Ep. 505: Seismology

Posted on Nov 8, 2018 in Planetary Science | 1 comment



We’re always interested in the surface features of the planets and moons in the Solar System, but that’s only skin deep. It turns out, these worlds have an interesting inner life too. Thanks to the science of seismology, we can peer into our planet and learn how it works… inside. And we’re about to take that technology to Mars.
(If you use the Raw Feed, and get the episode for Cassini instead of Seismology, just delete and redownload. I’ve replaced the incorrect file with the new one now. Apologies for the mistake!)
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Show Notes

Seismology
What Is Seismology?
What Are Seismic Waves?
Using seismology to locate dangers and determine where warnings are needed
S-waves and P-waves
Seismology in space
Lunar and Planetary Seismology
Using seismology to determine the composition of planets
Mars Insight Mission November 2018
Ceres is also another good candidate for seismology
Astroseismology

Transcript

Transcription services provided by: GMR Transcription

Fraser: Astronomy Cast, Episode 505, Seismology. Welcome to Astronomy Cast, our weekly backspace journey through the cosmos, where we help you understand, not only what we know, but how we know what we know. I’m Fraser Cain, Publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the Director of Cosmo Quest. Hey Pamela, how are you doing?
Pamela: I’m doing well. As an astute listener will have been able to notice, we’re moving institutions. There are a whole lot of different reasons for this. But, bottom line, I’m gonna be at an organization with roughly 100 other PhD scientists, all working on cutting edge research. And I’m getting back to my astronomical roots. Or at least planetary roots, in this case. And Astronomy Cast is going on this journey. We have a new 501C3. If you are aiming to make yearend donations, now is the time to do it, because we wanna be able to pay our staff.
And please, while we’re sorting the finances of the transfer, everything will transfer. Everything will transfer. But now, your donations would really be useful for making sure that Suzie gets paid, Chad gets paid, Server gets paid, Amazon kind of likes to get paid.
Fraser: Yeah, exactly. Well, congratulations.
Pamela: Thank you.
Fraser: And I look forward to working with the Planetary Science Institute. Especially, so much of the work that we do through Cosmo Quest is directly connected to a lot of really interesting Planetary Science research. I know you always go to the conferences every year, so it’s a great fit. And actually, I highly recommend their – I get a ton of news from the Planetary Science Institute’s news feed. It’s one of the best that’s out there. So, highly recommend it. I need to remind everybody that Dr. Paul Sutter and I are going to be going to Costa Rica with a few dozen of our best friends.
And we’ve reached the minimum for the number of people to come. So, we’re definitely doing the trip. And I think we’ve got about another six weeks. Right to the end of December for people who want to sign up to be able to commit to the trip. So, if you’re interested in joining us in Costa Rica, we’re gonna take telescopes, we’re gonna see volcanoes, we’re gonna see wild life. And then every night we’re gonna be doing sky watching. I’m gonna teach you the night sky with these beautiful dark skies in Costa Rica. So, if this is a thing that you want to be a part of, you should definitely go to astrotours.co and check it out.
Pamela: And if the timing and price, or both, is wrong for that trip, I’m taking people on a tour of the American southwest next end of summer, fall, depending on your point of view. And that’s astrotours.co/starstrider. And come see my favorite part of the world.
Fraser: Awesome. And we’ve been informed that it’s not just about space. This is the key. It’s mostly about tourism and adventure and experiencing these places. And so, it’s a great thing to bring your significant others. Don’t worry, it’s not gonna be overly spacey, just in case you’ve got someone who’s on the fence and like, “I don’t know. It’s gonna be a lot of nerds there.” Yes, there will be a lot of nerds there.
Pamela: I promise beautiful scenery. Beautiful rocks. You’ll have beautiful birds.
Fraser: Yeah. Absolutely. All right. So, we’re always interested in the surface features of the planets and the moons and the solar system. But that’s only skin deep. Turns out, these worlds have an interesting inner life too. Thanks to the science of seismology, we can peer into our planet, and learn how it works inside. And we’re about to take that technology to Mars. Pamela, seismology. What is it, and how does it work?
