«No living being on Earth can cross the 100 km of ice on Ganymede»

Frenchman Nicolas Altobelli is responsible for the development of science activities for Juice, the ambitious mission of the European Space Agency (ESA) to Jupiter and its three large icy moons: Ganymede, Callisto and Europa. The 6-tonne probe is expected to depart on April 13 from French Guiana and will reach its destination eight years later. There he will stay another four years to find out, with the help of his ten scientific instruments, if these worlds, which seem to house great oceans in their interior, can be habitable, and why they are so different from each other. In his journey, Juice it will become the first spacecraft to orbit a moon other than our own, Ganymede. Altobelli, who works at the ESA headquarters in Villanueva de la Cañada (Madrid), will follow the launch from the Kourou spaceport in French Guiana. Count the days.

-Why are you so sure that Ganymede, Callisto and Europa have oceans?

-The conditions of pressure and temperature inside the moons allow liquid water. Furthermore, from the Galileo probe measurements we know that Jupiter’s enormous magnetic field changes when you get close to one of its moons. For that, an electricity conductor is needed inside the icy moon, and the only possible one is salt in liquid water. On Ganymede it is more complicated, because it has its own magnetic field. Another reason we know there is liquid water is what the Cassini spacecraft has seen on Enceladus, an icy moon of Saturn no larger than Spain. There is liquid water below the surface, and the vapor escapes into space. It occurs by tidal forces exerted by the giant planet’s gravity. The orbit is eccentric, it is not a perfect circle, so the change in tides generates forces and friction that deforms the outer layers of ice. This generates a lot of heat that allows the water to stay liquid. This mechanism demonstrated on Enceladus also works on Europa and a bit on Ganymede, where we also have the heat of naturally occurring radioactive elements.

-How do you think those oceans are?

It’s what we have to find out. But models predict that those on Ganymede are at least 100 km below the surface and may be 100 to 150 km deep. That’s much more than Earth’s oceans. Because the Earth seems blue, an aquatic world: 75% of the surface is covered with water… But it is only 4,000 meters deep. By comparison, a moon the size of Mercury with oceans 100 km deep is an enormous amount of water.

-What is Juice going to look for there?

-We have three icy moons: Europa, Callisto and Ganymede. What is surprising is that they are completely different. For example, Callisto has much more ice than Europa, which is made of rock with some water. Why are they different even though they are together in the same area of ​​the solar system? Getting to know you is our main objective. It will help us understand other worlds that may harbor favorable conditions for life as we know it.

-Juice is not prepared to detect life.

No, and it’s important to say. There are no instruments on board that can detect life. It is not easy to get it. On Mars, many missions are necessary for that objective. In the icy moons we are in the stage of detailed characterization. Only following Juice might we know if there is a way to attempt a mission with specific instruments to search for life, or to search for conditions favorable to the emergence or evolution of life. The emergence of life does not need the same environment as its maintenance. Now life does not appear on Earth, there is too much oxygen. Ancient life on Earth developed without oxygen: single-celled bacteria that use carbon dioxide and hydrogen under special conditions. You don’t have to look for a world with oxygen in an atmosphere, that is the environment that complex organisms have made for ourselves. What we are looking for are conditions for the emergence of life as we suspect it occurred on Earth.

-But if there were microscopic life, not to fantasize regarding something more complex, wouldn’t Juice see it?

-What you might see, but in a very hypothetical way, are signs of metabolism. The Viking probe on Mars carried an instrument on board for this, to find out if there are products of life, bacteria or other organisms that are transforming the chemical elements of the environment into other chemical elements. It would take a lot of experiments in laboratories on Earth to figure out what kind of metabolism might give a signal that we might measure with the probe, and for this there is now no answer. Exceptional news requires exceptional evidence.

-If there are indications that these worlds may be habitable, what is the next step?

-We put ourselves in 2035… The first step would be a lander, but it is very complicated. It is very difficult to land on the surface of Europa because of the enormous radiation.

-The only moon that Juice will orbit is Ganymede. Is it more interesting than the others?

-No, the most interesting thing is to compare the moons. There are other parameters, such as the total dose of radiation that we are going to have during the mission. One goal is to have the complete tomography of the interior of a moon. And Ganymede, in its evolution, is a bit between Callisto and Europa. There is a young surface and a very old one. There is much to discover there.

-Juice will end up dying on Ganymede, might we contaminate the moon with an organism from Earth?

-It is another reason why we have chosen Ganymede. There is no known mechanism that allows an organism on Earth to traverse 100 km of ice. The probability of contamination is almost zero, according to the COSPAR (Commission for Space Research) rules that have evaluated the mission. In Europe, it would be different, it’s only 10 km.