Europe’s mission to Jupiter
The European Space Agency (ESA) is preparing for its first-ever mission to Jupiter’s moons. All About Space speaks to the mission’s project scientist, Olivier Witasse, to find out why the Galilean satellites are so appealing
Olivier Witasse, project scientist of JUICE, reveals why the ESA's spacecraft will provide the best views of the Jovian moons yet
How have the moons that JUICE will be visiting – Ganymede, Callisto and Europa – sustained a subsurface ocean despite being so far away from the Sun? Why haven’t they frozen over?
They are called ‘icy moons’ because they are made of frozen ice. But there is a mechanism inside the moons which generates heat, and then the ice can melt and become liquid. The question is, how is the heat generated inside?
Here we have two possible reasons. One answer is that because the moons are actually large, there are – like on Earth – some radioactive elements. Inside Earth, for example, there is heat generation coming from the interior from radioactive elements. But the other process, which is quite interesting, is that because the moons are relatively close to the very big planet Jupiter, and because of the characteristics of their orbits, there is a gravitational struggle. In fact, these moons don’t have the shape of a normal ball – like a football – they are more like a rugby ball. They change shape because of the gravitational attraction between the moon and Jupiter. That makes the moon change shape and keeps the interior moving. That also generates some heat, and this heat allows the ice to melt and become liquid.
How were these subsurface oceans discovered?
The discovery of subsurface liquid water was one of the greatest discoveries in planetary science, which was made about 20 years ago by NASA’s Galileo mission. The mission went around Jupiter, and the spacecraft needed a few flybys around the icy moons and Io. With its magnetometer it discovered that there is liquid water inside Europa, Ganymede and possibly Callisto.
That was a really great discovery because the measurements were not that obvious, because, you can imagine, we can’t see it. It’s inside the moon. It is done with the magnetic field. This liquid water is not pure water; it’s salty water. It can change the magnetic field around the moon. And because the magnetic field around Jupiter is greater it changes the magnetic field a bit, and we could measure this change with the magnetometer.
The measurement indicated that there is liquid water inside Europa. There is liquid water inside Ganymede, the biggest moon of Jupiter. And for the last icy moon, which is Callisto, there was an indication that there was liquid water, but the measurement was not so easy to interpret.
Why was JUICE chosen as the first Large-class mission for the ESA’s Cosmic Vision program?
The selection took place in 2012, and for any selection of missions, whether it’s NASA or the
ESA or any other space agency, it’s always a very complex process. There is the call for ideas for missions. Then there are some studies. Then the ESA selects the mission based on some committees, which give a recommendation, and then they make some evaluations. All in all there are a lot of discussions that address all these questions, and at the end there is a committee in the ESA which decides what is best, given all these topics.
In 2012 there were three missions in competition. JUICE was one of them. The two others were a mission to study gravitational waves, which is now called LISA [Laser Interferometer Space Antenna], and the third mission was ATHENA [Advanced Telescope for High-ENergy Astrophysics], which was an X-ray observatory.
These three missions were in competition, and after discussing the merits of each of them, JUICE appeared to be the best one. From what we see now, to me it looks like an obvious choice, because if you see the LISA mission on gravitational waves, it’s very complex from the technology point of view. You need three spacecraft with which to do laser interferometry in space. It’s very complicated. ATHENA poses a lot of challenges, and we see now that those missions are not ready to launch in 2022.
You’ve worked on many planetary science missions, such as the Cassini-Huygens mission, Venus Express, Mars Express and more. Based on your experience, how does the JUICE mission rank in terms of originality?
All missions are unique and interesting. Every mission is selected based on some science questions that are interesting at the time of their proposal and selection. All the previous missions on which I’ve worked are quite interesting, and they deliver a lot of amazing results. In the case of JUICE, what is very interesting is that for Europe, it’s the first time we will go to Jupiter. It’s a really challenging mission. We don’t go to Jupiter all the time. This kind of mission is done every 30 years or so. That’s why when we manage and develop such a mission we try to address all the possible science questions at Jupiter. In terms of science return, interdisciplinary science, it’s really great because we address all kinds of questions. We’re going to study the interior of the moons, their surfaces, their atmospheres, the link between the moons and Jupiter. We’ll study Jupiter and the dust around Jupiter and the question of habitability.
The most interesting question that JUICE will address is the question of why we have this liquid water inside these moons of Jupiter. That’s a question that we would like to understand better, because a very important objective of planetary science and astrobiology is to find possible places for life besides Earth. This is because we’d like to know if life can develop somewhere else besides Earth. That’s the big question in science.
There are going to be ten scientific instruments on this spacecraft. Would you be able to single out two or three that you think will yield some exciting results?
