Interior models imply the presence of carbon inside Io and Europa

Abstract

The Galilean moons of Jupiter have been of great interest over the past years and are the targets of the JUICE and Europa Clipper missions. Europa is an ocean world that may potentially be suitable for hosting life, while Io is the most volcanically active world of the solar system. Despite their apparent differences, Europa and Io are expected to have accreted in a similar environment in the circumjovian disk [1]. Determining the interior structure, chemical composition and thermal evolution of Europa and Io is crucial to understanding the origin and the history of the Jovian system and assess the habitability of Europa’s subsurface ocean. In previous studies, the low bulk density of icy moons and dwarf planets was classically associated with low Fe/Si ratios [2], [3], [4]. More recent studies motivated by space mission observations suggested that carbonaceous organic matter (COM) may be present in significant amounts in most planetary objects of the outer solar system [5], [6]. Carbonaceous matter in the rocky interior could indeed decrease the mean bulk density and thus explain the values observed for Io and Europa. The presence of COM within these bodies may have considerable implications on their bulk composition, internal structure and their thermo-chemical evolution.

In this study, the interior structure and composition of Io and Europa are modeled through a joint analysis using a MCMC scheme constrained on the mass, radius and Moment-of-Inertia factor retrieved during the Galileo mission [7], [8]. Density profiles are calculated using state-of-the-art equations of state for a large range of Fe/Si and Mg/Si ratios. We show that Io and Europa can have Fe/Si and Mg/Si ratios comparable to the solar composition only if a low-density component here taken as graphite is considered. The graphite content in Io's and Europa's refractory interior is estimated between 3-8 wt% and 1-22 wt%, respectively, which can exceed up to a factor five the typical carbon content in carbonaceous chondrites. The amount of graphite within Europa is anti-correlated with the thickness of the hydrosphere and positively correlated with the radius of the inner metallic core. A hydrosphere thickness larger than ~125 km would suggest an undifferentiated rocky core with low carbon fractions while a thinner hydrosphere (<125 km) would be indicator of a differentiated interior into an inner metallic core and a rocky mantle containing more than 5 wt% of graphite. Thermal metamorphism of the accreted carbonaceous matter in Europa leads to its graphitization and release of water and volatiles, which may contribute to a significant, possibly dominant fraction of its hydrosphere. Thermal degradation of carbonaceous matter may thus be a major contributor to the water and volatile budget of Europa. Future gravimetric, altimetric and magnetic measurements by Europa Clipper will refine the Moment-of-Inertia factor and determine the hydrosphere thickness. Such information are crucial to determine carbon content and differentiation state of Europa and hence the thermo-chemical evolution of Europa and the habitability of its subsurface ocean. The SUDA dust analyzer on board Europa Clipper will also provide constraints on the composition of Europa's ice grains and volcanic dust ejected by Io, thus allowing testing of the potential contribution of carbon to their bulk composition.

 

References

[1] Estrada, P.R. et al. Formation of Jupiter and conditions for accretion of the Galilean satellites. In: Pappalardo, R.T., McKinnon, W.B., Khurana, K. (eds.) Europa, pp. 27–58. University of Arizona Press, Tucson, AZ (2009)

[2] Sohl et al. (2002). Implications from Galileo observations on the interior structure and chemistry of the Galilean satellites. Icarus, 157:104-119.

[3] Kuskov, O. L. and Kronrod, V. A. (2001). Core sizes and internal structure of Earth’s and Jupiter's satellites. Icarus, 151(2):204–227.

[4] Petricca, F. et al. (2025). Partial differentiation of Europa and implications for the origin of materials in the Jupiter system. Nature Astronomy, pages 1–11. doi:  https://doi.org/10.1038/s41550-024-02469-4

[5] Néri, A. et al. (2020). A carbonaceous chondrite and cometary origin for icy moons of Jupiter and Saturn. Earth and Planetary Science Letters, 530:115920. doi: https://doi.org/10.1016/j.epsl.2019.115920

[6] Reynard, B. and Sotin, C. (2023). Carbon-rich icy moons and dwarf planets. Earth and Planetary Science Letters, 612:118172. doi: https://doi.org/10.1016/j.epsl.2023.118172

[7] Anderson, J. et al.  (1998). Europa’s differentiated internal structure: Inferences from four Galileo encounters. Science, 281(5385):2019–2022.

[8] Casajus, L. et al. (2021). Updated europa gravity field and interior structure from a reanalysis of Galileo tracking data. Icarus, 358:114187. doi: https://doi.org/10.1016/j.icarus.2020.114187

 

Acknowledgments

This work was supported by the Agence Nationale de la Recherche (ANR, project OSSO-BUCO, ANR-23-CE49-0003 to BR) and by the European Union (ERC, PROMISES, project #101054470 to CS). Views and opinions expressed are, however, those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. This research utilized the resources of the GLiCID Computing Facility (Ligerien Group for Intensive Distributed Computing, www.glicid.fr, Pays de la Loire, France). The work of M.B. was supported by the Czech Science Foundation (project No. 26-21877S). GT benefits from financial support from CNES for the preparation of the NASA Europa Clipper and ESA JUICE missions, for his participation to the Clipper/SUDA investigation, and the JUICE/3GM and MAJIS investigations.