Magnetic Field and Structure
Venus's crust appears to be 50 kilometres (31 mi) in thickness, and composed of silicate rocks. Venus's mantle is approximately 3,000 kilometres (1,900 mi) thick, but its composition is unknown. Since Venus is a terrestrial planet, it is presumed to have a core made of semisolid iron and nickel with a radius of approximately 3,000 kilometres (1,900 mi).
Pioneer Venus Orbiter data indicates that Venus does not have a significant magnetic field. The magnetic field of a planet is produced by a dynamo in its core. A dynamo requires a conducting liquid, rotation, and convection. Venus is thought to have an electrically conductive core, and although its rotation period is very long (243.7 Earth days), simulations show that this is adequate to produce a dynamo (Stevenson 2003). This implies that Venus lacks convection in its core. Convection occurs when there is a large difference in temperature between the inner and outer part of the core, but since Venus has no plate tectonics to let off heat, it is possible that it has no inner core, or that its core is not currently cooling.
Read more about this topic: Geology Of Venus
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