Two icy moons of Jupiter, though neighboring and of similar size, exhibit radically different internal structures. A new study suggests this dichotomy stems from their very formation, challenging current theories.
Callisto and Ganymede, two large icy moons of Jupiter, have puzzled scientists for decades. Although they are neighbors and comparable in size, their internal structures appear strikingly different.
Artist's view of the interiors of Ganymede and Callisto. Ganymede shows a layered, differentiated structure, while Callisto is thought to have a poorly differentiated interior, mixing rock and ice.
Data from the Galileo mission reveals that Ganymede, the largest moon in the Solar System, would have completely differentiated into a metallic core, rocky mantle, and icy crust. In contrast, Callisto would have retained a only partially differentiated internal structure, composed of a relatively homogeneous mixture of rock and ice.
A team including CNRS Terre & Univers researchers (see box) proposes a novel hypothesis: this divergence would have emerged during the formation phase itself, without the need to invoke major subsequent events.
By simulating accretion processes in the gas and dust disk surrounding Jupiter, the researchers developed a thermal evolution model accounting for all heat sources: radiogenic heating from short-lived radionuclides, impact heating during accretion, as well as thermal radiation from the circumjovian disk.
The study's results suggest the observed dichotomy between Ganymede and Callisto could be naturally explained by similar formation conditions, assuming identical composition and impactor size distribution.
Their simulations show Ganymede would have reached melting temperatures very early during its formation, while Callisto, formed in a colder region of the disk, would not have crossed the water ice melting threshold, though it may have incorporated a significant fraction of large impactors.
The study highlights that subtle differences, such as local disk temperature or orbital position relative to Jupiter, can suffice to explain radically distinct evolutionary paths.
Ganymede, more massive and formed closer to Jupiter, was exposed to more energetic impacts and a hotter environment, conditions sufficient to trigger global melting.
Callisto, accreted farther out in a colder region, thus preserved a poorly differentiated structure.
These findings challenge the dominant hypothesis that this dichotomy results from secondary processes, such as late bombardment or tidal effects linked to orbital resonances. The European JUICE mission, expected in the Jovian system in 2031, will play a key role in testing these hypotheses through high-precision gravitational measurements during its planned Callisto flybys.