Discoveries at the icy moons of Jupiter and Saturn | 91TV
Transcript
- Good evening, everyone, and welcome. My name is Sheila Rowan and I'm the physical secretary
- for the Royal Society. A few housekeeping rules before we start this evening. Could I ask that
- mobile phones are switched to silent mode? There are no planned fire evacuations. So in the event
- of a fire alarm sounding, do please vacate the building via the nearest fire exit. You'll see
- them marked and the assembly point is at the top of the Duke of York steps to the right of
- the Society. There will be an opportunity this evening to ask questions at the end of the talk,
- so do think of that as we go along. This evening we have the Bakerian medal and lecture being
- presented, and it's the premiere lecture in the physical sciences. The lectureship was established
- through a bequest by Henry Baker FRS, of £100 for an oration or discourse on such part of natural
- history or experimental philosophy at such time and in such manner as the President and Council
- of the Society for the time being shall please to order and appoint. So the lecture series began in
- 1775. The medal is of silver gilt. It is awarded annually and accompanied by a gift of £10,000.
- Tonight's lecture, entitled Discoveries at the Icy Moons of Jupiter and Saturn,
- will be given by Professor Michele Dougherty, winner of the Bakerian Medal and Lecture 2024.
- Michele Dougherty is a professor of space physics at Imperial College London. She is
- leading unmanned exploratory missions to Saturn and Jupiter and was the principal
- investigator for the magnetometer instrument onboard the Cassini Mission to Saturn, as
- well as being the principal investigator for the magnetometer for the Jupiter Icy Moons Explorer,
- better known as JUICE of the European Space Agency that launched in April 2023. She is head of the
- physics department at Imperial College London, is a fellow of the Royal Society, was awarded the
- Royal Astronomical Society Geophysics Gold Medal in 2017, a CBE in the 2018 New Year Honours list,
- and was awarded the Institute of Physics 2018 Richard Glazebrook Gold Medal and Prize.
- Ladies and gentlemen, it is an absolute pleasure to present Professor Michele Dougherty.
- Good evening, everyone. Really nice to have you here. So what I'm going to talk about this evening
- are two different spacecraft missions that I've been involved in. One of them focusing on Jupiter,
- and that's the most recent one, that one was launched last year, and one which focussed on
- Saturn, and that ended in September 2017. One of the reasons I like to show this particular
- set of images is really I want to focus on the picture of Saturn on the right here. I was told
- it would take a bit of time. Here we go. One of the things I'd like you to notice is that it looks
- as though there are little plumes of something coming off this ring that we have around Saturn,
- and that's one of the discoveries that I'm going to talk to you about this evening. Now,
- this picture was put together over a very long period of time. Lots and lots of different images
- were taken from the camera on board Cassini to put this image together and those of you with good
- eyesight will see, if you see just where my little red dot is there, that is the Earth and I always,
- the hair on my arm stands up on end when I think about that, because there was a spacecraft out at
- Saturn and essentially looking back at the Earth. So let's talk a little bit about Cassini to begin
- with. So this particular view here shows you what the spacecraft looked like. You can see
- it was covered with a lot of gold foil. That gold foil was a thermal blanket. It helped
- keep the instruments and the spacecraft at the kind of operating temperature that they needed.
- There is a large white umbrella shaped object here. That's the high gain antenna. Once a day,
- the spacecraft would turn to the Earth and would send all the data down that it had collected the
- day before, and then from my perspective, the most important part of the spacecraft is this very long
- boom here. Oh, this is a really strange mouse, I don't have control of it yet, but this very long
- boon is where you put the magnetometer instrument. So the instruments we build at Imperial College
- measure the magnetic field of the environment that we're in but we want to make sure the magnetic
- field we measure is due to the environment and not due to the spacecraft. So we need to get
- ourselves as far away from the spacecraft as we can. So we had two different instruments on
- board. We had one halfway down that boon and then we had one right at the end of the boon as well,
- and all of those flags are from the countries that were involved in in
- building the instrument. So you can see it was a very large international mission.
- This shows you a view of the Cassini spacecraft in the test chamber before launch, and the reason I
- like to show this is it gives you an impression of how big the spacecraft is. So you can see
- the people standing in the test chamber before launch. This chamber allowed us to test that the
- instruments in the spacecraft were able to operate in the vacuum of space that they were going to
- find once we were out in space, but also that they could survive the temperatures that we would
- see on the Cassini tour out to Saturn. So we went into Venus on the way out to Saturn to give us a
- gravity assist to get us out to Saturn, and there, the temperature's about plus 50 degrees Celsius.
- Once we were out at Saturn and it was about minus 170 degrees Celsius, so we needed to be able to
- test that the instruments were able to operate in all of those ranges. You might also notice
- that the long boon, that our instrument on can't be seen there, and the reason for that is it was
- folded away into this little canister that you can see there, because you can't launch a spacecraft
- with a large 11 point metre boon sticking off from the side, and so that was deployed after we
- were launched. So I'm going to talk to you about magnetic field observations and what I thought
- is I'd give you just an understanding about what it is that we measure, so that when I talk to you
- about the first discovery that we made, you'll get an understanding about how it was we were able to
- make that. So this shows you a view of the Earth. It could be Jupiter. It could be Saturn. But what
- it's showing us is what the magnetic field lines look like. So any planet that has an internal
- dipole in the interior generates a magnetic field. Now we can't see it with the naked eye,
- but the instruments we build measure the magnetic field, and if you could see what they looked like,
- they looked like those red lines that you can see there. One way to think about it is if you've got
- a piece of paper, you put a bar magnet underneath that piece of paper and iron filings on top of
- the piece of paper, those iron filings will lie along the lines of force of the magnet. So that's
- essentially what we're seeing there. Something else to keep in mind is as the planet rotates,
- so as the Earth or Saturn, as I'm going to talk about in a short while, as it rotates once a day
- on its axis. So for the Earth, that's every 24 hours. For Saturn, it's just under ten hours.
