In Our Time - Jupiter

Episode Date: July 27, 2023

Jupiter is the largest planet in our solar system, and it’s hard to imagine a world more alien and different from Earth. It’s known as a Gas Giant, and its diameter is eleven times the size of Earth’s: our planet would fit inside it one thousand three hundred times. But its mass is only three hundred and twenty times greater, suggesting that although Jupiter is much bigger than Earth, the stuff it’s made of is much, much lighter. When you look at it through a powerful telescope you see a mass of colourful bands and stripes: these are the tops of ferocious weather systems that tear around the planet, including the great Red Spot, probably the longest-lasting storm in the solar system. Jupiter is so enormous that it’s thought to have played an essential role in the distribution of matter as the solar system formed – and it plays an important role in hoovering up astral debris that might otherwise rain down on Earth. It’s almost a mini solar system in its own right, with 95 moons orbiting around it. At least two of these are places life might possibly be found. WithMichele Dougherty, Professor of Space Physics and Head of the Department of Physics at Imperial College London, and principle investigator of the magnetometer instrument on the JUICE spacecraft (JUICE is the Jupiter Icy Moons Explorer, a mission launched by the European Space Agency in April 2023)Leigh Fletcher, Professor of Planetary Science at the University of Leicester, and interdisciplinary scientist for JUICECarolin Crawford, Emeritus Fellow of Emmanuel College, University of Cambridge, and Emeritus Member of the Institute of Astronomy, Cambridge

BBC Sounds, music, radio, podcasts. Thanks for downloading this episode of In Our Time. There's a reading list to go with it on our website, and you can get news about our programs if you follow us on Twitter at BBC In Our Time. I hope you enjoyed the program. Hello, Jupiter is the largest planet in our solar system, and it's hard to imagine a world more alien and different from Earth. It's known as a gas giant.

Our planet would fit inside it 1,300 times, but the stuff it's made of is much, much lighter. When you look at it through a powerful telescope, you see a mass of colourful bands and stripes. These are the tops of ferocious weather systems that tear around the planet, including the great red spot, probably the longest-lasting storm in the solar system.

Jupiter is so enormous that it sort of played an essential role in the distribution of matter as the solar system formed, and it plays an important role in hovering up astral debris that might otherwise rain, down on Earth. With me to discuss Jupiter are Michelle Doherty,

Professor of Space Physics and Head of the Department of Physics at Imperial College London, and Principal Investigator of the Magnetometer Instrument of the Juse spacecraft. Juice is the Jupiter icy moon explorer. Lee Fletcher, Professor of Planetary Science at the University of Leicester, an interdisciplinary scientist for Juice, and Carolyn Crawford, Emeritus Professor of Immanuel College University of Cambridge, an emeritus member of the Institute, Astronomy at Cambridge.

Carolyn Crawford, one of the first people to make a detailed study of Jupiter was Galileo. Why was he drawn to study it, and how did it help him formulate the heliocentric model of the solar system? Jupiter's been observed since antiquity because it's the third brightest thing in the night sky. Only Venus and the moon are brighter, so it was an obvious target for Galileo to turn his telescope to. And when he did this in January 1610, he was surprised first to see that Jupiter was accompanied by three little bright stars. And as he observed from night to night, he sort of watched the progression of Jupiter across the sky

and discovered there were four of these little stars in a line above Jupiter's equator. So first of all, he's discovering the first new thing in our solar system and something that was invisible to the in aided eye, but is obviously visible only through the telescope. And even though these four little stars tracked with Jupiter against the fixed backdrop of stars, they would from night to night change position relative to Jupiter. They would shuttle from one side to the other, sometimes disappearing altogether.

And his interpretation, which was correct, was that these were an orbit around Jupiter. There are moons and orbit around Jupiter in the same way that our moon orbits the Earth. and in a way this is behaving like a mini solar system and the observation of things that were not in orbit around Earth went counter to the prevailing idea that all of the heavens rotated around Earth and Earth was at the centre. So the observation, just of these four little moons

in orbit around Jupiter was a key indicator that the Earth didn't need to be at the centre of everything and a very positive move towards supporting the heliocentric, the sun-centred version of the solar system. Why do we call it Jupiter? It's named Jupiter because it's such a prominent object in the night sky. It's named after the king of the gods in Roman mythology. Okay.

The astronomer Giovanni Cassini also made some important observations. What did he discover? Cassini was observing Jupiter with, obviously, by the mid-1660s, a much better telescope than was available to Galileo. And he noticed when he looked at the disk of the planet that it was banded, They had coloured stripes across it. There were features on the disc, including a large red spot, which we now call the Great Red Spot. And he discovered this, and apparently Robert Hook also discovered it around the same time.

