What’s Shaking Below Mars’ Surface?

ROXANNE KHAMSI: This is Science Friday, I’m Roxanne Khamsi. You’ve seen the effects of earthquakes on our planet, the ground shakes, the Earth trembles, and if it’s strong enough, damage and devastation. But it turns out that ours is not the only quaking planet around there are Mars quakes too.

Seismologists have been studying these quakes on Mars and they’re giving scientists exciting first clues as to what’s going on below the Martian surface. Several new papers based on the data from the Mars InSight Lander were recently published in the journal Science. Joining me now to talk about that mission and what it is revealing is Bruce Banerdt, he’s principal investigator for the Mars InSight mission. And Sue Smrekar, she’s deputy principal investigator for the Mars InSight Lander and the principal investigator for the planned VERITAS space probe to Venus. They’re both based at NASA’s Jet Propulsion Laboratory in California. Welcome to Science Friday.

BRUCE BANERDT: Hi, how are you doing?

ROXANNE KHAMSI: Great. Sue, it’s good to have you too.

SUE SMREKAR: Thanks.

ROXANNE KHAMSI: Bruce, I’d like to kick it off with you. Can you describe these quakes for us? Like how big are they on the Richter scale?

BRUCE BANERDT: Well, these are actually fairly small quakes by our kind of Earth standards. These are less than about magnitude four on the magnitude scale. And that’s a quake that you would feel pretty well, you’d feel it shaking you around if you were within 10 or 20 miles of the epicenter on the Earth or on Mars for that matter. But if you got much farther away from that you probably wouldn’t feel very much. So these are not very big quakes but Mars is a small planet, so they don’t have to go very far to get through the inside and we’re able to use these very small quakes to probe deep into the planet.

ROXANNE KHAMSI: You and Sue and others were detecting these quakes with the seismometer from InSight. Can you tell me a little bit about how you use the technology to find out what’s going on inside the planet?

BRUCE BANERDT: Well, the science of seismology is basically taking the wiggles on a seismogram, which are the displacement waves that come through the planet and using them to pull out the information that they picked up as the waves have traveled through the planet. So when a fault breaks on the other side of the planet it sets up vibrations and those vibrations move through the planet much like sound waves move through the air.

And as they move through the planet the properties of these waves are affected by the materials they move through. They can get reflected off of boundaries, they can get refracted just the same way as light is refracted in a prism as they go from one kind of material into another kind of material, they get attenuated the waves die out, they lose some of their energy as they go through materials. And some materials attenuate them more than others, especially hot materials which are a little bit softer, tend to kill off the waves more quickly than cold, brittle materials. And so all these things are basically they’re pieces of information about the deep inside of the planet that get encoded into the signal.

And so we’ve used things like the travel times of different waves to infer the different paths that they’ve taken. We look at their frequency content. We look at their polarization. There are just a myriad of different ways that you can attack these signals with the same kinds of processes that have been developed for radar, radio, and things like that, and even acoustic recordings to pull this information out of our seismograms.

ROXANNE KHAMSI: That’s amazing. And you’ve been working on this lander for around a decade. Is it as straightforward as taking an Earth seismometer and getting it to another planet?

BRUCE BANERDT: Nothing in space is straightforward. It’s actually a super complex and difficult endeavor to get something that works on the Earth and make sure that you can actually do the same thing on the very harsh surface of another planet like Mars.

ROXANNE KHAMSI: So it sounds a little tricky. You’ve got to package it up and somehow deliver it in a way that’s unique?

BRUCE BANERDT: Right. I mean first of all, you have to get it on the ground on another planet. And it’s hard enough just landing on Mars I’m sure you’ve seen the videos about how difficult and how hair-raising it is to land on Mars but once you’ve landed on the planet your instrument’s sitting on top of your lander, which is about a meter away from where you’d like to be which is on the ground and so we had to include a robotic arm that would pick it up off the deck of the lander, place it on the ground, and then pick up another shield to put over it to protect it from the wind. So that was a pretty complex operation in and of itself.

And then we had to shield it against the temperatures. On Mars the temperatures can go up and down by more than 100 degrees Celsius. And you know, how things expand and contract with temperature, when we’re measuring displacements of the ground, vibrations of the ground, some of those vibrations are no larger than the size of a hydrogen atom. And so you can imagine even small temperature variations down to 1,000th of a degree can make signals on our seismometers so we have three different layers of thermal insulation, we have a windshield to keep the wind from blowing on it. We do all kinds of things to cut down on other sources of vibrational noise so we can see these extremely small vibrations that have traveled for thousands of kilometers through the planet.

