Powering fire and ice with iron: Strong metallic core dissipation within the Galilean satellites

UP-FRONT SUMMARY

Canonically, researchers explore the outermost viscoelastic shells of the Galilean satellites as the layers primarily responsible for the diurnal dissipation of gravitational potential energy stored within their eccentric orbits around Jupiter. In this study, we find that tidal heating within solid, inner iron cores may be a first-order control on the heat budget of the resonant Galilean satellites.

This has widespread implications for body evolution, sustained geologic activity within the core and rocky mantles of these worlds, episodic resurfacing, magmatism, and their future exploration. Understanding the role of metallic core dissipation is key to understanding the habitability of Europa and Ganymede, the potential for a magma ocean on Io, and the history of Callisto. Future measurements by NASA’s Europa Clipper and ESA’s JUICE spacecraft will help to unravel the contributions of core dissipation to these global heat budgets.

DISSIPATIVE CORE PLAUSIBILITY

Observations of Io indicate a metal-rich core (e.g. Van Hoolst et al., 2020). At Europa, Schubert et al. (2009) infer that a metallic core is both permitted and likely, though a core is not uniquely required (Bills et al., 2009). Ganymede’s intrinsic magnetic field provides strong evidence for a metallic core (Schubert et al., 1996). Callisto, however, is likely only partially differentiated, with a dense rocky/metal core and a lighter silicate mantle (e.g. Bagenal et al., 2007). Figure 1 shows constraints on core state for Europa.

 

Figure 1: For an example Europa, Bulk density (thick) and Moment of Inertia (thin, dashed) constraints (Casajus et al., 2020). Colors show tidal heat flux at the CMB due to only core dissipation.

On Earth, there is a high plausibility that the solid inner core exhibits viscosities ranging from ~10¹³–10¹⁸ Pa s (Ritterbex and Tsuchiya, 2020), reflecting seismic (10¹³ Pa s), geodynamic (< 10¹⁶ Pa s; Buffett, 1997), and orbital constraints (~10¹³–10¹⁴ Pa s; Koot and Dumberry, 2011).

For the range of iron shear moduli applicable to the Galilean cores (~15–45 GPa; Birch, 1952), the viscosities that maximize dissipation are of the order ~10¹⁴–10¹⁶ Pa s. Newtonian creep likely dominates at the low pressures of the Galilean satellites cores (Van Orman, 2004), and pure iron viscosities (Birch, 1952) are in the range of 10¹⁰–10²⁰ Pa s, depending on temperature and grain size (Figure 2).

Figure 2: Pure iron rheology (Birch, 1952) as a function of temperature and grain size.

We estimate dissipated power from the tidal response of Io, Europa, and Ganymede using a Maxwell rheology (Figure 3). We find that core dissipation at Io and Europa can exceed that of their outer viscoelastic layers. At Ganymede, the heat flux contribution from the iron core is less than the radiogenic contribution. However, concentrated at the core-mantle boundary, the heat flux is likely sufficient to sustain continued or episodic melting of the metallic core.

Figure 3: (Left) contours of surface-averaged tidal heat flux as a function of layer viscosity. For Io, white lines indicate constraints from observed dissipation (Lainey et al., 2009).  (Right) Phase lag offset between h₂ and k₂.

ADDRESSING MYSTERIES / SEARCHING FOR ANSWERS

Io’s Magma Ocean: To match observed dissipation, Io’s average mantle viscosity should be ~10¹⁴–10¹³ Pa s (Segatz et al. 1988). However, these values are many orders of magnitude lower than the lowest inferred for Earth’s partially molten asthenosphere (~10¹⁹ Pa s; e.g. Dixon et al., 2004) and cannot support the mantle convection associated with high magma eruption temperatures (Moore et al., 2003). The presence of a pervasive “magma ocean” has been proposed to explain discrepancies (e.g. Khurana et al., 2011).

If Io experiences significant core heating, then a rock viscosity consistent with that of Earth’s mantle (10¹⁹ Pa s) and core viscosity of ~10¹⁵ Pa s can explain the observed heat flux without requiring a magma ocean or low-viscosity mantle (Figure 3).

Periodic Resurfacing and Dynamos at Europa and Ganymede: Europa’s young surface and Ganymede’s terrain dichotomy reflect episodic resurfacing. While dissipation within ice shell would allow rapid changes within those layers, heat conductively diffuses from the core slowly (~10⁰–10¹ Gyr). Rapid transport may occur through solid-state mantle convection and/or rock melting and ascent from the CMB (Figure 4). Additionally, core melting causes the tidal response to decouple from the solid body (Henning and Hurford, 2014), decimating dissipation.

The timescales for core melting and freezing, the onset and cessation of thermal convection, and the onset and ascent of melting at depth may compete to create an episodicity in seafloor heat flux, and may modulate the response to episodic changes to the orbital configurations of the satellites. Changes in heat flux may drive resurfacing through induced changes in the ice shell thickness and geodynamic state (Nimmo, 2004), and rotational coupling to the interior.

Callisto’s lack of evidence for resurfacing and sustained (un-forced) orbital eccentricity may reflect the absence of a dissipative iron core, and inefficiency of icy dissipation within the satellite.

Observational Predictions: If core dissipation is meaningful today, it may be observable in the phase lag difference between k₂and h₂ (Figure 1). In the absence of a phase lag offset, we predict that the present-day core may be partially molten, decoupling it from the solid body and decreasing dissipation, potentially producing an intrinsic magnetic field. If an iron core is responsible for Io’s observed high heat flux, we expect the real part of k₂ to reflect a solid body (~0.09) rather than a body with an internal fluid layer (~0.5) per Bierson and Nimmo (2016).

 

Figure 4: Europa’s (surface-averaged) seafloor heat flux (orange dashed line) responds to periodic changes in core dissipation (solid orange line). CMB temperatures (blue) vary from melting across the critical temperature for mantle convection. Heat transfer occurs through (a) diffusion, and (b) self-consistent melt migration and convection.

ACKNOWLEDGEMENTS

This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA, and supported through the NASA Habitable Worlds (80NM0018D0004), the Europa Clipper project, and the NASA Postdoctoral Program, administered by Oak Ridge Associated Universities under a contract with NASA (80HQTR21CA005).