And we know now that this is a giant storm within the clouds of Jupiter. It's the biggest storm in the solar system, isn't it? It's a biggest storm in the solar system. And it's been observed for the last four centuries. It's probably existed a lot longer. So first of all, he discovered the band. ending in Jupiter, he saw the great red spot. But he also went further and looked how these tracked across the disc

and made a very good estimate for the time that Jupiter took to revolve and estimated Jupiter's day is just under 10 hours. So even though it's this enormous planet, it's the fastest rotator within the, of all the planets in the solar system. And not just that, he also noticed it undergoes what we call differential rotation in that it rotates slightly slower at the post. than round the equator. And this is not what you see in a solid body like the Earth or Mars,

where everything rotates as one coherent body. But it's the first indication this is a different kind of planet. Lee, Lee Fletcher, it's called a gas giant. So what does that mean and how is it formed? So I often think that the word gas giant is possibly not the best one to apply to these enormous giant worlds, because this is not just a cloud of matter that one might be able to fly through with a spacecraft. In fact, I think a better name might be fluid worlds

because these have phases of matter from gases to liquids to even more complicated and exotic phases down at great depth. The majority of Jupiter is made of hydrogen with a smattering of helium and then a little sprinkle of other compounds and materials mixed in. So what you're looking at is a fluid world made primarily of hydrogen and helium that gets compressed and compressed when you get compressed and compressed

when you get down into the deep abyss at the centre of Jupiter. And how do we think it was formed? So Jupiter would have formed at the same time as the other planets in the complicated and chaotic early days of the solar system. Maybe we would have had small, rocky, icy bodies known as proto-planets. How long ago is used? We're talking four and a half billion years ago, really back at the dawn of our solar system.

Now, only the largest of those proto-planets would have survived, sort of hoovering up and eating up all of the smaller objects. And when they got to a critical mass, around about 10 times larger than our own planet, Jupiter, or at least the proto-planet that became Jupiter, was able to start to suck in and accrete all of the surrounding nebula gases, the hydrogen, the helium, and all of the ices that were present in the birth of our solar system. They got bigger and bigger and bigger, hoovering up more and more material to grow to this gargantuan size that we see today when we look out at Jupiter.

Why did it grow to this Gargaguan science and not on the planets? It was simply the winner of that chaotic experience in the early days of the solar system. In fact, Saturn and Uranus and Neptune, the other three giant planets in our solar system, were similarly forming in this chaotic environment in the early days of the solar system, but they didn't form quite as fast, quite as quickly. So they didn't have the opportunity to suck in all of that surrounding nebula, hydrogen and helium, which means that you have something like Uranus and Neptune, which are maybe only three or four times as big as Earth,

whereas Jupiter is 11 times the diameter of Earth is absolutely enormous. When you say suck in, what does that mean? So it means that the gravitational field of the rocky, icy ball of material that was the proto-planet, is starting to pull in all of the other solids and gases that were present surrounding that forming planet. It's a process known as accretion. We see accretion happening in Saturn's rings, around gas, galactic discs within the planes of galaxies,

and we have an example of the end product of that accretion process in these giant planets of our solar system. What's the composition of Jupiter? You've touched it, but if you could go over that again, because I'd like to talk about the element and how the range, especially the metallic phase. The main thing that comprises Jupiter is hydrogen and helium, with then a smattering of other chemical elements mixed in,

things like you have oxygen, carbon, nitrogen and salt, and they're all present in what chemists refer to as their reduced forms, i.e. they're combined with hydrogen to form water, methane, hydrogen, sulphide and ammonia. Now, that is a very tiny percentage of Jupiter. The majority is hydrogen. And as you go deeper down into the abyss of Jupiter, down towards the interior, that hydrogen is being compressed and compressed and compressed. And the electrons within hydrogen can literally be squeezed out of the atom, meaning that you have free electrons and a sea of protons.

And that is an exotic form of matter that we call metallic hydrogen. It's only produced on Earth for fleeting instance in very high-pressure experiments in laboratories. And within that metallic hydrogen, you can have currents flowing. And it's those enormous currents down maybe within the interior 50% or so of Jupiter that allows it to have the biggest magnetic field that we have anywhere in. in our solar system for a planet, that is. Thank you. Michelle, Michelle Dukaday. Can you tell us about the various flyby missions?

People obviously were attracted to Jupiter, the Voyager, Galileo, Ulysses and Juse and so on. Can you just give the listener an idea of when they happened, what they brought back, if anything? Of course. So the Jupiter system is essentially the most visited system in our own solar system. Started off with two flybys by the pioneer spacecraft,

Pania 10 and 11, back in the late 1970s. There were then the two Voyager spacecraft, Voyager 1 and 2, in the late 80s. And then there was the Ulysses spacecraft that actually used Jupiter as a gravitational assist to get up out of the equatoral plane and went into orbit around the sun. Then there was the Galileo spacecraft which went into orbit around Jupiter. So this was the first spacecraft that spent a long time in the system. A long time being what? Four years.

That's a long time. And one of the reasons an orbiting spacecraft is so much more interesting is that you can separate temporal and spatial features from each other. If you've got a single flyby, it's much more difficult to actually do that. We then had the Cassini spacecraft that flew past Jupiter on its way to going into orbit around Saturn. The New Horizons spacecraft flew past Jupiter as well. We now have the Juno. spacecraft in orbit around Jupiter.

So it's a system where people like to go and learn lots about. What lots have they learned? We know much more about the internal processes that generate the magnetic field that Lee referred to. We know much more about the atmosphere. We also know that there are four large moons of Jupiter. But for me, I think the most interesting thing about Jupiter is its very large internal dynamo field.

