Water makeup of Jupiter's Galilean moons

 A new study finds that birth conditions fixed water contrast on Jupiter's moons

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 an icy crust. A new international study co-led by Aix-Marseille University and Southwest Research Institute finds that this stark contrast in water content was imprinted at birth as the moons formed around Jupiter, rather than arising from later evolutionary processes. Study was co-led by Aix-Marseille University and Southwest Research Institute (SwRI) and reveals that the striking contrast in the water contents of Jupiter's Galilean moons was established at birth, as they formed around the gas giant. Within Jupiter's circumplanetary disk, hydrated materials forming Europa remained water-rich, while the same materials dried up when crossing the dehydration line before reaching Io, producing an intrinsically arid moon. 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. 

Since the first spacecraft explored the Jovian system in the late 1970s, scientists have recognized that Jupiter's large moons display strikingly different characteristics, with Io and Europa providing the most pronounced example. Io is a dry, intensely volcanic world, whereas Europa is ice-covered and widely believed to conceal a deep subsurface ocean of liquid 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. "Io and Europa are next-door neighbors orbiting Jupiter, yet they look like they come from completely different families," said Southwest Research Institute scientist Olivier Mousis, second author of the new study. "Our study shows that this contrast was not written over time; it was already there at birth."

To test these ideas, 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.  "Io has long been seen as a moon that lost its water later in life," Mousis said. "But when we put that idea to the test, the physics just refuses to cooperate: Io simply cannot get rid of its water that efficiently." The same modeling shows that Europa would also retain its water even under extreme conditions, contradicting the notion that subsequent evolution alone sculpted their different compositions. Since the first missions in the late 1970s, scientists have known that Jupiter's moons exhibit markedly different characteristics. Io and Europa provide the most striking example. "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 The Astrophysical Journal paper detailing these findings. "Our study shows that this contrast wasn't written over time, it was already there at birth."

According to the study, the compositional contrast between Io and Europa is therefore not the outcome of long-term volatile loss, but a direct legacy of the primordial environment in Jupiter's circumplanetary disk when its moons formed. Within that disk, hydrated materials destined for Europa remained water-rich, while similar materials crossing an inner dehydration line before reaching Io dried out and produced an intrinsically arid moon. The results indicate that Io and Europa were already fundamentally different at birth, with Io assembling from dry materials and Europa accreting from ice-rich building blocks. "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." 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. "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." 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.

"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 conclusions challenge the long-standing assumption that Io's high density reflects massive volatile loss after formation. Instead, they point to spatial variations in temperature and chemistry within the disk around young Jupiter as the key factor in setting the initial water budgets of its large moons. Upcoming missions will provide critical tests of this new picture. 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.  NASA's Europa Clipper mission and the European Space Agency's Juice mission will explore Jupiter's large moons in detail, examining their interiors, surfaces and space environments. By probing plume activity and the isotopic fingerprints of water in material erupting from fractures in Europa's icy shell, these spacecraft are expected to offer powerful clues to the early conditions of Jovian moon formation and further refine models of how Io and Europa diverged as primordial ocean worlds.