On 13 April 2023, ESA launched JUICE, the Jupiter Icy Moons Explorer. It is currently on its way to Jupiter. But what is so special about Jupiter, and what have the Icy Moons to do with it? Let’s take a look at Jupiter and the Galilean moons…
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What do we know about Jupiter?
Jupiter is the fifth planet from the sun. It is by far the largest planet in our solar system. In fact, it is more than twice as massive as all other planets combined.
It formed about 4.5 billion years ago from the same ingredients that make up our sun. But it didn’t grow massive enough to ignite, so it became a gas giant.
As a gas giant, it has no surface. Actually, we still don’t know if it has a solid core at all.
We know, however, that its atmosphere is made up mostly of hydrogen and helium and likely has three layers of clouds. However, those clouds are made up of ammonia and water.
Going deeper into the atmosphere, pressure and temperature increase, so that at some point the hydrogen is compressed so much, that it becomes a liquid. Fun fact, this gives Jupiter the largest ocean in the solar system.
The stripes and swirls we see in images of the planet are actually the clouds moving in jet streams and storms through the atmosphere. Storms can be quite durable and stay for centuries. Probably the most famous storm is the Great Red Spot.
The jet streams and storms are fueled by the planet’s fast rotation. One rotation – so a Jovian day – takes only 10 hours.
A Jovian year, however, is 12 Earth years long, as that is the time it takes Jupiter to complete one orbit around the sun.
This is because it is 5.2 Astronomical Units (AU) away from the sun. As one AU is the distance between the Sun and the Earth, this means Jupiter is 5.2 times farther from the Sun than Earth.
Just to get a sense how far this is, the light from the Sun needs 43 minutes to reach Jupiter. It only takes the light 8 minutes to reach Earth.
But there are more commonalities between Earth and Jupiter. Both have a magnetosphere. However, Jupiter’s is 16 to 54 times as powerful as that of Earth. And it causes some of the solar system’s most spectacular aurorae.
Closer to the planet, the magnetic field traps charged particles and accelerates them to very high energies. These lead to intense radiation that bombards the inner moons.
Speaking of moons, Jupiter has more than 75 of them. Over 50 have already gotten names.
So, how did we learn all this about Jupiter and what more can we learn at and about Jupiter?
How did we learn it?
Jupiter itself was already known to ancient civilizations on Earth. They already knew it was different from a star. In fact, the word “planet” itself derives from the Greek word πλανήτης, meaning “wanderer”.
Both the Greeks and the Romans made it the king of the planets by naming it after their gods’ fathers – Zeus and Jupiter.
The invention of telescopes – first mentioned in 1608 – marked a milestone in studying Jupiter. These optical instruments enabled astronomers to observe the planet in greater detail, unveiling features such as its distinctive stripes and some of its moons.
As telescopes improved over the following centuries, more and better observations were made from Earth. The invention of rockets, however, made it possible to send spacecrafts directly to Jupiter.
In December 1973, Pioneer 10 became the first spacecraft to visit Jupiter, providing humanity with the initial close-up images of the planet and some of its moons. During its flyby, it also discovered Jupiter’s magnetic field and radiation belts.
A year later, in December 1974, Pioneer 11 flew past Jupiter, capturing images of the Great Red Spot and making the first observation of the polar regions. By observing Jupiter’s moon Callisto, it made it possible to calculate its mass.
Subsequently, Voyager 1 (March 1979) and Voyager 2 (July 1979) further enhanced our understanding of Jupiter. These spacecraft obtained detailed images of Jupiter’s atmosphere, revealing the majestic Great Red Spot as a storm. They also unveiled previously unknown features, such as tiny rings encircling the planet.
Moreover, Voyager 1 and Voyager 2 discovered two new moons, marking the first time a spacecraft had made such a lunar discovery. They also were the first spacecraft to observe volcanic activity on Io and detected water ice on the surface of Europa.
The Ulysses spacecraft conducted flybys of Jupiter in 1992 and 2004, focusing on the study of its magnetosphere.
In 1995, the Galileo spacecraft became the first to enter orbit around Jupiter, commencing its mission to gather valuable data about the planet, its surroundings, and its moons.
Galileo provided evidence for the presence of liquid water beneath the surfaces of some moons. In December 1995, it made history by sending a probe into Jupiter’s atmosphere. It’s the only spacecraft to have done so far.