It makes the environment around Jupiter a rather difficult place to go to for spacecraft and instruments. The radiation environment is really intense, so you have to be really careful how much time you spend in that environment. And I think for me, one of the other interesting things about Jupiter is there's a tilt between the rotation axis and the magnetic axis. And that means the magnetic field wobbles up and down, and that has a real impact on the moons of Jupiter,

and we're going to study that with juice, which was going to take some time to get there. When you talk about fly past, do you literally mean fly past, or do they go into this haze of water and cloud? It's flying past at a distance. It's flying past at a distance. We had the Galileo spacecraft had a probe that actually traveled down through the atmosphere of Jupiter.

So that's the only source of our understanding of the atmosphere of Jupiter in situ. a sense. Can we go to the moons now? Start with the main moons if you want. We'll be talking about them for a few minutes now. How many are there and what are their main features? So I checked on this because every time you look it up you find someone's found a new moon of Jupiter. So as of March of this year there are 95 moons orbiting around Jupiter. A lot of them are really small. You can't see them.

What's small? Tens of kilometres in diameter. are the ones that we are going to focus on with the upcoming mission, Juce, and also the NASA Europa-Clipper mission, are the four Galilean moons, the ones that Caroline talked about earlier, the ones that Galileo saw back in 1610. And so that is Io, which is closest to Jupiter.

That's a really strange world. We've got sulfur volcanoes erupting off the surface of Io. We then have Europa, which is the moon that the NASA, Europa Klipper mission is going to focus on. And at Europa, there's a liquid water ocean underneath the surface. And we think that water might be quite close to the surface of Europa. We then have Ganymede, which the Juice mission is also going to focus on.

Ganymed is a very strange body. It's the only moon in the solar system that has an internal planetary field. And we don't quite understand why. But it too has an ocean, we're almost certain of. And then last but not least, the last of the four large moons is Callisto. Callisto has a very old surface, and we think its interior is undifferentiated. So we don't think there's a solid core, and we don't think they're different layers above that core. And we don't quite understand why the other large moons are and Callisto isn't.

So that's something that we want to try and understand. Thank you. Carolyn, can you talk about the atmosphere and the various levels of the atmosphere or Jupiter? As Lee said, it's mainly hydrogen, maybe 86% hydrogen, 13.6% helium, and then you've got the compounds made of these other elements. But it has this structure where it's molecular, so molecular hydrogen as we understand it, perhaps for the first thousand kilometres in depth. And then the weight of the overlaying layers of atmosphere starts squeezing the hydrogen. and it becomes as incompressible as a liquid. So we have a layer which is sort of liquid hydrogen. It acts like a liquid.

And then you get down to this phase that Lee mentioned where you've broken the hydrogen apart and you've got free electrons sort of swirling around behaving much like a metal. So that is the bulk of the atmosphere. Maybe at the centre there is a core. Maybe about... Why maybe?

We can't see it. It's buried underneath all this atmosphere. You have to infer the composition from the gravitational effect on the satellites that Michelle has mentioned that fly past. It's possible that there's a core, because there's got to have been some planetesimal for all this gas and this ice material to have accreted on right at the beginning of the formation of Jupiter. But it would perhaps have about 10 times the mass of Earth, maybe be comparable to the size of Earth. And that's right at the centre and that's the seed from which this enormous planet has grown around. One of the successes of the Juno spacecraft, which is currently in orbits around Jupiter as we speak,

is that it was able to very sensitively measure the gravitational field of Jupiter. And that was important because of exactly what you were just saying. It allowed us to distinguish between whether Jupiter has a solid core or a core that was more mixed with the environment. And actually the latter seems to be true. You can think of Jupiter as having a, a solid core, fluffy core where that heavy material that was the seed of Jupiter back in the early solar system is now well mixed with the material surrounding the core.

Jupiter seems to have suffered from or to enjoy great weather storms. What do you make of this? Jupiter's weather is readily visible through a telescope. If you're lucky enough to get out there on a dark night and look at Jupiter through a telescope, you can see the stripes of Jupiter. You can see the great red spot. And sometimes, times you can see other storms. The Great Red Spot is actually twice as large as planet Earth would be swallowed twice. It's twice as large. It's not actually a hurricane. Now, if you're familiar with weather maps on Earth, you'll see highs and lows as they move around over the oceans and over the continents. And you can think of the Great Red Spot like one of these antipsychonic highs, the H that appears on a weather map. The weather systems on Jupiter are occurring very near to the topmost layers. In the time,

tiny fraction, maybe 1% of the surface layers of Jupiter, what's going on in the deep abyss down below those stormy layers is largely a mystery, simply because we can't see down that far into the Jovian interior. The really cool thing about the weather that we can see is that it's powered by the same process as we have here on Earth. It's the condensation of water, forming huge cumulonimbus clouds and associated lightning strikes that is driving all that bubbling, churning change within the Joviant atmosphere. And if you were unlucky enough to be there in a balloon, say, descending into the darkness of Jupiter, you'd see these