ROXANNE KHAMSI: Well, this does not sound easy peasy but I’m glad that you guys figured out the robotic arm and all those different protections, fantastic. So Sue, can you tell us a bit about what’s going on with these quakes? On Earth as far as I understand there’s tectonic plates pushing into each other but that’s not the case on Mars, right?

SUE SMREKAR: Right. No plate tectonics on Mars but that doesn’t mean that there isn’t tectonics. So on Earth we have our plates that are sinking into the mantle and sliding past each other and colliding to form mountain belts. So we have measurable velocities of these plates at the surface constantly causing geologic activity. And basically, all of our earthquakes. But on Mars, it’s a so-called single plate planet but that doesn’t mean that there’s not fracturing, deformation, faulting, volcanoes.

Now of course, most of the surface of Mars is quite old, billions of years old but there are still a few places on the surface that are recently, from a geologist standpoint active. And one of the places is actually pretty close to our land about 1,000 miles from our lander and it’s called Cerberus Fossae. And it’s got these 500 kilometer, few hundred mile long fractures that are related to volcanism. There have been flows that have come out in the past few million years. So for Mars that’s super recent. And so there’s still geologic activity on Mars.

ROXANNE KHAMSI: And as I understand, it’s the planet is cooling as well, that adds something to the whole picture.

SUE SMREKAR: Absolutely. Yeah. Yeah, and in fact, before we sent InSight to Mars, people did calculations to try to estimate the amount of fracturing of faulting that would be occurring due to that cooling of the planet. I mean, all planets are cooling and Mars is perhaps dominated by that process of cooling.

And maybe one of the interesting things we discovered is certainly some of the overall activity is due to cooling but perhaps a surprising amount is coming from these particular fractures. And the other thing that we found is that some of the other fractures that are pretty recent and kind of the other side of the planet, were actually because Mars is a much bigger core, we’re actually in the shadow of the core with respect to the other side of the planet and how the seismic waves travel through the planet. So we can’t actually pick up all the quakes from some of the places that we think should be tectonically active on Mars. So yeah, we have the cooling and fortunately, have this great local seismic source too.

ROXANNE KHAMSI: How is this different from what’s here on Earth, what we know about the inside of our planet?

BRUCE BANERDT: Well, as Sue was talking about how Mars is cooling off. And actually the way a planet cools is really fundamental to its geology, to the features on the surface and the way they evolve. On the Earth, the planet loses its heat mostly through the process of plate tectonics, when hot material rises at mid-ocean ridges and as it spreads out it can cool itself through the floor of the ocean. And that’s a very efficient way of cooling a planet and it lends itself to a lot of dynamics, a lot of action. There’s a lot of motion, a lot of forces that are built up. And so we have a very active planet with lots of seismic activity, lots of volcanic activity, and accompanying hydrothermal activity and so forth.

On Mars, since it only has one plate essentially, there’s no plate tectonics you essentially have one single plate covering the entire planet, it cools more slowly, it cools by conduction through the surface. And so most of the geologic activity is dominated by either localized volcanism or in some cases, there are some rising and falling of that one plate as hot plumes from deep in the mantle rise up and can push up on the bottom of the crust or maybe pull down where they descend back into the mantle. And so it’s a very different kind of set of forces and processes that occur on Mars. And that’s– to some extent that explains a lot of the differences in the surface features that we see on Mars compared to the Earth.

ROXANNE KHAMSI: Do all the rules that we’ve learned for geology on Earth necessarily hold true on Mars? Are these things that are surprising you both?

BRUCE BANERDT: I would say, for the most part, the same rules apply. The really interesting part for us as scientists is when you have the small deviations not necessarily from the rules but from the way the rules are applied. And so you have the same physics, the same physical laws, the same general geology but the details, that’s where the really interesting stuff is.

For example, on Mars the crust is a little bit thinner than it is on the Earth. The core is a little bit bigger relatively speaking, and those differences between the Mars and the Earth are due to differences in either the starting conditions of the planet’s formation or in the path of evolution that it took from those very earliest years till today. And so we’re looking at those differences and using them to fine tune our models for understanding how these planetary processes work.

