New study about Jupiter’s Galilean moons

 Water differences set during formation of Jupiter’s Galilean moons set at birth were studied in detail

While Io, the most volcanically active moon in the solar system, appears completely dry and devoid of water ice, its neighbor Europa is thought to harbor a vast global ocean of liquid water beneath its icy crust. A new international study co-led by Aix-Marseille University and Southwest Research Institute (SwRI) reveals that this striking contrast was established at birth, as they formed around Jupiter, not from later evolutionary processes. How long did it take to establish the water content within Jupiter’s Galilean moons, Io and Europa? This is what a recent study hopes to address as a team of scientists from the US and France investigated the intricate processes responsible for the formation and evolution of Io and Europa. This study has the potential to help scientists better understand the formation and evolution of two of the most unique moons in the solar system, as Io and Europa are known as the most volcanically active body in the solar system and an ocean world estimated to contain twice the volume of Earth’s oceans, respectively.

Since the first missions exploring the Jovian system in the late 1970s, scientists have known that Jupiter’s moons exhibit markedly different characteristics. Io and Europa provide the most striking example. While Io is a dry and intensely volcanic world devoid of water, Europa is icy and thought to conceal a vast subsurface ocean of liquid water. “Io and Europa are next-door neighbors orbiting Jupiter, yet they look like they come from completely different families,” said SwRI’s Dr. Olivier Mousis, second author of an Astrophysical Journal paper detailing these findings. “Our study shows that this contrast wasn’t written over time, it was already there at birth.” For the study, the researchers used a series of models to simulate the early formation history of Io and Europa billions of years ago, which occurred when Jupiter was much brighter than today. The goal of the study was to ascertain how Io lost its water whereas Europa gained large amounts of it. The researchers suggested both moons initially had water but atmospheric escape cause Io to lose its water while Europa retained its water.

The team tested two main hypotheses to explain the differences. The first suggests that the extreme conditions prevailing close to Jupiter during satellite formation prevented water ice from being preserved, depriving Io of this component from the outset. The second hypothesis proposes that Io and Europa initially formed with similar amounts of water, but Io subsequently lost most of its volatiles over time through atmospheric escape and erosion processes. The study notes, “Despite the assumptions adopted in this work, Io was likely unable to lose its initial water inventory. After the dissipation of the accretion disk and the fading of Jupiter’s luminosity, the residual ice shell would not have been removed by tidal heating over geological timescales. This suggests that Io accreted primarily anhydrous [non-water] silicates and that the compositional contrast between the two inner moons reflects the thermodynamic structure of Jupiter’s CPD [circumplanetary disk] at the time of their formation, rather than divergent evolutionary or atmospheric loss processes.”

The international team reconstructed the earliest evolutionary stages of Io and Europa, assuming that the moons’ water originated from hydrated minerals incorporated during formation. Using an advanced numerical modeling framework, the study coupled the internal thermal evolution of the moons with volatile escape processes, accounting for all major heat sources active in the young Jovian system, including accretional heating, radioactive decay, tidal dissipation and Jupiter’s intense radiation. While Ganymede and Callisto comprise the third and fourth Galilean moons, respectively, the researchers note they were not included in this study for numerous reasons. These include higher surface gravities, colder formation from the greater distances from Jupiter, and the decreased tidal forces which are substantially stronger on Io and Europa. Unlike Io and Europa, Ganymede and Callisto’s are comprised of much larger amounts of ice, and the decreased tidal forces from Jupiter has enabled both moons to retain a primarily icy state, unlike the more active Io and Europa.

“Io has long been seen as a moon that lost its water later in life,” Mousis explains. “But when we put that idea to the test, the physics just refuses to cooperate: Io simply can’t get rid of its water that efficiently.” For that matter, Europa would not lose its water either, even under extreme conditions. The findings indicate that Io and Europa were already fundamentally different at birth, Io forming from dry materials and Europa accreting from ice-rich building blocks. The tidal forces enacted on Io and Europa come from the constant stretching and compressing that both moons experience as they orbit the much larger Jupiter, with their orbits being somewhat elliptical (oval-shaped). When they are closer to Jupiter, they are stretched by its massive gravity and are subsequently compressed as they travel farther away from Jupiter. This phenomenon is also called tidal flexing, and it results in the interiors of both moons heating up from friction over great timescales, with Io being volcanically active and Europa containing a large body of salty liquid water.

“The simplest explanation turns out to be the right one,” Mousis said. “Io was born dry, Europa was born wet, and no amount of late-stage evolution can change that.” These models indicate that the compositional contrast between Io and Europa is not the outcome of subsequent evolution, but rather the direct legacy of the primordial environment surrounding Jupiter at the time its moons formed. These conclusions challenge the long-standing assumption that Io’s high-density makeup resulted from a massive loss of volatiles after its formation. In the end, the researchers proposed that instead of both moons forming with water and Io later losing its water, the Io initially formed without water while Europa formed with water. Essentially, the present-day environments on both moons were established during their initial respective formations. This study comes as NASA’s Europa Clipper is currently en route to Europa to study its potential habitability, with an estimated arrival date of April 2030. During its scheduled 4-year mission, Clipper is slated to conduct approximately 50 close flybys of Europa using elongated orbits. The reason for this is so the spacecraft doesn’t stay in Jupiter’s massive magnetic field for too long, and its intense radiation could damage the spacecraft components and compromise the mission.

What new insight into Io and Europa’s formation and evolution will researchers make in the coming years and decades? Only time will tell. Beginning in 2031, NASA’s Europa Clipper mission and the European Space Agency’s Juice mission will study Jupiter’s large moons, providing critical new data to further test these conclusions. In particular, sampling plumes of water ice expected to be erupting from cracks in Europa’s icy surface will provide historical context. “By probing plume activity and the isotopic fingerprints of water, these missions will help us reconstruct the early conditions of Jovian moon formation,” Mousis said.