tremendous cloudscapes in front of you with lightning strikes illuminating those bulbous white, fluffy clouds. Thank you, Michelle. Can you tell us about Jupiter's magnetic field? Yes, of course. So it's the largest magnetic field in the solar system. And as I alluded to earlier, that makes the environment rather difficult for spacecraft because what the magnetic field does is it accelerates the plasma ions and electrons and accelerates them to very high energies. And so if you're close to I.O., you have to be really careful because the spacecraft and the instrument can't

survive very long. But what the magnetic field does is it essentially controls the environment around Jupiter, and that means it has an influence on how the moons behave. And that's one of the things we're going to use with the due spacecraft. We're going to use the fact that the magnetic field of Jupiter changes all the time. That changing background field at the moons induces electrical currents that flow in conducting oceans underneath the surface. Those electrical currents generate a field of their own, and that's what we're going to measure. And that's going to allow us to work out not only what the depth of the ocean is,

but what salt content is as well. You described that very clearly, but what does that mean in the overall? What it's telling you is that the interior is still warm. There are processes taking place in the interior of Jupiter that are generating the field, which Lee described earlier. so you've got the electrons being stripped off the hydrogen that's allowing electrical currents to flow and to form and that's what's generating the magnetic field that you see outside. Because you've got the magnetic field, you get aurora, you get the northern and southern lights that form in the atmosphere of Jupiter,

just like they do on the earth, and that then allows you to get a better understanding about how these particles spiral down the field lines and how they change over time. Carolyn, in July 1994, that's it. Fragments of the comet Schumacher Levy 9 collided with Jupiter. What do we learn after that collision? The great thing about this collision was that it was predicted.

A comet had passed by Jupiter a couple of years earlier and got sort of ripped apart by the gravitational forces into these 20 fragments, which then went into orbit around Jupiter. And so scientists were ready for the collision when it occurred, We knew what day, we knew kind of what time was going to happen. And there was a concerted observing campaign all across the world to watch these impacts as they happened. And the thing that was interesting, you have each of these fragments successively.

They'd be vaporised. They'd only get perhaps a couple of hundred kilometres down below the surface. And vaporise in a hot bubble that would then rise and then leave these dark scars that stayed on Jupiter for several months. afterwards, you could see them. And what was interesting was what these cometary particles were possibly delivering. I think the thing you can liken the Schumacher-Levy-9 impacts to are what are known as airburst events within the Earth's atmosphere. So you might be remembering Chelyabinx, about a decade ago,

or the Tunguska event back in the early 20th century. These are events where a cometary impact would come in, explode and cause shockwaves that would spring. out over enormous areas in the Tunguska case, flattening trees for hundreds of miles wide. But this on Jupiter was causing this tremendous scar of dark, sooty-like material that was left over from those impacts.

Now, for an atmospheric scientist like me, that was perfect, because it's like putting a tracer into Jupiter's atmosphere to then watch as it moves around because of the winds that are prevalent within Jupiter. It's like putting cream into a cup of coffee that's then being stirred, and you can see all the filaments as the cream is spreading out within the coffee.

Now, the Schumacher Levy Nine event began a renaissance in looking at Jupiter and trying to track these air bursts, because we have now detected several smaller than the Schumacher Levy Nine collision, but we can actually see the flashes in videos that are taken by amateur astronomers. And we now know that Jupiter is being hit by smaller objects. Think of them as shooting stars within the atmosphere. atmosphere of Jupiter regularly, at least once a year, if not more frequently than that, then every so often, maybe once a century, will get something as enormous a Schumach and

Leary 9 going in and causing those huge scars. Let's talk about the main moons in more detail. Of course. Michelle, let's start with Io. So, I.O. is a volcanic moon. If you are... What does that mean? It has volcano...

It has sulphur volcanoes exploding. off from the surface. And so if you're on a spacecraft that's flying past Io, if you were fortunate enough, you can see one of these large sulfur dioxide volcanoes going off. And that means, I think it's producing a ton of second of material through its volcanic activity. And that means there's a huge amount of material in the vicinity of Io that's picked up by the magnetic field. And this material is then accelerated, and this is where the very high energies come from. And that's why the environment around Io is not the safest place for a spacecraft to spend quite a lot of time in.

Carolyn, when you look at Io, it's got these fantastic colours of oranges and reds, which are in connection with the sort of sulphur compounds that are coming out from the plumes of these volcanoes. But one of the curious things about Io is that it doesn't have craters. It doesn't look like a moon, perhaps your perception of what a moon should look like. And that's because these volcanoes and the lava outflows are continually resurfacing the little moon, filling up any craters, smoothing out the terrain, and just reforming it continually. And Io is so volcanically active because of its position between Jupiter and the other three large moons.

And it's caught in a gravitational tug of war. And this causes tides within the moon, flexing the surface. It keeps through friction, it keeps the centre of the moon warm and molten, and powers a lot of this volcanic activity. After I.O. comes Europa. Staying with you, Carlin, what are its main features? Well, Europa is the smallest moon, and it couldn't look more different from I.O.

It's the second one out. But it is shiny, it is bright, and it is white. and it has a surface constructed, mainly of water ice. And this is water ice, admittedly, at minus 160 degrees C when you're at the surface. So it's acting a bit more like rock. It's like granite. So you have this surface, but it doesn't show craters. Again, it's very smooth.

