Venus has no magnetic field...but Venus is also a planet..Why?
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The Earth's magnetic field is formed by an internal "dynamo" inside the core. A dynamo requires the following: a conducting liquid, rotation, and convection. The rotation of Venus may be slow but simulations showed it was enough to produce a dynamo. Some possibility are that Venus doesn't have a solid inner core like the Earth, or that it's core isn't cooling, or it's core has completely solidified.