This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. The most volcanically active body in the solar system is Io, the innermost of Jupiter's Galilean moons. Since the 1970s, Jupiter and its moons have been visited by a number of space probes, including Pioneer, Voyager, Galileo and Juno. Each time one of these probes reached Io, and even between successive orbits of a particular probe, Io's surface had changed. In December 2023 and February 2024, Juno flew by Io at a distance of 1,500 kilometers, seeing, among other things, twin erupting plumes and lava lakes. What drives this intense volcanism, and what can Juno and its predecessors tell us about the structure of Io and how it interacts with Jupiter?

Scott Bolton has a prolific research career that focuses on Jupiter and Saturn and the formation and evolution of the solar system. He has led a number of science investigations on the Cassini, Galileo, Voyager and Magellan missions and is the principal investigator of the Juno mission. He's director of the space sciences department at Southwest Research Institute in San Antonio, Texas. Scott Bolton, welcome to Geology Bites.

Scott Bolton: Thanks for having me. Happy to be here.

Oliver Strimpel: Let's start with an overview of the Jovian system.

Scott Bolton: Jupiter is the largest of all the planets in our solar system, and therefore a lot of scientists believe it must have formed first. Not only is it the largest planet, it's spinning around in 10 hours. So you have this planet that's 10 times the size of the Earth (1,000 Earths fit inside of Jupiter), and yet it's spinning faster than the Earth is. So it's moving around very, very quickly. And then it has the four Galilean moons that were discovered by Galileo, the astronomer, back in the Italian Renaissance, when he pointed the first telescope up there. And Io is one of those. And then you have Europa, Ganymede and Callisto, and they're moving around at different rates. Io goes around in a little less than a couple of days. And the magnetosphere of Jupiter, which comes out of the magnetic field of the planet, comes out of the poles. It's spinning very, very fast. It has a collection of charged particles, and that is whipping around, banging into these moons, because it's tied to Jupiter's spin rate. So it moves around in 10 hours, all of these charged particles. So they bang in and sputter these moons and knock off particles.

Oliver Strimpel: Juno is the latest space probe to have approached Jupiter. Could you give us a summary of the Juno mission and how it compares with the prior missions that have reached Jupiter?

Scott Bolton: Juno is somewhat unique for two reasons. One is it goes over the poles of Jupiter. Previously, the spacecraft have visited around the equator. And the other thing that's new about Juno is we go very, very close to Jupiter. We're basically scraping just over the atmosphere as we fly over. And so we're getting a really close view of Jupiter, and we're getting this global view. We basically can map the planet. Each orbit comes down over a different longitude. Voyager and Pioneer flew by, and Cassini even flew by on its way out to Saturn, and we got a view from the equator, not that close to Jupiter. And Galileo orbited around Jupiter, but not as close as Juno, and also confined to the equatorial regions.

Oliver Strimpel: When Juno first arrived at the Jovian system, you placed it in a 53-day, highly eccentric orbit, coming as close as, you said, really scraping the top, 5,000 kilometers above the top of Jupiter's atmosphere. How did you then change the orbit to get it close to Io?

Scott Bolton: Actually, Jupiter did that work for us. We were in an elliptical orbit. When you get close to Jupiter, that's called perijove, and when you're far away, it's called apojove. And our perijove originally was near the equator of Jupiter, and we were going over the poles. And Jupiter's gravity field is asymmetric, and it kind of twists the orbit of Juno, so that the perijove moves more and more north with each orbit, almost 1 degree per orbit. And as we do that, when we're coming in toward the North Pole, it starts getting closer and closer to the planet. And as that moves closer to the planet, first you'll encounter Ganymede, then you get to Europa, and finally Io. It's basically the orbit, what they call the line of apsides, that's being twisted around. And we actually didn't have enough propellant to do that; we're just mere mortals. But Jupiter, of course, with its giant gravity field, really controls the orbit.

Oliver Strimpel: So as I said, you got as close as 1,500 kilometers from the surface of Io. What instruments did you use to study it?

Scott Bolton: We have a visible camera; we have an infrared camera and spectrometer. We also have a full set of fields and particles instruments that look at the magnetosphere. So we have charged particle instruments, like for plasma, energetic particles, plasma waves and radio emission. We have a microwave radiometer that we basically invented to look into Jupiter's atmosphere and to measure how much water there is and how the dynamics of the atmosphere work below the clouds, and we used that to look into the surface of Io. And we also have magnetometers, and we also have the gravity science, which looks at the interior structure and measures the gravity field. And we have an ultraviolet instrument.

Oliver Strimpel: You have a microwave instrument. Are we looking at black body emissions or absorptions or reflections? How does that work to see the atmosphere and even below the surface?