- Those magnetic field lines are frozen into the planet and they're going to rotate as well, and so
- you've got a moon somewhere out at some distance from the planet. As the planet is rotating, those
- field lines are going to be moving over the moon, and that's something I want you to keep in mind
- as I talk to you about one of the moons of Saturn. So this shows you a view of the Saturn system. You
- can see Saturn off to the left. You can see the visible rings beautifully. The A, B, C,
- and D ring. You can also see some of the moons at the distance that they orbit away from Saturn and
- you can see a rather diffuse ring. Let's wait for it to come up. There we go. This diffuse ring that
- you can see here and that ring, when the Voyager spacecraft flew past Saturn, the spacecraft, the
- instruments on the spacecraft measured that the material in that ring is made up mainly of water,
- ice, and the moon that I want to talk to you about is Enceladus. Enceladus is orbiting there,
- and you can see it looks as though it's embedded right in the centre of the E ring. We've got Mimas
- orbiting inside of Enceladus and Tethys orbiting just outside, and I'll mention both of those in
- the next couple of slides as well. So this is a view of Enceladus. So when Cassini first arrived
- at Saturn, it arrived in July 2004, and the plan was we would have three flybys of Enceladus in
- early 2005. Before we got there, we knew something about Enceladus. One of the things that we knew
- is that its surface was made up of water ice, because when the Voyager spacecraft flew by,
- it had an instrument that was able to remotely sense what the surface was made up, and it was
- made up mainly of water ice. Now, this particular view that you can see here was taken on by the
- imaging instrument on board Cassini. They're two different photographs that were taken,
- and the first thing you'll notice is there are deep cracks on the surface, but the other thing
- you'll notice is there are not many craters, and that was a real surprise because Mimas and Tethys,
- the two moons I pointed out to you in the previous slide, their surface is covered
- with lots and lots of craters. So the implication is that something was going on at Enceladus that
- was making its surface look younger than it should have. So something was potentially resurfacing it,
- and we weren't quite sure what that could be, but because we knew the surface was mainly water ice
- and the E ring is made up mainly of water ice, there was always the question, what is it that
- is the source of the E ring? And the question was, is Enceladus the source of the E ring? But
- we weren't quite sure how it could have been. So I mentioned we had three Cassini flybys planned. The
- first one took place in February 2005, and you can see the flyby distances on the bottom left of the
- slide there. So the first one was quite far away. It was just under 1300km away from the surface.
- The second one took place a month later and that was much closer, that was 500km away. The third
- one was due to take place in July of the same year and the plan was it would be about 1000km
- away from the surface, but based on what we saw on those first two flybys, we were able to persuade
- the Cassini project to take us much closer on the third flyby. So the third flyby was just
- 173km above the surface. Something else to keep in mind about Enceladus is it's really quite small.
- Its diameter is only 500km. So that gives you an idea about how far away those different flybys
- were. So what we found on the first two flybys was something rather strange in the data. We weren't
- expecting to see it and the best way that we could visualise and describe to people what
- we thought we were seeing is in the following way. So there are two different views I'm going to show
- you. So the first one, your eye is looking down on the north pole of Saturn. So here we are looking
- down on the north pole of Saturn. Saturn's the yellow ball. The gold rings are the visible rings
- of Saturn. The blue lines are the magnetic field lines of Saturn that are rotating with Saturn once
- a day, and the orange ball is Enceladus as it's orbiting around Saturn. Now, if Enceladus was a
- dead body, the blue lines, the magnetic field lines wouldn't see it at all, and they'd just
- go straight through it so you would see no sign of any disturbance in the magnetic field. But what we
- seem to be seeing in our data was that instead of that happening, the following was going on. So now
- your eye is - where is it? Here we go. Now your eye is looking side on at Enceladus. So here's
- Enceladus, the magnetic field lines, the blue lines from Saturn are moving towards Enceladus,
- but instead of just going straight through them, they seem to be held upstream of Enceladus, almost
- as if Enceladus was a much larger obstacle than its physical size. One way that you can do that,
- and it happens on the Earth, is that if you have an atmosphere, the upper regions of the atmosphere
- become ionised by solar radiation. They form a plasma, and that stops the magnetic field lines
- and plasma of the solar wind penetrating down onto the surface. So this was one way in which we could
- describe the observations that we were seeing. Now, I want to preface this by saying that we were
- not sure if we were seeing something real, because we were still calibrating our instrument. We had
- got the trajectory of the spacecraft back from the project, and it wasn't the final trajectory.
- So we were still a little uncertain as to whether we were actually seeing something real, but the
- fact that we saw it on two flybys, separated by a month, told me that something was going on. So
- what we did is we put the following schematic together to describe what it was we thought we
- were seeing. So what we thought those two flyby datasets were telling us was that Enceladus was
- covered by a diffuse atmosphere covering the entire surface. The outer regions of that
- atmosphere were becoming ionised, and they were having an effect on the magnetic field of Saturn.
