Partial Differentiation of Europa and Implications for the Origin of Materials in the Jupiter System

Academic Background

Europa, one of Jupiter’s icy moons, has long been considered one of the most promising candidates for extraterrestrial life in the solar system. Its surface is covered by a thick ice layer, beneath which a liquid water ocean may exist. The internal structure and evolutionary history of Europa are crucial for understanding its habitability. However, despite the valuable data on Europa’s gravity and magnetic fields provided by the Galileo mission, many uncertainties remain regarding its internal structure, particularly the degree of core differentiation, ice shell thickness, ocean depth, and mantle composition.

Previous studies based on Galileo’s gravity data suggested that Europa might have a differentiated metallic core. However, recent analyses indicate that the gravity field data may support the hypothesis of partial differentiation, implying that Europa may not have fully differentiated a distinct metallic core. This discovery raises new questions about Europa’s formation and evolution: Did Europa indeed undergo complete differentiation? What was the thermal evolution process in its interior? These questions directly impact our assessment of Europa’s habitability.

To address these questions, researchers combined geophysical, geochemical, and thermal evolution models to reanalyze the Galileo mission data, aiming to reveal Europa’s internal structure and evolutionary history. Additionally, the study explored the origin of Europa’s building materials, particularly its relationship with the early distribution of materials in the Jupiter system.

Source of the Paper

This paper was co-authored by Flavio Petricca, Julie C. Castillo-Rogez, Antonio Genova, Mohit Melwani Daswani, Marshall J. Styczinski, Corey J. Cochrane, and Steven D. Vance. The authors are affiliated with the Jet Propulsion Laboratory, California Institute of Technology; the Department of Mechanical and Aerospace Engineering, Sapienza University of Rome; and the Blue Marble Space Institute of Science. The paper was published online on December 11, 2024, in the journal Nature Astronomy, with the DOI 10.1038/s41550-024-02469-4.

Research Process

1. Data Collection and Gravity Field Analysis

The researchers first reanalyzed the radio science data from the Galileo mission, particularly Europa’s gravity field data. Using Bayesian inversion and Markov Chain Monte Carlo (MCMC) methods, they generated models of Europa’s internal structure. These models were based on Europa’s mass and moment of inertia (MOI), combined with geophysical and geochemical constraints.

2. Internal Structure Modeling

The researchers assumed a multi-layer internal structure for Europa, including a metallic core, rocky mantle, ocean, and ice shell. Using the MCMC method, they explored the possible ranges of parameters such as core radius, core density, ocean depth, and ice shell thickness. To describe the mantle structure more precisely, the mantle was divided into 50 equally spaced sublayers, and the pressure, temperature, and density of each sublayer were calculated using Perple_X software.

3. Thermal Evolution Modeling

To understand Europa’s thermal evolution history, the researchers developed a thermal conduction model, considering radiogenic decay and tidal heating as the primary heat sources. The model assumed that Europa underwent cold accretion after formation and simulated the temperature changes in the interior by tracking the thermal evolution of the rocky mantle.

4. Possibility of Metallic Core Differentiation

The researchers also explored the possibility of metallic core differentiation in Europa. By calculating the melting point of iron-iron sulfide (Fe-FeS) alloys and their melt volume under different pressures and temperatures, they assessed the conditions for metallic core differentiation. The results showed that the high-temperature regions in Europa’s interior were limited, making complete metallic core differentiation unlikely.

Key Findings

1. Europa’s Internal Structure

The results suggest that Europa’s internal structure may be partially differentiated, meaning its metallic core is small or absent. Based on the latest MOI data, Europa’s core radius may range from 49 to 269 kilometers, significantly smaller than the previous estimate of 425 kilometers. This finding aligns with the hypothesis of Europa’s cold evolution.

2. Mantle Composition

The researchers found that Europa’s rocky mantle has a low density of approximately 2,750 kg/m³. By comparing with different carbonaceous chondrites (CV, CI, CM) and cometary materials (e.g., 67P/Churyumov-Gerasimenko), they concluded that Europa’s primary building material may be CV-type chondrites.

3. Water Content

The study estimated Europa’s water mass fraction to be 7.4±1.3%, consistent with the expected range (7.0–9.0%). This suggests that part of Europa’s water inventory may have originated from external sources, such as comets.

4. Thermal Evolution

The thermal evolution model indicates that Europa’s interior temperature was relatively low, and the temperature conditions required for metallic core differentiation (>1,600 K) were not fully achieved during its evolution. Potassium leaching further limited the internal heating process.

Conclusion and Significance

This study reveals Europa’s internal structure and evolutionary history, particularly its partially differentiated nature. The results suggest that Europa may have undergone cold evolution, with a small or absent metallic core. Additionally, part of Europa’s water inventory may have originated from external sources, such as comets. These findings are significant for understanding Europa’s habitability.

Research Highlights

1. Partial Differentiation Hypothesis: The study proposes for the first time that Europa may be partially differentiated, challenging the previous assumption of complete differentiation.
2. Cold Evolution Model: Through thermal evolution modeling, the researchers revealed Europa’s cold evolutionary history, explaining the limitations of metallic core differentiation.
3. Material Origin Analysis: Using geochemical models, the study inferred that Europa’s primary building material is CV-type chondrites and explored the external sources of its water inventory.

Future Prospects

Future missions to Europa, such as Europa Clipper and JUICE, will provide more precise gravity and magnetic field data, further validating these findings. These missions will help us better understand Europa’s internal structure and its habitability.