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Fig. 9 | Progress in Earth and Planetary Science

Fig. 9

From: Iron snow, crystal floats, and inner-core growth: modes of core solidification and implications for dynamos in terrestrial planets and moons

Fig. 9

Precession dynamo: the total power deposited into the lunar core. The solid line shows the total power deposited into the lunar core, P Σ, as a function of semimajor axis (bottom x axis) for a precession dynamo: continuous mechanical stirring caused by the differential motion, due to Earth-driven precession of the lunar spin axis, between the solid silicate mantle and the liquid core below. The adiabatic threshold value is marked as P th (dashed line). The dash-dotted line is for P Σ = 3 × 1011 W, which represents an estimate of the power available to drive the dynamo in the Earth. The time before the present is plotted at the top. There is 7 × 1028 J of energy available for dynamo generation between the time of the Cassini state transition (at a lunar semimajor axis of 34 R E) and the cessation of the dynamo (when the power decreases to below P th, i.e., ~2.8 Ga b.p.). The results suggest that a mechanically driven dynamo could persist for at least ~1.6 Gyr. The inset shows the geometry of the situation. The mean motion, n, of an orbit is related to the semimajor axis, a, through n = (GM e /a 3)0.5, where M e is the mass of Earth. Figure after Dwyer et al. (2011)

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