The Revolution of the Four Galilean Moons

The successful five-year study of Jupiter (2016-2021) by the JUNO space probe had already yielded a treasure trove of new information about the largest planet in our Solar System. Therefore, NASA decided to extend the mission for another 5 years (until 2026) to study three of its four Galilean moons.  

It was the initial observations of these moons that set a revolution in motion. 

The following link is a video NASA put out about Junos’ approach to those moons: Watch Juno approach Jupiter and the Galilean moons.

Since 1973 there have been a good number of missions to study Jupiter and its moons (see the following link for more details: List of missions to the outer planets).

To start with, note the table below, outlining the major properties of these moons in comparison to our own.

Like our moon, all the Galilean moons are tidally locked; meaning they always show the same side to Jupiter. 

The table reveals that despite the enormous difference between the Earth and Jupiter, the Galilean moons seem to possess geophysical properties not that dissimilar from our own moon.

One other remarkable feature of the three closest moons is that they possess gravitationally generated orbital resonance. For every one orbit of Ganymede, Europa does two and Io does four. Callisto does not share in this resonance. It possibly drifted out of resonance early on or simply was never as gravitationally affected as the closer moons.

Here are highlights of some of the most recent significant findings uncovered by Juno.

Poor Io is the most stressed of all Solar System moons. Its torment is created by opposing gravitational forces from Jupiter, Europa, and Ganymede. This tidal stretching/squeezing generates enormous heat leading to the creation of the largest volcanoes in the solar system (over 400). The dust from these volcanoes becomes so ionized that it doubles the reach of Jupiter’s powerful magnetic field (science.nasa.gov).

Juno’s flyby detected an enormous volcanic eruption in Io’s southern hemisphere; unquestionably, the largest volcano in the solar system. It covers an area of more than 40,000 square miles (think Lake Superior). The energy released by this one spot exceeds that of the combined energy of all the power plants on Earth (jpl.nasa.gov, 2025).

The sailor in Coleridge’s poem, The Rime of the Ancient Mariner, might feel right at home on Europa (as well as Ganymede and Saturn’s Enceladus). It possesses an underground salt water ocean (40-100 miles deep), double that of the Earth. On it, rests a 15-20 mile thick icy crust. Here is a theoretical schematic of Europa’s interior structure:

Juno discovered that this crust is not static.The  geological deformations in the cracks in Europa’s icy surface indicate:

1. The crust is decoupled from the rocky interior. It floats on the liquid ocean. This allows the crust to exhibit “polar wander”.
2. The plumes that emerge from cracks in the ices demonstrate that liquid water and brine can reach the surface: evidence of an active ocean (Jet Propulsion Laboratory, 2024).

NASA’s Europa Clipper is planned for launch later this decade to further explore the possibility of extraterrestrial life on this moon.

The moon has several firsts to its name. It is the largest moon in the solar system ((NO…Saturn’s Titan is not!). Its 100 mile deep crust hides an ocean twice as large as Europa’s, though currently is considered a less likely candidate for extraterrestrial life. The irregular pattern of dark and light areas continues to baffle astronomers. Most significantly, it is the only moon in our solar system to possess its own magnetic field. This is most probably generated by its liquid metallic core.

The singular Juno discovery concerns the “reconnection” of two magnetic fields. The intersection of the two fields generates energy that boosts electron activity. Understanding this process helps scientists better understand how both the moon and planets generate their own magnetic fields. (See figure below).

Furthermore Juno found that the highly complex magnetospheric interaction produces auroras around the northern and southern poles of Jupiter. (See the figure below) These auroras arise directly from the generation of highly charged particles produced from this interaction (Hue et al., 2022).

Unfortunately Juno was not programmed to study Callisto. Previous missions have established that it is the most cratered object in the solar system.This would also indicate that it is the oldest Galilean moon. Furthermore, lack of surface fractures indicates minimal tectonic activity. Because it is so stable with a mostly homogenous surface and because it is beyond Jupiter’s dangerous radioactivity, it is also being considered as a potential spaceport for future space explorations (science.nasa.gov). To study these matters further, both ESA’s JUICE and NASA Europa Clipper are planned for multiple flybys in the next 6-9 years.

Of course, these explorations raise more questions than answers. The rich tapestry of features in these four Galilean moons continues to revolutionize our understanding of how our Solar System came to be and our own place in it.