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- Title
Birth and Decline of Magma Oceans in Planetesimals: 1. Experimental Study of Erosion and Deposition of Particles in an Internally Heated Convecting Fluid.
- Authors
Sturtz, Cyril; Limare, Angela; Tait, Stephen; Kaminski, Édouard
- Abstract
This paper is the first of two companion papers presenting a theoretical and experimental study of the evolution of crystallizing magma oceans in planetesimals. We aim to understand the behavior of crystals formed in a convective magma ocean, and the implications of crystal segregation for the thermal and structural evolution of the convective system. In particular, we wish to constrain the possibility to form and preserve cumulates and/or flotation crusts by sedimentation or flotation of crystals respectively. We use lab‐scale analog experiments to study the stability and the erosion of a floating lid composed of plastics beads lying over a convective viscous fluid volumetrically heated by microwave absorption. We propose a law for erosion and re‐entrainment that depends only on two dimensionless numbers that govern these phenomena: (a) the Rayleigh‐Roberts number, characterizing the vigor of convection and (b) the Shields number, that encompasses the physics of the flow‐particle interaction. We further consider the formation of a cumulate at the base of the convective layer by sedimentation of beads that are denser than the fluid. We find that particle deposition occurs at a velocity that scales with the Stokes velocity, a result consistent with previous experimental studies. We build up a model that describes the transient evolution of the convective system's thermal state and the fraction of particles that segregated from the flow or that remain in suspension. Plain Language Summary: At early times in planetary formation, the heat supplied by 26Al was large enough to generate massive melting episodes in planetesimals and to produce magma oceans. Following melting and iron core differentiation, the proto‐mantle of these 10–100's km radius rocky bodies was a magma ocean that behaved like a convective fluid bearing solid crystals. The goal of this study is to understand the crystal‐melt segregation in such a magma ocean. In order to characterize the behavior of crystals in a convective flow we build a theoretical model based on lab‐scale experiments. We use a viscous fluid heated by microwave absorption to simulate the convection regime expected in magma oceans. We use plastic beads as analog of crystals in the experiments and we study the formation of either a flotation lid or a sedimented basal cumulate. We first demonstrate that both types of deposit form only if the buoyancy of the particles balances the shear imposed by convection, we then propose a complete model to account for the transient evolution. Key Points: We study experimentally the transient behavior of a convective fluid bearing particles that can float or sediment to form depositsThe erosion process is described by a modified Shields number that compares the convective shear to the buoyancy of the beadsThe deposition rate scales with the Stokes velocity of beads in the absence of convection
- Subjects
EROSION; PLANETESIMALS; MAGMAS; HEAT radiation &; absorption; CONVECTIVE flow
- Publication
Journal of Geophysical Research. Planets, 2022, Vol 127, Issue 12, p1
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2021JE007000