And like IO, it has been resurfaced continually, but not by lava, but by liquid water. Now, Michelle has already mentioned how we know there are. subsurface oceans on at least three of the Galilean moons from the electroconductivity that they exhibit. So we think that Europa has an ice shell, perhaps about 10 kilometres thick. Underneath that, there is a slushy water ocean. And the ice shell shows fishes and cracks. And every so often, Europa, Lycao undergoes some tidal stretching and flexing. these fissures will open

and material from the ocean underneath will spray out and we've seen this from Fly Pass from the Hubble Space Telescope we've seen plumes of material being issued out from these cracks and this can then resurface again fill in the craters and re-coat the surface

and keep it shiny and bright and smooth Michelle you want to come in Yes please so if you have a look at some of the surface images of Europa that have been taken by Voyager and Galilee They remind me very much of what it looks like if you're on an aeroplane flying from the UK to the US. And you look out of the window as you're going over Greenland at the edge of the ice shelf. You can see almost these large bits of the ice shelf falling off and being moved by the slushy ice underneath.

And that's exactly what I thought of when I saw images of the surface of Europa. I think when you're talking about Europa and some of the other satellites in the Jupiter system, the reason that planetary scientists get so excited about this is because you have a combination of liquid water, which can act as an ideal solvent for the chemistry of potential life to work within that environment. Now, the key question is whether those huge, deep, dark oceans beneath the icy crusts are actually in contact with rocky silicate material down at depth. Because if they are, it starts us thinking about the situation that we have on Earth at the very bottom of our oceans,

where we often see these things called black smokers, which are towering rocky structures, giving off material into their environment, powered entirely by geothermal energy. It's dark down there, so there's no sunlight reaching them. And yet they have entire ecosystems forming around these black smokers. Life has found a way to exist in darkness.

Now, if somewhere like Europa or Ganymede or maybe the other satellites in the outer solar system have similar conditions, that's extremely exciting for our quest to one day find life potentially in our solar system. And then we come to Ganymede. Ganymed is the largest satellite in our solar system. And in fact, it's so large

that it's about 50% bigger than our own. Earth's moon and about 10% larger than the planet Mercury. So if it wasn't in orbit around Jupiter, you'd think of it as a planet in its own right. It's also one of the only places other than the Earth in our solar system that has its own internal magnetic field. And that's really amazing for people like Michelle studying magnetic fields because you've got Ganymede's magnetosphere sat within Jupiter's magnetosphere and the two interacting and exchanging material with one another. On the surface, which is primarily made of, say, 50-50 rock and ice,

you see dark terrains than are known as Reggionez, overlaid by brighter terrains called sulci. And that's evidence that like Io, like Europa, this moon is undergoing some kind of tidal flexing, which is leading to tectanism, lighter material sitting over on top of ancient darker material. And it's quite a fascinating variable environment on the surface. because it has polar caps of water ice frost that sits for some time during the Ganymede's orbit, seven-day orbits around Jupiter, over the polar terrain of Ganymede.

Do you want to come in, Michelle? Absolutely. So for a magnetometer person like me, Ganymede is an ideal place to go. It has its, as Lee has pointed out, it's got its own internal dynamo field, a real surprise, only moon in the solar system that does. It's embedded in this ever-changing background field of Jupiter. And then you've got this, we're almost certain there's a liquid water ocean at Ganymede. We think it's a global ocean. And we're able to measure or will be able to measure these electrical currents that are flowing in the ocean.

So it's a way of enabling us to work out where the Ganymede is a place where life could form. The way we like to describe it is we're searching for potential habitability. So are the ingredients, there for life to form. We think you need three things. You need liquid water. You need a heat source and you need organic material. And that's what we're going to confirm with juice is whether Ganymede has all three of those, Europa as well. And it's whether those three ingredients are stable enough over a long enough period of time that something can actually happen, something like the black smokers that Lee has referred to. I think for me, maybe one of the most important

realisations that planetary scientists have come to in the last 30 years or so is the fact that if you're looking for liquid water, which you do if you want to see whether life can form, you don't have to look close to the sun to be able to do that. You know, there's been a real focus on Mars. And of course, we are interested in whether Mars did have life on its surface or underneath the surface in the past. But we've realized that you can go much further away from the sun and you can find liquid water. It's just. just not on the surface, it's underneath the surface. And so you go beyond the snow line, where if there is liquid water on the surface, it's going to be ice. And so that's why we're

focusing on these moons of the outer solar system. And just to give a sense of how much water there is, if you think the Earth's oceans at its deepest point are maybe 10 kilometres thick, on Ganymede, that's more like 100 kilometres thick. So the amount of salty, dark ocean water that we're talking about is simply vast. If there's anywhere in our solar system where the conditions that Michelle mentioned, that potentially habitable conditions may be found, it's on these icy satellites of the Jovian system. Is it right that there's more liquid water on Europa than in total on Earth? I think the Earth could almost be considered as a dry environment compared to some of these icy satellites, yes. One of the things that excites me about the upcoming

missions to Jupiter, so the NASA's Europa Clipper mission and Issa's juice spacecraft, is we're going to have two spacecraft in the system at the same time. We will be able to make images of the outgassing of water vapor from Europa while one of the spacecraft is really close and actually measuring what's coming off. And that's something that you don't get to do very often. I think a lot of the missions are assessing the potential for life. We're not necessarily going to find life, but we're trying to characterize the properties of the ocean, of the surface, the nature of the interior of the moons, all of these things that build a picture of whether we might in the future be able to send missions that will look further for life.

