The mystery of the largest marsquake ever recorded

Sarah Al-Ahmed: What caused the largest Mars quake ever recorded? We'll find out this week on Planetary Radio. I'm Sarah Al-Ahmed of The Planetary Society With more of the human adventure across our Solar System and beyond. NASA's InSight Mars Lander reached the end of its mission on the Red Planet about a year ago, but the science never stops. Benjamin Fernando, a post-doctoral fellow at the University of Oxford, joins us this week to talk about the largest Mars quake recorded by InSight and the international effort that it took to pinpoint the cause of the quake. Then Bruce Betts, the chief scientist of The Planetary Society and I will talk about our earthquake experiences in this week's what's up? Apparently we're the kind of nerds that run for the seismograph the moment we fill the earth move under our feet. If you love Planetary Radio and want to stay informed about the latest space discoveries, make sure you hit that subscribe button on your favorite podcasting platform. By subscribing, you'll never miss an episode filled with new and awe-inspiring ways to know the cosmos and our place within it. Honestly, I have a strange fascination with earthquakes. I grew up in a town that bordered California's San Andreas Fault where earthquakes happen all the time. Our world's earthquakes are caused mainly by the movement of our planet's tectonic plates. The Earth's lithosphere, which is the rigid outer layer, is divided into several large and small plates that float on this semi-fluid material beneath them. The plates are constantly moving, albeit very slowly. Faults like the one that I grew up next to are fractures in the Earth's crust where the plates slide against each other. Over time. Stress builds up along these fault lines, and when it's released, it causes earthquakes. These quakes can range and scale wildly. Sometimes they're barely perceptible and other times they're big enough to bring down buildings and devastate entire communities. As scary and tragic as some earthquakes are, the perpetual motion of tectonic plates on earth is part of what makes our world so good for life. Plate tectonics not only helps shape the diverse habitats on our world, but it also helps regulate things like our climate and the circulation of our oceans. This process is so crucial to the way that our small terrestrial world functions that it's only natural that we would seek to understand similar processes on other worlds like Mars. Mars has shown evidence of tectonic activity in the past, but it doesn't have the same kind of dynamic plate tectonics that we have on earth. That's part of why scientists were so amazed that NASA's InSight Lander detected a 4.7 magnitude Mars quake on May 4th, 2022. The quake, which was dubbed S 1,222 A was so much larger than anything we'd recorded on Mars in the past that it launched an international effort to understand the cause of this Mars quake. NASA's InSight mission, which was the Mars lander that detected this Marsquake operated from November, 2018 to December, 2022. The lander was designed to unravel some of the mysteries of the Red Planet's interior by studying everything from Mars quakes to heat transport within the planet. With the spacecraft's Mars quake data in hand, space agencies around the world work together to determine the quake's source did an impact cause it or was it seismic activity? Only time and science would tell. This week's guest is Dr. Benjamin Fernando, a postdoctoral fellow at the University of Oxford. To get to the bottom of the mystery, he coordinated the first ever collaboration of all Mars orbiting missions. He's also the lead author on the research. His team's paper called a Tectonic Origin for the largest Mars quake observed by InSight was published in the Geophysical Research Letters on October 17th, 2023. Hi Ben. Thanks for joining me.

Benjamin Fernando: Thanks for having me.

Sarah Al-Ahmed: I tell you, every once in a while I'm going through my space news logs and I see a headline that makes me literally double take, and I feel like this is one of them. But before we get into all of the crazy details, what is it that InSight detected on May 4th, 2022?

Benjamin Fernando: So what we ended up detecting was the largest Mars quake that we've ever seen on Mars with InSight. And we were interested in whether it might be from a meteorite impact like the other big events that we've seen on Mars. But after search for a new crater, we were able to conclude that actually this was a tectonic event, so caused by processes similar to what caused earthquakes here on Earth, and it just so happened to be the largest one we'd ever seen on Mars.

Sarah Al-Ahmed: How big was it? What was the magnitude?

Benjamin Fernando: Yeah, magnitude of 4.7. So small compared to the sorts of things you might get in California, but big enough that you would probably notice if it would've happened here on Earth.

Sarah Al-Ahmed: I mean, for context, that is just the largest Mars quake we've ever detected and InSight found more than 1,300 of these events. So this is actually quite surprising to me. And am I remembering correctly that this Mars quake actually lasted for quite a while?

Benjamin Fernando: It did, yeah. So the ringing, the reverberations of the planet after the quake went on for well over an hour in the lower frequencies were even longer than that, which is a lot longer than you'd expect something to go on for on earth. So this was an unusual event in multiple ways.

Sarah Al-Ahmed: What could cause it to last that long?

Benjamin Fernando: What we expect is that waves, so seismic waves that travel through the planet, they get what we call scattered. That is they bounce off stuff and that sort of delays some of them. And we think on Mars that the surface waves, those are the really high amplitude waves. The ones that bring down buildings in California when there's a big quake, they're particularly strongly scattered because they propagate in the crust of Mars and that crust over billions of years has become very fractured. It's been impacted by thousands, thousands of micrometeoroids and larger things to the point where it's basically just dust. And all of that fragmentation has produced this really strong scattering. So when the seismic waves propagate through them, some of them are significantly delayed. On earth, that happens occasionally, but it's very, very rare to get a seismic event that would go on for anything closer that long.