- So what I did is I went out to the Jet Propulsion lab. There was a science meeting that was being
- held by all of the teams, and I was going to make the case to the project that we wanted to go much
- closer on the third flyby. I remember I got there and I was jet lagged, eight hour time difference,
- and I was standing in a line for coffee, rather nervous because I wasn't sure if I was going to be
- able to persuade the project that it was important to do this, and there was a man standing in the
- line in front of me, and he turned around and he said, 'Michele, what are you doing here?' And it
- was the person who was responsible for essentially driving the spacecraft. I told him what we thought
- we were seeing, and he rubbed his hands together in glee and he said, 'Cool. I've always wanted to
- go closer to a planetary body than anyone else.' So I knew I had one vote in my pocket when we
- went into the meeting. So it was a difficult case to make because we had planned all of the
- observations we were going to make in the first four years that we were there. So every instrument
- knew when they were taking data and what they were going to do. So if we changed the flyby altitude,
- it would change everyone's data sets and what they could get, and also we would use fuel,
- and you never know when you're going to need extra fuel at the end of the mission to extend it or do
- other things. So we were taking a chance if we did that, but we managed to get most people to
- agree that it was worth doing, and so the project agreed they would use the fuel to change the third
- flyby altitude to 173km above the surface, and fortuitously, and you'll see why I say that in
- a moment, we came up from below the south pole of Saturn and flew… Oh, I don't have control of
- the mouse. I'm sorry. We flew just below the south pole of Enceladus. I didn't sleep for two or three
- nights before that third flyby, because if we had seen nothing, no one would ever have believed
- anything I said again, but luckily, this is what we saw. Instead of there being an atmosphere
- covering the entire surface of Enceladus, there was an outgassing of water vapour from the South
- Pole. It was like a cometary plume coming off from the south pole of Enceladus. But because
- we went so close, all the other instruments were able to take great data as well, and I'm going to
- talk you through some of that data. So this here shows you data from two different instruments. In
- the top left, you can see data from the camera on board, you can see something that hadn't been seen
- before, and that is very deep cracks at the South Pole, which the imaging team called tiger stripes.
- Bottom left is data from an instrument that was able to remotely sense the temperature of a body
- that it was looking at, and they expected to see the largest temperatures at the equator, because
- that's where the solar radiation is strongest, and they found that the temperatures were quite hot
- there, but they also found that there was a hot spot right at the South Pole. If you overlay those
- two data sets on top of each other, the right hand image shows you that the hottest spot was right
- over one of the cracks at 91 degrees Kelvin. Now, that's not really very hot, but out at Saturn,
- that's pretty hot, and so the implication was that not only did we have this outgassing of water
- vapour coming through the cracks at the South Pole, but we had internal heat leaking out from
- Enceladus, and that was a real surprise because Enceladus, as I said, is really quite small. We
- had always thought it had long since cooled down from when it first formed. So that was the second
- surprise. The next surprise, and the clincher, really, for Enceladus being a really interesting
- place to study is the following. So this was data that came from an instrument that was
- able to almost taste what it was flying through. It was called the ion neutral mass spectrometer,
- and as we flew through the plume, it was able to measure what was in the plume. So they found water
- vapour, which we knew was there because we had seen that in our data set. They found methane,
- carbon dioxide and carbon monoxide, but they also found organic material, and when you search for
- places in the solar system where life might be able to form, you need four different ingredients.
- You need liquid water, which we have at Enceladus. It's escaping through the water vapour plume. You
- need a heat source. You need organic material, and then you need those first three ingredients
- to be stable enough over a long enough period of time that something can happen. So this was
- when the focus of planetary scientists really moved on to Enceladus and we had a long extended
- mission with Cassini, and we had many, many more flybys of Enceladus as a result of this.
- There's also another interesting moon at Saturn called Titan. Titan was not only studied by the
- Cassini orbiter, but it was also studied by the European Space Agency Huygens probe, which
- actually travelled down through the atmosphere of Titan and landed on the surface. The reason
- we were so interested in Titan is that we think, or we thought then, that its atmosphere was very
- similar to what the Earth's atmosphere was like when it first formed. So by studying Titan's
- atmosphere, it was almost like going back in time and seeing what our atmosphere was like,
- but we had never seen down to the surface of Titan because it's covered in this orange hydrocarbon
- haze, and so even with the best cameras on board the previous spacecraft, and even with the visual
- camera on board Cassini, we could not see through that hydrocarbon haze. So that's one of the
- reasons we flew a lot of different remote sensing instruments on Cassini, because they actually,
- as I'm going to show you, allowed us to see through the haze. This here is the first
- confirmation that there was liquid or is liquid on the surface of Titan. So as the Huygens probe, as
- ESA's Huygens probe travelled down to the surface, it was taking pictures and we saw what looked like
- old riverbeds, dry beds. We didn't see any liquid, but there clearly had been liquid on the surface,
- and so we spent the first two and a half years of the Cassini orbiting mission searching for
- liquid on the surface, and we never found it. Then suddenly, two and a half years after we got there,
- there was this glint of sunlight, or from an ocean at the North Pole. Turns out it had been
- the dry season on Titan before that, there hadn't been any rain and so it had been completely dry,
- and then suddenly the season changed and the lakes at the North Pole began to fill up with liquid.
- Not with liquid water, but with liquid ethane and liquid methane. So the next slide shows you a view
- of what the surface of Titan looks like. This was not taken by the camera, the visual camera
- on board Cassini. It was taken by the Visual Infrared Mapping Spectrometer which was able to
- see through the haze. And so this… Sorry. Wrong way. There we go. This large blue region here
- was the only part of the surface that we'd just been able to glimpse through the haze before.