In subsequent years, other missions continued to explore Jupiter. Cassini, during its flyby in 2000, captured more detailed images of the planet’s atmosphere. New Horizons, in its brief visit to Jupiter, made improved measurements, enriching our knowledge of Jupiter’s moons.
Finally, in July 2016, Juno successfully entered orbit around Jupiter and remains operational, collecting data on the planet’s formation and evolution. If you would like to learn more about Juno and New Horizons, take a look at my post about spacecrafts beyond Mars.
That’s quite a long list of spacecrafts, which have visited Jupiter. They have gathered a lot of information about Jupiter, but also about its moons. So, let’s take a closer look at some of them.
Learn more about the exploration of Jupiter / References:
What are the Galilean moons of Jupiter?
I already mentioned them a couple of times before, so let’s take a look at the Galilean moons.
The Galilean moons are the four largest of moons of Jupiter. They are named Galilean moons after their discoverer Galileo Galilei.
He discovered them between 7 January and 13 January 1610 using a self constructed telescope. He named them after his patrons, the Medicean planets. For himself, he used roman numbers.
Today we use the names given to them by Simon Marius, who discovered them around the same time as Galileo Galilei. The names – Io, Europa, Ganymede, and Callisto – were suggested to him by Johannes Kepler.
Going from the closest to the farthest, they are Io (about 422,000 kilometers), Europa (about 671,000 kilometers), Ganymede (about 1,070,000 kilometers), and Callisto (about 1,883,000 kilometers).
Three of them – Ganymede, Io, and Europa – are in what is called a resonance. Every time Ganymede completes one orbit around Jupiter, Europa completes two orbits and Io completes four.
Together, they form kind of a “mini solar system” as they formed from what was left over after the formation of Jupiter. This is similar to how the planets formed after what was left over after the formation of the Sun.
Not only that makes them an interesting target. But scientists found evidence of liquid oceans of water beneath the surface of three of the moons.
So, let’s take a closer look at each of them.
Ganymede
What do you know about Ganymede? Have you even heard its name? If you’ve read or watched, for example, The Expanse, you might have. So let’s take a closer look at it and what we know so far.
With a diameter of 5262 km, Ganymede is the largest moon in our solar system. That’s a larger diameter than the planet Mercury, which has a diameter of 4881 km. Looking at their mass, however, it’s the opposite. Mercury has a mass of 3301 · 1020 kg, while Ganymede has a mass of 1481 · 1020 kg.
Ganymede is composed of three main layers. At its center, it has a metallic iron core, surrounded by a shell of rock. Which in turn is surrounded by a shell of mostly ice. Scientists, however, found evidence, that in this layer is an ocean of liquid water – estimated to be 100 km thick.
This not only means that Ganymede’s ocean could have more water than all the water on Earth’s surface, but also, that life might have developed in this ocean. An about 150 km thick layer of ice protects the ocean from the very low surface temperatures between 90 and 160 Kelvin (-183,15 to -113,15 degree Celsius).
Above the surface, Ganymede has a faint oxygen atmosphere. However, it’s far too thin for you to breathe in.
There’s one more thing that differentiates Ganymede from the other moons, it has, like Earth and Mercury, a magnetic field. This was discovered by the Galileo spacecraft and was the first time a magnetic field around a moon was observed.
Ganymede’s day is just over seven Earth days long, the same amount of time it takes Ganymede to orbit Jupiter once.
Europa
What comes to your mind, when you here about Europa? No, not Europe the continent but Europa, the moon of Jupiter.
Europa is, by size and mass, the smallest of the four Galilean moons of Jupiter. It has a diameter of about 3121 km and a mass of 480 · 1020 kg. This is, by the way, less than the Earths Moon, which has a diameter of about 3476 km and a mass of 734 · 1020 kg.
Europa’s surface is mostly made up of frozen water ice. Visible long, linear fractures crisscross it. Scientists found evidence that beneath the 15 to 25 km thick surface, there is an ocean of salty water. Below the ocean, Europa probably has a rocky mantle and an iron core.
If the ocean exists and is about 100 km thick, as proposed, it contains twice as much water as Earth’s oceans combined. As Europa is closer to Jupiter, tidal heating might be enough to lead to volcanic or hydrothermal activity on the seafloor. This might provide the proper conditions for life, making Europa the most promising place for life beyond our planet.
Above the surface, Europa has an extremely thin oxygen atmosphere. Don’t even think about being able to breathe there.