Sarah Al-Ahmed: The only thing I can think of in comparison was when I was younger, there was an earthquake that went through my town, but I was at the high school at the time and everybody else was on the other side of town. The high school was on top of a very sandy kind of material, whereas the rest of the town was on solid granite. So when that earthquake came through, it was just kind of like a mild shake for everyone else, but everyone in the whole high school got motion sick because it went on for over a minute. It was a really strange experience.

Benjamin Fernando: Absolutely. It can make a big difference what the sort of subsurface composition is to how much shaking you get from seismic waves.

Sarah Al-Ahmed: And that's part of the magic about studying Mars quakes is that it can tell us more about the interior of the planet, which is really kind of what InSight was designed to do among other things. Would you mind telling us a little bit about the InSight mission and its goals and the instruments that it had on board?

Benjamin Fernando: Yeah, so InSight was a NASA mission with technology provided from a host of other nations whose primary goal was to study the interior Mars and give us insights into its composition, its origin and its evolution. And we're interested in studying the interiors of other worlds, because they also teach us about how our own earth came to form came to be the way that it is today. And InSight was the first dedicated geophysical mission with a seismometer on it that landed on Mars. We tried to do something similar with the Viking spacecraft in the 1970s, but it didn't really work. So InSight many decades in the sort of planning and development. But it finally came together in 2018 when we landed on Mars and with that very sensitive seismometer, we're interested in detecting the quakes from all around Mars to try and use them to probe the interior structure and dynamics of the planet. Some of those quakes are very close by, they propagate mostly in the crust. Some of them are much deeper and they propagate through the deep interior of the planet and they've given us information on the core, for example, how similar it is to the earth in some ways, how different it is to the Earth's in other ways. And InSight was really wonderful, because this is the first time we've had access to a data set like that for Mars and the first time we've been able to get as detailed a picture of the interior of the planet. It's not quite as detailed as we have for the earth, but it's certainly probably the second best at this point.

Sarah Al-Ahmed: Clearly, Earth and Mars are very different worlds. Mars doesn't have the same kind of plate tectonic situation that we have here on Earth. So what are the tectonic forces that work on Mars that could cause a Mars quake? Obviously we were assuming right off the bat that it was some kind of impact, but it didn't turn out that way.

Benjamin Fernando: Absolutely. So the biggest quakes that we see on Mars, we think they're still caused by tectonic forces, just not driven by plate tectonics anymore. So Mars may well have had plate tectonics at some point in the past, but today it looks like it's pretty geologically dead in the sense that it's a single plate planet. It's not seeing subduction, it's not seeing new crust being formed anywhere. But the planet's still cooling, it's still extremely hot on the inside. A small amount of that energy is escaping out into space on a second-by-second timescale. And over billions of years, that loss of energy means that as the planet cools, it starts to shrink. That shrinking doesn't happen uniformly, it looks like some places accommodate more of that stress than others. And that produces a buildup of stress which is released during Mars quakes. So that's one way in which the planet relieves the pressure that comes about from cooling over billions of years. And some of the smaller Mars quakes we think may have slightly different origins. So they come about from sort of thermal expansion and contraction if you like. So over the course of Martian day, the surface heats up quite considerably in the sun and cools at night, and that tends to produce much smaller Mars quakes that we see when they're much closer to inside. But the biggest ones we think are probably these residual stresses that build up in the crust and then eventually let go in this sort of explosive way through a Mars quake. It's exactly the same as how stress is released in earthquakes on Earth, just on a much smaller scale.

Sarah Al-Ahmed: So overall, I mean we're saying that this is the largest Mars quake detected by InSight, but how much larger was it compared to all of the other ones that we found?

Benjamin Fernando: Yes, it was several times larger than even the next largest Mars quake that we've seen. And one thing that we're interested in on Mars is understanding the ratio between the number of large Mars quakes to small Mars quakes. And on earth we have a well-defined relationship for that. That tells us how many Mars quakes that we've given magnitude would you expect at say magnitude five versus magnitude four versus magnitude three, and you expect exponentially more small ones. And on Mars, we were kind of not just curious into how much larger this quake was, but did it occur with the same sort of frequency that we would expect? Was it likely that we would see a quake of this size over the duration of the InSight mission? And as it turns out, before landing, the sort of predictions had suggested the largest quake we might see over a two-Earth year, one-Mars year mission was about magnitude five. And we saw something that was magnitude 4.7, which is pretty close. So it suggests that we still have a reasonably good understanding of how seismicity on Mars might work.

Sarah Al-Ahmed: That's really great to hear actually, because you see an event like this that on the face of it seems very surprising, but in actuality it's validating our previous assumptions. Does that mean that on average Mars sees a quake of this size about every two years or we're just kind of lucky?

Benjamin Fernando: Yeah, it's pretty difficult to tell with any great degree of certainty because what we call these scaling laws that tell us how many we should expect, the further out you get from anything you've seen before, the less reliable they become. So we were thinking, yeah, every few years, something like that. With this new data, you'd probably still say it probably happens every few years that would be detectable by InSight, so it gets further away, quakes get less detectable. I think that's probably the best we could say at the minute, but obviously it opens a really obvious opportunity and avenue of extended research for going back to Mars to see if that kind of baseline holds true. Because it looks like the seismicity on Mars varies over the course of a Martian year as well, which it really doesn't do on Earth.

