Thermal and magnetic evolution of an Earth-like planet with a basal magma ocean

Victor Lherm, Miki Nakajima, Eric G. Blackman
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Abstract

Earth's geodynamo has operated for over 3.5 billion years. The magnetic field is currently powered by thermocompositional convection in the outer core, which involves the release of light elements and latent heat as the inner core solidifies. However, since the inner core nucleated no more than 1.5 billion years ago, the early dynamo could not rely on these buoyancy sources. Given recent estimates of the thermal conductivity of the outer core, an alternative mechanism may be required to sustain the geodynamo prior to nucleation of the inner core. One possibility is a silicate dynamo operating in a long-lived basal magma ocean. Here, we investigate the structural, thermal, buoyancy, and magnetic evolution of an Earth-like terrestrial planet. Using modern equations of state and melting curves, we include a time-dependent parameterization of the compositional evolution of an iron-rich basal magma ocean. We combine an internal structure integration of the planet with energy budgets in a coupled core, basal magma ocean, and mantle system. We determine the thermocompositional convective stability of the core and the basal magma ocean, and assess their respective dynamo activity using entropy budgets and magnetic Reynolds numbers. Our conservative nominal model predicts a transient basal magma ocean dynamo followed by a core dynamo after 1 billion years. The model is sensitive to several parameters, including the initial temperature of the core-mantle boundary, the parameterization of mantle convection, the composition of the basal magma ocean, the radiogenic content of the planet, as well as convective velocity and magnetic scaling laws. We use the nominal model to constrain the range of basal magma ocean electrical conductivity and core thermal conductivity that sustain a dynamo.
具有基底岩浆海洋的类地行星的热演化和磁演化
地球的地球动力已经运行了 35 亿多年。磁场目前由外核的热合成对流提供动力,这涉及内核凝固时轻元素和潜热的释放。然而,由于内核成核的时间不超过 15 亿年,早期的发电机不可能依靠这些浮力源。鉴于最近对外核热传导率的估计,在内核成核之前,可能需要另一种机制来维持地球动力。一种可能是在长寿命基底岩浆海洋中运行的硅酸盐动力。在这里,我们研究了类地行星的结构、热、浮力和磁力演化。利用现代状态方程和熔融曲线,我们对富含铁的基底岩浆洋的成分演变进行了随时间变化的参数化。我们将行星的内部结构整合与耦合核、基底岩浆海洋和地幔系统的能量预算结合起来。我们确定了地核和基底岩浆洋的热构成对流稳定性,并利用熵预算和磁雷诺数评估了它们各自的动力活动。我们保守的名义模型预测,在10亿年后会出现瞬时的基底岩浆洋动力活动,然后是地核动力活动。该模型对几个参数很敏感,包括地核-地幔边界的初始温度、地幔对流的参数化、基底岩浆洋的组成、行星的放射性含量以及对流速度和磁力缩放定律。我们利用名义模型来约束维持发电机的基底岩浆洋导电率和地核热传导率的范围。
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