- You can see it both in this view, here and here, too. You can see sand dunes, essentially in an
- equatorial belt. It's filled with brown organic sand. There is an ice volcano just in the left
- there and there's an impact crater just over here, too, and the purple colours that you can
- see show water ice. So now we have a much better understanding about what the surface of Titan
- looks like but it's not only the surface that is interesting, from gravity observations at Titan,
- and also from some hints from some of the data that was taken by the Huygens probe, there is
- clearly a global subsurface ocean underneath an ice crust, but based on the modelling work that
- has been done, it is a liquid water ocean. So the implication is that there are two moons of Saturn,
- at least, there might be more, Enceladus and Titan that have got these liquid water oceans.
- Let's move on to Jupiter for a bit. This shows you a view of our understanding of the interior
- structure of the four Galilean moons of Jupiter. They were named after Galileo Galilei, who was the
- first scientist to actually see them. He built his own telescope, and he saw that these objects were
- changing in position around Jupiter and that's when he realised they were moons. We have Io in
- the top left. Io is covered with volcanoes. Sulphur dioxide volcanoes. Europa top right,
- Callisto bottom right, and Ganymede bottom left, and based on magnetic field observations from the
- NASA Galileo spacecraft, we are almost certain that these three moons have got liquid water
- oceans under the surface, and you might think, how can a magnetometer tell you that there's liquid
- water? The way that we do it is if you've got a conducting body that has a magnetic field that
- is changing over it, that changing magnetic field will induce electrical currents in the conducting
- body. Those electrical currents generate a magnetic field of their own, and the magnetometer
- instrument can measure them. So that's what the magnetometer instrument on Galileo did. They made
- measurements which could be described by having these liquid water oceans under the surface. So
- the discoveries of that Galileo spacecraft made of Jupiter, and the discoveries that Cassini
- spacecraft made at Saturn, led us on to being able to put together this rather a complicated slide,
- but for me, it shows what I think is the most important realisation that planetary scientists
- have come to in the last 30 years. So what we're looking at here is on the left, we're looking at
- the mass of the star relative to the sun but let's say the sun is here, and then we're looking at the
- planets in the solar system and their distance away from the sun. Prior to the Galileo and
- the Cassini observations, the focus by planetary scientists if you're searching for liquid water on
- other bodies other than the Earth in the solar system, the focus had always been on the inner
- solar system. So inside of what we call the snow line, where if you have water on the surface,
- it's liquid. But the realisation we've come to is you can be searching for liquid water and we know
- there's liquid water out there much further away from the snow line. So at Jupiter, at least three
- of the moons have got liquid water oceans under the surface, and at Saturn, at least two of them
- are. So now the focus for searching for potential habitability in our solar system has moved to the
- outer solar system, and in fact, this was a slide that we showed to the European Space Agency 12
- years ago now, when we were trying to persuade them that the Jupiter spacecraft mission that is
- now on its way to Jupiter should have been chosen as the first one to go, and I see we've got at
- least one ESA, former ESA person in the audience. So we took you in, Fabio, but JUICE is now on its
- way and it was based on putting together our understanding of the Galileo observations,
- but also the discoveries we made at Saturn. The reason I like to show this is it just reminds me
- to tell you how big the environment around Jupiter is. So Jupiter has an internal planetary field.
- Those yellow lines are the magnetic field lines of Jupiter. They interact with the solar wind,
- and essentially the magnetic field protects Jupiter from the effects of the solar wind.
- So this very large cavity that you can see here forms in the night sky. If we could see it,
- that's how large it would be compared to the moon, and so you get an understanding about how vast
- the environment is that the JUICE spacecraft is going to. The other reason I like to show it is
- the telescope that you have there, it reminds me, my first view of Jupiter and Saturn was
- through a telescope that my dad built. I saw the rings of Saturn and I saw the moons of Jupiter,
- and I remember my sister and I mixed the concrete for the base of the telescope. We, of course,
- thought that was the most important part, and it was because it wouldn't have stood upright if it
- hadn't been for us, but in those days, I never thought I would end up doing what I do but I've
- rather enjoyed it. So let's talk about JUICE now. So this is what the JUICE spacecraft looks like.
- I've got some proper images of the spacecraft just before it was launched, but of course, you can't
- take a picture of the spacecraft with the large solar panels deployed on the ground and so that's
- one of the reasons I want to show you this. So the way in which we will be powering the instruments
- is via solar power. We've got solar panels of 76m² in area. By the time we get out to Jupiter,
- the power will be going down to about 80 watts, but it will be able to drive the instrument.
- The magnetometer boon on which our instrument is placed you can see is down below here, and we've
- got three different instruments on the boon. We're having to fly an additional instrument,
- which only measures the magnitude of the magnetic field to allow us to calibrate our instrument when
- we orbit around Ganymede, and there are some other instruments on the boon as well, and
- then the large high gain antenna, similar to the one I showed you on Cassini, can be seen there as
- well. So let me show you some images. It's really a call out to the magnetometer engineers who were
- in the room. We were in the midst of building the first model of the instrument when we were told to
- go into lockdown, and so it was a very difficult time. Everyone was sent home, and there we were
- supposed to be building the instrument. We were able to get back into the lab about three months
- later, and so this shows you a view of what the instrument looks like. So there are three
- different parts to it I mentioned, the one that we built at Imperial College, this fluxgate,
- is shown there. We have one which was built by our colleagues in Germany, which is what I'm showing
- you now, and then the new instrument, the third one is a scalar sensor. That one is right at the
- end of the boon. You can see all the cabling that runs from the instruments along the boon onto the
- main body of the spacecraft, and then you can also see the electronics box where all of the
- electronics that are driving the instrument are and that's placed on the body of the spacecraft.