Scott Bolton: You see all of those that you just mentioned, but you predominantly see black body emissions. So when you look at Jupiter, you have different wavelengths. We have six channels, and the short wavelengths, or high frequencies, look at the top of the atmosphere, and the longer wavelengths can see in, and how far into the atmosphere they see is driven by the opacity of the atmosphere, which is driven by ammonia and water. Now, when we looked at Io with that, we weren't sure what we were going to see, because no instrument like this has ever been used to really look at lava and rocky things like that. We actually think we're seeing into the lava a little bit, because we see at the longest wavelength channels that the temperature is going up. That means that maybe you're seeing in, where it's warmer. But of course, even though there are tons of volcanoes all over, it's not completely covered with volcanoes. It's mixed with rock and sulfur dioxide ices and things like that, so it's a mixture, whatever we see. We're still trying to figure out exactly how to interpret it.

Oliver Strimpel: Just fascinating. To follow up on a point you made, do microwaves actually penetrate the lava? Can you actually see a little bit below the surface?

Scott Bolton: We're not sure. We think so. We think that's what we're seeing, but we have no calibration of this type of measurement, because there's no experience taking an instrument like this and looking at rock and lava very much. I think that we probably are seeing into the lava, but we definitely see the temperature going up, which means that you're probably seeing into the surface, where it's warmer.

Oliver Strimpel: You mentioned that Juno's trajectory took us for the first time over the poles of Jupiter rather than the equator, and I wonder if its trajectory also gave us a view of parts of Io's surface that had not been visible to us before, and if so, whether you were able to map a distribution of the volcanism according to the latitude on Io.

Scott Bolton: Yes, in fact we did. In the orbits approaching the first flyby, which was at the end of December this last year, we got the first real views of Io's North Pole, and so we made a map of the volcanoes around the North Pole. In the next few orbits, we'll get the first views of the South Pole, and so we're making maps of how the volcanoes are distributed at both poles and, of course, around the equator.

Oliver Strimpel: I know it's early days yet as far as receiving all the data and processing it and so on, but so far, what kind of volcanic activity have we seen in the latest Juno imagery of Io?

Scott Bolton: Io must be an amazing place to visit. It's covered with volcanoes that are going off all the time. And if you go to the poles, there are still volcanoes everywhere. So if you were a hiker, and you were lucky enough to land on Io, and you had the equipment or some sort of natural way to survive this (because it must be really smelly there, like Yellowstone), no matter where you were, you would have a view of many volcanoes. And we also saw these incredible mountains, because we got some imagery along the terminator. And you can see these mountains are also all over Io, and they're coupled to the volcanoes, but they're not necessarily volcanic. It has its own capability to make mountains. And some of them are quite sharp; they look like cliffs. It's just an amazing place. Giant lava lakes. Some of these are reflecting with specular reflections, so at least the lava lakes must have a very, very smooth surface. In fact, most of Io must be relatively smooth. We compare it to Europa and Ganymede, which we flew by years earlier, and that surface is much more rough. The ice is rough, and the reflection is diffuse scattering, whereas on Io it's specular, almost like it's glass.

Oliver Strimpel: So the frozen surface of the lava, you're saying, that's shiny? On Earth, of course, it gets very matte when we have cooled lava.

Scott Bolton: It might be smooth, more like obsidian. If you look sometimes near Earth's volcanic regions, you'd find obsidian, which is a very smooth kind of glass. So whatever's forming there might be relatively pure and very smooth. And then a lot of lakes are cooling and get a crust over them, and then where the lava is coming up might make a ring around the lake. And sometimes there are protrusions out of that, which represent natural terrain elevation changes, almost like islands. And then of course you have the volcano itself. Sometimes we see some of that. If you go to Hawaii or Iceland right now, you'll see this bubbling ring of lava, and there's a cooled-off lake, and the material can get quite smooth. I don't think it's liquid, because it's very cold on the surface of Io, so the lava must be cooling very, very quickly.

Oliver Strimpel: So you said it would be smelly on the surface of Io. I presume you're referring to sulfur compounds like hydrogen sulfide or sulfur dioxide. But Io's very colorful surface has been likened to that of a well-stocked pizza. So do we know what generates all those many reds, yellows and the dark greens and grays and blacks?

Scott Bolton: Sulfur compounds are certainly associated with the volcanoes as they erupt, and those are predominantly the thing that's causing the yellows and the reds. Now, when the sulfur gas comes out of a volcano on Io, it instantly freezes, so it turns into snow, and it's probably colored snow, or dirty snow. So you get the yellow, reddish gook that's being laid around. And the molten lava, as soon as it comes out, also starts cooling. So when that cools off, it produces a lot of the grays and black.

Oliver Strimpel: What drives all this intense volcanism on Io and makes it the most volcanically active body in the solar system?