Pamela: It is the study of things that shake, rattle, and roll.
Fraser: So, we’re all mostly familiar with this idea seismology as a way to detect earthquakes.
Pamela: Exactly. So, our world, like many worlds, for a variety of different reasons, will have waves passing through it. And it’s the study of how these waves move through a world that is seismology. And we’re used to thinking of it in terms of earthquakes, which, on our planet, earth, in the quake, these are usually triggered through plate tectonic shifts. This is where the crustal plates are moving under each other, over each other, and beside each other. And as they move, they don’t do it in a smooth and continuous motion. Rather, they periodically release a whole bunch of energy and jump anywhere from millimeters to meters in distance. And as they make these jumps, as they release this energy, you end up with both compression waves, like sound waves that we sense when our eardrum gets rattled, we get sound waves moving through the soils, the rock, the liquid in the crust of the earth, and we also get the more up and down sinusoidal waves that we’re more used to thinking about when we think of how a guitar string vibrates.
Fraser: So let’s go into that. I know that scientists call them S waves and P waves, right?
Pamela: Yes.
Fraser: If you were experiencing an S wave, what would feel like, in terms of an earthquake?
Pamela: Well, some of the worst earthquakes out there, you actually catch videos where you can see the land doing the up and down thing.
Fraser: Yeah, I’ve seen that.
Pamela: In general, it’s not going to be that dramatic. And what seismometers detect is much smaller. We’ll just go with smaller. So, here what you’re looking at is individual particles, as they get vibrated, will get oscillated spatially, and that oscillation spatially that is perpendicular to the motion of the waves. So, the particle moves perpendicular to the motion of the wave. That is the S wave, which actually stands for secondary waves. Because these waves, these up and down vertical waves, they move slower through the crust. And they only move through the rocky bits of the planet earth.
When they hit the liquid under the mantle, they’re like, “Nope, I’m done. I’m not gonna oscillate that liquid.” And so, they only travel through the outer, rocky levels of our planet. And they travel slower and get to the recipient of the wave second. Thus, S.
Fraser: And the P wave, that’s the primary wave, I’m guessing.
Pamela: Exactly. So, the P wave is the primary wave. This is the compression wave. And in this case, particles that are getting moved, move in the exact same direction that the wave is oscillating. So, you’ll have particles that get compressed, and they move in the exact same kind of wave that you get with stop and go traffic. Stop and go traffic is actually a form of compression wave. So, the motion of your car, one hopes, is in the direction of motion of the wave.
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Fraser: Those who have experienced earthquakes, and we get them all the time here on Vancouver Island, everyone is always like, “Are you Okay?” Whenever there’s an earthquake, I get these emails, I’m sure you get them for tornadoes and stuff, right? But people will mention me on Twitter whenever there’s an earthquake on Vancouver Island. And they happen all the time. And people are like, “Are you all right?” Yeah, I’m all right. We feel them, though, for sure. And it is this really unsettling feeling that the word is moving back and forth. And it’s this hard to explain – it’s just this feeling like someone’s grabbed you, and is just pulling you back and forth. It’s really unsettling.
Pamela: There’s a really easy way to explain it. So, we get earthquakes here, we’re near the San Andreas Fault, and we have a constant background of magnitude threes. And periodically they create much higher. And the first time we had one of these larger ones after I moved here, I woke up in the middle of the night and went to kick the dog off the bed. Because it felt like there was a dog doing the scratch, scratch, scratch, scratch, scratch thing on the bed, shaking the bed. And there was no dog. So, earthquake, dog scratching on bed, similar for the correct magnitude of earthquake.
Fraser: I think that’s exactly right. When you go through one of these, it is a lot faster than you think it’s going to be. It is like, shake, shake, shake, shake, shake, shake, shake, shake, shake, shake. That’s how it feels, that speed is how it’s shaking. Anyway, we’re going a little too far down the, what earthquake rabbit holes feel like. So then, seismology, the science, what is seismology doing? What is it looking for, and how are scientists using this?