- One of the things we need to do before we're allowed to launch an instrument is we need to test
- it can survive the rigours of the launch. So the bottom view shows you a view of the electronics
- box on a vibration table, which vibrates like hell and you cross your fingers and hope that
- the box is going to survive, and then this one here is the shock test. You essentially drop a
- heavy weight on the electronics box and hope it survives, and it did but it's always a bit nerve
- wracking. I don't have the stomach to watch it, but I get told afterwards if all is okay. So let's
- talk a little bit about launch. So this shows you a view of the Antonov aircraft, that is the
- largest - it's the only aircraft that's large enough in the world to transport spacecraft,
- and so the way in which you get them out of the aircraft, here, it's landed in French Guiana. The
- nose cone opens up and this shows the spacecraft being transported off of the Antonov. This shows
- you a view in the launch preparation area. You can see the people that are standing there to
- give you an idea of the size of the spacecraft. Of course, the solar panels are folded away, the
- magnetometer boon is folded away as well, and in fact you can actually see the magnetometer boon.
- So here the thermal blanket is black compared to what we had on Cassini but the magnetometer
- boon is folded away here. So there were three different parts to it that folded up away against
- the side of the spacecraft, and those of you with good eyesight can just spot the instruments. So
- there's one here, there's one there, and I can't make out the third one. I think it's just there,
- but for the launch itself, they needed to be folded away in that manner. I can't talk about
- a spacecraft without showing launch pictures. So this shows you the launch itself. So we went out
- to French Guiana for the launch. It was due to go on 13th April. We were sitting just behind the
- mission control room, and you could see everything that was going on, and ten minutes before launch,
- they usually send up a balloon into the atmosphere just to check what the weather is like above the
- launch site. And we saw a lot of talking going on after that, and they then scrubbed the launch.
- The reason they scrubbed the launch is they found lightning in the atmosphere above the launch site,
- and you don't send a 1.5 billion spacecraft into a lightning storm. So they delayed the launch by
- a day. We went back in the following day and it went off beautifully. Arianespace did such
- a spectacular launch that we didn't need to use any fuel to get ourselves into the right place.
- So we've now got a lot of extra fuel, which is going to allow us to have an extended mission
- once we get to Jupiter, and once we get to the moon, that we're really going to focus
- on Ganymede. So I'm going to play the launch for you. It's a little bit loud, so don't get a
- fright. Your heart is really in your mouth. You've spent 15 years building the instrument,
- and then it's someone else's responsibility to get it up into outer space, and I've been
- told I'm a bit of a control freak. So I don't like someone else being in control. So let me
- talk to you a little bit about the science that we're going to do and then, in fact,
- I want to go back to Cassini. The reason I'm going to do that is there's a spectacular movie I want
- to play, and that's a really nice way to end the presentation. So that's why I'm jumping around
- a little bit. So this shows you the three moons that we're going to focus on with JUICE. For me,
- the most interesting one, and as a person who's interested in the magnetic field, it makes sense,
- and I'll describe to you why because Ganymede is the only moon in the solar system that has
- its own internal dynamo field. So if you stood on the surface of Ganymede, you had a compass,
- the compass needle would point to the north pole of the magnetic field. It's the only moon in our
- solar system that has that. It's also the largest moon in the solar system. So this
- is the one on the right here. You can see what we think the internal structure looks like as well.
- So we've got this internal dynamo but we also have we think this liquid water ocean where
- the Galileo spacecraft made measurements of the induction currents in that liquid water ocean.
- Now with JUICE, what we plan to do is we plan to, when we go into orbit at the end of the mission
- around Ganymede, we plan to measure not only the depth of the ocean, but what its conductivity,
- what its salt content is, and also get an understanding about how deep the ice crust
- is above the ocean. We're not going to do it by ourselves. There are many other instruments on
- board JUICE that are going to be able to let us do that. Callisto, which is this one here,
- it's got a very old surface and so it's really the best place in the solar system to study the kind
- of impacts that have taken place over the history and over time. The real surprise about Callisto is
- its internal structure is not differentiated. You can see there are different layers to Ganymede.
- Callisto, we don't think there are, and that's a surprise. These moons are orbiting around Jupiter,
- quite close to each other. They formed at around about the same time. So one of the questions we
- want to try and understand is why is the internal structure so different? Also with Callisto,
- magnetometer observations from Galileo pointed to the fact that there is this global ocean under the
- surface as well. We have 25 flybys by the JUICE spacecraft of Callisto. So we plan
- to make those measurements and confirm and get an understanding, again, we hope potentially of
- how deep that ocean is. In the bottom right, we have Europa. Europa has a deep liquid ocean and
- we are almost certain that the silicates and the interior are in touch with the liquid environment,
- and so organic material could potentially be leaking out onto the surface. We will only have
- a couple of flybys of Europa, partly because the radiation environment at Europa is rather intense
- and we want to focus on Ganymede, but also because there's a NASA mission called Europa Clipper that
- is going to be going to Jupiter around about the same time as us, and Europa Clipper is going to
- have at least 50 flybys of Europa. So the fact that those two spacecraft are both in orbit at the
- same time allows us to take two point measurements to really much better understand the system.
- Now, one of the things I lose sleep over is whether we're going to be able to measure these
- tiny induction signals. So the instrument can do it. It's a fantastic instrument,
- but we're going to have to really model this really well and understand the data really
- well. So these are the different measurements that we are going to have to make and we're
- going to have to separate them each out from each other. So the… Oh, it's gone and it's back again.
