Role of carbon in the interior structure of Jupiter’s moons Europa and Io

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 believed to have accreted in a similar environment in the circumjovian disk [1]. In addition, their thermo-chemical evolutions are modulated by the nature of the accreted materials [2], and tidal heating forces induced by Jupiter and the Laplace resonance. Thus, 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 as a whole. Previous studies attempted to conciliate the internal structure of Io and Europa with their chemical composition in order to constrain the origin of the accreted materials. The relatively low density of their rocky interior suggested that Europa and Io may be depleted in iron relative to the solar compositions and that iron-poor ordinary chondrites may be the most suitable accretionary materials [3], [4], [5]. More recent studies however preferred volatile-poor carbonaceous chondrites for Europa [6], which highlights the lack of consensus. However, these studies do not simultaneously consider Io and Europa while the comparison between the two bodies is necessary to better assess their chemical composition and origin. Organic matter has been proposed to be incorporated in large amounts in the interior of large icy moons like Ganymede and Titan [7], [8]. Organic matter may also constitute a significant amount of Europa’s interior [9]. 

In this study, the internal structure and chemical composition of Europa and Io are constrained through a joint analysis. The method relies on a Monte-Carlo Markov Chains inversion scheme fitting the mass and moment of inertia of the two bodies [10], [11]. Using state-of-the-art equations of states for the densities of the metallic core, silicate mantle, and integration of the internal structure, elemental ratios Fe/Si & Mg/Si are computed and compared to that of chondrites. Two endmembers are used for the numerical modelling, with a carbon-free interior and another with a mantle incorporating a mass fraction of graphite. Different temperature profiles are also tested to take into account uncertainties on the present thermal-state of the two bodies. In the carbon-free scenario, the results show that only the elemental ratios of iron-poor L/LL chondrites are reached for both Europa and Io, which is consistent with the conclusions of the aforementioned studies. These chondrites are, however, almost water-free and thus cannot explain the hydrosphere of Europa on their own. With the addition of several weight% of graphite in the silicate mantle, elemental ratios of iron- volatile-rich carbonaceous chondrites, whose compositions are close to the solar photosphere, are reached. In the case of Io, the amount of graphite is systematically higher than the bulk carbon content of carbonaceous chondrites for any temperature profile. For Europa, while the water content is systematically lower, the amount of graphite is strongly anti-correlated to the thickness of the hydrosphere. This suggests that Europa and Io have accreted from materials enriched in refractory organic compounds and reduced in water relative to carbonaceous chondrites. This favors an accretion scenario where ice and organic rich pebbles are delivered from the outer solar system and are progressively ablated the more they move towards Jupiter, explaining the volatile gradient observed in the Galilean satellites [1], [12]. The results further support the idea that carbon under the form of organic matter is a major component in the bulk composition of outer solar system objects and may have strongly affected the chemistry of Europa’s ocean. 

 

References

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Acknowledgments 

This study has been co-funded by the European Union (ERC, PROMISES, project #101054470). Views and opinions expressed are however those of the author(s) 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.