If you're going to send landers, you need to have a much better understanding of where you would land on the surface of moon, where would be the best target area. That's essentially what I was going to come in and say is, you know, I'm often asked, why don't we simply send a lander? And my question in return is where do we land? If you want to get underneath the surface and see what's under the surface, if you land on part of the surface

where the ice crust is 100 kilometres in depth, there's no ways you're going to get under the surface. So I see Juice as a way of us deciding where for the future spacecraft missions, which are going to be landers, where we will want to land. And crucially, Juice and Europa carry ice penetrating radars that we'll be able to not just look at the surface,

but we'll be able to probe down several kilometres, if not 10 kilometres, into those icy crusts. And that would tell us where the thin points are, where the thick points are, and really guide in, say, 10, 20, 30 years' time, where should we be sending our robotic rovers to the surface of these icy satellites? Is the next step in exploration? How thick do we think the icy shells of these moons are? Is it of the order of 10 kilometres?

or thicker? Certainly 10 kilometres for something like Europa, although I have seen several tens of kilometres being listed, and then something like 100 kilometres plus for Ganymede. So it's thick. They are thick icy shells. The last of the major moons is Callisto. Do you want to kick up about that?

Yes, of course. So Callisto is large. It's not as large as Ganymed is, but its surface looks really old. and one of the things that we plan to do with juices, I've lost count how many flybys we've got, Lee. I think we've got about 22 flybys of Callisto. And we plan to get an understanding about how the surface has aged,

how it might have been different from previous spacecraft. But also, as I alluded to earlier, we want to try and get an understanding about why the interior of Callisto doesn't seem to be differentiated. We don't seem to have a solid core, and we don't have different layers above. that call. Although I was at a conference a couple of weeks ago and someone was talking about the fact that they don't think the Galileo observations were good enough to be able to confirm that it's

not differentiated. So that's something we're going to try and get a better understanding of. But the other reason I'm interested in Callisto is we think there is a global liquid water ocean underneath the surface again. And so the signatures that we're going to be able to measure of currents flowing in that ocean will allow us to work out whether it's a global ocean, but also how deep it is and what its salt content is. So another thing about Callisto is that the surface is really ancient. It bears witness to bombardment over eons of solar system history. Multaneon. So multiple billions of years of solar system history. And it has these incredible multi-ring basins where something like a comet or fragments of an asteroid must have hit it.

and you get a huge chain of craters that scar all across the surface of Callisto. It must have been a very violent and unwelcoming place in its early days. And today it sits there as a relatively dark and cratered object on the outer edge of these Galilean satellite system. Can we talk about juice, please? Juice stands for Jupiter icy moon explorer. And we first started thinking about a juice-type mission. about 15 years ago, and it was selected by the European Space Agency 12 years ago,

and we spent the last 12 years building the spacecraft and building the instruments. But the focus of juice is twofold. It's going to allow us to better understand the Jupiter system, as we've described some of our understanding of the Jupiter system so far today. But the other focus is the three moons, Europa, Ganymede and Callisto. and Ganymede is going to be a real focus at the end of the mission we're going to go into orbit. As Lee mentioned earlier,

this is the first spacecraft that will go into an orbit around an icy moon. It's going to take us six and a half years to get there. Another reason why you need a huge amount of patience to do this. So we're going to get there in late 2031 and we will spend at least three years in the Jupiter system and we'll end by going to orbit around Ganymede. Colin, what role did you?

Jupiter play in the shaping of the solar system as we know it. As Lee described near the beginning of the programme, Jupiter formed very fast alongside the rest of the solar system. But because it formed so quickly and it is so massive, it has a profound effect on the rest of the solar system. And in particular, you have to realise the early solar system is a fairly chaotic place. There's a lot of debris. There are planetesimals crashing into each other, coalescing or shattering. And Jupiter has this fantastic gravitational effect that can stir up all this debris. Now, most obviously, we have the asteroid belt between Mars and Jupiter,

because Jupiter's gravitational field kept stirring up objects within, at about that radius from the sun, and preventing them from coalescing into a planetesimal. But more widely, Jupiter acts to both deflect and direct a lot of the icy debris that was left over from this early stage of the solar system. And gravitational interactions of these icy bodies with Jupiter can send them a number of directions. I mean, obviously some may land in Jupiter like the Shemakela Levi-9 comet. Some may be held out of our solar system. So it's like a clearing debris out of our solar system, throwing them out to,

the old cloud, which is a reservoir way out about light year from the sun of icy bodies, where comets come from, or out beyond Neptune into the Kuiper belt. So it's tossing these rocky bodies out of the main part of the solar system. It's also deflecting some towards the inner parts of the solar system. And so a lot of this is responsible for what we call the late heavy bombardment. And this happened when the inner rocky planets were maybe 500 million, years old. So they're still quite young. The rocky debris peppers the surfaces. This is how we get the craters on Mercury and the moon

