In early 1610, using his refracting telescope, Italian astronomer and physicist Galileo Galilei detected what he believed were three, then four, stars surrounding the planet Jupiter.

Within a week, he realized they weren’t stars but moons orbiting Jupiter. 

His discovery challenged the reigning belief—vehemently upheld by the Catholic Church—that the Earth was the center of the universe. In fact, Galileo’s moons supported the Copernican theory that the Sun is the center of the universe, and that our planets revolve around the Sun.

Three of those four moons—Ganymede, Europa and Callisto—are at the heart of a recent European Space Agency mission to Jupiter launched in mid-April. Along with studying Jupiter’s atmosphere and magnetosphere, the primary goal is to study the trio of moons and the water thought to be lying beneath their icy crusts, with a special focus on Ganymede, the largest moon in our solar system.

In 2015, NASA’s Hubble Space Telescope detected evidence of an underground saltwater ocean on Ganymede, leading to the possibility of life beyond Earth.

The JUpiter ICy Moons Explorer, aka Juice, will take eight years to arrive within Jupiter’s orbit; NASA's own “Europa Clipper” mission to Jupiter is expected to launch in October 2024. NASA's journey has a different mission: it will explore Jupiter's moon Europa to investigate whether that moon could harbor conditions suitable for life. 

We asked physics Professor Phil Kesten for his take on the newest journey to the largest and most massive planet in our solar system. And with 92 moons, Jupiter can also boast having the most moons in our planetary system.

Scientists are hoping the Juice mission might detect a form of life existing in the salt water under the surface of Ganymede. Where does the evidence of salt water come from? 

Ganymede is the only moon in our solar system known to have its own magnetic field, and this field interacts with the magnetic field of the Sun. Observations of that interaction suggest that there is something else that also affects the fields. Our models tell us that that “something else” must be a salt water ocean beneath Ganymede’s icy surface. Also, non-salt water would affect the electromagnetic field surrounding Jupiter and Ganymede in a way different from what we observe. The presence of this magnetic field means that Ganymede has a molten core, in which ions swirl around as the moon rotates. So Ganymede is not a solid rock. And why? It’s hot, hot, hot deep inside the Moon. And “hot” means there’s energy. And “energy” is necessary for life to form!

What’s the significance of a salt water ocean? 

Salt water plays an important role because sodium chloride can break down other minerals into nutrients, and salt played a key role in how we believe life formed on Earth.

If there is a form of life in the salt water on Ganymede, scientists say it would have to include carbon. Why carbon?

We believe that carbon is key to the formation of life. Carbon atoms have four electrons in their outermost atomic “shells,” which means carbon atoms can form bonds with four other atoms. So carbon is at the heart of CH4, for example, which is  methane. Carbon is at the heart of sugar molecules and at the heart of DNA. Oh, and these carbon-based molecules hold a large amount of stored energy. All of this points to…life!

Mars is more Earth-like than any other planet in our system, one of the reasons Elon Musk is hoping to build a city there by 2050. Could that happen on Jupiter, or one of its moons?

We can’t live on Jupiter because Jupiter is a “gas giant” planet—no hard surface (the gas just gets denser and denser as you go down!)  Could we live on one of Jupiter’s moons? Maybe. We might be able, for example, to find a supply of water or water ice, which we could melt, and that would be important. But Mars is closer, and has a mean temperature more amenable for a life-sustaining habitat. So I would imagine we would aim for humans living on Mars before we would go to one of Jupiter’s moons.

The problem with Mars is its thin atmosphere, and there’s not much water there. And it’s so cold we’d either need to live underground or wear space suits above ground. There’s a lot of infrastructure that would need to be built for us to survive there. 

Jupiter is 547 million miles away from Earth, but by the time the Juice mission reaches Jupiter in July 2031, it will have traveled 4 billion miles. Other missions have made the trip to Jupiter more quickly. Even NASA’s Europa Clipper mission to Jupiter, which launches in October 2024, will only take 5.5 years to get to the gas giant planet, arriving before Juice does. What gives?

The key is in the path taken to get from Earth to Jupiter. Because both are moving—in orbits around the Sun—you can’t take a straight-line path. Which means that it matters where Earth is and where Jupiter is when the mission is launched. 

Juice was launched by the European Space Agency, Europe’s counterpart to NASA. Why should Americans care about this mission?

We should care about it because it’s great science. There's a long history of the various countries in Europe and the United States doing science together. In the field I started in, high energy particle physics, there are basically two organizations that operate the most prestigious particle physics labs in the world: Fermilab outside of Chicago, and CERN, in Geneva. Lots of European groups and American groups collaborate on both experiments. That’s true throughout the history of big science. People aren’t thinking about nationalities; they’re thinking about the cool science they can do.

You talk about cool science that could take place on this mission. In your opinion, what would be the coolest discovery?

I hold out hope that we’ll find evidence for life.  When you think about it, sure, we have lots of examples of life on our planet, and it’s certainly not all like the mammals and plants we see. But it’s all, nevertheless, Earth-based life. Finding life that did not form on Earth could help us understand how life forms in ways that we could not otherwise even guess.