Pamela: So, seismology doesn’t just measure the earthquakes, it literally measures all the things that shake, rattle, and roll. So, a good seismometer is gonna notice that semi-truck that goes rattling down your street. It’s going to notice the explosion of the local frackers. It’s going to notice a landslide happening on a nearby mountain. Now, these are all surface events. And also, we’ve used seismometers to notice when North Korea – which is the most recent nation to have done underground nuclear testing, we could measure where it occurred using seismometers. And the cool thing is, since these are waves travelling through rocks, they’re moving at a finite speed.
And well, different waves going through different parts of our planet will end going slightly different velocities. Some parts of the planet are a little differently textured than others. But by having seismometers scattered all over the world, having experienced decades of earthquakes, explosions, and other such things that go boom and bang, we can now reverse engineer exactly where a wave started, to figure out, okay so, we had an undersea earthquake, one kilometer below the ocean floor, off the coast of Indonesia, we need a tsunami alert versus we had an earthquake 10 miles underneath the Andes mountains, everyone be calm.
Fraser: Right. But I guess using seismometers to detect earthquakes, and the strength of earthquakes, and that kind of stuff, is very important. It tells you what’s going on right now. But one of the parts that I find so fascinating is that, these waves, as they move through the planet, have allowed scientists to, really, understand what’s inside the planet. To map it out by listening to it.
Pamela: And this all comes down to the fact that we have the primary waves. The compression waves will travel through anything. They’re happy to compress liquid. They’re happy to compress rock. Just like sound waves, you can still hear, although badly, a radio if you’re under the pool. Sound waves move through liquids. Sound waves move through solids. And these P waves generated within our planet, or on the surface of our planet, move through all parts of the planet. Now, the S waves, the ones that oscillate things, since they give up the ghost when they hit liquids, they have to travel around.
And by measuring travel times of these two different kinds of waves, you can get at the different kinds of routes they had to take to get from where the earthquake, the landslide, the nuclear reaction occurred to wherever the seismometer is. And by scattering these things all over the place, we can do things as wild as mapping out the distribution of lava underneath a Hawaiian island.
Fraser: Right. So, you’ll get this really powerful earthquake that’ll happen in Indonesia, or in Iran, or in Haiti, and on the one hand, they are absolutely human catastrophes, where there’s significant loss of life, but every one of these events, they’re so powerful that they generate these waves that roll through the planet and bounce off of the different layers in the planet in different ways.
And so, every time there’s one of these events, scientists are able to make these really large connections to see how these things are moving around, and understand more about what are the layers. That’s literally how they figured out the core of the earth, the outer core of the earth, they figured out how many cores the earth has. The mantle, the crust, how thick these things go, just by how these waves are moving through and bouncing and reflecting and refracting and all of this, through the planet.
Pamela: And it’s not just the earth that seismology works for. And this is where the Apollo astronauts tried their own hand at mapping out the guts of the moon using seismometers that were placed on the lunar surface.
Fraser: Moonquakes.
Pamela: Exactly. Back with Apollo 11, this is Buzz Aldrin here that we’re talking about; they actually took a seismometer and placed it on the moon’s surface to detect moonquakes. Now, the moon isn’t geologically active the way the earth is. But it does get hit by rocks from space. It does have the periodic landslide. And these different things that it experiences create seismic waves. And they were able to detect 100 to 200 hits of meteorites during the life of the seismometer that they had on the moon. That’s kind of amazing, if you think about it.
Fraser: And so, the problem, at this point is, with a lot of the other worlds – we’ve only had a working seismometer on earth and on the moon, but it’s a big solar system. And we really want to understand the inner lives of all of the worlds in the solar system. And we’re gonna get our next one in just like three weeks now, when NASA’s Mars InSight Lander arrives at Mars. What are we gonna learn? And what’ve they got planned for this?