Sarah Al-Ahmed: I mean that makes a lot of sense if a lot of it is caused by that temperature variation, seasonal variations could cause earthquake seasons even, I mean depending on the hemisphere, but that in and of itself is a very alien thing.

Benjamin Fernando: Yeah, I mean it turns out that Mars actually is closer to the asteroid belt at the same time in each of its years. So we look to see if that might explain it. So were some of these unexpected events, impact events? I don't think we have any good evidence for that at the moment. In fact, some of the evidence points in exactly the opposite direction. But it just shows how much we've got to learn about Mars quake season, which is really not a term you would use on Earth, even in the equivalent.

Sarah Al-Ahmed: It sounds like we need an InSight too or just a million million more missions to do this. I mean, I'm sure as a lot of our listeners know, there are a lot of things that InSight wished it could accomplish but couldn't just because of issues with certain instruments, particularly the mole instrument that was trying to dig under the surface. There's so much left that we still wanted to learn from that mission. So maybe round two is worth it once we maybe try digging down on the moon a little bit first and then try it again on Mars. But pinpointing the cause of a Mars quake like this is a huge undertaking. Even if you have an assumption, I think it's an impact crater. How do you even validate that? And in order to figure it out, you had to mobilize space agencies around the world. In fact, this was the first collaboration between all Mars orbiting missions going on at the same time on a single project. How many space agencies were involved in this?

Benjamin Fernando: So we had this huge turnout. So we had NASA, we had the European Space Agency, we had the UAE Space Agency, the Chinese Space Agency, the Indian Space Agency. So those are the five big players in Mars research at the moment. And between them they have sort of eight missions with cameras that are able to image the surface in some form or another. And they were all able to contribute to this exercise, which was really cool. I don't think anything like that has ever been done before on Mars.

Sarah Al-Ahmed: As the lead on this paper, I'm sure you had to coordinate a lot of this. So what were the challenges of trying to communicate across that many space agencies?

Benjamin Fernando: In the first instance, even finding the right people to talk to is a challenge. These are not natural avenues for collaboration. They're not necessarily nations or partners that you tend to see working together in space research particularly frequently. So coming up with that program, coming up with a set of requirements of what we needed from them and then being able to have them go through the data, have us go through the data and really exclude there being a new impact event, a new impact crater on Mars was a huge undertaking because all of these instruments function slightly differently. We had images taken at slightly different times and it just required a lot of, shall we say, logistical effort to put it together even before we start thinking about the science that it enabled.

Sarah Al-Ahmed: Just personally, I've had that same issue trying to context certain space agencies. I'm just saying Isro, if you ever want to talk.

Benjamin Fernando: I think that's about where we are as well. It can be quite challenging if you don't have an established relationship to get in touch.

Sarah Al-Ahmed: But that's part of what's so fun about space exploration. Now we have a reason to make those connections and we can in fact help each other out in a real meaningful scientific way. Other than looking at the ground from space to try to find impact craters, what other kinds of data were we using to learn more about this?

Benjamin Fernando: So the seismic recordings themselves were actually a really rich data set. For a lot of the Mars quakes there's information there, but it really looks very different to what you might call a terrestrial seismogram and earth seismogram. With this event, we had a beautiful clear set of seismic phases, so a P wave and an S wave, which you might've heard about on earth before. And the P wave, primary wave, pressure wave that travels fast through the earth and then the S wave, the secondary or shear wave, which is a transverse wave which travels a little slower and then after those come the surface waves and those are the big high amplitude destructive phases that propagate only in the crust. And we had the full spectrum of those for this event, which meant that we could really use some techniques that we thought about for a long time, but never had the data quality to work with on Mars. So the seismic data was really enlightening for this event.

Sarah Al-Ahmed: If we make the assumption that this was actually an impact event, was this enough information to guess how big the impact crater was or even where it was located on the planet?

Benjamin Fernando: We had a reasonable guess at where it was located somewhere relatively close to the equator and to the east of InSight, but that box is still huge. It was on the order of 10 degrees by 10 degrees, which translates into 10s, 100s of 1000s of square kilometers. Not an easy area to search in any meaningful way without a concerted effort. The other thing is of course, how big the crater would've been. And the challenge there is it's like the Mars quakes. We've only got what we call the scaling relationships that predict crater size from Mars quake size or the other way around Mars quake size from crater size for much, much smaller craters on Mars things that are in the 10-meter range to the 100, 150 meter range. And even then we weren't expecting to see anything in 150 meter range, but we did. For an event of this size, we were sort of taking the relationship and extrapolating it far, far out beyond any point where we had data, but it looked like we'd probably be looking at a crater that would be somewhere in the 300 to 500 meter diameter range, and that would probably have produced a blast zone that would've been about 100 kilometers, so 60 miles across.

Sarah Al-Ahmed: That would be kind of hard to miss, I feel.