As I said, we don’t go to Jupiter every year, so once we reach such a mission we try to put the best instruments on the spacecraft. When the ten instruments were selected we tried to select the best possible payload for such an ambitious mission. In this case the payload was selected so that all the instruments could complement each other.
If you want me to select two or three, I would say the camera, JANUS, will be very interesting because it will deliver fantastic images not only of Jupiter, but of the surface of the moons at high resolution. It can image the surface better than ten metres (33
feet) per pixel, which means someone will really know what’s going on on the surface, and it’s always nice for the general public and the geologists and scientists in general.
Because we discussed the internal subsurface ocean, we have the magnetometer [J-MAG]. With the magnetometer we will detect the tiny variations of the magnetic field due to this ocean and we’ll be able to – certainly in the case of Ganymede – know the depth of this ocean. We don’t know if the ocean is ten kilometres [6.2 miles], 40 kilometres [24.8 miles] or even 50 kilometres
[31 miles] deep, and we want to know the amount of liquid water. The magnetometer will give us this information, and information on the composition.
Another instrument which is also quite interesting is the laser altimeter, GALA. We will send lasers to the face of Ganymede and understand the little variations in altitude of the surface. This can help identify mountains or canyons or so on, but also it will see the variation of the change in the shape of the moon.
How much water is estimated to be in these icy moons compared to the amount on Earth?
What is amazing, surprising and fascinating is that we think there is more liquid water inside each moon than on Earth. We know the depth of our ocean, approximately ten kilometres [6.2 miles] maximum. But inside the icy moons the oceans are believed to be very big. They could be 50, 100 or even 200 kilometres [30, 60 or 125 miles] deep. That’s much more than the little ten kilometres from the surface of Earth.
For example, at Europa it’s possible there is twice the amount of liquid compared to Earth, and for Ganymede maybe five times.
What does this mission mean for our understanding of how the Solar System formed, and do you think this will completely change our definition of what a habitable world is?
First we are going to collect images and all the data in order to understand the system as it is now. Then, based on that, as a group of scientists we can do what we like to call the ‘big picture’. We collect all the data and then extrapolate it through time, do some modelling, think about it and come up with some theories. Then we can get information on the origin of the moons.
It’s very difficult to know what we can get out of the JUICE data, but I’m pretty sure that when we have the data from all the systems there will be an interesting outcome in terms of moon formation and origin.
As for the habitability, that will depend firstly on confirming that they have liquid water. Once we have confirmed that there is liquid water, then with the data set we are going to characterise these oceans. For example, how deep are the oceans?
What are the compositions? Where are they under the surface? Are they very close to the surface, or are they very deep inside the moons? All this information will be fed into the study of habitability, because the habitability of the ocean will depend on all these different factors.
The Cassini mission famously went through the water plumes found to be erupting from the surface of Saturn’s moon Enceladus to study them. Is this something that you plan to do with the JUICE spacecraft?
Regarding water plumes in the Jupiter system, at the moment we have a hint that they possibly occur at Europa. It’s not yet 100 per cent sure for a few reasons. The detection of those plumes is a bit complicated. Although in the case of Enceladus and the Cassini data, the plume is very clear. It was imaged. It was detected in all the data.
In the case of Europa, it is not yet clear. And for the other moons we have not detected anything. But with JUICE we are going to perform two close flybys at about 400 kilometres (250 feet) from the surface. Depending on the plume, we might go through the plume or not. But if JUICE does go through a plume, it’s not a big problem because the plume is not something that can damage the spacecraft. I mean it will likely be a liquid vapour, so there will be no harm to the spacecraft. The problem would be that we’d need to adapt the trajectory to go through this plume, and that’s a bigger challenge. This is because the trajectory of JUICE is constrained by many, many topics. But if we can, we will do it.
This year marks the 410th anniversary of
Galileo Galilei discovering the four ‘Galilean moons’ of Jupiter. In this time we’ve gone from seeing the moons through a telescope to actually planning a mission to go there. What do you think will be the future of the exploration of these moons?
You never know what the future will bring, but I think the next step after JUICE, depending on the results, of course, I think the next mission to Jupiter and to its moons will be a mission which lands on the surface of either Europa, Callisto or Ganymede. I think that could be the next step.
And if there is good information on the liquid water inside, what I would foresee is some element that would dive into the crust to reach the liquid water and explore it with a submarine, or something like that. I think that in the next 50 years we will see this kind of mission.
And then you’ve got technology advancing quite fast, so we are going to have to wait for ten more years to see the next human on another surface, which seems quite like science fiction. After the Moon, and after Mars, the next logical step will be to send humans to the surface of those moons. Maybe 100, 200 or 300 years from now, that could be the future.
“WHEN WE MANAGE AND DEVELOP SUCH A MISSION WE TRY TO ADDRESS ALL THE POSSIBLE SCIENCE QUESTIONS AT JUPITER”