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Music of the Spheres

Season 1Oct 29, 2018

The Earth "rings" when earthquakes strike, just as vibrations through a bell create a ringing sound. The InSight mission will see if tremors cause Mars to chime with its own planetary song.

On a Mission podcast cover image of tracks on Mars with NASA JPL title text

[0:00] Narrator:Did you know that the Earth rings like a bell?

(bell tone)

[0:05]When there’s an earthquake, vibrations travel through rock, just as vibrations move through a bell to create a ringing sound. We know our planet rings like this because of seismometers, instruments that measure the vibrations of the Earth moving beneath our feet. We don’t hear the Earth ringing because these rock vibrations, called seismic waves, have a frequency spectrum below one Hertz. The lowest frequency human ears typically can hear is 20 Hertz. By speeding up seismic waves hundreds or even thousands of times, you can hear what an earthquake sounds like. This, for instance, is the altered sound of an earthquake in Japan.

[0:43] (altered sound of earthquake)

[0:51]It’s not just the Earth that rings when the ground is disturbed. The Moon also rings.

[0:55]Apollo 12: “Hey, I just threw something and it hit the ground, it must be going up 300 feet. Boing!” “Stop playing and get to work!”

[1:05] Narrator:Astronauts put seismometers on the Moon during the Apollo missions in the 1960s and 70s.

[1:10] Apollo 12: “Looks like it’s going real good Houston. This seismometer, I’m having trouble making the skirt lie down, but other than that it looks good. Doesn’t want to just lay flat like it does on Earth at 1 G, it sorta wants to…”

[1:26] Narrator:Rather than wait around for a moonquake, or for a meteor to hit the Moon, astronauts set off small explosives to create seismic waves.

[1:34]Apollo 14: “5-4-3-2-1… Fire. This thing has a pretty good kick to it.” “Ok, good shot Ed.” “It’s like firing both barrels of a 12-gauge shotgun at once.” “Roger.”

[1:55] Narrator:To create bigger vibrations, they needed a bigger bomb. On the Apollo 12 mission, after they’d left the surface of the Moon and returned to the command module orbiting above, they sent their lunar module back down to the Moon’s surface. It hit the ground about 40 miles away from the Apollo 12 landing site, with the force of one ton of TNT. As recorded by the seismometer, the shockwave lasted for an hour.

[2:19] Although the Apollo 13 mission is better known for “Houston, we have a problem,” and the struggle to get the astronauts safely back home, when they circled the Moon they dropped their S-IVB, one of the stages of their rocket, and it hit the Moon 85 miles from Apollo 12’s seismic instrument. The rocket booster hit with the force of 11-and-a-half tons of TNT, and the shockwaves lasted nearly three-and-a-half hours.

[2:44]Apollo 13:By the way, Aquarius, we see the results now from 12’s seismometer. Looks like your booster just hit the Moon, and it’s rocking it a little bit. Over.”Well, at least something worked on this flight.” “I’m sure glad we didn’t have a LEM impact too.” “Right.”

[3:07] (intro music montage)

[3:39] Narrator:Welcome to “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory in Pasadena California. I’m Leslie Mullen, and this season we’re taking a close look at the InSight mission to the planet Mars. One of the things the InSight mission will do is place a seismometer on Mars to feel for vibrations. InSight is actually an acronym. Here’s Bruce Banerdt, lead scientist on the mission.

[4:03] Bruce Banerdt: It stands for Investigations of… Oh man, Internal Exploration of Mars using Seismology… Nah, I can’t visualize the letters… S-I-G-H-T. To be honest, I figured out the acronym after I had the name. In the end, I came up with InSight because we really are looking into the planet. We’re trying to see into Mars with these geophysical instruments… InSight kept going back, and I kept trying to put together an acronym that would work. It’s like: Internal Exploration using Seismic Investigations Geodesy and Heat Transport. There you go. And the exploration just kind of keeps its lowercase E. We don’t actually get that into the acronym.

[4:52] Narrator:Ok, sometimes you have to be creative to make an acronym work.

[4:56] Bruce Banerdt: We almost called it Tremors. Tremors is a crazy set of monster movies that are set in California. My kids wanted me to call it Tremors because they think those movies are really funny.

[5:06] Narrator:Part of the InSight acronym is “seismic investigations.” We want to see if Mars has quakes, but there’s more to it than that. Vibrations going through a planet or a moon are a way to see the different layers deep inside.