Benjamin Fernando: Yeah, so we thought it would pretty hard to miss. The two exceptions to that would've been if the meteorite had exploded in the air, it wouldn't have left any crater. So you may have read about the Chelyabinsk Air Burst, which was the biggest event that's happened on the earth in modern times or in the last 100 years, shall we say. And that was a complete atmospheric disintegration. There was no crater at the end, but it produced a huge seismic signal. The other thing that we weren't sure about is this event, we've located it quite close to some very steep topography. And it's possible that if a crater happens on the edge of a hill or the edge of a mountain somewhere where there's what we call a big topographic gradient, meaning the ground is very rough, it can mean that you either don't see the crater or that it's buried in shadow or if it forms in the side of the hill, it's possible the crater collapses as soon as it forms. So whilst the fact that there wasn't a huge new crater that was obvious was an indicator that maybe this wasn't an impact event, it wasn't enough to completely exclude it as a possibility until we'd done a more thorough search to make sure that we weren't looking at an air burst. So where the whole thing disintegrates in the atmosphere or there's only very small bits left, or conversely something that impacted into a very steep hill.

Sarah Al-Ahmed: I hadn't even really considered that it could have been an air burst. And those events on earth have been absolutely destructive both in Chelyabinsk and as we think with the Tunguska blast, which flattened an entire forest. In this situation, could you get an earthquake of that magnitude from an air burst that seems like it would be huge?

Benjamin Fernando: Yeah, I mean a lot of the seismic signals that we've seen from impact events are actually identified through the sound propagating in the Martian atmosphere, which then shakes the seismometer and we record that. In some ways air bursts can actually be more efficient ways of generating seismic signatures on Mars we think. Potentially because when you hit the ground, a lot of that energy goes into deforming the ground, it goes into making the crater, it goes into heating the crater. Only a very small fraction of it, probably a lot less than 1% goes into seismic energy from an air burst. We expect that to be much higher. So yes, you heat the air and you do deform, but a larger fraction of the energy we think goes into what we call acoustic energy, so sound waves in the atmosphere. So we don't necessarily have an exact constraint on it, but it does look to us like air bursts are a very efficient way of generating seismic signals on Mars. As they are on earth, there's only one impact created that's ever been recorded seismically on earth. The rest of the signals we have are all at us.

Sarah Al-Ahmed: And that works even with the thin nature of that Martian atmosphere.

Benjamin Fernando: In some ways that's better. So the atmosphere's thinner, which means that things tend to get a little bit lower before they explode in the atmosphere than they would on earth just because on earth they're heated higher up because the atmosphere's thicker. The other advantage is the Martian atmosphere. It's thin, so it heats up and cools down a lot and that sounds like it would be a bad thing, but it means that at certain times of day you get what we call wave guides and they're basically passages in the atmosphere where the sound is refracted and sort of bent in such a way that it propagates a long, long distance because reflecting off different parts of the atmosphere. That does happen on earth, but we think it's perhaps more reliable and easier to predict on Mars and that can allow you to hear air bursts or bowl-like events we call a media coming in and exploding in the atmosphere from a much bigger distance than you otherwise would be able to.

Sarah Al-Ahmed: So given it might be a little difficult to figure out whether or not this was actually an impact based on the data or even finding the crater, in the terrain that you were looking at, was there anything that could indicate that this was an area that was higher in seismic activity? Because on earth you can kind of make guesses based on the terrain.

Benjamin Fernando: Yeah, absolutely. On earth you look for active faults, you look for volcanoes. All of those are things that indicate that you're probably in a seismically active area. There are areas on Mars which before InSight landed had been identified as potentially more seismically active from orbit, though what looked like might be active rift systems, areas where the surface was undergoing lots of deformation or equivalent areas actually where we see a lot more avalanches and rockfalls, which might indicate the ground is shaking more often there. The area that this event occurred in for most of it actually it looks pretty, I don't want to say uninteresting because it's not interesting. It looks pretty unspectacular. There's nothing there that's kind of a smoking gun for the event that we saw. There's nothing that makes us think, wow, this is a highly likely progenitor Fault. Of course, what we can't see is below the surface on Mars, we don't have any sense of what the tectonics look like deep within. So it's potential that there's something buried there which is given rise to it, say a Fault system, which is just really not exposed on the surface at all. And therefore we have no indication that it might even exist.

Sarah Al-Ahmed: But in the end, the easier assumption is probably to think that this might be an impact. And in fact, there were previous seismic events detected by InSight that were similar-ish that were caused by these impacts. At least in your paper you suggest that there are two that you were mostly comparing them to. What were the similarities and differences between these different seismic events?

Benjamin Fernando: So the two big seismic events, we'll call them ten-ninety-four and a thousand. That's just the identifiers that we give them. And then the new one was twelve-twenty-two, we were not very good at coming up with names. The project I have is to try and name these, but it's not taken off unfortunately, maybe you can help with that.

Sarah Al-Ahmed: Are they doing them an order of their detection?

Benjamin Fernando: No. The number refers to what Martian Day, which SOL they were detected on, so 1000 happened on SOL 1000. The first two 1000 and 1094, they both had these big craters, they were very obvious once we knew to look for them. But the seismic events were also quite similar in that they had these strong surface waves as are the waves propagating in the crust. They had very clear and distinct phases. 1222 had strong surface waves as well, but there were also perhaps some slight indications of a difference in that there was more shear wave energy. So that is not what we'd expect from an explosion and impact on the surface we'd expect more pressure energy. So that was maybe one difference between the two. But also just in terms of the scale, we knew it was possible to get big Mars quakes, but the two next largest events that we'd seen or the two of the three next largest events we'd seen had been impact events. So it meant that it was quite a reasonable hypothesis based on the kind of likelihood of the rate and then also of the waveforms themselves that this might've been an impact event. And that's what made us go look for it.