and also hitting Mars, the Earth and Venus. A lot of the debris falls into the sun. And so Jupiter is kind of moving, shuffling stuff around the solar system. The exciting thing for the debris that may have landed on Earth during this bombardment is that these are icy bodies. And this could be a source of the liquid water that we have on Earth because Earth was molten when it formed it was too hot it would not have had water on the surface

a huge amount of icy debris sent to the surface could bring the liquid water we need for our oceans, possibly even organic compounds that we need for life Thank you. We're coming to the end of the programme now but Lee amateur astronomers are particularly praised by all of you

why is that? So as a professional astronomer we would love to be able to use enormous telescopes out in Hawaii and Chile on a nightly basis to track the changing weather patterns, the stripes, the storms, the great red spot. But we simply can't. There's not enough time and money in the world

to be able to do that. So what we do is we rely on the army of amateur citizen scientists who have backyard telescopes that go out night after night and they observe Jupiter using a process called lucky imaging. What that means is they take a video of Jupiter

through the eyepiece of their telescope and then they save a... only the sharpest frames from that video, and they create astonishingly detailed observations, which then crucially, they upload to a central website repository. Honestly, the science of the Jovian atmosphere wouldn't be as far forward as it is today without the contribution of amateur observers.

My first view of Jupiter was through a telescope my dad built when I was a young child in South Africa. I remember seeing Jupiter in its red spot and the four large moons. never thought I'd be involved in doing what I'm doing today. What about you, Karwood? Jupiter, as we've learned in this program, it's fascinating in its own right, the environment of Jupiter itself and the moons.

But we also study it as an example of what many exoplanets, the planets around other stars are like. Many of these, or the ones that we first discovered, are giant planets. Jupiter is the nearest analog we have to perhaps have a clue about what other exoplanets are like. Thank you all very much. Thanks to Carolyn Crawford, Michelle Loughity and Lee Fletcher,

and to our studio engineer Andrew Garrett. We take our annual break now and we'll be back on the 14th of September. Please join us then. In the meantime, on BBC Sounds, you can listen again to the almost 1,000 programmes we've made so far. Thanks for listening. Have a good summer. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin

and his guests. If you can just say what you hadn't time to talk about, that you'd like to talk, Lee. So I'd like to tell you a bit more about the juice spacecraft because it is a marvel of engineering that we launched from French Guyana in April of 2023. It's a 27 metre wide spacecraft when you get these two enormous arrays of solar panels either side.

And you need more than 80 square metres of solar power to produce the electricity that we need to power all the instruments that we've got that we're sending to Jovi in orbit in the next few years. Now, it's on its way now, but it's not going direct to Jupiter. That would take a huge amount of fuel, not only to get there, but to slow yourself down when you arrive. So it does a clever looping maneuver around the inner solar system,

flying past the Earth, flying past Venus, coming back by the Earth again, and continuing to go ever faster and faster and faster until in 2029, we finally start the journey out towards Jupiter orbit. Isn't it burning up fuel when it's whipping around the Earth are going to get in a 29 times? We're using fuel a tiny amount,

but the beauty of it is, is that we don't need as much fuel as we would need to go direct to Jupiter. You're using the gravitational fields of all of the inner planets instead of having to take a bucket load of fuel with you out to Jupiter. Now, you do need that fuel when you get to Jupiter,

because we have a critical maneuver called the Jupiter orbit insertion. which is quite a nerve-wracking moment when the main engine has to fire in order to get us into Jupiter orbit, where we'll then stay for the remainder of the four or five or fingers crossed, even longer mission in orbit around Jupiter and Canamede. So that fuel is crucial for us later on.

If I may follow up on that, I think for me, one of the most important things about missions like this is how long they take to happen and then how long it takes us to get there. When I give public lectures, I look into the audience and I look at the schoolchildren in the audience. And I say to them that you are going to be the people who are going to analyze the data once we get there. Just like with myself, I wasn't involved in the Cassini Spacecraft mission when it was built, but I was fortunate enough to be moving into this area when we first started taking the data.

So it's almost handing on from one sort of generation to the next. And on that note, I don't know if you remember this, Michelle, but when the mission was first selected back in 2012, I was a new father. I'd got a six-month-old at the time. And I remember sort of talking to my family and talking to my colleagues about the fact that by the time we launched Juice, my daughter would be leaving primary school, which indeed she is in the next few days.

And by the time we arrive at Jupiter, I will have a teenager, which none of this Jupiter science has really prepared before, I should say. So it is a generational thing, and it might be children who weren't even born when Juice was first being formulated will be the ones that would be making those exciting discoveries in the years to come. What can you tell us about the Great Red Spot? So the Great Red Spot is often known as the largest storm system in our entire solar system. In fact, you could fit the whole of planet Earth inside that storm twice over. It's gargantuan.