[5:18] Bruce Banerdt: An earthquake is almost like a little flashbulb. It illuminates the inside of the planet with seismic waves, and the seismometer is like a camera that picks up those waves and helps put together a picture. Pixel by pixel we get to put together a 3D picture of the inside of a planet.

[5:34] Narrator:That’s also why astronauts put seismometers on the Moon: to peek under the surface and see how the Moon is constructed.

[5:40] Bruce Banerdt: So seismology turns out to be the primo method for finding out what’s inside a planet. Like 90% of what we know about the inside of the Earth is all mostly because of seismology.

[5:53] Narrator:The layers inside tell you about the history of a planet, says Sue Smrekar, deputy principal scientist of the InSight mission.

[5:59] Sue Smrekar: We know that any rocky planet it starts out molten. From little blobs of material floating around in this disc around our Sun, those planetesimals, those little bits of matter and rock, they come together and they crash. And that creates a lot of heat energy just from the impact of those planetesimals with each other.

[6:19]Then what? Then it forms these layers inside the planet. The heavy stuff sinks to the bottom and forms a core. The light stuff rises to the top and forms the initial crust. The layers that form on the inside, they have to be different because rocks of a given composition, they’ll go through a phase transition. If you take a rock from the surface and you put it down a depth of 500 miles, that pressure and temperature can cause it to transform, you know like from graphite to diamond, right? This is like kind of a classic example. But virtually all rocks will change their shape, their crystalline shape, when you apply pressure and temperature, if you apply enough.

[6:58]So we’ll get this very new picture of what happens after planets…after they’re molten, and how these layers start to form. We’ll know the size of the crust from seismology, know the size of the core, and we’ll get a much clearer picture of what happened immediately after Mars formed. And from that we can learn about not only that initial phase, but also how it’s changed over time.

[7:23] Narrator:Unlike the Apollo missions, the InSight mission will not be blowing up explosives or crashing rockets down on the surface of Mars to create seismic waves. Here’s Bruce Banerdt again.

[7:33] Bruce Banerdt: The idea of having a controlled energy release is a common method in seismology, because then if you know exactly where your event is, your energy release, which is either a quake, an explosion, an impact, anything that bangs the ground, if you know exactly where it is, if you know exactly what time it is, then you have extra information that you can use to measure the speed at which the waves have gone from that point to your seismometer, and that speed is one of the key things that we’re measuring in seismology and using to probe the material.

[8:06] The rocket that brought Apollo to the Moon basically stayed with it until it got close to the Moon, and so it was in the right place at the right time. The way that a planetary spacecraft works is that you give the big boost with the Centaur upper stage and then you separate them and then you do sort of the fine tuning for the next six months using smaller rockets on the much less massive interplanetary spacecraft, in this case InSight. In order to get that Centaur to actually hit Mars exactly where you want it, you would have to put a whole attitude control system on it, you’d have to do trajectory correction maneuvers, you’d basically have another spacecraft which would be extremely expensive, and something that big would be very difficult to navigate all the way to Mars.

[8:51] What we did consider is actually sending smaller spacecraft containing basically cannonballs. If you take tungsten, which is something that doesn’t erode very much from a high-speed entry into an atmosphere, you can take a tungsten ball that’s about six inches across and if you target it right at the planet and have it impact at interplanetary speeds, which are about 10 kilometers per second, even a small ball like I said about six inches across will make a crater that’s about 15 or 20 feet across on the surface and that creates a seismic signal that you could see many hundreds of kilometers away. One of the co-investigators on our team actually put together a proposal to NASA to do that and NASA considered it and decided that…I think it’s going to be about another 50 or 60 million dollars… and they said, “Oh, you guys should be happy with what you’ve got.”

[9:42] Narrator:The InSight mission will rely on Marsquakes to set off their seismometer.

[9:46] Bruce Banerdt: A marsquake is caused by the same thing that causes earthquakes. It’s caused by stresses, by forces that build up in the crust of a planet, and finally break the rocks. Like you pull a rubber band, and once it breaks, it snaps. On Mars, we think that the forces are due to basically thermal activity inside the planet, just the contraction of the planet itself as it gets older and older. As it cools, the planet gets smaller, and the crust around the planet has to kind of crinkle a little bit to fit on a smaller and smaller sphere as the planet thermally shrinks. The other things that can happen is if you have a particularly warm spot in the mantle, it can actually buoyantly push up on the crust, and that will bend the crust and cause stresses and cracks to form, and those cracks will be marsquakes.

[10:31] Narrator:Meteorites also smash down onto the martian surface from time to time, creating big impact craters and shaking the ground.