Sarah Al-Ahmed: We'll be right back with the rest of my interview with Ben Fernando after the short break.

Bill Nye: Greetings Bill Nye here, what a year it's been at The Planetary Society from advocating for missions that matter, to funding new technology, to grants for planetary research and biology and space, to sharing the wonder of space exploration with the world. You made it all happen. Many of the great scientific missions we've dreamed of are happening and it's thanks to your support, but we can't stop now because tomorrow's discoveries must begin today. With your help, we can keep our work going strong into 2024 and continue to explore worlds, find life and defend Earth. Plus, when you make a gift today, your gift will be matched up to $100,000 thanks to a special matching gift challenge from a generous Planetary Society member. Your year-end contribution, especially when doubled will go far to help launch us into all the exciting opportunities tomorrow will bring. Visit planetary.org/planetary fund. Thank you.

Sarah Al-Ahmed: How do you actually look for something like this? Because you've mobilized all these different space agencies, you have a bunch of different instruments all over Mars. How do you even go about that?

Benjamin Fernando: Yeah, I want to say it's more sophisticated than just looking for it, but honestly for the most part it isn't. So we look for it in a bunch of different scales, and what I mean by that is, we have some cameras which take really high resolution images of the surface, like 30 centimeters a foot per pixel, something like that. And then we have other cameras which take really low resolution images, say 20 miles per pixel, but the ones at a lower resolution obviously capture more of the surface and they tend to take images much more often. So you might get an image of the whole disc of Mars at a resolution of a few miles per pixel every day, but you would only get an image down to sort of meter scale resolution on a very, very irregular basis. Most of the surface of Mars hasn't been mapped at that resolution yet. So what we did is we started by looking at images that are taken at very low resolution immediately after the event. So we are not going to see small things that are on the scale of meters or even 100s of meters, but we'd see big things. So we're looking for a fresh blast zone, we're looking for a potential dust cloud. We don't see either of those. And then we look on a slightly smaller scale, sort of a medium scale. This is sensitive to 10s, 100s of meters, and what we are looking for there is either a blast zone or a fresh crater. Is it possible that there's stuff there that we haven't seen or that the crater's smaller than we would expect? We don't see anything there either. The final thing that we do is look in the really high resolution, so down to sort of sub meter per pixel. It's incredible for Mars to see stuff at the scale, but we can't map the whole area. So what we do is we look in almost a few random areas just to look for any signs of one of those air bursts. So that might be very small fragmented craters that have formed and they're clearly new. That might be a sign that something big has exploded in the atmosphere and all that's left is this very small holes at the ground. We don't see anything like that either. So then having gone through this data at a bunch of different scales, and obviously the high resolution images take longer to get just because there are more of them and you can't do the whole planet in one go. So once we've looked through it on a bunch of different space scales and a bunch of different time scales, we were able to conclude we think it's probably not an impact crater.

Sarah Al-Ahmed: How do you actually compare all of these images? Because there's so much of them, and I know in the past, even with a single spacecraft's data, just the different lighting and the angles as you go around the planet can make it really difficult to detect changes.

Benjamin Fernando: Especially when they're images from different instruments that have different resolutions and different parameters, even different colors. Some of them are black and white, some of them aren't. One of them even actually images in the ultraviolet, which is great, but not going to see the crater itself. There's two ways you can do it in general with impact craters. One is you just look for new craters and Mars is covered in craters, but there is some giveaways that a crater is fresh, so you would expect a blast zone around it. You wouldn't expect sand to be forming ripples at the bottom of the crater. That's quite difficult because it might tell you that a crater's formed within the last 10 years, but it wouldn't tell you that a crater's formed in the last 10 days. The other smoking gun is what we call differences, so you see it in one image but not in an image that was taken sometime before. But of course that's much harder because then you require two images of the same portion of the surface with constraining dates. So if you saw a crater in an image from May 2023, and it wasn't in an image from May 2014, it might be the crater we're looking for, but it could also be something completely unrelated that formed in the intervening decade.

Sarah Al-Ahmed: But in the end, you didn't end up finding that crater. No,

Benjamin Fernando: We didn't find anything of the right sort of size and scale that we were looking for. So that's a strong indicator that it wasn't an impact event.

Sarah Al-Ahmed: But as you said earlier, that's a good indicator, but not all the evidence that you need to really say for sure. So what is the missing puzzle piece here?

Benjamin Fernando: I think the honest truth is we can't say with 100% absolute guaranteed certainty that it wasn't an impact event, but both the seismic data and the orbital imagery data suggest it was more likely to have been a tectonic event. There's only so much we can do to piece it back together after the fact. I think that if you really wanted to answer this question, the next thing to do would be to go back to that location on Mars with more seismometers and see if you see similar events coming from the same location. Because you wouldn't expect one-off events of this size with no other seismic events around them from the same area. But obviously that's a huge undertaking and not really something that was in the feasibility of this study to complete.

Sarah Al-Ahmed: How unfortunate is that? I know we have limited funding and time, but-

Benjamin Fernando: Absolutely.

Sarah Al-Ahmed: Honestly, can you imagine the things that we would discover about these worlds if we had a seismograph on all the rocky worlds or even out to some of the moons? That is not this day, we'll just have to hope and build for that future. What was kind of the turning point for your team when you guys realized that it probably wasn't an impact? And what did you take the steps in the aftermath to really solidify that?