- The hardest one is the one on the far left. So that's what I was talking about earlier.
- If you've got a conducting body and you've got the white lines, the magnetic field lines of Jupiter
- changing over that conducting body, they are going to generate currents in that little blue ocean you
- can see that generate a magnetic field, and they slightly change the magnetic field as a result,
- but these signals are really small and they're changing all the time and they're different
- frequencies that are doing the inducing. The middle one shows you the internal planetary
- dynamo at Ganymede and so we need to be able to model that, measure it properly so we can
- subtract it away from the data, so we can tease the signals out that we need in the left hand
- image. The positive thing about the dynamo field is it doesn't change over time. So it's static.
- So once we've measured it and we know what it is, we can subtract it from the data. Then last
- but not least, on the right hand side, Ganymede is embedded in the magnetic field of Jupiter and
- so you've got all these plasma currents flowing and you've got a huge amount of things going on.
- So those are the things we need to be able to separate out, and one way I'd think about it
- when I'm losing sleep is it's like trying to find needles in a haystack and they're
- changing shape and colour all the time, and that's what we need to be able to do. So one
- of the things we need to make sure of is that people's expectations are not too high. We're
- not going to resolve the ocean characteristics of Ganymede when we first get there. We will only do
- it by the end of the mission, and it's always difficult to have patience and wait for that.
- That red circle and red ellipse shows you what the orbit of JUICE looks like. So right at the end of
- the mission, we're going to go into this 200km circular orbit above Ganymede. Okay, so before I
- move back on to Cassini, I just wanted to really place JUICE and Europa Clipper into context. So
- from my perspective, we've moved from the original discovery of the Galilean moons by Galileo back in
- 1610 to an understanding that the deep habitats on our Earth, where the pressures and the
- temperatures are really difficult to survive in, bacteria has been found in those environments. We
- have had exploration by the Voyager spacecraft, but also by the Galileo spacecraft to Jupiter,
- by the Cassini spacecraft at Saturn. The discovery of the first extrasolar water world in 2012 and
- then in the 2030s, we will have JUICE and Clipper, and they will be characterising those environments
- and helping us decide where we want to land in the future. People say, why don't you just
- go and land on one of the moons and dig under the surface? Well, where do we land? You don't
- want to land where the ice crust is 100km in depth, because you'll never get underneath the
- surface. So that's part of the exploration that we're doing. So in the last ten minutes or so,
- let me go back to Cassini. I wanted to describe to you very briefly the end of the Cassini mission.
- So we spent in the end, 13 years orbiting around Saturn, but before the end of the mission, we knew
- we were running out of fuel, and we wanted to make sure that Cassini did not crash land on Enceladus,
- for example, because if we ever find life on Enceladus in the future, we don't want to have
- a man made object have landed or crash landed on the surface. You might want to ask me how we could
- have landed on Titan with the Huygens probe later. So what we decided to do is we decided
- to burn Cassini up in the atmosphere of Saturn, and one of the things that we had not been able
- to get our heads around at Saturn is the internal planetary field of Saturn. The way that planetary
- dynamos work, we think, is that if you've got the rotation axis of the planet and the magnetic axis,
- there has to be a tilt between those two axes, because otherwise the planetary dynamo can't
- continue to be generated, and all the data we had got at Saturn, even after 13 years, showed those
- two axes lying on top of each other. So we weren't understanding something. So what we decided to do
- is we would end the mission by focusing on Saturn, and so we spent the last year of the mission
- getting ourselves ready. So there was what was called the ring grazing orbits. So we use Titan,
- which is in orbit around here, we use the gravity of Titan to get us up out of the equatorial plane
- and we had six months of these ring grazing orbits where the closest approach of the spacecraft was
- just beyond the edge of the visible rings. Then after six months of that, we used Titan again
- to get us into these, what we called the grand finale orbits, where we were inside the gap. Well,
- we were inside the inner edge of the ring and the top of the atmosphere of Saturn. Then the
- final orbit in orange is where Cassini plunged into the atmosphere. This just shows you a closer
- up view of how close we were getting. We were about 2000km above the top of the atmosphere,
- and one of the reasons I like to show this is it just confirms that the rings are not solid.
- They're made of countless individual particles, each in their own orbit around Saturn, and so if
- we had been able to be there, we would have been able to see the spacecraft through the rings.
- So this really talks to what I mentioned earlier, the main driver for why we were doing this,
- so that we could get as close as possible, make measurements of the magnetic field lines outside
- the planet to allow us to understand what was actually going on inside, and by the end of the
- mission, we realised that there was no dipole tilt between the rotation and the dipole axis.
- It turns out there's a secondary dynamo. There's a shallow dynamo close to the surface of Saturn,
- which is masking the effect of the dynamo that's coming from the interior, and if we hadn't done
- these end of mission orbits, we would never have been able to confirm that. So this shows you an
- artist's impression of the first dive in that gap. I remember we were out at the Jet Propulsion
- Lab to do this. We've been told there was a large empty space in this region, but we weren't sure.