[10:39] Bruce Banerdt: We know that there are impacts on Mars, not only do we see old impacts but Mars Reconnaissance Orbiter’s detecting new impacts every month, just about. It sees new craters where they weren’t there before. And so those will give us seismic signals.

[10:52] Narrator:For the Apollo missions, even with the controlled explosions to set off the seismometers, scientists at first weren’t sure what they were seeing when they looked at the data.

[11:01] Bruce Banerdt: Apollo 11 had measured seismic activity for a few weeks. The seismologist saw the data and it just looked like a bunch of kind of random noise. And then when they went back with, I believe Apollo 12 had a seismometer on it, and then they crashed the upper stage of the rocket and that signal looked like one of those noise things that they’d seen before. When they looked at it more closely, they realized that this actually was a seismic signal, but it wasn’t like a seismic signal you see on the Earth. It was something that was kind of a gradual increase in the amplitude and then a long die off because the Moon transmits seismic waves differently than the Earth does. There’s a lot of head scratching before they kind of figured out how that worked, but once they knew what a seismic event looked like on the Moon they could go back and say, “Oh, we’ve seen dozens of these before.” And then they were off and running on lunar seismology. But without that one event that they say, “Oh, that’s what a seismic event looks like,” they were kind of hosed for a while.

[12:02] Now, on Mars, of course, we’re not going to be able to do that, so we come pre-armed with the realization that a seismic wave might not look like what we expect it to — although there are lots of reasons to believe that the similarities between the Earth and Mars are much more important for seismology than the differences that we see between the Earth and the Moon. We’re expecting things that look more Earth-like, but we also have the understanding that it’s easy to fall into a trap of seeing something that you haven’t seen before and just dismissing it out of hand.

[12:33] Narrator:Another way to think of this is: what will Mars sound like? After I learned about the Apollo mission’s seismometers, I mentioned to Bruce that the Moon rings like a bell. He quickly corrected me: “Earth rings like a bell,” he said, “but the Moon rings more like a gong.” There is a gong in the InSight strategy room, and he not only agreed to illustrate the concept for me, he took apart his own doorbell at home in order to bring in one of the chimes.

[12:58] Bruce Banerdt: So there’s lots of musical instruments based on kind of this bell sound. (DING!) Very pretty sound. A gong is a little bit different. It has a little bit scruffier sound. If you look at it real closely, you can see there’s lots of kind of dents in a gong and those dents actually keep the sound waves from running free. So when the sound waves hit these various dents, it sort of scatters them. It makes them kind of bounce off and go in different directions. If you hit the gong (BONG), you can see it has kind of a little bit of an edge to it. (BONG) So you can hear that tone but you can also hear it kind of sizzle on top of it and that’s all the waves kind of getting scattered around.

[13:42]The Moon has a characteristic much like a gong, in that the top 50 kilometers, maybe 100 kilometers or so, is all broken up from all the impacting. And because it’s so dry, there’s almost no water at all in the Moon’s crust, those cracks never heal up again. On the Earth, you have water, you have heat, and it tends to make those cracks kind of heal over millions of years and basically becomes more like solid rock again. But on the Moon, when a wave comes it tends to bounce off the crack, it bounces here, bounces there, and bounces back and back, they start running into each other. The waves go on and on for a long time because the Moon is very dry, they don’t get damped out. It “rings” for a long time, but it doesn’t ring like a bell. It rings more like (BONG) a gong. (laughs)

[14:31] Narrator:Like the Moon, Mars is dry, but Mars has other aspects more in common with Earth.

[14:36] Bruce Banerdt: We think it’s gonna’ ring more like the Earth because even though Mars is dry on the surface, it does have moisture. We have polar ice caps which are largely CO2 but partially water as well, and so there’s a water cycle. There’s certainly evidence of water in Mars’ history on the surface… (DING) yeah, there’s your Mars, (BONG) There’s your Moon. So we think that Mars is going to act more like the Earth, but it’s a mystery until we actually go and measure it. That’s one of the big questions.

[15:20] Narrator:Bruce told me that scientists have wanted to put a seismometer on Mars for a long time. And they sent one — in the 1970s — on the Viking landers.

[15:29] (NASA short video: “Vikings to Mars”)

Mars. It’s waiting out there. Just as it always has. Reluctant to give up its secrets, even to earlier spacecraft flying by its sometimes dust-shrouded surface.

Roger, we just switched to single subcarrier (garbled). Copy that.