Pamela: So, if all goes well, on November 26, the Mars InSight Lander is going to get to the red planet. It’s going to go and settle itself into a rather boring part of the Martian surface.
Fraser: The boringest part they could find on Mars.
Pamela: Yeah. Well, sometimes that’s what you want. So, they’re gonna go land somewhere exceedingly boring. They’re going to watch this all transpire from MarCo A and MarCo B, the two suitcase satellites that are accompanying Mars InSight. And Mars InSight, it’s going to deploy its seismograph. And it’s going to look to map out landslide noise, rock impact noise, and there may be Marsquakes, we don’t know for sure, that are generated on the interior of the red planet. And with all these different kinds of things that are going to generate our primary waves and secondary waves, these compression waves and oscillatory waves, we’re gonna finally be able to figure out, what is the interior structure of Mars?
And one of the things we didn’t talk about is how, not just the Apollo 11, but all the subsequent Apollo seismographs, putting all of their data together. Data that spanned all the way up to 1977. We were able to figure out that the core of the moon has a partial melt layer, that it’s inconsistent, asymmetric. All of the oddities of the lunar interior we got at, initially, just through seismography. We’ve been able to help improve our understanding through the gravity mapping that we’ve done, that we talked a little bit about a few episodes back, but it was really from measuring how waves move through the moon that we got a detailed understanding. Now, we don’t have this for Mars.
Fraser: Right. And so, this is the big question. And I love how carful this Rover was built and the seismology that it’s going to be able to do. I don’t know if you heard, when they were constructing it they created a vacuum chamber where it was going to be running its needle, or however it works. And there was a slight leak in the vacuum chamber. And the atmosphere going into it on Mars would be enough to make it not accurate. So, they had to tear the chamber apart and rebuild it, because in their tests they learned that it wasn’t gonna be sealed property. And their data would have been, mostly, not the level of precision that they were going to need.
Because, as you said, Mars is old and probably dead. And there could be some activity, but how much activity? And are there Marsquakes happening? But not only that, it’s gonna detect meteorites smacking into the ground within a few kilometers of the spacecraft. It’s going to provide that final close, interior look at Mars that we’ve never had. And so, they really made the most precise machine for this one job. It drills a hole that is meters deep, and drives the probe down into the regolith on Mars. This has never been tried before.
Pamela: And they’re going to be measuring the temperature. And this is the thing I’m most looking forward to, because people have been talking about what they think the temperature structure beneath the surface is. But we don’t know for sure. Now, this is going to be another one of those scary landings. It’s not quite the sky crane that we had with Mars Curiosity. That was frankly terrifying. This is much more of a Space X style landing, with the retro rockets and everything. But yeah, we’re 16 days-ish away. And we want everyone to understand what the seismology’s good for before we get there. Because people, like you said, have been talking about how red and dead Mars, and, well, red galaxies are.
And with Mars, there is still that chance that there might still be active volcanism. And if we can detect any kind of a liquid interior, that unfortunately starts to say, “Well, that’s seasonal methane we’re seeing. Maybe it’s interiorly produced and is just stuff melting.” Now, if we can completely eliminate any geological activity, that’s super cool. And then, just a general, curiosity for curiosity’s sake, what is the inside of Mars? We want to know.
Fraser: Yeah. There’s a lot of great questions that this is gonna answer. Now, what are some other places that you think could really benefit from a similar seismology mission going to explore them? Ceres for sure.
Pamela: Yeah. So, Ceres we know has had active cryovolcanism. There have been a large number of extinct, and looking like they’re active, cryovolcanoes spotted all across its surface and mapped out in some detail. And we can, by using modeling of the slump rate how quickly the volcanoes go from pointy volcano to hilly volcano, by using various models they’ve been able to, somewhat, date how old they think the different dead volcanoes are. It would be amazing to add seismographic information to try and see if we can map out the liquid pockets the same way we do with the magma pockets on Hawaii and on Iceland. So, I would love, love, love to get seismometers all over Ceres.
