"Clipper" and "Juice" are to explore the four moons of Jupiter

More than 400 years ago, Galileo Galilei discovered four moons orbiting the planet Jupiter with his telescope. That was a scientific sensation at the time and astronomers are still very interested in the “Galilean moons” Io, Europa, Ganymede and Callisto.

Jupiter's moon Europa in particular fires the imagination of astronomers. Enormous fountains of water vapor and fine ice particles shoot about 200 kilometers into space from the surface of the 3,122-kilometer celestial body. This is suggested by images from the Hubble Space Telescope.

Researchers therefore suspect an ocean of liquid water beneath Europa's ice sheet. From an earthly point of view, this in turn would be an important prerequisite for the existence of life. And so one can speculate, until proven otherwise, that there could be forms of life on the moon Europa, within our planetary system.

Last year, 2022, the NASA research probe "Juno" flew past Europe at a distance of only 352 kilometers. This provided the opportunity for some snapshots of the planet's surface.

However, there was only a time window of two hours available for this, because the probe raced past Europa at a speed of around 24 kilometers per second.

One can hope for more information from the NASA research probe "Clipper", which will set off for Europe in October 2024. With a special radar, the probe should be able to look under the ice shell of the moon and possibly detect liquid water there.

As early as this year, the European space agency Esa wants to launch the research probe “Juice” (“JUpiter ICy moon Explorer”) with an Ariane 5 rocket towards Jupiter. Among other things, she is to explore the planet Europa in more detail there.

The launch window for "Juice" is scheduled for April 5-25, 2023. With ten measuring instruments, the probe is to explore the planet more closely - in particular the spectacular geysers and the chemical composition of its surface. The researchers also hope to be able to use radar to determine the thickness of the ice sheet for the first time.

However, “Juice” will not only target Europe. He should also explore the other "Galilean moons". In fact, Ganymede is the ultimate destination for the probe, as it is scheduled to enter orbit around this moon of Jupiter. But before that, she can also explore Europe and Callisto during several flybys. A total of 35 flybys are planned. However, the moon Io will only be able to view the probe from a greater distance.

With a diameter of 5262 kilometers, Ganymede is the largest moon in the solar system. And another superlative: no other moon has such a pronounced magnetic field of its own. It is probably caused by a liquid iron core.

Ganymede's surface is also covered by a thick layer of ice. So there is similar speculation as in Europa that a subsurface ocean could harbor life forms.

Researchers can easily explain that liquid water can still exist inside moons far away from the sun, which is covered by frosty ice: The enormous gravity of the giant planet Jupiter creates strong tidal friction inside the moons. The resulting heat can be enough to bring temperatures above the freezing point of water.

What is also exciting about Ganymede is the existence of oxygen in its thin atmosphere, which the Hubble space telescope was able to prove. This oxygen is probably created when water ice is split into oxygen and hydrogen by sunlight.

While the volatile hydrogen can escape into space, the more massive oxygen can be held in place by Ganymede's gravity. All in all, Ganymede is also an exciting research object.

Of the "Galilean moons", Callisto is the farthest from Jupiter, so the effect of tidal friction is weakest here. Its ice crust is therefore likely to be significantly thicker than that of Europa or Ganymede and therefore the probability of being able to discover traces of life there is correspondingly lower.

In contrast, Io's tidal forces are particularly strong, leading researchers to assume that it contains a lot of molten rock in its interior. In any case, the strong volcanic activity on its surface is visible.

In this respect, Io is surpassed by no other celestial body in the solar system. Even if Io is a pretty safe candidate for life, the researchers are still hoping for new insights into volcanism.