Benjamin Fernando: I think we would be surprised after we got the low resolution images of the surface if it wasn't impact just because there was no big blast, but it wasn't a sort of guaranteed certainty at this point. I think the turning point is once we finally got the data from all our partner agencies and overlaid it, and there's really nothing that looked particularly obvious there. So it was a very gradual process rather than a sort of Eureka, oh, there's no crater there because of course we're looking to exclude something as a possibility rather than confirm it. And that's naturally more difficult than being able to point to a smoking gun.

Sarah Al-Ahmed: And that's less exciting than, "Oh my gosh, look at this Mars quake we detected." I mean, what was the team thinking when they first looked at the data and found this 4.7 magnitude Mars quake?

Benjamin Fernando: Everyone was shocked. This was in the last few months of InSight. We weren't expecting the mission to still be operating at this point. But it was, the seismometer was only on for a small fraction of the day, so we were actually very lucky that we caught it in the first place. I think that, yeah, we were all just pretty shocked, because it was in a very noisy part of the year as well. We weren't expecting strong events, and we did see some afterwards, but this was really the mission's last hurrah. We were very lucky to capture something of this magnitude.

Sarah Al-Ahmed: That's what was so funny about it. I remember having that same reaction being like, wow, that's really surprising that they detected this so close to the mission's end and then to learn that it wasn't an impact, that's very strange. But now that we have all of this information, what can it tell us about Mars and its seismic activity broadly?

Benjamin Fernando: Again, we're trying to draw conclusions from a single event, but it tells you that Mars place hosts a pre hefty mass quakes. If you wanted to go live there one day, sure, this thing isn't going to bring your house down, but you wouldn't want to park your spacecraft on top of a Fault system, which routinely gives off magnitude five Mars quakes if you don't have to. So that's another kind of open question is it's highly unlikely that this is the largest event that ever occurs on Mars. If you see a magnitude fire quake and you've only been there for a few years and you are kind of only listening part of the time at this point. It's likely that there's much larger stuff that happens that we haven't seen yet. So if you wanted to go live on Mars, I think this has helped us to quantify the seismic hazard, the seismic risk. It's not like moving to LA, you don't need to worry about that, but it's more active than moving to the East Coast.

Sarah Al-Ahmed: And imagine you're trying to build a habitat on Mars. The things that we worry about are making sure that you're airtight and you're not getting perchlorates from the soil into your air or any of those things. But the last thing on my list would've been make sure that your building is okay for earthquakes.

Benjamin Fernando: Yeah, absolutely. And if your people have often talked about living in cave systems or lava tubes on Mars because they provide a natural shielding, great. But if you're doing that in a seismically active area, that sounds like a recipe for disaster all of a sudden. So I don't think we're at the stage of producing a map of Mars that says, don't live here if you want to live in a cave. But it's something that's worth thinking about because the secondary effects of Mars quakes, rocks, falls, avalanches, cave collapse. I couldn't tell you how often they happen, but of an event of this size happening on earth, it wouldn't be unusual for something like that to occur.

Sarah Al-Ahmed: Especially with habitats, I know some people have been considering using Martian regolith in combination with other things to cement it together, but we're going to need to make sure that those materials have enough spring and sway to them that they don't snap or accidentally leak our air out into space as we're trying to live on Mars.

Benjamin Fernando: Absolutely. If you live in California and your house shakes a little bit and one of the walls caves in, I mean it's obviously not great. It can be dangerous, but it's fine. The air outside is not toxic. On Mars your tolerance for that sort of thing. It's got to be much, much more tolerant to it because if the wall does collapse, that would be bad, very bad.

Sarah Al-Ahmed: And we've seen unfortunately, the difference in how different nations have to deal with earthquakes because here in California we have them so often we've retrofit all the buildings for it, but in nations where they happen less often or they have less preparation for it can be absolutely catastrophic when these earthquakes come through. And thankfully, we are a long time out from building entire civilizations on Mars, so we have time to think about it, but this is just Mars. I understand that you've worked on lunar missions in the past and you're currently working on the Dragonfly mission to go to Saturn's Moon Titan. Is there anything that this mission and this process has taught you that we can take forward to these other missions going forward on the moon and Titan?

Benjamin Fernando: Yeah, I think just that science is hard and it's expensive and it takes a long time. InSight people conceived of seismology on Mars in the 1960s and 70s. It took until 2018 until we had the first real useful data back. Dragonfly is going to be amazing, I'm looking forward to it, but it's going to be 2030s, late 2030s by the time we start to get data back. So it's a long-term process. In terms of what we've learned, obviously we've developed a whole host of new modeling techniques. I do a lot of computer-based modeling of impact events, which we'll take forward and apply. The other thing that we're thinking about is how similar might Mars quake signals be to moon quake or Titan quake signals? In some ways, they're probably quite similar in other ways, they're probably entirely different. Along that vein of thinking, is it likely that impact signatures on Mars and we have impact signatures from the moon back from the Apollo days, do they look similar to impact events on Titan? How many impact events do we expect on Titan? The answer is probably not very many, but there are all these open questions in seismology that we probably should have a good grasp of before the next generation of missions go to the moon this decade and then Titan in the next.

