A moon can sit next door to another and still feel like a different world.<\/p>\n
Around Jupiter, that contrast shows up fast. Io looks scorched and bone-dry, yet it is the most volcanically active moon in the solar system. Europa, just one orbit farther out, wears a shell of ice and is thought to hide a global ocean of liquid water beneath it.<\/p>\n
A new international modeling study co-led by Aix-Marseille University<\/a> and the Southwest Research Institute<\/a> argues that this split did not emerge later. The difference was there from the start, baked into the materials each moon accreted as they formed around Jupiter.<\/p>\n \u201cIo and Europa are next-door neighbors orbiting Jupiter, yet they look like they come from completely different families,\u201d said SwRI\u2019s Dr. Olivier Mousis, second author of an Astrophysical Journal paper detailing these findings. \u201cOur study shows that this contrast wasn\u2019t written over time \u2014 it was already there at birth.\u201d<\/p>\n A new international study co-led by Aix-Marseille University and Southwest Research Institute (SwRI) reveals that the striking contrast in the water contents of Jupiter\u2019s Galilean moons was established at birth, as they formed around the gas giant. Within Jupiter\u2019s 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. (CREDIT: SwRI) Two competing origin stories<\/p>\n For years, researchers have weighed two broad explanations for the Galilean moons<\/a>\u2019 water gradient.<\/p>\n One idea ties it to Jupiter\u2019s circumplanetary disk, the swirling material that fed the growing planet and its satellites. In that picture, temperature ruled everything. The inner region stayed too warm for ice to survive, so moons that formed closer in started out drier. Farther out, beyond the snowline, water ice could condense and accumulate.<\/p>\n A second idea flips the story. It suggests that all four large moons began water-rich, perhaps even as ocean worlds. Then the inner moons, especially Io, lost their volatiles later. Mechanisms proposed for that loss include hydrodynamic atmospheric escape during warm early phases, plus ongoing heating that could keep water in a vulnerable state.<\/p>\n The new study set out to test which story the physics supports for Io and Europa<\/a>. The team focused on a specific pathway for \u201cstarting water,\u201d rather than assuming piles of surface ice. They assumed water entered the young moons mainly through hydrated minerals, rocks that contain water bound in their structure.<\/p>\n Putting early Jupiter to work in a model<\/p>\n To replay the earliest stages, the researchers coupled two linked pieces of physics: the moons\u2019 internal thermal evolution and the loss of volatiles from the surface.<\/p>\n They accounted for major heat sources expected in the young Jovian system. Those included accretional heating during growth, radioactive decay, tidal dissipation, and Jupiter\u2019s intense radiation. As the models warmed the interior, hydrated minerals could dehydrate, releasing water that would migrate toward the surface and build a hydrosphere.<\/p>\n Simulation of an Io-like protosatellite. Left: evolution of the internal temperature profile. Top right: evolution of radius (red), mass loss M l (yellow) hydrosphere mass Mh (blue), and fluid masses released from hydrous minerals. Bottom right: evolution of density (red), surface temperature (black), and disk temperature (dotted). Blue dashed lines and labels A, B, and C mark the successive model phases. (CREDIT: The Astrophysical Journal) <\/p>\n Then the escape model took over. If surface temperatures rose enough to form a surface ocean, that ocean would feed a vapor atmosphere. Under low gravity and warm conditions, the atmosphere could flow off in a thermally driven, hydrodynamic wind<\/a>.<\/p>\n The study explored two end-member growth styles because the size of incoming material matters. In one, the moons accreted \u201cpebbles,\u201d smaller particles that deliver energy differently. In the other, they grew through kilometer-scale impactors, called satellitesimals, which can punch deeper and deposit more heat inside the body.<\/p>\n \u201cIo has long been seen as a moon that lost its water later in life,\u201d Mousis explains. \u201cBut when we put that idea to the test, the physics just refuses to cooperate: Io simply can\u2019t get rid of its water that efficiently.\u201d<\/p>\n When water refuses to leave<\/p>\n The models showed a stubborn result. Once a body like Io develops an established hydrosphere and then cools enough to freeze, removing the remaining water becomes extremely hard.<\/p>\n The work argues that neither tidal heating nor other late processes can efficiently eliminate a fully formed ice-rich shell over long timescales. The authors also note that sputtering by Jupiter\u2019s magnetospheric plasma<\/a>, while real today, would need to be vastly stronger to strip a meaningful fraction of a primordial ocean within about 10 million years. Their estimate says it would take sputtering rates far above what existing studies suggest.<\/p>\n Europa, in the same framework, also resists losing water. Even when the team pushed toward conditions that maximize escape, Europa generally retained volatiles under most scenarios.<\/p>\n Final density of protosatellites accreting pebbles as a function of formation distance a and accretion timescale. (CREDIT: The Astrophysical Journal) <\/p>\n That matters because it undercuts the simple \u201cboth started wet, one dried out\u201d narrative. If Io begins with substantial water, the model struggles to empty it. If Europa begins with substantial water, the model says it should still have most of it.<\/p>\n So the cleanest way to match what we see today is to start them different.<\/p>\n \u201cThe simplest explanation turns out to be the right one,\u201d Mousis said. \u201cIo was born dry, Europa was born wet \u2014 and no amount of late-stage evolution can change that.\u201d<\/p>\n A narrow window for a water-stripped Io<\/p>\n The paper does not claim Io could never lose volatiles. It outlines conditions that can reproduce Io\u2019s present density, but the window looks tight.<\/p>\n