Seven decades of exploring planetary interiors with rotating convection experiments

Abstract

The interiors of all the planets in the solar system consist of layers, most of which are made out of fluids. When these layers are subject to superadiabatic temperature or compositional gradients, turbulent convection takes place that transports heat and momentum. In addition, planets are fast rotators. Thus, the key process that underpins planetary evolution, the existence of dynamo action or lack thereof, the observable flow patterns, and much more, is rotating convection. Because planetary interiors are remote and inaccessible to direct observation, experiments offer crucial, physically consistent models capable of guiding our understanding and complementing numerical simulations. If we can fully understand the fluid dynamics of the laboratory model, we may eventually fully understand the original. Experimentally reproducing rotating thermal convection relevant to planetary interiors comes with very specific challenges, in particular, modelling the central gravity field of a planet that is parallel to the temperature gradient. Three distinct classes of experiments have been developed to tackle this challenge. One approach consists of using an alternative central force field such as the electric one. This comes with the caveat that these forces are typically weaker than gravity and require going to space. Another method entails rotating the device fast enough so that the centrifugal force exceeds and effectively supersedes Earth’s gravity. This mimics the equatorial and lower latitude regions of a planet. Lastly, insight into the polar and higher latitude regions is gained by using the actual lab gravity aligned with the rotation axis. These experiments have been continuously refined during the past seven decades. Here, we review their evolution, from the early days of visualising the onset patterns of convection, over central force field experiments in spacecraft, ultrasound velocity measurements in liquid metals, to the latest optical velocity mapping of rotating magnetoconvection in sulphuric acid inside high-field magnets. We show how innovative experimental design coupled with emerging experimental techniques has advanced our understanding of planetary interiors and helped us paint a more realistic, detailed picture of them, including Earth’s liquid metal outer core.

Keywords

• Rotating convection
• Experimental fluid mechanics
• Measurement techniques
• Planetary interiors
• Turbulent convection