The magnetic field of the Earth is produced by the magnetic dynamo. What conditions must a rocky body possess in order to exhibit the dynamo?
Clearly, the planet's interior must not be too cold. However, Venus for example, according to Wikipedia, also lacks the dynamo although its interior is hot:
The lack of an intrinsic magnetic field at Venus was surprising given it is similar to Earth in size, and was expected also to contain a dynamo at its core. A dynamo requires three things: a conducting liquid, rotation, and convection. The core is thought to be electrically conductive and, although its rotation is often thought to be too slow, simulations show it is adequate to produce a dynamo. This implies the dynamo is missing because of a lack of convection in the Venusian core.
They offer explanation in which either:
- Lack of plate tectonics reduces heat gradient and prevents convention in mantle.
- Inner core is solidified, missing or lacking temperature gradient.
I am interested in more complete list of conditions that are necessary, like for example in this answer about plate tectonics.