ROXANNE KHAMSI: I’m Roxanne Khamsi and this is Science Friday from WNYC Studios. I’m talking with Bruce Banerdt and Sue Smrekar about investigating the seismology of other planets. As I understand, you’re working on plans for the VERITAS mission to Venus, which could also try to figure out things about the geologic processes working on that planet but from orbit. So how do you do that?

SUE SMREKAR: Well, we’re going to take data from a couple of different instruments, we’re going to get topography at high-resolution, radar images. For Venus it’s shrouded in this thick cloud layer, so anything that we do from orbit has to be able to penetrate through that cloud layer. So we use radar to do that. And we also have a spectrometer that sees the surface around 1 micron, like a thermal part of the spectrum. And with that we’re able to look for things like variations in the iron mineralogy that tell us that a volcanic eruption has been there recently, it hasn’t yet chemically equilibrated with the atmosphere.

We can also look for actual active eruptions but you have to be super lucky to see active eruptions because on Venus, on Earth, everywhere, basically when super hot lava comes to the surface it starts to form a crust very quickly. And so it’s hard to see that thermal signature from orbit for more than a few weeks or so.

ROXANNE KHAMSI: So do you have a timeline for VERITAS?

SUE SMREKAR: Well, we’re negotiating with NASA headquarters on exactly when we’re going to launch. We’re hoping it will be towards the end of 2027.

ROXANNE KHAMSI: Great. Both of you have said some really interesting things and what I’m curious about is what would you both hope to learn from either of your missions?

BRUCE BANERDT: Well, in terms of InSight we’ve really with these three papers kind of hit on the main goals of the mission. I mean this is really what we started out 10, 15 years ago to do, which was to delineate the size of the core the thickness of the crust and the structure of the mantle of Mars.

On that level, we could sit on our laurels now and say we’re done but of course, we’re still alive on the planet, we’re still alive on the surface taking seismic data as we speak. And we’d like to first of all refine those measurements, get them down to more precise values. And we’re looking at new things, we’re looking at the possibility that seismic activity on Mars might have a seasonal variation, which is we have some hints of that now which would be very strange and very different than what we see on the Earth. There’s lots of different weird kinds of quake signals that we’re seeing that we don’t understand yet. So there’s a lot to still to understand about Mars.

SUE SMREKAR: Well, I’ll tell you the things that I’m hoping to learn about Venus. For me, the fascinating thing about Venus is that it is so similar in size to the Earth but it doesn’t have plate tectonics. And we’ve been talking all about how planets lose their heat and how what’s going on inside with the loss of that heat affects what’s going on the surface, Earth has plate tectonics, Mars has these big volcanoes, and it still has faulting and so forth. Venus is this crazy place, it has a young surface, it’s somewhat similar an age to the surface of the Earth and it’s so big, it has this giant amount of heat, this heat engine that should be churning and producing something like plate tectonics but it doesn’t.

So the big question for me is how is it operating, what’s the process? We think that it may have a lot to do with the volcanism. There’s just 80% of the surface is COVID in volcanoes and so maybe there’s some kind of intermediate process where it loses a lot of heat through volcanoes that never erupt on the surface. And the other thing that’s truly fascinating to me is that we believe it has subduction zones, where one of these fixed plates is sinking into the mantle. And that is how everyone thinks plate tectonics started on the Earth. Earth didn’t start out that way, it didn’t form with plate tectonics. It formed with a single plate. So this huge question is, how those plate tectonics start and that crust is like billions and billions of years old. So we have little data to actually tell us how the Earth made this massive transition to plate tectonics, which has so dominated the evolution of the Earth. But on Venus we think we can study the process of subduction occurring today and to be able to see how a planet maybe starts down the path of plate tectonics to me is super fascinating.

ROXANNE KHAMSI: Well, as our planet is turning we’ve run out of time, unfortunately but Bruce Banerdt, thank you so much. Susan Smrekar, thank you so much.

BRUCE BANERDT: Oh you’re very welcome. I’m really thrilled to be able to talk about this.

SUE SMREKAR: Yeah, a pleasure. Thanks.

ROXANNE KHAMSI: Bruce Banerdt is principal investigator for the Mars InSight mission, and Susan Smrekar, she’s deputy principal investigator for the Mars InSight Lander and the principal investigator for the planned VERITAS space probe to Venus.

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