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

TL;DR

The paper investigates why Io and Europa show different water inventories by testing whether Io could have lost its water while Europa retained volatiles. It achieves this by integrating a 1D interior evolution model with a hydrodynamic atmospheric escape framework inside Jupiter's early circumplanetary disk, incorporating radiogenic heating, accretion heating, and Jupiter's primordial luminosity. The results suggest Europa likely retained most volatiles across plausible histories, whereas Io would require specific conditions (e.g., proximity to Jupiter, rapid accretion, or dominant large-impactor accretion) to achieve complete devolatilization; the observed contrast can be explained by formation location relative to the phyllosilicate dehydration line in the Jovian subnebula rather than solely by atmospheric escape. The findings support a scenario in which the Galilean moons experienced formation-location–dependent evolution, with implications for isotopic signatures and future JUICE/Europa Clipper constraints.

Abstract

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.

Figures (4)

• Figure 1: Simulation of an Io--like protosatellite forming at $a = 6~R_{\text{Jup}}$. The body accretes only pebbles, growing from an initial radius $R_0 = 100$ km (at $t_{\text{start}} = 3$ Myr) to a final radius $R_f = 2000$ km over $\tau_{\text{acc}} = 0.6$ Myr. Left: evolution of the internal temperature profile. Top right: evolution of radius (red), mass loss $M_{\text{l}}$ (yellow), hydrosphere mass $M_{\text{h}}$ (blue), and fluid masses released from hydrous minerals $M_{\text{f}}$ (blue, dotted), all normalized to the total water mass fraction $w M_{\text{tot}}$ with $M_{\text{tot}}$ the total mass accreted. 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: disk dissipation, dehydration of hydrous minerals, and the end of atmospheric mass loss.
• Figure 2: Final density (color scale) of protosatellites at the end of the simulation for $R_f = 2000$ km, shown as a function of formation distance $a$ (sampled every 0.5 $R_{\text{Jup}}$ and interpolated using cubic interpolation) and accretion timescale $\tau_{\text{acc}}$. Accretion starts at $t_{\text{start}} = 3$ Myr, from either pebbles (left) or satellitesimals (right), each with a density of 3000 kg m$^{-3}$. Dotted lines indicate the present orbits of Io and Europa for reference.
• Figure A1: Final density of protosatellites accreting pebbles as a function of formation distance $a$ and accretion timescale $\tau_{\text{acc}}$ with $t_{\text{start}} = 2.5$ Myr.
• Figure A2: Final density of protosatellites accreting satellitesimals as a function of formation distance $a$ and accretion timescale $\tau_{\text{acc}}$ with $t_{\text{start}} = 2.5$ Myr.