Vikings to Mars. The stage is set, the spacecraft are well underway, and the people on the ground are nearly ready for this country’s first Martian landing.

[16:00] Narrator:Bruce says the Viking seismometers didn’t work as planned:

[16:04] Bruce Banerdt: I was following the Viking mission back when I was a graduate student. It had a seismometer on it that we had high hopes for, but turned out to be just not quite sensitive enough. They weren’t able to give it the resources to be able to put it on the ground on Mars, and so all it ever ended up measuring was the wind blowing the lander around, and we were all disappointed. But then they shut off the Apollo seismometers in ’77, and Viking didn’t send back useful seismic data, and so, most of the terrestrial community said, “Well, there’s no data coming back. There’s no science to be done.” They went off to do other interesting things that had to do with the Earth. I kept on working on these things, and trying to press them forward. And so, I became identified as the planetary seismology guy, at least in the United States.

[16:50]Back in the late 1980s, there was a guy at the microdevices laboratory here at JPL who had a really sensitive accelerometer. It’s something that measures motion really sensitively, like at the picometers, which is 10 to the minus 12 meters. He thought, “Hey, this would be a great seismometer.” Something that measures ground motion. So he came to talk to me and he says, “Hey, would you like to work with this? We can make a seismometer. It will only weigh 30 grams, or 40 grams, or something crazy small. And then you’ll be able to put it on a rocket, and send it anywhere in the solar system you want.” I said, “Wow, that sounds pretty cool.” So that sent me down the path of trying to get these seismometers on a Mars mission.

[17:34] Narrator:Bruce has been trying to put a seismometer on Mars for over 30 years. But there’s limited money for space missions, and there’s a lot of places to explore.

[17:42] Bruce Banerdt: The thing is, you’re competing against probably 25 other really great proposals, and so trying to get your nose above the pack or ahead of the pack. It’s really a tough problem when you have so many great things to do in the solar system.

[17:54] Narrator:Every time it seemed like he might finally get to send a seismometer to Mars, he was disappointed.

[17:59] BruceBanerdt: It was actually on NASA’s official road map for planetary exploration to have this, what they call the Mars Network Mission, which was going to send 16 landers to Mars all with seismometers, dot them all over the surface, and we’re going to do this great seismic experiment. And then, started looking at the bill for something like that, and it’s a billion and a half to two billion dollars. And this is back in the ’90s when that was worth a lot more money than it is today. And so NASA got a little bit… You know, let’s put the brakes on this a little bit.

[18:29] Narrator:Scientists in other countries also want to send a seismometer to Mars. The Russians tried with their “Mars 96” mission, but the rocket failed and it came crashing back down to Earth. Bruce later recruited the French seismometer team from that Russian mission to work with him. And then…

[18:45] Bruce Banerdt: I figure this is the last proposal I probably had time to make because a proposal takes two or three years. Once they select you, it takes another five years, six years, and then your mission is another two or three years. So you really have to have about 10 years in your pocket when you do one of these proposals, if you want to do the whole thing before you retire. So we put in this proposal, and then NASA chews on it for about eight or ten months. I think we put in the proposal in January. In the summer time, NASA’s saying, “Well, we’re getting close. We’ll be making our selection pretty soon. When are you going to be available? So that if your mission’s selected, we can involve you in the press conference,” and stuff like that. This was August, I’ll be available any week, except for this one week. Because I’m going camping and there’s no internet, there’s no availability. Of course, they pick that week to make their decision.

[19:38] I was actually up in the mountains when they made the announcement, this was the Sierra Nevadas, just south of Yosemite. Beautiful, beautiful area. I was there with my whole family. We were driving down the mountain, and we finally got into phone contact. Suddenly, the phone in my pocket, as I’m driving down, starts going, “Ding, ding, ding,” my email alert. It’s just like going on constantly. I’m going, “Whoa, something’s going on here.” And of course I’m thinking in the back of my mind, “This could be, this could be selection,” but I don’t want to really commit to that emotionally, right? SoI’m going down the mountain, and my phone is dinging away. I think, “Well, maybe my iPhone is just on the fritz. Maybe it got in the water when I was playing in the river or something.” I don’t know. So we get down to the bottom of the mountain.

