Why Europa Stayed Wet While Io Dried Out: Tracing the Early Lives of Jupiter’s Inner Moons
The paper investigates a long-standing puzzle in planetary science: why Io, Jupiter’s closest large moon, is extremely dry and dense, while Europa, just one orbit farther out, contains a vast amount of water. Bennacer and collaborators start from the idea that all Galilean moons may have initially formed from similar materials, possibly including water-bearing (hydrous) rocks. The key question they address is whether differences in later evolution, especially heating and atmospheric escape, could explain the present-day contrast between Io and Europa, or whether the moons must have formed differently from the start.
Background and Competing Ideas
In the introduction, the authors lay out two main explanations that have been proposed previously. One possibility is that the moons formed in different temperature regions of Jupiter’s circumplanetary disk. In this picture, Io and Europa formed inside the snow line, where temperatures were too high for water ice to survive, while the outer moons formed in colder regions. The alternative hypothesis suggests that all the Galilean moons originally formed as water-rich “ocean worlds,” and that the inner moons later lost their water. For Io and Europa, this loss could have occurred through hydrodynamic atmospheric escape, where intense heating produces a thick water-vapor atmosphere that can flow away into space. Europa’s present ocean could then have formed later, after water was released from heated, hydrated rocks once Jupiter had cooled.
Modeling the Early Evolution
In the methods section, the authors describe how they test these ideas using detailed numerical models. They simulate the early thermal evolution of Io and Europa by combining an interior heat model with a model of atmospheric escape. The simulations track how heat builds up inside the growing moons due to radioactive decay, tidal heating from Jupiter’s gravity, and energy released during accretion. A key process is the dehydration of hydrous silicates: when temperatures exceed about 873 K, these minerals release their water. This released water is assumed to migrate to the surface, forming a temporary ocean and atmosphere that may then be lost to space. The authors also explore two different growth modes, accretion of small pebbles and accretion of large kilometer-sized satellitesimals, because these deliver heat to the interior in very different ways.
Results for Io and Europa
The results show a clear contrast between the two moons. Europa is found to be very good at retaining its water under nearly all plausible formation scenarios. In the models, large-scale dehydration of Europa’s interior typically occurs only after the first ten million years of evolution. By that time, Jupiter’s luminosity has declined enough that atmospheric escape is inefficient, allowing Europa to keep most of its volatiles. Io, on the other hand, is much more difficult to dry out. Even when the authors assume conditions that strongly favor atmospheric escape, Io does not easily lose a large primordial water inventory. Only very specific scenarios, such as forming extremely close to Jupiter, accreting very rapidly, or being heated by large impactors, produce an Io-like, fully devolatilized outcome.
Sensitivity to Formation Conditions
The paper then examines how sensitive these results are to key parameters such as formation distance from Jupiter, accretion timescale, and impactor size. Europa retains volatiles across almost the entire tested parameter space, even if it formed near its current orbit. In contrast, reproducing Io’s high density requires fine-tuned conditions. The overlap between scenarios that dry out Io while leaving Europa wet is narrow, suggesting that it is unlikely both moons started with similar water contents and diverged solely because of later atmospheric escape.
Discussion and Conclusions
In the discussion, Bennacer and colleagues argue that the simplest explanation is that Io never accreted much water to begin with. They propose that Io formed interior to the phyllosilicate dehydration line in Jupiter’s circumplanetary disk, a region where water-bearing minerals were already dehydrated before they were incorporated into the moon. Europa, forming farther from Jupiter, could accrete hydrous minerals and later release their water to form its subsurface ocean. In this interpretation, the present-day contrast between Io and Europa reflects differences in the temperature structure of Jupiter’s disk during formation, rather than dramatically different evolutionary or atmospheric loss processes afterward. Future measurements from missions such as JUICE and Europa Clipper may help test this idea by measuring the chemical and isotopic properties of Europa’s water.
Source: Bennacer