Scott Bolton: Io's volcanic activity is really driven by Jupiter, its tidal forces. So it's the same thing that raises the ocean here on Earth: our Moon pulls and changes our tides. Jupiter is doing that on Io. It's relentlessly squeezing and literally torturing this poor moon as it orbits Jupiter. It's getting squeezed. The gravity field is so strong, it's basically changing its shape, and it feels a stronger gravity on one side versus the other, and that is churning inside and producing all of the volcanic activity.

Oliver Strimpel: Do we know anything about the mechanism by which this heating leads to the many volcanoes that pepper Io's surface?

Scott Bolton: For that, you have to look inside of Io. And the question is whether there's a global magma ocean, or pockets of lava that just produce local volcanoes. In other words, if I go down deep into Io, is there a layer that's all lava? And in fact, we chose to do two flybys in order to get the gravity field very, very precisely, to distinguish a global magma ocean from one that is not global. I think that in the next few months we will probably resolve that question and put very good constraints on how Io works inside and how it produces all of the volcanoes that we see.

Oliver Strimpel: We see volcanism on other bodies apart from the Earth, such as our own Moon. So not all volcanism is caused by tidal heating, then?

Scott Bolton: Not all of it. Tidal forces are much smaller here, because the Moon can only do so much to us. Venus is also volcanic. Mars, you can see all the volcanoes. So tidal forces play a role. Some of them have small moons going around them, or, in the case of Venus, it's closer to the Sun, so it gets some tidal forces. But a lot of it is geothermal as well. There's material deep down in the planet that's keeping it warm, radioactive material, radiogenic heating. People see it on Enceladus. We see cryo-vents, where water is spewing out of the ice, so something cracks the ice. People think that maybe Europa might have plumes of water coming out of it; the ice might be cracking there as well. It must be feeling some tidal forces from Jupiter too, even though it's further away than Io. Io is the one that's really getting tortured, because it's really close.

Oliver Strimpel: Juno has a magnetometer on board, you said. How does Io affect Jupiter's magnetic field, and what can we learn from that interaction?

Scott Bolton: Juno has a very, very sensitive magnetometer. It's a very powerful tool to investigate the interior of Jupiter and actually see how the magnetic fields are produced, and to map out the magnetic field of Jupiter. Now, Jupiter's magnetic field is enormous. In fact, it's the largest structure in the solar system. It makes what's called a magnetosphere, which is filled with charged particles. So the magnetometers on Juno were designed to measure Jupiter's magnetic field and map it. Io's impact on Jupiter's magnetic field and magnetosphere is the fact that it's filling the space with charged particles, material that comes out of the volcanoes. It gets ionized through sunlight or collisions and then starts whipping around and becomes part of Jupiter's magnetosphere. That also feeds the aurora. When they get caught up on these magnetic field lines, it's almost like an umbilical cord between Io and Jupiter. There's a spot in Jupiter's aurora that actually tells you where Io is, connecting Jupiter with Io through its magnetic field.

Io is so powerful, in the fact that it's distributing all of these charged particles from the volcanoes, that there's a ring around Jupiter that's called the Io torus, and it's filled with charged particles. And we've known about this for a long time. Previous missions have studied this. You can study this with some telescopes, even from the Earth. And this torus of charged particles around Io is very, very high in radiation. It's a very hazardous place to go for spacecraft. And that torus also makes a little bit of aurora. So when you look at the aurora of Jupiter, you can see sort of a tail coming out of Io that's coming from this torus ringing around. And you can also see this spot.

Oliver Strimpel: You mentioned that you saw some topography on the surface of Io that was not volcanic in nature, cliffs and so on. Are we talking about tectonics on Io, or what do you think might cause non-volcanic topography there?

Scott Bolton: Yeah, you need some kind of mountain-building capability, not necessarily plate tectonics. So far we haven't really uncovered plate tectonics on another body other than Earth. But something is moving around and you're forming mountains, and we see mountain ranges formed in other places besides the Earth.

Oliver Strimpel: What's next in the flight plan or science plan for the Juno mission?

Scott Bolton: We will continue to go under Io for the next few orbits, relatively close, tens of thousands of kilometers away. So we're hoping to continue to monitor now the South Pole and the volcanoes, and we're trying to tie the volcanic activity to activity that we see in the aurora and the magnetosphere, to really understand how Io is driving this whole system. But also we're doing occultations. We actually go behind Jupiter. We're solar powered, so it's a little bit nail-biting, because when you go in behind Jupiter, you don't have power going onto your solar arrays for a little while. But we're getting incredible measurements of the atmosphere by doing occultations. We pass the radio beam through the atmosphere, and we watch it blink on and off as we go into an occultation, as we're going behind Jupiter, and you can learn a lot about the density and the properties of the upper atmosphere.