Fraser: Yeah. Ceres is one of these icy worlds. More of a rocky icy world I would love to see some of the outer stuff. Enceladus, Europa, and I know the New Horizons team would love to have had a seismometer that they could use to understand what was going on with Pluto. How is it getting these glaciers of nitrogen, and ammonia, and methane? And how did the mountains of water ice form? There’s one last place that I want to talk about seismology, briefly, and that’s the concept of asteroseismology, which people will hear that term. What’s going on there?
Pamela: So, asteroseismology is talking about the way waves propagate through stars. Stars, for a whole different variety of reasons, will end up with oscillations in their atmosphere. Pulsating variable stars, which is like my happy place, will build up massive oscillations where they, essentially, expand and contract like a beating heart. And they can do this, either en masse, with one massive oscillation of the entire surface. They can do this with harmonics, where different parts of the surface are expanding while others are contracting. And by studying these large scale oscillations, we can actually measure over the fullness of time, by which I actually mean over a couple generations of human observations.
We can start to see changes in a star’s density. Changes in what way through the Hertsprung-Russell Diagram that star is evolving. Now, beyond the massive pulsating variable stars, we find all kinds of smaller, more complicated, harmonics in the atmospheres of stars, even like our sun. Where you have all across the surface, pockets that are rising and falling in a grid-like formation. With these complex, highly interfering waves. And there was a project in the ‘90s, GONG that got set up to constantly watch for monitors all across the surface of the planet, these fine scaled oscillations, that up until then, had been predicted, but hadn’t really been observed.
GONG was able to recover all of the things that were predicted. And today, we study the solar surface, and the solar oscillations in higher resolution than, it sometimes seems like we should be able to deal with. With terabytes of data per day coming down from the solar dynamic orbiter, allowing us to study waves moving through the solar atmosphere.
Fraser: You get starquakes. And they create waves that move through them, because you don’t have the rock, the whole thing is a plasma. But in the interior, the levels of density are so high that you do have layers. You have the radiative zone, you have the core, and the density can be much more dense than rock, than any metal that we have, but it’s hydrogen mashed together. And so, it’s the exact same process where you get these waves moving through the various layers of the sun. And then you see the ripples on the sun, and that tells you more about what’s inside of it. It’s a stunning science.
Pamela: One of the coolest things, for me, in variable star astronomy, is that some of the dramatic period changes that we see are thought to potentially be linked to convective overshoot. And the way that I’ve always envisioned this is, imagine if your lava lamp has that blob that’s rising. And it just keeps going, and overshoots into your room, and sends ripples through your atmosphere. That is an oversimplification of what might be happening in stars. But it’s still that same idea that sometimes what’s going on just decides to go on with a little bit more enthusiasm than it normally does.
Fraser: That’s awesome. Just to let everyone know, next week we are going to be at a convention, but together.
Pamela: Yes. And there’s going to be rockets involved.
Fraser: Yes. So, we’re going to try to record, somehow, live. I’m not really sure what the plan is. So, anticipate a strange time, but we will try to make it happen. Because we’re gonna be in the same place, and that’s always a lot of fun. All right. Thanks, Pamela.
Pamela: Thank you, Fraser.
Male Speaker: Thank you for listening to Astronomy Cast, a non-profit resource provided by Astrosphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at AstronomyCast.com. You can email us at info@astronomycast.com, Tweet us @astronomycast, like us on Facebook, or circle us on Google+. We record our show live on YouTube every Friday, at 1:30 p.m. Pacific, 4:30 p.m. Eastern, or 20:30 GMT. If you miss the live event, you can always catch up over at cosmoquest.org, or on our YouTube page. To subscribe to the show, point your pod catching software at astronomycast.com/podcast.xml.
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[End of Audio]
Duration: 28 minutes

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  1. If you use the Raw Feed, and get the episode for Cassini instead of Seismology, just delete and redownload. I’ve replaced the incorrect file with the new one now. Apologies for the mistake!

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