1. A Critical Core Size for Dynamo Action at the Galilean Satellites.
- Author
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Trinh, K. T., Bierson, C. J., and O'Rourke, J. G.
- Subjects
- *
ELECTROMAGNETIC waves , *ELECTRIC generators , *LIQUID metals , *HEAT flux , *REYNOLDS number - Abstract
Ganymede is the only known moon with an active dynamo. No mission has discovered intrinsic magnetism at the other Galilean satellites: Io, Europa, and Callisto. A dynamo requires a large magnetic Reynolds number, which in turn demands, for these moons, a large metallic core that is cooling fast enough for convection. Here we quantify these requirements to construct a regime diagram for the Galilean satellites. We compute the internal heat fluxes that would sustain a dynamo over the wide ranges of plausible radii for their metallic cores. Below a critical radius, no plausible heat flux will sustain a dynamo. Europa likely sits on the opposite side of this limit than Ganymede and Io. We predict that future missions may confirm a small (or absent) core, meaning that Europa could not sustain a dynamo even if its interior were cooling as quickly as Ganymede's core. Plain Language Summary: In the late 1990s, the NASA Galileo spacecraft visited the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. Many studies interpret the Galileo gravity and magnetic field data to mean that: (a) Ganymede's intrinsic magnetic field arises from convection in a liquid metal core (i.e., a "dynamo" that converts mechanical energy into electromagnetic energy), (b) Europa has a metal core that is somehow incapable of generating a detectable magnetic field, and (c) Callisto may not have a metal core at all. Indeed, Ganymede is the only moon in our solar system known to sustain an active dynamo. Confirming the presence (or absence) of core convection provides us with valuable insight into the moons' structural and thermal history. In this study, we show that Europa's and Callisto's metal core (if they exist) may be too small to produce a dynamo, even if they convect. Crucially, no celestial body is known to have a convecting metal core without sustaining a dynamo too, but Europa might be an exception. Recently launched missions such as NASA's Europa Clipper and ESA's JUICE may test whether a small core size can explain the absence of dynamos at Europa and Callisto. Key Points: Dynamos in the Galilean moons would require both a dynamo‐producing region thicker than ∼250 km and core heat fluxes above ∼3 mW m−2Ganymede and Io probably have larger metallic cores than Europa and Callisto, but all satellites likely have similar core compositionsA small core could explain the absence of a strong dynamo at Europa even if Europa and Ganymede have similar core heat fluxes [ABSTRACT FROM AUTHOR]
- Published
- 2024
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