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Basal magma oceans develop in Earth and Venus after accretion as their mantles solidify from the middle outward. Fractional crystallization of the basal mantle is buffered by the core and radiogenic and latent heat in the magma ocean. Previous studies showed that Earth's basal magma ocean would have solidified after two or three billion years. Venus has a relatively hot interior that cools slowly in the absence of plate tectonics, which reduces heat flow through the solid mantle. Consequentially, the basal magma ocean could remain as thick as similar to 200-400 km today. Vigorous convection of liquid silicates could power a global magnetic field until recently while a core-hosted dynamo is suppressed. The basal magma ocean may be a hidden reservoir of potassium and other incompatible elements. A high tidal Love number could reveal a basal magma ocean and would definitively establish that the core is at least partially liquid.

Plain Language Summary Venus is Earth's nearest neighbor but arguably the least-studied planet in the inner solar system. Although there are no direct constraints on its deep structure, the mantle of Venus is assumedly solid by analogy to Earth's current condition. However, recent models of Earth focus on the prospect that a thick layer of melt called a "basal magma ocean" persisted in the lowermost mantle for billions of years. This layer cools orders of magnitude more slowly than a magma ocean near the surface because the solid mantle acts as an similar to 3,000-km-thick blanket. Moreover, the solid mantle itself remains hot in Venus compared to Earth because its surface is scorched and desiccated. This study argues that the lifetime of the basal magma ocean in Venus plausibly extends to the present. Detecting a thick, molten layer with future spacecraft missions would support the hypothesis that Venus and Earth formed under similarly energetic conditions.