If you were unlucky enough to be in a balloon, being blown around the periphery of the Great Red Spot, it would take you about four days to do a complete lap of the storm. And I think the way to think of it is that that very powerful jet stream that encircles the Great Red Spot keeps the air inside the spot separated from the air outside of the spot. That means interior to the Great Red Spot, you can get the atmosphere literally being cooked by the red spot,

rays of UV light from the sun. And that cooking process splits apart the bonds within the molecules that are present there and recombines them into some kind of material that is very good at absorbing blue light and great at reflecting the red light. To this day, despite all of the fly-by spacecraft and the orbital spacecraft that we've had, we still don't actually know what the red material is. We've got lots of ideas, but we don't know what that compound is that creates the red colour. Could be phosphorus, could be nitrogen-based, could be sulphur-based, something that's dredged up from the interior. But the storm itself is the most eye-catching feature that you can see when you look at the atmosphere of Jupiter. And Carly? I was just going to touch on the other moons,

because as Michelle said, we've got well over 90 moons in orbit around Jupiter. But in truth, only about eight of those probably formed alongside Jupiter. And they're the ones that go, they rotate round Jupiter in the same direction as Jupiter spins and they orbit above the equator. Most of the rest of them are much smaller and are probably bits of icy debris that Jupiter has captured with its gravitational field. They will go very eccentric orbits, very sort of oval rather than circular. They'll go the opposite direction to Jupiter's spin. And so there's quite a coterie of moons, but they're all very different and they're sort of late comers to the When you say smaller, what's smaller?

A few tens of kilometres across at most, you know, nothing comparable to the four Galilean moons, which are all comparable to our moon, or in the case of Ganymede, larger than Mercury. But the other thing that people often don't realize is that Jupiter has rings. So they're just as extensive as Saturn's rings, but they're not shiny, they're sort of dark,

for it sort of dust-like. They only reflect about 5% of the light. In fact, if I remember right, I don't think we discovered them until Voyager flew past, so at the late sort of 1970s. And these, unlike Saturn's rings, which are icy and bright,

and they're the remains, we think of a small icy moon that got shattered a bit. The rings from Jupiter are generated from sort of micrometroids impacting on the four inner moons and throwing dust into orbit and creating these rings. Anything else? Mervin, could I pick something up that Carolyn mentioned and I should have done earlier? And that was, you know, where you can use the Jupiter system to understand exoplanetary systems.

And you were talking about Jupiter, I think. Ganymede, we think, is a water world. And we think there are many exoplanetary bodies that are water world type bodies. And so by focusing on Ganymede with juice, we're going to understand a whole number. new class of planetary body. And I should have mentioned that earlier and I didn't. And indeed, many of these exoplanets that we see around other stars may well have exo moons of their own, which resemble Ganymede or Europa, some of these systems we're seeing around Jupiter.

That's right. And so if we can demonstrate that the Galilean satellites of Jupiter are habitable environments, then one day when we do detect these exo moons around exo giant planets, we can start to imagine that the habitable zone is prevalent, across our galaxy. And who knows, maybe there's someone out there looking back at us and wondering what's happening in our solar system. I hope they don't. Perhaps this is frivolous correction, but how's the sun reacted to this competition of Jupiter? One would think the sun is so enormous, it hasn't really cared too much about what Jupiter is doing in the outer solar system. Nevertheless,

the sun's influence on Jupiter is still tremendous, even though it's out at five astronomical units away from the sun. Now, Carolyn, you mentioned that Jupiter wandered about in the early days of the solar system. It moved in close to the sun. And the Jupiter that would be close to the sun, say at Earth orbit, would be very different from the Jupiter we see today. It'd be much hotter. All of those overlying clouds we see today would be completely evaporated, and you'd have a water cloud Jupiter. It would be almost perfectly white with water storm clouds visible everywhere on the planet itself. And the sun is critical for the due spacecraft mission

because that's how we get our power. In some ways, the sun is there and Jupiter will be very glad it is for us to be able to get our data. I was going to touch on the stripes of Jupiter again. Stripes and zones. There's zones and belts as we know. So the zones are very reflective. They appear white.

The belts are much darker. They're sort of a reddish-brownish colour. But what you're seeing, again, if you look through a telescope and you see these stripes, are photons of light from the sun that have travelled all the way out across the solar system and have then interacted with icy crystals of ammonia and water and been reflected back. Then those photons that have been reflected by the clouds of Jupiter travel all the way back across the solar system again, into your telescope, into your eyepiece, and into your eye. That's quite the journey of some photons that have literally touched and been scattered

by the icy grains within Jupiter's cloud tops. And that red in the belts, do you think it's the same red as we see in the great red spot? Actually, I think it's subtly different. I think when you see the dark brownish colours of the belts, it's because you're seeing deeper cloud layers, whereas those white things are more, think of them like cirrus clouds that are condensing high up in the upper troposphere, or the higher levels of the atmosphere, whereas the brown belts are much deeper down. The Great Red Spot is really unique because it's definitely red.

It's definitely red and that means it's a different chemical compound that's forming that. The producers coming in now, Luke, to put an end to this. At the height. We could talk. We could talk about it. Anyone like a bit of tea? Oh, good, thank you. I'm okay, thanks.

Do we know how deep the Great Red Spot is? Bun. So Juno has suggested it goes about 500 kilometres down. I told you it's impossible to stop them talking. This is a new five-part drama from Radio 4 called An Eye for a Killing. The true story of Scotland's most notorious serial killers, burke and hair. You might have heard their names. You might also have heard that they were 19th century grave robbers.

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