Callisto
Next on our list of moons is Callisto. It is the second-largest moon of Jupiter, and in fact the third-largest moon in our solar system. With a diameter of 4821 km, it is only a little smaller than the planet Mercury. Comparing the masses, Callisto with its 1075 · 1020 kg is lighter than Ganymede.
We’re not very knowledgeable about the structure of Callisto, which is probably a reason to visit the moon again and find out more. We know that Callisto has a rocky, icy surface covered by craters. In fact, it is the oldest and most heavily cratered surface in our solar system.
Because plenty of old craters are still visible, Scientist believe it has little geologic activity, which would constantly change the surface.
Data collected by the Galileo spacecraft appeared to indicate Callisto has a subsurface ocean. However, newer research revealed, that this ocean probably is deeper beneath the surface or doesn’t exist at all.
Its very thin atmosphere mainly contains carbon dioxide.
Io
The last of the moons we take a look at is also the most active of the four Galilean moons – Io.
Io has a diameter of 3643 km, which is a bit less than Earth’s Moon. It has a mass of 893 · 1020 kg. This makes Io the third largest of Jupiter’s moons.
The orbit of Io is the closest to Jupiter of the four Galilean moons. Because of Europa and Ganymede, it’s an irregularly elliptical orbit.
The proximity to its planet and the pull from Europa and Ganymede also lead to tremendous tidal forces. In fact, these force cause its surface to move up and down by as much as 100 meters. They also generate a lot of heat within the moon, keeping the subsurface crust liquid.
This in turn leads to active volcanism, which constantly renews the surface. Some of those volcanoes soar up to 18 km height. They also lead to floodplains of liquid rock on the surface. In fact, Io is the most volcanically active world in the solar system.
As Io orbits close to Jupiter, it cuts the planet’s powerful magnetic lines of force. This turns the moon into an electric generator. And a powerful generator as well, as it can develop 400,000 volts across itself and create an electric current of 3 million amperes.
Because of these condition, Io has no water and an atmosphere made up of Sulfur dioxide.
Keeping this in mind, I doubt we could build a base on Io, as depicted in the sci-fi series The Expanse.
Why go to Jupiter?
As you see, we know quite a lot about Jupiter and its moons already. So, why should we send another spacecraft to study the Jovian system?
The short answer is, there is still a lot more we can learn about the moons, the planet and its surrounding environment. Let’s take a look at some examples.
Jupiter is not only an important part of our solar system, but it is also a great example for gas giants orbiting other stars in our galaxy. So, by studying how it (and its moons) formed and evolved, we can better understand gas giants elsewhere in our galaxy.
Understanding the processes that drive Jupiter’s weather, chemistry, and climate allows us to search for these processes at other gas giants and learn more about them.
Yet, there is more, as the planet and its moons are like a solar system, we might even learn how star systems for and evolve.
I previously mentioned the magnetosphere, but didn’t get into how complex the magnetic environment around Jupiter really is. In fact, understanding it remains one of the Solar System’s outstanding mysteries.
We know the Galilean moons orbit within Jupiter’s intense magnetic and radiation environment. But how do they interact with this environment, and what are the consequences?
And how does Ganymede interact with its surroundings, as it has its own magnetic field? Only two other solid bodies in the solar system generate a magnetic field like that. Can you name them? That are Mercury and Earth.
As we are talking about the moons. We still don’t know if Ganymede, Europa, and Callisto actually have subsurface oceans. Much less how big they are or how they are composed.
And finally, some Galilean moons are good candidates for having all it takes to support life. But did they ever get this far, or could they maybe support life?
You see, we still have a lot to learn about Jupiter and its moons. And therefore, ESA is sending the Jupiter Icy Moons Explorer (Juice) to Jupiter and especially its moons. But more on that another time.
Learn more about spacecrafts related to Jupiter
Do you want to learn more about active spacecrafts related to Jupiter? I briefly mentioned them above, let’s take a closer look at them.
Learn more about NASA’s Juno mission
We’ll take a brief look at NASA’s Juno mission, which is still in orbit around Jupiter.
Lucy’s visiting the Trojans
Have you heard about the Trojans? Not the people from the ancient city, but the Trojan asteroids? Find out more about them and NASA’s Lucy mission.
Or find out more about What’s happening beyond LEO?
Credit for the image at the top: NASA, ESA, A. Simon (Goddard Space Flight Center), and M.H. Wong (University of California, Berkeley)