Sarah Al-Ahmed: It's funny because on Mars, I would assume that most of the things were caused by impacts, but on Titan, I would guess it would just be seismic activity. That atmosphere is so thick trying to get through that it would be a very big event.

Benjamin Fernando: Yes, I think there clearly are impact events that happen on Titan. Dragonfly is going to visit a crater. But the surface is far less cratered than even the Earth's in terms of number of identified craters. And obviously we haven't searched this thoroughly as we've searched the earth, but it looks like there's a much stronger filter of impact events provided by both the thickness of the atmosphere. And it is not just, it's very dense, but it actually extends over a much bigger range. So what we call the scale height, the atmosphere is much thicker for a much longer duration on Titan than it is on Earth, and that means that stuff just burns up higher in the atmosphere is much less likely to reach the surface.

Sarah Al-Ahmed: I bet there are some spectacular meteor showers around there.

Benjamin Fernando: Yeah, I think unfortunately the atmosphere is not very transparent, so I'm not sure how many of them you'd see, but they're probably are spectacular.

Sarah Al-Ahmed: How did you get involved in studying Mars quakes? I mean, it's not like we've had a seismograph on Mars for very long. How did you branch into that?

Benjamin Fernando: It was during grad school actually. I was working on developing computational methods for simulating acoustic waves, sound waves, coupling to seismic waves on the earth. So thinking about atmosphere to ground and ocean to ground, and it seemed like a very natural and obvious extension to start thinking about applying that to Mars. And I guess that sparked off this whole sleigh ride of looking for impact radars on the surface of Mars.

Sarah Al-Ahmed: I feel like it's so funny because when you've talked to people who are deeply embedded in these new realms of science out on other planets, it's always something like, "I fell in love with collecting rocks as a kid, and then I just started studying meteorites and now I'm the head of Mars sample return." It's such an interesting life journey, so I always love hearing about that.

Benjamin Fernando: Yeah, I'd love to say I had it all planned, but no.

Sarah Al-Ahmed: But in the end, despite not planning it out, you get to be the head of paper that discovered the largest Mars quake ever. Did you celebrate afterwards?

Benjamin Fernando: I think we did. There was a lot of media interest. It was actually a very busy few days when that paper came out, but we just celebrated the fifth anniversary of InSight's Landing on Mars, so I think we'll go out for a drink this week to mark that.

Sarah Al-Ahmed: Well let everybody on the team know that they did a great job. And I'm so excited by this. I cannot wait for a future where we have even more data on this. There are so many mysteries hiding inside Mars.

Benjamin Fernando: More questions to be answered than I think we started with.

Sarah Al-Ahmed: It's always that way, isn't it?

Benjamin Fernando: Yeah.

Sarah Al-Ahmed: Well, thanks for coming on the show, Ben, and telling us all about this.

Benjamin Fernando: Thanks so much.

Sarah Al-Ahmed: Let's be real for a second. How cool is it that we live in an age where we can actually solve mysteries like this? Think about how many scientists and engineers it took to make all of those spacecrafts, and then the level of coordination it took to actually piece together this puzzle. It's a beautiful moment of learning and collaboration that makes me feel genuinely hopeful for the future. I feel like we should send a little love to NASA's InSight mission team. If you go to their website, you can actually make a digital postcard that you can send to InSight, which will be read by the mission team. You can pick your favorite image from the mission and then write a little note to customize your postcard to the team. You can make your postcard to the team at mars.nasa.gov/insight. I'll put a link to that on the show page for this episode of Planetary Radio, along with other resources so you can learn more. Now, let's check in with Bruce Betts, the chief scientist of The Planetary Society for what's up. What's up, Bruce?

Bruce Betts: What's up?

Sarah Al-Ahmed: What's up?

Bruce Betts: A lot of things Sarah, do you really want to know what's up? I'll tell you, but never mind. What do you got?

Sarah Al-Ahmed: Unless it's Mars. But I don't know. I think it's so cool that they finally figured out what caused that Mars quake and the fact that it wasn't an impact was not what I was guessing. What is the biggest earthquake you've ever been in? I mean, we live in California, so we've been through quite a bit of them, but.

Bruce Betts: Well, in terms of the intent of the question, roughly 5.8 or so, 5.9, but it was just not very far away shortly after I came down to Caltech, so that made it more fun. And then a nice aftershock during a planetary science seminar. So it's always good. And I actually thought, I'll take a little side note, she can cut out if you like, but I was in the building with the seismometers that they always show for any Southern California earthquake. They were on the upper floors, we were on the bottom floor, and it was supposed to be this designed to survive an earthquake more. So it rolls, theoretically, it's rocking. And then there was the earthquake, and I went to work thinking, oh, this'll be good. I'll be good. And then people come by. Oh, there's a fluorine gas leak possibly on the top was like, fluorine gas. Holy crap.

Sarah Al-Ahmed: Let's get out of here.

Bruce Betts: So I became friends with the middle of the Caltech athletic field, so I figured I'm good, I'm good now. That's my earthquake story. But I felt the seven ish earthquake, that was a Northridge, but I was down in South Orange County at the time, so it was just a rolling motion down there, so it didn't have the impact. Scary as hell, but not that dangerous was the level I got to.