- Turns out it wasn't completely empty. There was a very energetic dust particle that made a hole
- in the high gain antenna, but I remember we were really nervous because we didn't know if
- the spacecraft was going to survive. What I find really interesting about spacecraft missions is
- when they first launch, it's almost like driving a new car. You have to understand how it works and
- what it's capable of. We would never have planned the end of mission orbits that we had at the start
- of the mission, because we didn't know if we could do it. Whereas by the time we got to the
- end of the mission, we knew what the spacecraft was capable of. So this here shows you a view of
- the resting place of Cassini, where it burned up in the atmosphere of Saturn. So this is again from
- the VIMS instrument. It shows you where Cassini went into the atmosphere. The red you can see
- is the internal heat that's leaking out from the interior of Saturn. To put it into a more global
- context, it went into the atmosphere just above the equator in the Northern hemisphere. This shows
- you a view at the end of the mission, and it was really interesting, those last six months of orbit
- were happening every six and a half days, and we were exhausted. So some of us were really almost
- relieved that the mission was over, because we couldn't keep that pace up, and then once we came
- to the end of the mission, we suddenly realised that the last 25 years of our life, it wasn't
- over, but it was going to be different because we were no longer involved in Cassini. So this
- is a great image and it always takes me back to that time, but I think a much more poignant
- image is the next one. This shows you a view in the mission control room, and it's this graph
- here that I want you to look at. As long as there was a sharp spike in that graph, it was
- telling us that the spacecraft was talking to the Earth. When that spike disappeared, it meant the
- spacecraft wasn't talking to the Earth anymore, and what happened was that spike disappeared and
- then it came back again briefly, because no one had told the spacecraft it was about to burn up
- in the atmosphere, and the spacecraft had been told, its computer told it, if you lose the Earth,
- you've got to move around so that you can find it, and so it desperately tried to talk to the Earth
- again, and then it disappeared. So let me show you this movie, which was made by NASA nine months
- before the end of the mission and everything that they said was going to happen happened.
- There was a spectacular end to the mission. Let me stop talking and play the movie. Nope.
- Sorry. I'll put my glasses on. Let's try again. It worked earlier when I practised. Here we go.
- A lone explorer on a mission to reveal the grandeur of Saturn, its rings and moons. After
- 20 years in space, NASA's Cassini spacecraft is running out of fuel, and so to protect moons of
- Saturn that could have conditions suitable for life, a spectacular end has been planned for
- this long lived traveller from Earth.
- of the Cassini spacecraft on a billion mile trek to Saturn. We have cleared the tower.
- In 2004, following a seven year journey through the Solar system,
- Cassini arrived at Saturn.
- The spacecraft carried a passenger, the European Huygens probe, the first human made object to land
- on a world in the distant outer Solar System.
- For over a decade, Cassini has shared the wonders of Saturn and its family of icy moons, taking us
- to astounding worlds where methane rivers run to a methane sea, where jets of ice and gas are
- blasting material into space from a liquid water ocean that might harbour the ingredients for life,
- and Saturn, a giant world ruled by raging storms and delicate harmonies of gravity. Now Cassini has
- one last daring assignment. Cassini's grand finale is a brand new adventure, 22 dives
- through the space between Saturn and its rings. As it repeatedly braves this unexplored region,
- Cassini seeks new insights about the origins of the rings and the nature of the planet's
- interior closer to Saturn than ever before. On the final orbit, Cassini will plunge into Saturn,
- fighting to keep its antenna pointed at Earth as it transmits its farewell. In the skies of Saturn,
- the journey ends, as Cassini becomes part of the planet itself.
- I'll stop there. Thank you.
- that was a fabulous lecture. Thank you very much indeed. I think you've
- kindly agreed to answer a few questions, if we have them. So if you have questions,
- please raise your hands. We have some folks with microphones so we will direct them towards you.
- So if you put your put your hands up, there we go. I can see in the third row, we have a hand up.
- Hi. Thank you. That was a really fascinating lecture. Thank you so much. My question is,
- as you mentioned, the Huygens probe obviously landed on Titan. I wondered
- how it was that you could justify not accidentally contaminating with…
- I primed you for that question, didn't I?
- So we put it through a planetary protection programme beforehand. So a simple way of
- describing it as you zap it with X-rays just before you launch to kill it of any bugs,
- or to kill any bugs that might be on it. It's really expensive to do that, though,
- and so there are different bodies in the solar system that are designated as places where you
- shouldn't take an unzapped spacecraft, and Titan is one of them. So that's how we were able to. But
- thank you for asking the question.
- Other questions? Yes, on the right here again, I think hands in the air.
- Hi, and thank you for the lecture. It was really interesting. You mentioned when you were talking
- about the materials coming out of Enceladus, that one of the big ingredients for it to turn
- into life would be time, but that Enceladus was showing signs of being quite young. Do
- we know how old it is or if it's had the sort of requisite time period for that to make?
- We don't know whether it has, but when I said Enceladus was young,
- its surface looks young. So Enceladus has been there as long as the other moons and Saturn have.
- They all formed at the same time but the question for us was, why does the surface look as young
- as it does, and what we think is happening is as the water vapour plume is coming out,
- some of the material is falling back onto the surface and that's resurfacing it in
- that sense, but no, we don't know what the duration needs to be for life to form.
- Thanks very much, and I think at the back, do you have some questions? There we go.
- We have a question from Alex, who's watching the live stream in the Netherlands. Does the activity
- on Enceladus show seasonal differences, like what you told us about on Titan?
- It's a very good question, Alex. It doesn't show seasonal differences,
- but it shows orbital differences and so depending on where Enceladus is in its orbit around Saturn,
- the orbit is not perfectly circular, it's slightly elliptical, and so on. Some places on the orbit,
- it's closer to Saturn than on others, and so the gravitational force of Saturn is
- stronger in some places than others, and that's known as tidal forces. Some parts of the orbit,
- when it's closer to Saturn, a lot more activity is taking place, and so the plumes,
- there are always plumes there, but there are different sources for plumes and so
- the activity changes in the orbit, but we haven't seen any seasonal effects.