[20:26] We have a tradition in my family. We go to the In-N-Out in Clovis right outside of Fresno when we get down from camping. I told my family to go ahead and go on inside and I’ll see what’s going on here. I’ve got a couple of phone messages. So I thought, “Well, before I look at my email, I’ll look at the phone messages.” There was one from a contact at NASA. And he says, “Oh, hey there, Bruce. Say, if you get a moment sometime, you might give me a call. I’ve got something to talk to you about.” And that was the message right? It was so noncommittal. The second message was from my project manager that I was working for on the proposal. I got his message and he says, “Hey, Bruce. We’re in! Give me a call.” And I just started jumping up and down, and jumping up and down in the parking lot right there in the In-N-Out. And my kids came screaming out of the In-N-Out Burger, because we kind of talked about what it might be. It was kind of crazy, but a lot of fun.

[21:22] We raced down to Pasadena. They already put together a party outside the Athenaeum, just on the spur of the moment. I showed up, of course all dirty and scraggly from a week of camping. And everybody else is like all dressed up in their summer finest. But nobody cared. That was August 20th, 2012. My birthday actually. Great birthday present.

[21:42] Narrator:Bruce and his team set to work on building the lander and all the instruments. Three years and a mountain of work later, InSight was on the launch pad. But then, the seismometer showed signs of trouble.

[21:53] Bruce Banerdt: We’d already overcome several technical problems in making such a delicate and sensitive instrument. And so, we were rushing to the end trying to get the instrument all finished in time to put it on the lander, and get it to the rocket. And our very last test, when we were doing what we call the thermal vacuum testing, where we actually tested in the conditions that it’s going to see at Mars, suddenly we saw some kind of a leak letting the vacuum out of this container. We finally traced it to the, what we call the feed throughs, the places where we actually feed the wires through the wall of this vacuum container, so we can get signals out, and power into these seismometers. The material around those wires had cracked a little bit when we went down to very cold temperatures. We found out later that the connectors that we used here were actually not really designed to withstand the super cold temperatures on Mars. They were designed to stand maybe minus 30 degrees C, and we had to go down to minus 100, 120 degrees C. And so, just the thermal expansion coefficient was such that it causes a very, very tiny crack. I mean, it was a really small leak. If you had that leak on your car tire, it would take about 50 years to lose 1 PSI out of your car tire. But these seismometers are so sensitive that, that was enough that by the time we got to Mars, we would have lost about a factor of five to ten in our sensitivity, which would make our experiment pretty much useless on Mars.

[23:18]We’d actually already sent the lander to Vandenberg, and it was sitting there, already checked out and ready to accept the seismometer. So the seismometer was just the last little cherry up on top of the sundae that we needed to get there. We tried to fix it, but we couldn’t find a way to patch it that would also withstand those big temperature variations, going from warm to cold and back again, as you have to do on Mars every day. In the end, our last attempt to patch it, we did the testing, and it failed. There just wasn’t any more time.

[23:49]We had to stand down from our launch in December of 2015, which was incredibly disappointing. It was really a dark time. It was almost Christmas, and then we had to give up suddenly on this mission.

[24:03]I went home and I moped for about two weeks, and kind of recharged my batteries, and started back up after New Years, told NASA, “We’d like to get a second chance to do this. If we have a little bit of time, we know how to do the repair right. And they said, ok well, take about two or three months, put together a detailed plan, and we’ll consider the path forward.”

[24:24] Narrator:InSight went back to the drawing board.

[24:27] Bruce Banerdt: We went completely through the entire system. We redid those connectors. And we went through all the other systems on the spacecraft and the other instruments as well and made small improvements. It was really a horrible experience to delay, but I think on some sense we’re better off, we have a stronger mission now than we did two years ago.

[24:46] Narrator:Last May, InSight finally launched from California’s Vandenberg Air Force Base and headed for Mars. It launched in early morning darkness, with a thick fog blanketing the area. Those of us who came to watch the launch could see nothing… but we heard the rumble of the rocket blasting off, we felt the vibration of it in the ground beneath our feet. And that seems the perfect send-off for a mission going to Mars to detect vibrations. To feel the heartbeat of Mars, and to listen for its song.

[25:23] Narrator:Next time, On a Mission:

[25:24] Marleen Martinez Sundgaard: We come in at night, and we call ourselves the little test-bed gremlins, just because no one sees us. Stuff happens in the night, and miraculously all the data is there in the morning.

[25:34] Narrator:If you like this podcast, rate like and share us on your favorite podcast platform. We’re hashtag: nasaonamission. Also check out NASA’s other podcasts: Gravity Assist, Rocket Ranch, What’s Up, NASA in Silicon Valley, and Houston We Have a Podcast. They can all be found on NASA’s podcast page. We’re “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory.

(end music: finis)

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