Global constraints to the properties of convection-driven magnetic fields in Super Earths

Physics

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[5440] Planetary Sciences: Solid Surface Planets / Magnetic Fields And Magnetism

Scientific paper

Starting from numerical models of planetary interiors of Super Earths (Valencia et al. 2006, 2007), i.e. rocky planets with composition and interior structure similar to the Earth, and using empirical scaling laws for numerical dynamos (Olson & Christensen, 2006), we develop global relationships between the properties of an hypothetical liquid iron core in planets with different masses and rotation rates and the critical parameters of a convection-driven planetary dynamo. The obtained global scaling laws are independent of particular details of the iron core physics and its thermal history, that, although very important in the determination of the magnetic dynamo properties (Gaidos et al. 2010), are presently very uncertain. We discover that although Super Earths with an arbitrary mass can develop a core convection-driven magnetic field at some point in its thermal history (modulo very uncertain factors that could avoid the development of an external liquid core in the hole history of the planet), the regime of the magnetic field will be constrained by the planetary mass and its rotational velocity. In a particular case planets with rotation rates similar to the Earth could have dipole dominant fields if their masses are not larger than 2 terrestrial masses. Beyond that limit magnetic fields are developed at some point but are always multipolar in nature (see figure 1). The maximum mass to develop an intense dipolar field that could be able to protect the atmosphere against the action of the stellar wind, strongly dependens on the rotation period, a relationship that it is translated on field regime regions in a mass-rotation diagram. Assuming extreme values for parameters that depends on the unknown planetary thermal history we compute regime regions in the mass-rotation diagram that allow us to globally constrain the masses of Super Earths that could generate magnetic fields compatible with an atmosphere and therefore with life.

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