Sarah Al-Ahmed: That's lucky. Yeah, I think it was 2019. There was an earthquake over in Ridgecrest that was like a seven-pointer, and we were actually in the middle of a show at Griffith Observatory. We were in the theater underground when it happened. And we're trying to pretend like everything's cool, and it is, it's one of the safest places to be in LA in an earthquake because they built it for that. But everyone started freaking out, and we're like, "Just be calm. We're totally safe." And then immediately afterwards we ran down to the seismograph and brought up the tracing, and it was really cool to see it in real time, but we were very lucky to be far enough away that it wasn't completely devastating. And it makes me glad that if we're going to be having Marsquakes of that scale, that at least for now, no one's living there to have to go through that. Maybe we'll have to retrofit our buildings someday on Mars.

Bruce Betts: We could just fit them.

Sarah Al-Ahmed: Just fit them. That brings up something interesting to me, which is one of these days we're going to have to have building permits and laws around how to build buildings on Mars. I mean, assuming we get there someday and however long that takes, but imagine being a building inspector on Mars.

Bruce Betts: Once again have been able to think farther in the future than my little brain can go. I never got to building inspectors. That's-

Sarah Al-Ahmed: But speaking of building a sea on Mars, it's actually quite topical. A few weeks ago, we did a show on November 18th with the Wienersmiths, the author of the new book City on Mars, and one of our listeners, Gene Lewin, wrote in a poem about that book that I wanted to share.

Bruce Betts: Oh, please do.

Sarah Al-Ahmed: All right. Says, "If you're looking to relocate to the moon or planet Mars, an orbiting built space station or something that's on par, you may find a few impediments, many fine points to discern. What do you do with all that poop? There's a lot to learn. We've tried it here on earth you see, biosphere did not work out. A distant human terrarium, success would be a doubt. Supplies would be a sticking point. Radiation comes into play. Low gravity that snaps your bones, the paths not come what may. And then there's all the legal stuff, space treaties have to be signed. You can't just claim a plot of land and take the resources you find. So city life away from earth on Mars or somewhere moonish, you may not want to hold your breath. It's not likely to happen soon-ish, though I have faith in humankind, our curiosity is a plus because when we're un-incentivized space brings out the best in us.

Bruce Betts: Really cool. I have to go back a moment. Did you just laugh hysterically at a poop joke?

Sarah Al-Ahmed: I did.

Bruce Betts: Okay.

Sarah Al-Ahmed: But come on, man. Space poop is funny. No matter who you talk to about space poop, it's always one of the most fascinating and embarrassing topics. The Apollo poop stories are some of my favorite

Bruce Betts: For once you've rendered me speechless. We covered some Apollo, but not poop. I think I've discussed it. Doesn't matter. Let's do this one. I'll ask it as a question first for your amusement. Name the only astronauts who flew an Apollo mission and a space shuttle mission. There were two John Young, who's the only person to have flown Mercury, Gemini, Apollo and Shuttle, and Ken Mattingly who flew Apollo was the command module pilot for one of the moon flights and also flew some of the shuttle flights. A little side note, asterisk for Fred Hayes who flew on Apollo-thirteen, but he actually did atmospheric flight tests of the shuttle where they did drops from an airplane. And so he did gliding tests but did not fly space and Ken Mattingly passed away not long ago. Apollo astronauts. So we remember him.

Sarah Al-Ahmed: Could you imagine being one of those people that has such a history with all of those space missions? Even just seeing a shuttle in a museum somewhere is a big thing for me. But being a part of the Apollo program and being on a space shuttle, what is their life? What was their life? I know I've asked you whether or not you would go to Mars, but clearly not. But if you could just go to space on one of those quick trips, would you do it?

Bruce Betts: I don't know. I certainly would've in the past, because I tried to be an astronaut and cooperative here. But I know, I don't think I would. It depends on the situation, but it depends. Once you have really established avenues, then I would be more inclined than the early phases. But I'm really glad there are people doing it. That's awesome. And I can't believe I said that, because I was very devoted to trying to fly to space in the past, but somewhere around having kids becoming an old fuddy-duddy, curmudgeon. But I would happily send you to Mars, Sarah, and I'm guessing you'd go.

Sarah Al-Ahmed: I would totally go. But in the meantime it's okay. Well, we'll create some kind of one-off space D&D campaign. I'll play in the office and just imagine.

Bruce Betts: Cool, I'm in.

Sarah Al-Ahmed: Let's do it.

Bruce Betts: That I would be willing to do, although it is risky. All right, everybody go out there, look up the night sky and think about poop jokes. Thank you. Good night.

Sarah Al-Ahmed: We've reached the end of this week's episode of Planetary Radio, but we'll be back next week with Mat Kaplan's excellent adventure with planetary geologist Kirby Runyon. You can help others discover the passion, beauty, and joy of space, science and exploration by leaving a review or a rating on platforms like Apple Podcasts or Spotify. Your feedback not only brightens our day, but helps other curious minds find their place in space through Planetary Radio. You can also send us your space, thoughts, questions, and poetry at our email, at Planetary radio at planetary.org. Or if you're a Planetary Society member, leave a comment in the Planetary Radio space in our member community app. Planetary Radio is produced by The Planetary Society in Pasadena, California and is made possible by our members around the world. You can join us as we puzzle over the Red planet at planetary.org/join. Mark Hilverda and Rae Paoletta are our associate producers. Andrew Lucas is our audio editor. Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. And until next week, ad astra.