered in Yellowtone’s last catastrophic eruption stretch all the way down to the southern states?
Yes. Tuff is the name that geologists give to rocks that are produced from volcanic ash. Some of the recent tuffs from Yellowstone have been found in the Gulf of Mexico, Missouri, Iowa, and Louisiana.
When was Yellowstone’s last catastrophic eruption?
It’s called the Lava Creek eruption, and it was about 600,000 years ago. You’ve probably seen pictures of the Mount St. Helen’s eruption that happened in 1980? That eruption ejected 0.3 cubic miles. Yellowstone’s Lava Creek eruption was 1,000 cubic miles.
Wow. So I guess that brings us nicely to the next topic, which is, what sort of a timeline is at play here?
There isn’t any evidence right now that one of these catastrophic caldera eruptions is imminent. In fact, most of the data suggests the opposite. We have evidence that the kind of magma in the sub-surface that would create these very explosive eruptions is actually solidifying and crystalizing.
OK… I don’t know why, but I almost feel disappointed.
Now, there is data that suggests a different kind of magma is actively being injected into the magma chamber. It’s called basaltic magma. The Yellowstone caldera system has a history of having these big, wild-factor, catastrophic eruptions, and these are followed by a whole bunch of basalt. So for example, Idaho, which used to be on top of where Yellowstone is now, is covered with basalt. That’s why it grows good potatoes. I don’t know that it would be a surprise to anybody if there were basaltic lava flows erupting in Yellowstone, in the short term.
How serious can those be?
They’re not going to be explosive. They’re going to be more along the lines of what we see in Hawaii Volcanoes National Park. But there are a couple of greater risks in Yellowstone. One of them is catastrophic earthquakes and one is steam-type explosions.
Let’s start with the earthquakes.
The Yellowstone area is the second-most seismically active in the lower 48 states, after California. Yellowstone has, on average, a couple of thousand earthquakes every year. Most of them are pretty small and not really felt. But in 1959, there was a massive earthquake right next to Yellowstone that had a number of human tragedies associated with it. It generated significant landfalls and landslides. Nobody in the scientific community has the ability to predict earthquakes, but I think that most everybody recognizes the potential for large, significant earthquakes in Yellowstone.
And it’s much more of an imminent possibility than the caldera going off?
And what can you tell me about steam explosions? I haven’t really read much about those.
In the field, there’s been a relatively recent recognition of the hazard associated with those. The whole idea is that the geyser systems and the hot spring systems in Yellowstone deposit this stuff called sinter on the surface. It’s sinter that builds the cones around the geysers and hot springs. Sometimes it can form a seal over the hot water, and it becomes like a pressure cooker. The water can be super-heated to the point where it can blow its top. Now, we know they have them, we know they exist, and we see them all over the park. Some little ones have happened in the last decade or so. There is even evidence that some of them are on the floor of Yellowstone Lake. That’s where some of the attention is focused now, because in the park the lakes can be one hundred feet deep and that water already applies a big pressure to the bottom. If something were to happen, like an earthquake, that reduced the level of Yellowstone Lake instantaneously, it would be like taking the lid off the pressure cooker. The water could flash to steam and cause a significant explosion. In fact, there is some evidence around Yellowstone Lake of tsunami deposits—tsunamis that could have formed as a result of these lake bottom hydro-thermal explosions.
Are Yellowstone’s geysers sort of symptomatic of the caldera beneath the surface?
That’s exactly right. They are how we know Yellowstone to be an active volcano. There are gases that are emanating from the magma that are being discharged in Yellowstone.
Even though it is not a possibility for a very long time, could you humor me and tell me what the potential doomsday scenario would be if the caldera were to go?
Life as we know it would cease. Food would be in short supply because agriculture would fundamentally change in the United States. Any large scale agriculture would cease at least temporarily, throughout the mid-continent. There would be a period of climatic cooling because the particulate material ejected into the upper levels of the atmosphere would induce incoming solar radiation. Even the Mount Pinatubo eruption in 1992 generated measurable global cooling. Are you familiar with that one?
Only a bit.
It was relatively large eruption at the Clark Air Force Base in the Philippines, and it only generated 7 cubic miles of erupted material.
Compared to a potential 1,000 from Yellowstone.
I remember once Yellowstone Park received a letter from a senator from Illinois. This was after a couple of big doomsday films came out in the late 1990s. They generated a lot of fear and federal representatives were being contacted by their constituents who were saying, “We are going to lose our farm if this thing erupts!” So a senator mailed a letter to the Superintendent of Yellowstone.
Saying what? “You guys need to handle this shit”?
Basically, yeah. They’d say that their constituents were concerned and ask what they could tell them and what we were doing. The Superintendent turned to the park geologist and said, “Write me a letter in response.” But you can’t do anything. There’s nothing that could stop the caldera. In fact, yeah, agriculture in Illinois probably would not happen if the caldera were to erupt on the scale it has in the past.
Yellowstone is incredibly volatile in so many ways.
It’s wild, man! It’s wild! It is the epitome of wildness, with all of its beauty as well as all of its risks.