With that, we're also getting closer and closer to Jupiter's polar region. As the perijove of Juno moves further and further north, the altitude that we pass over the poles is dropping. And so we're studying these polar cyclones very, very closely. And we're able to see now, with the microwave radiometer, that the beam is getting to the point where we can resolve these polar cyclones. And so we're studying those with multiple wavelengths and looking at how they look to microwave eyes, at the same time as we're seeing them in infrared and visible light, and really for the first time looking underneath those polar cyclones, to see how they compare to each other, how deep the roots are, and to compare those to other kinds of vortices that we see all over Jupiter. And of course, we're also continuing to map out the magnetosphere and the aurora. We're getting closer and closer to the North Pole, so we're really getting to the point where the auroral accelerations are happening, and so we're going to learn a lot about how the aurora on Jupiter gets created. And we're finally going to get to the point where we're crossing into the rings of Jupiter, which are very, very close to Jupiter. Very hazardous; these are very dangerous places to go. We're kind of an armored tank, and our shields are still holding, so we're getting closer and closer, and we're going to study the rings of Jupiter as well.

Oliver Strimpel: You're saying you're an armored tank, but if you get hit by a particle in the ring, can you survive that?

Scott Bolton: We're hoping that we won't get hit. But it's not just the particles hitting you; it's also the radiation. We're getting more and more radiation with every orbit.

Oliver Strimpel: What is the ultimate fate of Juno going to be?

Scott Bolton: Either it will run out of propellant, which will mean that we can't point the antenna down to the Earth to downlink our data (we use the propellant to steer the spacecraft a little bit, and can change its orbit a little bit, but mostly to turn the spacecraft and point the solar arrays or the antenna at the Earth), or we will have suffered so much radiation that some of the systems will stop working. And so the end of Juno will probably be defined by one of those. At one point, we were designed so that we would purposely move Juno into Jupiter to dispose of it, to make sure it didn't crash into Europa. But now we're past that, and we can't crash into Europa, really. Jupiter's gravity field is too powerful. So we will just die naturally, and eventually the orbit will cause it to fall into Jupiter, but we won't necessarily control that.

Oliver Strimpel: Thinking about future missions, what kind of mission would best advance our knowledge of the interior of Jupiter's moons? Not just Io, but some of the other ones where we think there might be a subsurface ocean. And of course, we speculate that they might harbor life.

Scott Bolton: Right. They may be habitable. So there are two missions, one that's already on its way out. That was the European Space Agency's mission called JUICE, and it's on its way out to study Europa and Ganymede and Callisto. And they'll get a little bit of Io; they don't get too close to it. Their target is mostly Ganymede. They'll eventually orbit Ganymede, and that will really tell us a lot about the interior of Ganymede. They'll get phenomenal data on the gravity field and the magnetic field of this body, as well as radar instruments to see into the ice. And then NASA has a mission that will launch later this year, called Europa Clipper, that will study Europa very closely. It doesn't go into orbit at Europa, but it gets many flybys of Europa, and eventually Ganymede, and they'll be there together studying these two moons. They were being designed around the same time as Juno, but they just got later starts. But they're follow-ons, and much of the design of the spacecraft is similar to Juno. So we have a radiation vault; they both have something like that. They're both solar powered, like Juno was.

Neither of them gets really close to Io, so some future mission to go to Io would be very interesting, either to orbit it, or, even better, to get maybe a sample of the plume and bring it back. What you'd really like is to get a step further, like OSIRIS-REx got material from an asteroid. It'd be great to get samples of the volcanic plumes or the ices and bring them back to Earth laboratories someday. Maybe you could land on Europa or Ganymede, and the ultimate would be to go down in with something and actually sample the ocean. They're quite deep, probably kilometers deep, maybe many kilometers, so it's not so easy to do.

Oliver Strimpel: If you could decide what a future space probe would be, where would you go and what would it look like?

Scott Bolton: Well, I'm kind of a greedy scientist, so I would want more than one. But comparing the planets is really important. Juno uncovered real puzzles about how Jupiter formed and how its inside works. We didn't really know that, and I think to fully understand that and interpret that, we need to get other examples. So we need to do the same thing at the ice giants, Uranus and Neptune. They have moons as well, which may also be ocean worlds. You could do a Juno-like mission. We've shown that you can study the body really close and at the same time get flybys of satellites. If you could do something like that at Uranus and Neptune, that would be phenomenal. Even going back to Saturn's system and studying Titan and Enceladus and Saturn would be beneficial. Enceladus may also be habitable, and it's sitting there sending its water plumes out, making it easy for us to sample them. And these giant planets, Jupiter, Saturn, Uranus and Neptune, represent giant planets that we see in exoplanetary systems. If we're going to understand these other planetary systems that we're discovering now, we really need to understand what's in our own backyard.

Oliver Strimpel: Scott Bolton, thank you very much.

Scott Bolton: Thank you.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you'll also find transcripts and a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.