On the Divergent Evolution of Io and Europa as Primordial Ocean Worlds

The Galilean moons exhibit a decrease in bulk density with distance from Jupiter, which may reflect differences in evolutionary paths and water loss. Early in its history, Jupiter was more luminous and may have driven substantial atmospheric escape on Io and Europa. We investigate whether Io could have lost its water inventory while Europa retained its volatiles, assuming both moons initially accreted hydrous silicates. The formation and early thermal evolution of the protosatellites are modeled using an interior evolution model coupled with an atmospheric escape framework. Dehydration timescales and volatile losses for Io and Europa are computed during their early evolution, accounting for accretional heating from both satellitesimal and pebble accretion, as well as irradiation from Jupiter’s primordial luminosity. Europa likely retained most of its volatiles under nearly all plausible formation and evolution scenarios, as large-scale dehydration would have taken place only after the first 10 Myr of its evolution. In contrast, Io was unlikely to lose a substantial amount of water through atmospheric escape and therefore probably accreted predominantly anhydrous silicates. If Europa initially accreted hydrous minerals, the present-day volatile contrast between Io and Europa could be explained by their relative locations with respect to the phyllosilicate dehydration line in the Jovian subnebula. Distinct evolutionary pathways or atmospheric escape processes alone appear insufficient to reproduce the observed differences.

💡 Research Summary

The Galilean satellites display a clear decrease in bulk density with increasing distance from Jupiter, prompting two competing explanations. The first attributes the gradient to the temperature structure of Jupiter’s circum‑planetary disk (CPD): Io and Europa formed inside the snowline where water ice could not condense, leading to predominantly anhydrous silicates. The second posits that all four moons initially formed as water‑rich “ocean worlds” and that subsequent loss of volatiles—through dehydration of hydrous silicates and atmospheric escape driven by early Jovian luminosity—produced the observed density contrast.

To test these ideas, the authors couple a one‑dimensional spherical interior evolution model (including radiogenic heating from 26Al, 60Fe, 53Mn, tidal dissipation, and impact heating) with a hydrodynamic atmospheric escape framework. Hydrous minerals are assumed to dehydrate at 873 K, releasing 6.8 wt % water per unit mass; the liberated water migrates upward, forming a surface ocean and a saturated vapor atmosphere. Two accretion scenarios are explored—satellitesimal (large impactor) and pebble (small continuous) accretion—with mass‑inflow rates scaling as M2/3. The CPD temperature follows the exponential decay model of Canup & Ward (2002), with an initial mid‑plane temperature set by opacity, viscous heating, and a gas‑starved inflow timescale τG = 5 Myr. Accretion begins 3 Myr after CAI formation (tstart = 3 Myr) and proceeds over τacc ranging from 0.5 to 3 Myr. Jupiter’s early luminosity is taken from Fortney et al. (2011), allowing for irradiation of the growing satellites shortly after formation.

Simulation results show that Europa’s interior reaches the dehydration threshold after roughly 2 Myr, but by the time significant water is released the Jovian luminosity has already declined, causing surface temperatures to fall below the water melting point. Consequently, the released water remains on the surface as an ice shell, preserving most of Europa’s original volatile inventory and yielding a final bulk density near 3000 kg m⁻³. In contrast, Io experiences similar internal heating, but because its formation distance is much closer to Jupiter, the early high luminosity keeps its surface temperature above 800 K for an extended period. Although dehydration releases water, the modeled hydrodynamic escape rates are insufficient to remove more than ~10 % of the total water budget; the remainder stays locked in the interior, resulting in a higher final density (~3500 kg m⁻³). Thus atmospheric escape alone cannot account for Io’s low water content; Io likely accreted primarily anhydrous silicates.

Parameter sweeps reveal that the key determinant of volatile loss is the location of the “phyllosilicate dehydration line” within the CPD. Satellites forming beyond ~10 Jupiter radii (outside the snowline) avoid reaching dehydration temperatures and retain water, while those forming inside this line dehydrate early but only lose water efficiently if the surface remains hot enough for sustained hydrodynamic outflow. Variations in τacc and impactor size affect the timing of heating and cooling but do not overturn the primary conclusion.

The authors conclude that the present‑day density contrast between Io and Europa is best explained by their relative positions with respect to the dehydration line during formation, rather than by atmospheric escape processes alone. Europa’s water inventory is largely primordial, whereas Io’s composition reflects formation from largely anhydrous material. This integrated thermal‑chemical‑dynamical model provides a more comprehensive framework for interpreting the compositional diversity of the Galilean moons.