- Thanks very much. I think there's another question at the back.
- Hi. If you were about to bet for one of the moons of Saturn or Jupiter for existence of
- life, where would you put your money?
- I would say Europa probably because we think its silicate mantle is in contact with the
- liquid water, but I'm now backtracking on that, Europa is in a very radiation high environment,
- so bacteria might not survive long on the surface. So as you can see, I'm hedging my bets.
- There's a question down here, person in the stripy top.
- Can you elaborate on the secondary dynamo you mentioned at Saturn?
- So yes, this was a real surprise. So there is a region quite close to the surface and when I talk
- about the surface at Saturn, there isn't a solid surface because it's a gas giant, where there are
- eddies of currents flowing and when electrical currents flow, they generate a magnetic field,
- and so there's this secondary layer where these eddy currents are flowing and that is enough to
- mask some of the effect from the interior. One of the things we thought of before we realised
- this was happening is we thought that maybe the dynamo at Saturn was decaying, maybe it
- was dying, because as it dies, it becomes more symmetric and so the dipole tilt will decrease,
- but it's the secondary dynamo.
- Don't know.
- question again in the front row.
- talk. Really emotional at the end. I can imagine what you must be feeling.
- The hairs on my arm still stand up on end and it's almost ten years ago.
- Can I just ask, data from the James Webb scope, is that going to influence how the
- JUICE missions and others are going to be taken, and are you taking that into
- consideration right now or are you waiting?
- James Webb Telescope focusing on Europa, where we think water vapour plumes might be happening
- on Europa, and I think it was JWST that showed an image. So the plan is that we very much hope
- that people will be asking for observing time on James Webb when JUICE and Clipper are there,
- so that we will be able to almost have three point measurements. So having Earth based or
- James Webb Telescope observations on top of the in-situ observations will make the science we get
- even better. So there needs to be coordination.
- Yes, a microphone will come to you.
- Here we go.
- When the thing went on to the moon, you know how the parachute deployed and when it showed,
- it seemed quite small in the picture and the parachute seemed quite big. I was wondering,
- I know you would kind of fold it into the probe that landed on the moon,
- and you know how it landed, I thought it might have a camera. So how could they both fit?
- It did have a camera. I'm going to step away from the mic. It was about that wide in diameter. I
- don't know how many instruments were there on Huygens. Maybe six or something. There
- was a camera because the thing that you saw in that final movie was the part of the video came
- from the camera, but you're right, they had to fold the parachute away. This was the first time
- anything has landed on a moon in the outer solar system, so we didn't know if it was going to work,
- but luckily it did. Well, not luckily. It was planned to work. I don't want to take away from
- ESA! One final question.
- Sorry, I shouldn't be chairing this, should I?
- It's great.
- Could I just ask a little bit more about extending the snow line? I thought the fact
- that you're extending this habitable zone out to Jupiter and Saturn, but I was wondering,
- how far can you go? Can you go out to Uranus, Neptune, and is there a more
- general relationship that you can come up with?
- last week where they were talking about a mission to Uranus to focus also on some of the moons where
- they want to make measurements to see whether, again, there is a liquid water ocean under the
- surface. I must confess, I switched off halfway through when I realised they'd get there in 2050,
- but there is the potential for the moons of Neptune and Uranus to also have liquid water
- under the surface, but it's probably way beyond our lifetimes to be able to work that one out.
- Just as well we have young people in the audience.
- Absolutely.
- be doing the data for us on Uranus, okay?
- questions. Thank you again to Michele for a fabulous talk.
- So we're not quite yet done. I think we have one final piece of business still to finish
- this evening, and that's to note formally the achievement that we are celebrating
- this evening and that is that, you know, as we have heard, the Bakerian Medal and Lecture 2024
- is awarded to Professor Michele Dougherty for her scientific leadership of the Cassini magnetic
- field instrument at Saturn, seminal research findings on potential life support on Enceladus,
- and leadership of forthcoming missions to probe Jupiter's icy moons. Congratulations.
- I have some presentations to do and I think pictures will happen. Many congratulations.
- That concludes the formal part of the evening, but I think we can continue to again congratulate
- Michele and ask any questions that you're still harbouring. If you want to make your way to
- the back of the room, I think there are some refreshments to be had. Thank you very much.
Join us for the Royal Society Bakerian Prize Lecture 2024 given by Professor Michele Dougherty CBE FRS.
One of the most important realisations that planetary scientists have come to in the last 30 years is that in the search for potential habitability in our solar system, the focus need not only be on planets close to the Sun, where water on the surface is in liquid form. Based on observations from instruments on the GALILEO spacecraft at Jupiter and the CASSINI spacecraft at Saturn, there are many potential places in our solar system where liquid water oceans may exist below the surface.
In this lecture, Professor Michele Dougherty will discuss discoveries made by CASSINI scientists, as well as future discoveries waiting to be made at Jupiter’s moons with the European Space Agency mission JUICE. The JUICE mission was successfully launched from Kourou in French Guiana in April 2023.
The JUICE spacecraft will spend at least three years making detailed observations of the giant planet Jupiter and three of its largest moons, Ganymede, Callisto and Europa, which all show hints of hosting liquid water oceans beneath their crusts. On Earth, life thrives in the deepest, darkest parts of our oceans near hydrothermal vents. Could life similarly evolve or survive in the ocean floors of these moons?
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